AP Biology The Circulatory System

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We will be continuing our discussion of animal physiology with the circulatory system.0002

The purpose of the circulatory system is to deliver oxygen, hormones and nutrients to the cells of the body0010

and to remove wastes including CO₂ as well as other waste products from metabolic processes.0017

In simple animals such as sponges and jellies, all the cells are in contact with the external environment,0024

which means that a circulatory system is not necessary.0031

These cells can pick up nutrients and oxygen directly from the water, and these components can enter the cells via diffusion.0035

And then, waste products can exit into the water the same way.0045

Animals such as sponges and jellies have bodies that are only a couple of cell layers thick, so they are in contact with the water on the outside.0050

And then, on the inside of the body, you may recall, they have a gastrovascular cavity.0059

As the name suggests, it combines both vascular system functions and GI tract functions and allows that inner layer of cells to be in contact with the water.0068

So, there is no heart, vessels, no circulatory fluid, no blood.0079

A similar idea occurs in flatworms. Flatworms, due to their very flat body structure, also have cells that are entirely in contact with the environment.0085

So, their bodies are in very close contact with the environment, and by diffusion, oxygen can enter.0097

However, most animals are larger and more complex than these, and they cannot exchange gas and nutrients directly with the environment0105

for every single cell in the body, because most of their cells are not even in direct contact with the environment.0114

In the section of respiration, we talked about gas exchange and the idea that specialized respiratory systems include structures like lungs and gills,0122

which provide a means of gas exchange.0132

However, the oxygen needs to be delivered to all the cells of the body, and some of these cells are distant from where gas exchange is taking place.0135

Gas exchange takes place in the alveolus of the lung. However, the alveolus is nowhere near your leg.0144

So, somehow, that oxygen needs to get to the leg or the arm or the brain throughout all the cells of the body.0153

Diffusion is one way that gases can move, but this would be far too slow.0160

Waiting for oxygen to diffuse down to your leg or brain would be so slow that the muscle or the brain cell would die waiting.0165

So, diffusion is much too slow. In order to bring large amounts of oxygen and nutrients to the cells quickly, the circulatory system evolved.0173

I have been focusing on oxygen, but recall that nutrients that are obtained by digestion in the GI system also need to be delivered to distant cells.0184

So, animals also require a circulatory system to carry out that function, and there are two general types of circulatory systems: open and closed.0194

Here is shown an insect as an example of an organism that has an open circulatory system.0206

And an open circulatory system means that the blood at one point leaves the vessels.0213

It does not mean that there are not any vessels. There often are some vessels, but the fluid is not contained within the vessels the entire time.0219

Here, this represents a pump, so a simple heart, and the pump, the heart, will move the circulatory fluid through vessels like an aorta.0229

From there, the circulatory vessels will branch out, and then, the fluid will be dumped out, released out, into cavities called sinuses.0246

So, these sinuses surround organs, so there will be an organ in here.0258

The respiratory, or excuse me, the circulatory fluid enters that cavity, and it bathes the organs and cells in this fluid.0264

Therefore, the organ can pick up nutrients and other substances from the fluid, and it can let waste products release them out into the fluid.0273

The fluid is, then, picked back up and returned to the heart.0287

In an open circulatory system, the fluid is called hemolymph. It is not blood, and hemolymph differs from blood in some fundamental ways.0292

In organisms with a closed - so, this is an open circulatory system - circulatory system, there are two types of fluid:0303

one in the vessels and one that bathes tissues and organs called interstitial fluid.0312

Here, there is only one type of fluid. They are one and the same.0318

It is called hemolymph.0321

The other thing is that frequently, hemolymph is not the means of delivering oxygen and picking up carbon dioxide.0322

Recall from the respiratory system, animals like insects have a tracheal system that they use to deliver oxygen directly to the cells of the body.0329

They do not take in the gas, give it to the circulatory fluid and have the circulatory fluid deliver the gas.0340

Usually, hemolymph is responsible for delivering nutrients and other substances, but it is often not the means of oxygen delivery or CO₂ transport.0348

This second type of system is the closed circulatory system, and here is an example of...here is an earthworm and annelid,0360

and that it will have a closed circulatory system, so most invertebrates have an open circulatory system.0371

There are exceptions such as annelids. Another exception is cephalopods have a closed circulatory system like squids and octopuses.0382

So, most invertebrates have an open system. There are some that have a closed system, and all vertebrates have a closed circulatory system.0398

In a closed circulatory system, blood remains within vessels the entire time.0409

The blood stays within the vessel. Here is the heart, blood vessel, artery, carrying blood away from the heart.0417

And these vessels, they branch out into capillaries, and oxygen and nutrients can diffuse across the walls of the capillary.0427

But, the blood does not leave the vessel. It stays in the vessel, and then, it is returned to the heart.0437

Therefore, in a closed circulatory system, the fluid within the vessels is the blood, and then, there is second type of fluid called interstitial fluid.0444

That is fluid that surrounds tissues in organs.0456

We are going to talk in depth now about mammalian circulation, again, with a focus on the human circulatory system.0463

I am going to start out talking about the different components of a circulatory system.0472

And the major components are vessels, some type of circulatory fluid - hemolymph in the open system, blood in the closed system - and a pump- the heart.0477

Starting out with blood vessels, there are three types of blood vessels that you need to know, and these are arteries, veins and capillaries.0493

Starting out with arteries, arteries pump blood or move blood away from the heart.0504

So, blood is pumped by the heart through the arteries, so they carry blood away from the heart, and they have thick muscular walls.0515

There are smooth muscles in the walls, and the walls are thick- thick-walled.0529

The blood within the arteries is under pressure. It is being pumped by the heart, so it is under a significant amount of pressure.0537

What happens is as the blood is pumped by the heart, it is under pressure when the heart contracts.0545

However, when the ventricles relax, the pressure decreases, but the thick walls of the artery prevent the pressure from the circulatory0553

system from dropping too low because once the pressure is decreased as the heart relaxes, the walls of the artery will spring back.0563

So, they are being pushed by the pressure that the blood is under, and then, they spring back.0573

