AP Biology The Respiratory System

Welcome to Educator.com.0000

Today is the first in a series of lectures on animal physiology.0002

Throughout these lectures, we are going to talk about various groups of animals but, the focus is going to be on human anatomy and physiology.0007

And we will be starting out with the respiratory system.0015

First, an overview of gas exchange in animals, respiration is the process through which gas exchange occurs.0019

And during respiration, animals take in a respiratory media such as air or water.0026

And they extract out the oxygen and release carbon dioxide back into the air or for aquatic animals, into the water.0033

This is a necessary process for aerobic respiration, so as you will recall, animals undergo aerobic respiration in which they generate ATP.0042

After taking in the gas, the oxygen, it will diffuse down its concentration gradient into the cells of the animal.0055

And the CO₂ will diffuse down its concentration to leave the animal's body.0064

Ventilation is the movement of air across a respiratory surface, so when we talk about ventilation in humans, we are talking about breathing,0070

so, breathing in and allowing the air to move across the respiratory surface of the lung.0080

When we talk about aquatic animals, then, we are talking about ventilating the gills, so movement of water across the respiratory surface of the gills.0089

Gas exchange occurs across different surfaces in different animals.0099

In some animals, it is the skin like in earthworms. In other animals, like I mentioned, the lungs, and still on others, it is gills.0105

Although the respiratory surfaces differ, there are some characteristics they have in common,0113

so general characteristics of respiratory surfaces such as lungs, gills or even skin.0119

Characteristics include the fact that the respiratory surface is generally thin to allow diffusion, to allow gas to diffuse across that surface.0134

Second, the surface is moist. Lung tissue is moist.0144

The skin of a frog, of amphibians, that use their skin as a respiratory surface is thin and moist. Finally, these structures have a large surface area.0149

In simple animals such a sponges, Hydra and flatworms, all of the body cells are in contact with the external environment.0165

So, gas exchange can take place across all the body surfaces.0174

However, as you are going to see in most animals, there are specialized surfaces for gas exchange.0177

We are going to start out by talking about gas exchange in aquatic animals.0187

Gas exchange in simple aquatic animals occurs via diffusion across the whole surface of the body.0196

That is what I just mentioned, and simple aquatic animals would include Hydra and jellies.0204

These are some of the simpler aquatic animals, and all of their cells are in contact with the water.0214

And therefore, oxygen can diffuse directly into the cells, and carbon dioxide can leave the cells and exit into the water.0221

However, in more complex aquatic animals such as fish, sharks, crustacean, gills are the means of gas exchange.0230

And the specific structure and location of gills varies among the different species. However, there are those common characteristics that I talked about.0240

Gills have a large surface area due to extensive foldings, so this increases the area available for gas exchange to occur.0251

As I also mentioned, ventilation is the movement of the respiratory media, in this case, water across the respiratory surface.0260

The gills are ventilated. Water is moved across the gills, so ventilation by movement of water across the gills, and this can occur is two essential ways.0271

One is that if we are talking about a fish, the fish moves, and water flows across the gills. The fish swims.0293

It keeps its mouth open. Water will enter the mouth, and it will flow across the gills.0307

However, even if a fish stays still, even if it stops swimming, it can still undergo ventilation. It still needs that flow of oxygen containing water.0311

So, this can occur a second way in that structures move the water across the gills.0320

In the fish, what it does is that the fish will draw water into its mouth, and then, the water will exit across the gills.0335

So, it is not just swimming, and the water is just passively entering the mouth. It is actually drawing water into the mouth.0343

And recall when we talked about the structure, the anatomy of fish, that there is a structure called an operculum- the bony structure.0348

It is a flap that protects the gills, and it also, helps keep the water moving through the gills.0360

In other aquatic animals, there are various other structures that push or move the water through.0365

An important concept in gas exchange in aquatic animals is that of countercurrent exchange.0373

Countercurrent exchange, this picture here, shows countercurrent exchange.0381

An example of it is the exchange of materials between fluids that are flowing in opposite directions, so exchange of materials or heat.0387

So, I am going to note that it can be an exchange of materials or heat between fluids moving in opposite directions.0404

In this case, we are talking about the exchange. We are going to focus on the exchange of oxygen.0420

This exchange occurs across a semipermeable membrane.0428

What happens is, there is water that enters the gills, and it is going to be flowing in this direction as shown.0432

Meanwhile, there is blood in the capillary, and this blood is returning from the tissues and cells of the body; so it is very low in oxygen.0444

It would not actually go all the way down to zero, but just for simplicity, I am going to show the water entering from the gills at 100% oxygen saturation,0455

and the water that is in the capillaries coming back from the body at 0% oxygen saturation and then, going on and increasing from there.0467

So, it is just for simplicity.0477

So, what happens is, the blood from the capillaries is depleted of oxygen, and it is flowing in this direction.0480