And that recoil maintains the pressure within the arterial system.0579

Arteries branch into arterials and then, finally, capillaries.0585

The second type of vessel is the veins. Veins are carry blood towards the heart, so blood returning either from body tissues or from the lungs.0596

Most veins, therefore, carry deoxygenated blood. There is an exception, though.0623

By that same measure, most arteries carry oxygenated blood.0637

And I say most because there is an exception that we are going to talk about when we talk about the heart.0646

But, when you say "define an artery", it is part of the definition that it is carrying blood away from the heart.0652

Usually, it is oxygenated but not always because the blood carried by the pulmonary artery is going away from the heart.0659

It is going towards the lungs. Yet, it is deoxygenated.0670

That is why it is going to the lungs.0672

So, anyways, veins carry blood towards the heart, and it is usually, but not always, deoxygenated blood.0674

The walls of the veins are much thinner, relatively thin, compared to the arteries, and the blood in the veins is under a lower pressure than in the arteries.0681

Now, blood is returned to the heart in part by muscle contraction.0695

So, for example, the veins in your legs, when you walk or when you run, you move around.0700

The contraction of your leg muscles pushes your venous blood back up towards the heart.0704

And that is why if someone wants to improve their circulation, they need to walk around or why if somebody is bedridden,0711

they are immobile, they are at a higher risk for blood clots because the blood pools, which makes it tend to coagulate and cause a clot.0719

So, anyways, movement is important for circulation.0726

Because the venous blood is not under a lot pressure, veins contain valves.0730

And what the valves do is they prevent backflow, so it prevents the blood from draining back into your feet towards the0736

dependent lower parts of your body and helps keep blood flowing upward or towards the heart, flowing in the right direction.0745

It prevents backflow.0754

Finally, capillaries: capillaries are very small vessels, so they are very small.0757

And they have cell walls that are only a single cell thick, which allows for diffusion of nutrients and gases across the cell wall.0764

The diameter of capillaries is so small that red blood cells actually have to go through single file, so these are extremely vessels.0777

Some terms that you should understand relating to blood vessels are vasoconstriction and vasodilation.0793

Vasoconstriction refers to the constriction or narrowing of the blood vessels.0800

And this is the result of the contraction of the muscles in the walls of the artery.0813

This will result in an increase in blood pressure, so vasoconstriction increases blood pressure. The opposite is vasodilation.0819

In vasodilation, blood vessels open up. They become wider, so instead of constricting, they dilate, so dilation of vessels.0836

And this is going to decrease blood pressure.0853

For example, if somebody is bleeding, they lose a lot of their blood, their blood pressure will drop.0860

And one way to compensate for that is for the vessels to constrict, so vasoconstriction can maintain the blood pressure.0866

Vasoconstriction and vasodilation also play a role in thermoregulation.0873

Thermoregulation is the maintenance of a constant internal temperature.0879

Cold triggers vasoconstriction, so if the blood vessels constrict, their diameter becomes smaller, then, less blood will flow through these vessels.0886

So, what happens is, it is specifically the vasoconstriction of superficial vessels, near the surface of the body.0902

When the blood vessels constrict, there is a decrease of heat loss from these vessels.0913

In contrast, heat triggers vasodilation, so on a hot day, your blood vessels will dilate.0921

That is going to increase the blood flow and allowing for increased heat transfer.0934

So, heat will be lost to the environment through those dilated vessels near the surface of your skin.0943

Vasoconstriction, vasodilation, also play a role in thermoregulation.0948

So, we have talked about vessels. The second component of the circulatory system is the circulatory fluid.0955

In mammals, blood is the circulatory fluid, and it contains a fluid component and a cellular component.0962

The average adult has about 5 liters of blood.0976

So, beginning with the fluid component of blood, that is plasma, and it contains many substances:0991

gases, proteins, hormones, antibodies, waste products being carried away from cells, various components of the plasma.0999

This is the fluid component of blood.1013

The pH of blood is around 7.4, and it is in part maintained by buffers - the buffer system that we will talk about - in the plasma.1016

In addition, so I mentioned proteins, some of the proteins are in the plasma, we are talking about the pH at 7.4.1033

There is a buffer system.1040

There are also clotting factors in the plasma that allow a blood clot to form when someone becomes injured, when they are bleeding.1041

So, that is just an overview of the plasma. We will talk more in a minute about clotting factors and the clotting system.1054

The second component is the cellular component. So, there is the fluid component, which is plasma and cellular components.1063

The major cellular component is red blood cells, so the cellular component: red blood cells and white blood cells.1072

First, red blood cells: the other name for these is erythrocytes.1079

The shape is that these are biconcave discs, so they are, sort of, collapsed in - biconcave discs - and they lack a nucleus.1093

This makes more room for them to do their job. Their job is to transport oxygen, and they contain hemoglobin.1109

The more hemoglobin, the more oxygen they can transport, so by not having a nucleus, there is more room for hemoglobin.1120

Hemoglobin is a protein that contains heme groups, and within the heme groups is iron.1125

And it is the iron that allows hemoglobin to bind oxygen and then, to release the oxygen, to reversibly bind oxygen.1136

We will talk about the structure of hemoglobin and transfer of oxygen in detail.1144

Red blood cells only use anaerobic respiration, so they only undergo anaerobic respiration.1150

And it totally makes sense if you think about it because their job is to transport oxygen.1159

If they were undergoing aerobic respiration, they would actually end up using up the oxygen that they are trying to transport,1164

So, that they are not using up the oxygen, instead, they undergo anaerobic respiration only.1171

Red blood cells have a life span of about 120 days. They live 120 days, and they are produced in the bone marrow.1177

So, the job of the red blood cells is to pick up oxygen in the lungs as the oxygen diffuses from the gas-filled alveoli into the capillaries.1194

They pick up the oxygen. They transport the oxygen into the body tissues.1204

Then, they release the oxygen, and the oxygen diffuses in the body tissues.1208

They also have a role in the transport of CO₂, and we will talk about that later, as well.1213

White blood cells are the second cellular component.1220

So, we have the fluid component and the cellular components, which includes red blood cells and white blood cells.1223

White blood cells or WBCs are also called leukocytes.1230

Leukocytes are produced in the bone marrow, and we will investigate these in detail when we talk about the immune system.1235

So, they are produced in the bone marrow, and they have a role in immunity. Their job is to fight infection.1245

If a person has a low white count, they are immunocompromised. They are at risk for serious infections.1251

If a person has high white count, if you test their blood and you say "wow, they have a higher than normal number of white blood cells",1257

that can indicate that they have an infection and that their body is fighting that infection.1264

We raise the number of white blood cells to fight pathogens/invaders.1270

Finally, there is a component that is not cells but actually cell fragments, and these are platelets.1275

Platelets are actually fragments of cells. They are produced in the bone marrow also, and they have a very important function in blood clotting.1282

I am going to talk briefly now also, as we are discussing blood, about blood types.1297

You have probably heard of blood types, or you may even know your blood type: A, B, AB and O and blood types positive and negative.1304

What this is referring to is antigens on the surface of blood cells.1314

If I look at a cell, I look at a red blood cell, and then, I test it; and I say, "OK, it has a particular antigen on the surface",1319

let's call the A antigen for the A blood group, and it is shaped like this, so this person is type A blood.1332

There is another antigen, the B antigen, and the B antigen, let's say, is shaped like this. That person is type B.1342

Type AB is an individual whose cells produce both the A and the B antigens.1356

Finally, type O: O is the absence of the A and B. They have neither, so they are type O.1367

The second system referring to positive and negative is the Rh. It has to do with the Rh factor.1374

So, there is another protein called the Rh factor, and the Rh factor we will say is shaped like this.1380

And if a person has the Rh factor, they are positive, so this individual is A+. If they don't have the RH factor, they are B-.1390

If they have the RH factor, they are AB+. If they do not have the Rh factor, they are O-.1400

Each type can be positive or negative depending on if they have the Rh factor.1406

Now, this is very important when it comes to transfusions because a person's1412

immune system will recognize antigens that are not present in their own blood.1417

So, if somebody is type A and you put B blood or AB blood in their body, they are going to recognize this and attack it.1425

This A+ person, if I transfuse them with B-, they will lyse. They will destroy those cells.1436

If I transfuse them with AB+, they will destroy the cells because they see the B is a foreign protein/antigen.1443

And actually, they are positive, so they will recognize this. They will recognize the A, but they will attack this B.1456

Therefore, for this person, I could give them blood that is A+.1463

I could give them blood that is A- because they just will not have the RH factor. They will not attack because of that.1470

I could also give them O, either positive or a negative.1476

Let's look at this person type B. If I give them something they do not recognize as part of their own antigen system, they will attack it.1484

So, if I give them this A+, they are going to attack it because of the A, and they are going to attack it because of this Rh factor.1493

So, I cannot give them that.1502

If I give them AB, they are going to attack the A part, and if it is positive, they will also attack this. I could give them B-.1504

Could I give them B+? Well, B+, they would have B, and they would also have this Rh factor.1516

Their body would be OK with the B. They would recognize it, but their body will not recognize the Rh factor, so they will attack.1523

One thing is that if you look at O-, it does not have A on it. It does not have B on it, and it does not have the Rh factor.1535

So, it is blank as far as these major antigens. Therefore, O- is the universal donor.1544

I could give it to A, B, AB, positive, negative. There is nothing on here to attack for the major antigens, so therefore, it is the universal donor.1555

Looking at the other way around, no matter what I give AB+, they are going to be OK with it.1566

If I give them A, great, he recognizes the A here. He recognizes the positive.1573

He will not attack it because he has got it.1577

If I give him B- or B+, they will be fine with that. He recognizes it.1580

He has all three antigens, so these all are familiar to the body. They will not attack it.1584

Therefore, AB+ is the universal acceptor.1590

So, what you have on your blood, you recognize as yourself, and you will not attack. What you do not have, you will attack.1596

So, you cannot accept blood from somebody who has got antigens that are not on your own blood.1603

You can accept O-. These are not there, but you cannot accept something that is not on your own blood.1609

So, that is blood groups, and I will explain a little bit about how blood transfusions are done.1617

OK, I mentioned that there are clotting factors in the blood. Blood vessels are lined with epithelial cells.1626

And this lining of epithelial cells is called the endothelium, so the wall of the blood vessel is the endothelium.1634

If a blood vessel is injured, the blood vessel wall, if the endothelium is damaged, platelets are activated.1651

So, damage to the endothelium triggers or activates platelets.1659

When the platelets are activated, they aggregate, and this aggregation forms an initial plug or an initial clot, and this clot starts to staunch the bleeding.1677

This is a plug to decrease the bleeding, and this is just the initial clot.1700

The clot needs to be reinforced, though, by fibrin, and so, it is later reinforced by fibrin.1711

And the fibrin formation is the result of a cascade, so there is a series of events.1723

One enzyme activating another, activating another, that culminates in the formation of fibrin to reinforce that initial plug formed by the platelets.1729

You do not need to know the whole long complicated clotting cascade, but I am going to just talk about the last steps that are important steps.1740

So, if you jump in later in the cascade, one thing you will see is that prothrombin, which is inactive, becomes converted to thrombin.1749

And this is the active form, so I will put a star. That is the active form.1759

Thrombin, then, acts on fibrinogen, which is the inactive form of fibrin, and it activates fibrin. It converts it to its active form, which is fibrin.1766

That fibrin is a filamentous thread-type structure, and it clumps up to form a clot at the site of the damaged vessel wall.1784

It reinforces that initial clot formed by platelets.1793

Now, these are floating around in their inactive form, which prevents clotting from just occurring at random times, which would be disastrous.1797

However, there are also anti-clotting factors that circulate in the blood.1807

So, the clotting cascade does not just occur randomly. It has to be triggered.1813

And platelets release clotting factors. The damaged cells can release clotting factors, and there are clotting factors in the plasma.1818

A deficiency in clotting factors leads to a disorder known as hemophilia, so hemophilia is a result of a deficiency in a particular clotting factor.1828