The water is entering the gills flowing in the opposite direction, and it is flowing across the gill, flowing across the gill; and oxygen is being extracted from it.0488

So, what you will see is that as oxygen diffuses across the membrane of the capillary from the water into the capillary,0497

what is going to happen is the oxygen saturation is going to drop.0509

So, maybe, this goes down to 90 and then, say 80, 70, 60, 50 and on and on down, but it is only going to go as low as 10- what I am showing here.0514

As the water flows across the gill, it is giving up its oxygen to the blood flowing in the opposite direction in a nearby capillary.0530

When the water enters the gill, it is going to pass very close to capillaries with blood flowing in the opposite direction, so this oxygen saturation is dropping.0540

Meanwhile, the blood in the capillary is picking up oxygen, so the saturation of the oxygen in the capillary is increasing.0552

At any point, though, in this exchange system, the percent of oxygen or the partial pressure of oxygen0564

- you can look at it that way - in the water is higher than the percent of oxygen in the blood.0575

So, even though this oxygen is increasing, and this one is decreasing, at any given point,0583

let’s say right here in the blood, there is 80 percent oxygen 70, 60, 50, 40, 30, 20, 10,0590

if I pick any point right here, even though this water has lost a lot of its oxygen, it still has more oxygen than the blood passing by at that same point.0603

So, oxygen will still diffuse down its concentration gradient.0616

The water in the gill essentially stays ahead of the blood in terms of oxygen concentration.0621

At any point in the system, the oxygen concentration in the water is greater than the oxygen concentration in the blood.0631

And what this allows is for there to be the exchange of gasses throughout the entire length of the system, the entire length of the capillary.0656

In this way, the blood is able to continually, as it moves through, pick up oxygen, and the water is able to give up its oxygen.0666

And if you look here, what this blood is encountering is water that has depleted of a lot of its oxygen.0678

However, this blood has even lower oxygen because it is just returning from the body.0689

So, it is set up so that the oxygen concentration of the blood at any point is lower than the oxygen concentration of the water.0694

And that is the essential point.0702

Now, as I said, heat can also be exchanged through a countercurrent system, and this is a mechanism for temperature regulation.0705

So, countercurrent exchange is used in thermoregulation. These systems are used in thermoregulation in some animals.0714

As an example, marine animals that have their legs or flippers submerged in cold water can use this system.0728

If an animal, say, has its legs submerged in cold water, the blood in the extremities is going to be very cold.0738

And what happens is, the vessels carrying the cold blood, the veins carrying the cold blood... so, let's show this here.0746

This is a vein, and this is going to be blood from the extremity; and this is cold because it has been submerged.0760

The animal's paw has been submerged in water. It gets very cold, and the blood returning to the heart is cold.0772

Meanwhile, arterial blood is going to be in nearby arteries that are running in the opposite direction.0779

The blood is flowing in the opposite direction, and this is blood coming from the heart. It is coming from their body core that is warmer.0789

And what will happen is heat will be transferred at every point along this countercurrent exchange system.0797

And that, then, warms the blood that is returning to the core of the body and cools the blood going out to the extremities.0806

And the result is that there is not as much heat loss from the extremities, so it maintains the body temperature- thermoregulation.0816

Alright, so, this is gas exchange in aquatic animals.0827

Now, we are going to talk about gas exchange in terrestrial animals, in land animals0831

starting out with simpler animals that rely on the diffusion of gasses across the skin.0839

Now, we talked about very simple animals where all cells are in contact with the external environment like in a flatworm.0849

The gas can just diffuse in, and oxygen can diffuse in. CO₂ can diffuse out.0858

If you look at a little bit more complex animal, an earthworm - so, let's talk about earthworms, which are annelids, segmented worms -0863

gas exchange occurs across the skin.0873

So, the skin is not just a surface for protection in these animals, it is also a respiratory organ, and this is an external respiratory surface.0881

In us, we have lungs. They are located inside our bodies.0896

Those are internal respiratory surfaces. This is an external respiratory surface.0899

And what happens is the oxygen enters the skin, and then, it encounters a rich network of capillaries.0904

And the oxygen enters those capillaries. Carbon dioxide leaves the capillaries and diffuses across the skin.0911

However, in many animals, the skin alone is not sufficient for gas exchange.0917

So, when we talked about amphibians like frogs, gas exchange across the skin is just a supplement for gas exchange via the lungs.0922

And then, in many larger, more complex animals, the skin is not a respiratory organ. In fact, there are just internal respiratory surfaces.0930

So, let's talk about internal respiratory surfaces like lungs.0939

One major advantage to an internal respiratory surface is that they minimize water loss, so an internal respiratory surface minimizes water loss.0945