And individuals with hemophilia are at risk for bleeding.1839

So, even a minor injury could be more serious in an individual with hemophilia because they are not able to stop the bleeding.1846

And they may need to be given these factors.1852

A thrombus is another name for a blood clot, so a thrombus is a blood clot.1856

And when we talk about the heart, we are going to talk about the role that blood clots can play in causing a heart attack.1861

Before we do that, let's talk about the final component of the circulatory system, which is the heart.1870

Amphibians and most reptiles have a three-chambered heart. Mammals and birds have a four-chambered heart, and that is what you see here.1880

It consists of two atria and two ventricles.1889

One side of the heart, the right side, is responsible for pumping the deoxygenated blood into the lungs.1895

The left side of the heart pumps the oxygenated blood to the systems of the body.1904

So, there are two different circuits. This is a double circuit system.1911

There is the pulmonary circuit, and there is the systemic circuit.1916

So, here, we have the right side of the heart, so if the person is facing you, this is the right atrium, the right ventricle, the left atrium and the left ventricle.1927

Let's go through the structure and then, the pathway of the circulation of the blood through the heart.1946

The heart is about the size of fist, and so here, I have the clenched fist, and then, here on the right side, we have the right atrium.1952

Between the right atrium and the right ventricle is a valve. It prevents backflow.1961

So, the blood from the atrium enters the ventricle, and we do not want it flow backwards into the atrium; so this valve prevents that.1968

And this valve is called the tricuspid valve.1974

Here, on the left side of the heart, separating the left atrium and the left ventricle, is another valve called the mitral valve.1981

These valves separating the atria and the ventricles are known as the atrioventricular valves.1997

Another name for the tricuspid valve would be the right AV valve and over here, we have the left AV valve,2008

tricuspid or right AV valve and the mitral valve or the left atrioventricular valve.2021

Now, the ventricles, the walls are thicker than the atrial walls, and in particular, the left ventricle is very strong and very forceful.2028

There are also valves between the heart chambers and the major vessels.2042

So, let's look at these different vessels. Here, if we have the lungs up here, then, blood is going to be oxygenated in the lungs.2050

And it is going to return to the heart via these pulmonary veins, so right here are the pulmonary veins.2062

Blood is coming from the lungs. It has picked up oxygen.2076

It enters the left side of the heart. The left atrium flows into the left ventricle and then, goes to the body.2078

This is actually oxygenated blood, so let me put that in. Let's actually change these.2088

This right here, this is oxygenated blood, and it is leaving the left side of the heart via the aorta.2095

Now, once the blood flows to the body, it is going to drop off its oxygen.2105

It is going to pick up waste products, and this deoxygenated blood is going to return to the heart.2111

It is going to enter the left side of the heart via the superior and inferior vena cava.2117

These two vessels are the vena cava and the superior and inferior vena cava.2123

The superior vena cava drains the upper half of the body: the trunk, the head, the upper extremities.2135

It returns the blood from the upper half of the body. The deoxygenated blood is going to go in here and here.2143

The lower half of the body is drained by the inferior vena cava.2151

The blood will go into the right side of the heart - the right atrium, the right ventricle - and then, to the lungs via this pulmonary artery.2156

This structure is the pulmonary artery, this structure right here.2171

So, tracing this route, let's start in the lungs.2184

The blood is in the lungs. It picks up oxygen, and it returns to the heart via the pulmonary veins.2189

So, this is oxygenated blood, so they are carrying oxygenated blood.2199

Remember, I mentioned that there are some veins and some arteries that are not what you would expect in terms of the type of blood they carry.2203

Most veins contain deoxygenated blood. The pulmonary veins are carrying oxygenated blood from the lungs to the left side of the heart.2210

That blood is going to pass through the left atrium, the mitral valves, enter the left ventricle and then, be pumped out of the aorta to the body.2221

So, it is going to the body from here.2233

Blood goes to the body. Body cells take the oxygen, give the blood the CO₂ and waste products.2237

That blood from the body enters the superior and inferior vena cavas. It is deoxygenated blood.2245

It enters the right atrium, passes through the tricuspid valve into the right ventricle and then,2252

is pumped out via the pulmonary artery to the lungs to pick up oxygen again.2261

So, for the right side of the heart, we have the blood being pumped to the lungs,2269

the pulmonary circulation and then, the systemic circulation on the left side of the heart.2274

The left side of the heart pumps the blood to the body.2278

A heart attack or a myocardial infarction is the death of the heart muscles.2284

So, what happens is the aorta carrying oxygen and blood branches of into what is called...2290

It has some branches that come of it that form the carotid arteries.2296

And the carotid arteries supply the heart with blood, with oxygen, with oxygenated blood.2300

So, the carotid artery is the responsible for supplying the heart with oxygen.2306

If the heart does not get enough oxygen, the result is a myocardial infarction, which is damage or death of the heart muscle.2311

Also, it is more commonly known as a heart attack.2324

Now, one risk factor for a heart attack is a disease called atherosclerosis, and this is a condition in which plaque builds up in the arteries.2328

Plaque consists of fatty deposits, and these build up in the walls of arteries.2346

So, if you were looking at a cross-section of the artery, and there were plaque deposits in it, then, these would narrow the artery.2351

If the blue is plaque, and there are these deposits, it is going to narrow the artery, so it is going to decrease the blood flow.2358

Then, when a serious problem can occur, even more serious is if the plaque ruptures.2365

If the plaque ruptures, a piece of it breaks off, and when a piece of plaque breaks off, it actually triggers thrombus formation.2370

So, a clot to form on the plaque, so now, you have this piece of ruptured plaque. It has got a clot on it, and it is floating through the bloodstream.2379

And it is floating through damaged arteries that are already narrow.2391

And what can happen is, the already narrow, now, you have this clump floating through.2395

And it can actually lodge in an artery, block it and cut off the blood supply.2399

If this occurs in the heart, the result is the heart muscle, which needs a lot of oxygen, is not getting oxygen, and it can be damaged. It can die.2404

And that is the pathophysiology underlying a heart attack.2414

We have looked at the anatomy of the heart, the blood flow through the heart, and I just want to emphasize these two circuits.2422

So, the system in mammals is known as double circulation, and some animals have a single circuit.2430

Mammals have a double circuit, so a double circuit system or double circulation.2438

There are two circuits that the blood travels through. I am going to put up here it is the pulmonary circuit, and below is the systemic circuit.2445

And the pressure drops quite a bit when the blood passes through the capillaries. These are capillaries.2457

But, since the blood returns to the heart and is pumped again before it goes to the second set of capillaries, it raises the pressure back up.2464

This is a very effective system in maintaining high pressure throughout.2473

And this is important for mammals because they are very active. They have a high metabolic rate.2479

They need a very good supply of nutrient and oxygen.2483

Right here, we have the right side of the heart, and here is the left side of the heart. This is the lungs up here.2489

What is happening is that the right side of the heart is going to pump the deoxygenated blood into the lungs.2498

And then, in the lungs, gas exchange will occur. The blood will become oxygenated again and then, drain into the left side of the heart.2508

The left side of the heart, the systemic circulation, the left ventricle, will pump blood out through the aorta to the body tissues and organs.2523

And in the capillaries of the body tissues and organs, the oxygen will be dropped off and CO₂ will be picked up.2535

So, then, that blood becomes deoxygenated and returns to the right side of the heart, so there are two circuits in the mammalian system.2547

The cardiac cycle refers to the alternation of contractions and relaxation that occurs in the heart.2556

This rhythmic cycle of contracting and relaxing of emptying the chambers during2564

contraction and filling the chambers during relaxation is known as the cardiac cycle.2570

One round, so the cardiac cycle, one cycle equals one round of filling and emptying of the heart chambers or one round of relaxing and contracting.2577

This would be equal to one heartbeat, so the heart beats.2600

What we call one heartbeat is one cycle of the heart contracts,2608

pumps the blood out, empties the chambers, relaxes and allows the chambers to fill again.2613

Some terminology you should be familiar with, systole is the part of the cycle when the heart contracts, so this is called systole.2619

Diastole is the part of the cycle when the heart relaxes.2637

So, during systole, the heart contracts, and blood is emptied. The chambers of the heart are emptied.2649

During diastole, relaxation occurs. The heart chambers fill back up.2659

The average heart rate in an adult, so average heart rate, is about 70 beats per minute, and you might just see it written as bpm- beats per minute.2665

During that minute, if the heart rate is 70, the heart pumps about 5 liters of blood.2683

And this is equal to the volume of blood present in an average-sized adult's body.2692

So, the total volume of blood in someone's body is pumped by the...that amount is pumped by the heart every minute.2697

Blood pressure: the typical blood pressure in an adult is about 120/80 with variations, but this is typical or normal.2707

It is considered normal around in this range.2718

And this top number is known as the systolic blood pressure. This is the pressure in the arteries during the contraction of the heart, so during systole.2721

This bottom number is the diastolic blood pressure, and it is the pressure in the heart when the ventricles relax.2734

Remember that it is the thick walls of the artery that maintain the blood pressure during diastole.2744

During systole, the walls of the arteries are pushed out by the pressure of the blood inside the vessels after the ventricle contracts.2749

When the ventricles relax and are refilling, the arterial walls spring back in, and that helps to maintain the blood pressure.2757

The second thing we are going to talk about now is how the heart rate is set and the maintenance of a regular rate and rhythm in the heart.2767

Heart muscle/cardiac muscle is inherently contractile. It has the inherent ability to contract.2777

If you take some heart muscle, put it in a test tube or an additional lab and looked at it, it contract. It has the ability to contract.2784

However, you cannot have just all these cells contracting at their own rate.2795

The heart would not move in a coordinated manner. It would not be able to pump blood.2800

If that happens, it is called a dysrhythmia, and it can even be life-threatening, so we need a way to maintain the regular coordinated rhythm of the heart.2805

And what the heart has is a structure called the SA node. This stands for sinoatrial node, and this is the heart's pacemaker.2816

It is the heart's natural pacemaker. It sets the heart rate.2830

So, this is a schematic diagram of the heart, but the SA node is located up here in the right atrium, just to give you an idea.2841

And it generates an electrical signal that travels through the walls of the atria.2850

And then, again, this is showing the right and left, sort of, separated out.2859

But in reality, there is the wall of the right and left atria that are next to each other.2863

And there is another structure, and it is called the AV/atrioventricular node.2868

And the signal gets delayed very slightly at the AV node, and what this allows is for the atria to contract slightly before the ventricles.2876

And the reason is you want to have the atria empty so that the ventricles can be filled before the ventricles contract.2886

So, the signal originates at the SA node. It is transmitted through the two atria to the AV node.2894

There is a little delay there while the atria is contracting. Blood flows into the ventricles, and the signal continues on.2901

So, it continues on through the ventricles.2910

And there are structures in the ventricles, the Purkinje fibers and the bundle of His, through which the signal is transmitted.2913

The signal travels from the SA node to the AV node through the bundle of His,2934

through bundle branches and via these structures right here, throughout the ventricles.2943

From the atria, SA node, a little pause in the AV node, then,2951

through structures in the ventricle that transmit the electrical signal so that the ventricles contract.2955

Because these electrical impulses, they are also detected elsewhere in the body.2962

These electrical impulses travel through body fluids and out to the skin, so we can actually detect the electrical impulses of the heart out at the skin.2967

And you have probably heard of a test called an EKG or sometimes ECG. This stands for electrocardiogram.2977

And in this test, what we do is place electrodes on the skin.2986

And that allows us to detect these electrical signals and to assess the functioning of the heart.2993

Now, although the SA node sets the rate of your heart. As you know, your heart rate can be modified.3001

If you are exercising, if your running, your heart rate speeds up. If somebody scares you, your heart rate speeds up.3008

If you are just resting, your heart rate slows down, and this is because the heart rate is affected by the autonomic nervous system.3013