Recall that respiratory surfaces need to be moist, and therefore, if the skin is a respiratory surface like in a frog, it needs to be moist.0957

And the animal is restricted to a moist environment, so an earthworm in a dry environment will dry out.0967

Gas exchange will not occur. It will die, so internal respiratory surface allows animals to live in drier environments.0973

The internal respiratory surfaces are moist, and they are lined with epithelium.0984

But as I said, this moist epithelial surface is confined to the inside of the animal's body.0993

Before we go on to talk about mammalian and specifically human respiratory systems, we are going to talk about arthropod respiratory system.0999

So, we are going to focus actually on insects and just use that as an example of a different type of respiratory system, although, it is an internal system.1008

If we look at gas exchange in insects, what happens is air enters the body through openings in the body called sphericals,1016

so, openings that are called sphericals, the gas enters.1029

So, air enters the sphericals then, goes into tracheae. Singular would be trachea.1037

The tracheae branch, and they become smaller and smaller; and these smaller structures are called tracheoles.1055

Air enters the sphericals then, goes to the tracheae and a trachea will branch into a tracheole.1064

And these will get smaller and smaller and smaller and branch more and more until they reach the various cells in the body.1072

And they will deliver the air directly to the cells.1081

So, gas exchange occurs at the epithelial surfaces that are at the end, so branching, branching, branching.1086

And then, here at the end of the tracheole, we have a spherical where the air enters.1093

And then, it is the trachea and then, tracheoles, and then, at the end of the tracheoles is an epithelial surface.1105

And the body cells, there will be an organ or a structure right here, and then, gas exchange occurs.1115

Notice that the gas, the oxygen is not going into blood like it does in us.1125

It is not going from a lung to blood and then, being delivered to cells. It is going directly from the respiratory surface to the cells.1130

And oxygen diffuses out to the cells. CO₂ is picked up.1140

Something important here, a fundamental difference between this system and, say, our system is that the circulatory fluid.1149

In arthropods, the circulatory fluid, and we will talk about this in detail on the lecture in circulation about the circulatory fluids.1155

The circulatory fluid in arthropods is called hemolymph, and although, hemolymph sometimes delivers oxygen in certain arthropods,1164

in most arthropods, the job of hemolymph, of the circulatory fluid, is to deliver nutrients, hormones, pick up waste products.1173

But, it is not a source of gas exchange or gas transport. Oxygen and CO₂ does not usually transport oxygen- like I said, not usually.1183

In some animals, it does, and we will talk about that later.1199

But for now, what you should know is that in insects, the respiratory surface directly delivers the oxygen and picks up the CO₂ from cells.1202

It does not use the intermediary circulatory system to deliver oxygen and pick up CO₂.1213

Now, in larger insects they need even more air to supply their tissues, and they sometimes will then have air sacs at the end of the tracheae.1219

And what happen is, as the insect moves, the air sac will expand.1232

And this allows the sac at the end - sort of like an alveolus - allows for a greater amount of oxygen to be delivered.1237

So, this is an example of a system in a non-mammal.1250

One thing, though, that is a commonality between various respiratory surfaces, as I said, is their large surface area.1259

And we talked about the folding of gills. Folding is one way to increase the surface area.1267

Here, there is branching, so these are highly branched structures.1272

Branched tubes and folded structures, those are commonalities in respiratory surfaces.1276

Now, we are going to talk about the human respiratory system.1282

And many of the structures that we are going to discuss such as lungs are found in mammals1286

other than humans and even non-mammalian vertebrates like reptiles and amphibians.1292

Even some of the invertebrates have lungs, but I am really going to be focusing on the human respiratory system.1299

And that is what is really important especially to know for the Advanced Placement Exam.1304

We are starting out at the top air containing oxygen is going to enter via the nasal cavity.1310

So, it enters via the nasal cavity, and a few things occur in the nasal cavity.1319

The air is warmed. The air is also humidified.1326

It gains moisture, so it is moistened or humidified, and finally, it is filtered. There are hairs in the nasal cavities that catch particulates such as dusts.1332

So, the air is warmed, humidified and filtered.1344

So, that air enters the nasal cavity, and the next structure that you need to be familiar with is the pharynx.1348

After passing through the pharynx, a little bit lower down is the larynx.1360

Within the larynx is the voice box, so in addition to being a respiratory structure, the larynx is important for speech.1368

And vibration of the vocal cords within the larynx is what produces speech.1377

There is a structure called the epiglottis.1384

And when we swallow food or fluids, then, the epiglottis covers the larynx so that food or fluid does not enter the lung.1389

Instead, the food or fluid goes into the esophagus, and we are going to look at the structure of the epiglottis more when we talk about the GI system.1398

But for now, just know that the epiglottis protects the airway and that it covers1406

the opening to the larynx so that food and fluid cannot end up in the lung.1411

So, the air has gone through the pharynx and larynx and then, enters the trachea, continues on down into the right and left bronchi.1419