The autonomic nervous system is responsible for involuntary actions.3023

The autonomic nervous system has two major branches to it: the sympathetic and parasympathetic systems.3033

The sympathetic nervous system speeds the heart rate up.3050

This sympathetic nervous system has to do with the fight or flight response, and it helps to speed the heart rate up; so this is increased heart rate.3056

Parasympathetic, if you are at rest, that will help to slow your heart rate down- decreased heart rate.3069

Hormones such as epinephrine or norepinephrine, that we will talk about in the endocrine section, also affect heart rate.3077

Now that we have covered the component of the circulatory system, the flow of blood through the heart, the components of blood,3087

I am going to talk in more depth about how oxygen is transported through the blood as well as how CO₂/carbon dioxide is transported through the blood.3094

Remember that red blood cells contain hemoglobin, and hemoglobin is the carrier protein for the oxygen.3103

It is what allows oxygen to be efficiently and effectively conveyed through the blood.3111

Oxygen actually has a low solubility than water.3118

It would be impossible to deliver enough oxygen to body tissues if we had to rely on just dissolving it in plasma.3122

In fact, only a few percent of oxygen is dissolved in plasma. The other 97% is carried by the hemoglobin.3130

Hemoglobin belongs to a group of proteins that are called respiratory pigments.3139

So, another respiratory pigment is hemocyanin.3147

Hemocyanin contains copper, whereas, hemoglobin contains iron, so we are going to talk about hemoglobin in depth.3154

Just to briefly talk about hemocyanin now, hemocyanin is found in the hemolymph of some arthropods and some mollusks.3164

Now, we talked about the fact that arthropods, for example, usually just deliver oxygen via the tracheal system directly to cells.3184

However, there are some arthropods as well as some mollusks that use the hemolymph as an important component to deliver oxygen to cells.3193

And the carrier protein that they use is hemocyanin.3201

The hemocyanin is not contained in cells in the blood. It is just in the blood fluid.3203

And this is a respiratory pigment, and it is actually a bluish-colored pigment.3209

Now, what we are mostly going to talk about is hemoglobin, and that is the respiratory pigment that is found in mammals and that is found in us.3214

Vertebrates use hemoglobin as their respiratory pigment. It is a protein consisting of four subunits.3223

Here is shown 1, 2, 3, 4 subunit. Each subunit contains heme, and within the heme is an iron molecule.3232

Each iron molecule can bind an oxygen, so one hemoglobin molecule can bind four oxygens.3245

Red blood cells are packed full of hemoglobin. Remember they do not even have a nucleus.3254

And they do not have a nucleus, which allows them to have even more room to just pack full of hemoglobin.3259

Each hemoglobin is bound to four oxygen, so red blood cells are very efficient at carrying oxygen from one site to another.3265

Now, this structure is very closely related to the function. Because it has four subunit, it actually allows for an allosteric interaction.3273

Remember that an in an allosteric interaction, what occurs in one part of a molecule can affect the conformation, can affect another part of the molecule.3283

And in fact, hemoglobin demonstrates cooperative binding, so hemoglobin demonstrates cooperative binding of oxygen.3292

What happens is the binding of oxygen to the first subunit causes a conformational change that makes it easier for the second subunit to bind oxygen.3305

That makes it easier for the third subunit, and it makes it easy for the next subunit.3315

So, it is toughest to get that first subunit to bind, but once that is bound to oxygen, binding is easier for the remaining subunits.3321

Now, this can be understood a little bit further by looking at a curve called the oxygen-hemoglobin dissociation curve.3331

And this is something you should be familiar with and that you should understand.3341

So, we are looking at this graph, and this graph has a sigmoidal or S shape.3346

The graph shows the percent saturation of hemoglobin with oxygen, so the percent of hemoglobin that is bound by oxygen.3353

And it is comparing it versus the partial pressure of oxygen in millimeters of mercury.3367

Over here, we have very low partial pressure of oxygen, and it is starting out at no hemoglobin being bound to oxygen.3373

This curve is actually flatter at very low partial pressures of oxygen, and then, the curve gets steep. It flattens out again.3385

So, exaggerating it some more, it would be an S-shaped curve.3394

Now, this has to do with that structure of hemoglobin with the four subunits and the cooperative binding.3397

At very low partial pressures, most of the hemoglobin is unbound to oxygen, and if I raise the oxygen a little bit, yes, some of the hemoglobin will bind.3404

But it is difficult because it is the binding of that first subunit, and it is difficult for that to occur.3415

Getting that first subunit bound is difficult, so a lot of the hemoglobins are not bound at all, and then, some of them start to bind a very first subunit.3421

However, if I raise the oxygen partial pressure even more, I take it up to, say, 20 here, I see that the curve is very steep.3430

And that is because once that first subunit is bound, it is easier for the second.3438

When the second is bound, it is easier for the third, so once we get past the first subunit binding, the curve becomes much steeper.3443

So, in this physiological range where most of our body tissues are, if you increase, say, I am here at 20,3452

and about 35% hemoglobin saturation with oxygen, if I increase to 30, that percent saturation goes to 60.3461

So, all this increase in binding of hemoglobin to oxygen by this slight increase in the partial pressure of oxygen.3475

At very low partial pressures, the curve is not steep. In the middle, the curve is steep, and then, the curve flattens out.3485

And the reason it flattens out is saturation has been reached.3491

I can add more and more and more oxygen, but the problem is there is no more hemoglobin sites left to bind oxygen.3495

So, at a certain point, adding oxygen is not going to increase the saturation. It is maxed out.3504

So, this is the shape of the curve.3509

And you should understand that this sigmoidal shape has to do with the four subunits of hemoglobin and the cooperative binding.3511

Now, low partial pressures of oxygen would be in active tissue.3520

So, if you look at your muscles when you are working out, they are using up a lot of oxygen.3527

Because they are undergoing aerobic respiration, they need to make a lot of ATP, a lot of energy.3532

So, they are going to use up oxygen that have low partial pressure of oxygen.3537

In the lung, there will be a high partial pressure of oxygen.3541

And the affinity of hemoglobin for oxygen can change based on conditions.3548

And this allows the hemoglobin to deliver oxygen where it is needed and pick up oxygen where it is needed.3555

One condition that affects the affinity of hemoglobin for oxygen is pH.3565

So, if we look at pH, a lower pH decreases the affinity of hemoglobin for oxygen.3573

A lower affinity can be looked at as the hemoglobin will let go of oxygen more readily.3593

Hemoglobin lets go of oxygen more easily, so what does this mean?3601

First of all, think about where pH is low. Recall that pH is going to be lower in metabolically active tissues.3610

I discussed this is the respiratory lecture, and I am going to discuss it again in a minute in more detail.3621

But for right now, just recall that in blood cells, carbon dioxide and water combine to form carbonic acid, which forms bicarbonate and hydrogen ion.3626

So, in a muscle or a body tissue that is very metabolically active, it is using oxygen, it is generating CO₂,3640

this reaction will be pushed to the right with the result of increased hydrogen ions and therefore, decreased pH.3647

In metabolically active tissues, the pH is low. The result is what we call a shift to the right of this curve, so this curve is going to change.3659

At low pH, this will be the curve: shift to the right.3681

This shift to the right, under certain conditions, is called the Bohr effect or the Bohr shift.3688

Let's look at what this means. Let's say I looked at here a PO₂ of about 30, and I say "Alright".3700

At a PO₂ of 30, I go up to my first curve, and hemoglobin saturation is about 60%.3714

Now, the curve has shifted to the right. At the same partial pressure of oxygen, 30, hemoglobin saturation is only 40%.3723

That means 20% has been let go. Instead of 40% or instead of 60% saturation, we only have 40%, so all of this has been let go.3736

Looking at it this way, it allows the hemoglobin to let go of oxygen or deliver oxygen to where it is needed.3752

And the reason for this is that the presence of these hydrogen ions changes the conformation of the hemoglobin molecule and gives it a lower affinity.3760

It does not bind as well to the oxygen.3768

This is how the red blood cells know "let go", or it helps them to let go of the oxygen.3770

So, when a red blood cell travels to the body, it arrives at a tissue that is in need of oxygen, and the pH there is low.3777

It will decrease its affinity for oxygen and let go of the oxygen and then, pick up CO₂, go back to the lung.3786

In the lung, pH is higher, so pH is back up at 7.4; so in the lung, this curve is going to shift back to the left.3793

And what is going to happen in the lung, then, is at any given PO₂, the hemoglobin is going to have a higher affinity for oxygen.3806

And it is going to grab oxygen, which is exactly what you want in the lung.3815

In the lung, you want a higher affinity of hemoglobin for oxygen, and so it will grab oxygen.3818

In the tissues of the body, you want a lower affinity of hemoglobin for oxygen so that it will drop the oxygen off.3825

If you look at curves for maternal and fetal hemoglobin, you will also see the differences in affinity here of hemoglobin for oxygen.3834

So, if I looked at a fetal hemoglobin curve, I am going to put it in black here, versus maternal, if this is fetal hemoglobin and the blue is maternal,3846

what the fetus needs to do is it needs to take oxygen from the mother to survive. That is where it has to get its oxygen.3859

So, at any given PO₂, if I look here at 40, what I will see is that the maternal hemoglobin binds, say, for 75% saturated.3865

But, the fetal hemoglobin is 90% saturated, so the fetal hemoglobin has a higher affinity for oxygen than the maternal hemoglobin.3876

Before going on, I also want to note that another factor that can cause a shift to the right, I said, it is caused by low pH.3886

Another thing that occurs in active tissue is the temperature increases.3896

If you are running, the temperature in your muscles is going to increase.3900

So, increased temperature also triggers the shift to the right because it is another signal that that tissue needs oxygen.3903

There is another hemoglobin-binding molecule called myoglobin.3914

Myoglobin has only one subunit. It does not have the four subunit structure.3919

So, it does not demonstrate this sigmoidal shape. It is just a linear graph for hemoglobin binding.3926

However, one thing about myoglobin is that it has a higher affinity for oxygen than hemoglobin, and because of this, it binds oxygen very effectively.3931

And there are marine mammals like seals that can dive and stay under water for 30 minutes even hours.3942

And what allows these mammals to do it when humans certainly cannot do this is that these mammals have large stores of myoglobin in their muscles.3950

So, stores of myoglobin allow certain marine mammals to remain submerged for long periods.3960

And it provides a store of oxygen for these mammals during that time.3980

Alright, we have talked about oxygen transport, and now, I am going to talk more about CO₂ transport.3992

So, the blood is traveling through the capillaries. It arrives at the body tissues.3999

It drops off its hemoglobin, and it picks up the CO₂ from those cells.4006

There is a small amount of the CO₂ that just dissolves in the plasma and is transported that way, so that is one way that transport occurs.4011

A second way that CO₂ is transported is it binds to hemoglobin.4020

A CO₂ can actually bind with the amino group on hemoglobin peptide, but these are not the major ways a CO₂ is transported.4026

The major way that oxygen is transported is binding within the hemoglobin molecule.4035

But, for CO₂, most transport is actually in the form of bicarbonate ion.4039

So, what happens is a red blood cell goes to the body tissues, drops off its oxygen, so here is a red blood cell.4046

The oxygen and this is the body tissues, and there is the capillary wall, of course.4054

It has diffused past the capillary wall into the fluids running the tissue and then, into the cells.4063

So, the oxygen is going to diffuse out, be dropped off to the body tissues. Then, carbon dioxide from the tissue is going to enter the red blood cell.4068

In the red blood cell, this CO₂ will recall the reaction where it is going to combine with water. It is going to form carbonic acid.4087

And this reaction is reversible and then, bicarbonate ion and hydrogen ion.4102

The conversion of CO₂ and water to carbonic acid, H₂CO₃, is catalyzed by the enzyme carbonic anhydrase.4110

So, the result here is that CO₂ is taken up, and reaction occurs that converts this CO₂ plus water eventually to bicarbonate and hydrogen ion.4130

So, what happens to this? Well, the bicarbonate can leave the cell and enter the plasma, and it becomes part of the plasma buffering system.4147

It serves as a part of the buffering system and also as a means of transporting this carbon dioxide in the bloodstream.4160

How does this serve as a buffering system? Well, let's say that the pH is too high.4171

So, if the pH is too high, this means we need more hydrogen ion.4180

What can happen, then, is this reaction will go the opposite direction. Actually, no, it will go in the forward direction.4187

If we need more hydrogen ion, then, more CO₂ will combine with water to form carbonic acid. That is H₂CO₃.4197

And so, CO₂ will combine with water to form carbonic acid, and that will, then, go on to form bicarbonate and hydrogen ion.4216

So, this is if the pH is too high.4241

Now, let's say the blood pH is too low. We need to decrease this level of hydrogen ion.4243

So, now, bicarbonate and hydrogen ions will react to form carbonic acid, which will, then, dissociate into carbon dioxide and water,4252

thus, lowering the concentration of hydrogen in the blood and raising the pH back up.4265

You can see this reaction going one way or the other helps to buffer the blood and keep the pH within a narrow range.4272

Once this blood travels to the lung, what is going to happen in the lung is that CO₂ is going to diffuse into the lung.4281

So, the CO₂ is going to go into the lung, which is going to pull this reaction back to the left.4292

As we draw the CO₂ off, hydrogen ions will combine with bicarbonate to form carbonic acid and then, carbon dioxide and water.4300

So then, more carbon dioxide can diffuse into the lung.4313

This reaction also helps to deliver the CO₂ into the lung once the red cells and the plasma reach the lung, OK?4318

Important points are that the majority of carbon dioxide is actually transported in the blood in the form of bicarbonate ion.4330

You should also be familiar with this reaction in which CO₂ and water combine to eventually form bicarbonate4340

and hydrogen ion and the fact that this allows for a buffering system in which to decrease the level of4347

hydrogen ions as needed if the pH is too low or increase the hydrogen ion concentration if the pH is too high.4357

OK, now, we are going to talk about some examples using the material from the circulatory system.4368

Example one: trace the pathway of blood through the circulatory system by placing the structures below in the correct order.4376

Begin with the oxygen-rich blood as it leaves the lungs to return to the heart.4384

We are starting in the lungs, so how is the oxygen-rich blood from the lungs brought back to the heart via the pulmonary veins?4391

So, this is a vein or veins that actually carry oxygenated blood.4404

We start out in the lungs. From the lungs, blood travels via the pulmonary veins, so I have used this one.4409

Oxygenated blood returns to the left side of the heart, so that would be first the left atrium.4422

The blood will pass through the mitral valve into the left ventricle.4430

When the left ventricle contracts, it pumps blood into the systemic circulation, and that occurs through the aorta.4437

The blood is, then, going to go into the body tissues, drop off oxygen, pick up CO₂ and then, enter veins to return to the heart.4449

In the upper part of the body, the superior vena cava drains this deoxygenated blood into the heart.4460

In the lower half of the body, it is the inferior vena cava, so we are just going to put "vena cava".4468

So, this blood is now deoxygenated. It is going to drain into the right atrium.4475

We used right atrium. We used vena cava.4487

Then, from the right atrium, the blood will go to the right ventricle.4491

It is deoxygenated, so it is going to be pumped from the right ventricle through the pulmonary artery to the lungs to complete the cycle.4495

So, this is the correct order: lungs to the pulmonary veins to the left atrium, left ventricle,4510

aorta, vena cava, right atrium, right ventricle and then, finally, pulmonary artery.4517

Example two: which veins carry oxygenated blood?4526

Well, from what we just discussed, you know that the pulmonary veins carry oxygenated blood from the lungs to the heart.4531

What is the name of the structure that functions as the pacemaker for the heart? That is the sinoatrial node, very commonly known as the SA node.4553

What are the cell fragments in the blood that function in clotting called? These are the platelets.4568

These are cell fragments found in the blood.4576

Describe the role of vasodilation and vasoconstriction in thermoregulation.4584

Recall that vasoconstriction is constricting of the blood vessels, so the diameter becomes smaller.4591

In vasodilation, blood vessels dilate. Their diameter becomes larger.4597

In the cold, vasoconstriction of superficial vessels occurs.4602

The result is decreased blood flow through those vessels, and then, the result is decreased heat loss via heat transfer through the skin.4619

Heat triggers vasodilation of the superficial vessels. The result is going to be a decrease in the vessel diameter or an increase.4636

Excuse me. Vasodilation is going to increase the diameter of the vessels.4654

The result is going to be increased blood flow through those vessels and increased heat loss through4660

heat transfer from the superficial vessels out in the environment, so this will cool the body down.4668

The result, then, increased heat loss, the body cools.4676

The opposite occurs with vasoconstriction. There is less heat loss through the skin, through the superficial vessels, and the result is the body warms.4681

And this helps us maintain our body temperature.4690

Hemoglobin, this dissociation curve is shown below. Sketch the expected curve following an increase in pH.4695

Think about what happens, the conditions, when pH is increased.4704

pH is decreased in active tissues. These are tissues that are using a lot of oxygen and generating CO₂.4710

And what we want to happen is decreased hemoglobin affinity for oxygen.4721

So, the result is if pH is decreased, we get the shift to the right that we talked about.4728

However, in the lungs, pH is going to be increased. That would be an example of higher, higher pH.4736

And what we want to do is increase the affinity of hemoglobin for oxygen because we want the hemoglobin to take the oxygen from the lungs.4743

Therefore, there is going to be a shift to the left, so the curve is going to be shifted to the left.4757

So, taking a look at this just to check, shift to the left is correct.4768

At a partial pressure of, say, 30, in my original curve, there is about 55% hemoglobin saturation.4773

In my new curve at a partial pressure of 30, there is actually a saturation of more like 90%. This is a difference of 35.4789

That much more oxygen was grabbed. It was picked up, which is what you want.4806

So, the answer is that there would be a shift to the left, so the curve would look roughly like this.4810

That concludes this lecture on circulation.4816

Thank you for visiting Educator.com.4819