So, here, we are just looking at one side.1433

If he is facing you, then, this is his left bronchus, so focusing on this one side, but the same structures are also found over here on the other side.1435

Air continues into the bronchus.1451

Then, the bronchus branches into these smaller structures called bronchioles,1456

which terminate in very small sac-like structures called alveoli or singular, alveolus.1464

So, this is the pathway of air through the respiratory system:1474

nasal cavity or mouth, pharynx, larynx, trachea, bronchus, bronchiole and alveolus, which is the site of gas exchange.1478

A couple things you should know is that the trachea contains cartilaginous rings, so rings of cartilage.1490

This maintains the strength, the structure and the support to hold this into a tube shape, so these cartilaginous rings.1501

The other thing you should be aware of is that the respiratory tract is lined with cilia, so the respiratory tract is lined with cilia and contains mucus.1511

So, what happens is certainly, the nose, the nasal cavity has filtered out some of the particulates but not 100%.1526

So, other particulates can be trapped in this mucosal layer, and then, the cilia move.1533

And what they do is they push the mucus upwards to remove any more particulates from the respiratory system.1540

So, the air has now made its way down to the alveolus, and this is the surface across, which gas exchange occurs.1551

The alveoli are microscopic structures, and taken together, they have a huge surface area.1561

They have a surface area of about - if you put them all together - 100 square meters.1567

Let's say this is an alveolus, and then, there will be a nearby capillary.1577

And oxygen will go down its concentration gradient from high concentration of oxygen in the alveolus to the lower concentration of oxygen in the capillary.1588

Meanwhile, CO₂ is going the other way, so gas exchange occurs across the membrane of the alveolus.1600

The alveolus is surrounded by fluid, and then, it will cross the capillary membrane and enter the blood.1611

Again, this is in contrast to the tracheole system that we talked about in insects.1617

In insects, the tracheae branched and branched into tracheoles that, then, deliver their oxygen right to the body cells.1622

So, it could be the leg of the insect. It could be the sensory organs of the insect, the nervous system.1632

It is all getting delivered directly there, whereas here, there is not a respiratory surface next to all parts of the animal's body.1643

There is not an alveolus over in the hand or going all the way down into the leg to directly deliver gas.1651

Instead, the respiratory surface gives the oxygen to the circulatory system to the blood.1658

And the circulatory system, then, delivers that gas to the tissues and organs in the body.1664

So, the alveoli are surrounded by tiny capillaries.1671

Oxygen diffuses out of the alveolus into nearby capillaries, and CO₂ diffuses in the opposite direction and then, can be exhaled out.1676

Pulmonary surfactants are substances found in the lung that lower the surface tension in the alveolus, in the alveoli.1686

What this does is it prevents alveolar collapse, so it prevents the collapse of the alveoli.1706

Premature infants sometimes suffer from a syndrome called RDS. This is respiratory distress syndrome.1719

And the reason is that surfactant does not developed until late in gestation until late in pregnancy.1729

So, if a baby is born prematurely, and there is not enough surfactant being produced, then, they are at risk for respiratory distress syndrome1736

because their lungs are not ready to be taken over that function of breathing, and the result is respiratory distress.1744

OK, before we go on and talk about ventilation, just a brief discussion of the circulatory system since it is very closely related to the respiratory system.1755

Although, there is going to be another lecture on that topic.1764

Once the oxygen is picked up by the capillaries, it enters red blood cells, and the red blood cells contain hemoglobin.1767

The hemoglobin contains iron, and the iron is capable of binding the oxygen.1780

So, the red blood cells use hemoglobin, and the iron within the hemoglobin, to carry oxygen to the cells.1787

And we will talk all about the structure and function of hemoglobin in the next lecture.1796

Carbon dioxide: some of it is transferred by hemoglobin.1800

But, most carbon dioxide is transferred in the blood plasma via a buffering system, so most CO₂.1805

So, hemoglobin transport oxygen, whereas, most CO₂ is actually transported in the form of bicarbonate ion in the plasma.1813

Alright, we have talked about the flow of air through the respiratory system and how gas exchange occurs.1824

Now, we are going to talk about ventilation in the human respiratory system.1829

Recall that ventilation is the process of moving air - in this case air - across the respiratory surface, which is the lung.1834

Ventilation occurs during the process of breathing.1843

So, breathing consists of inspiration, taking the air in, and expiration in which the air is moved out of the body.1846

And ventilation or breathing occurs differently in various species.1857

For example birds lack diaphragms, and we are going to talk about the diaphragm muscle.1863

Birds do not have diaphragms, yet, they still breath very effectively.1868

We are going to focus on mammalian ventilation, and in mammals, there is a sheet of muscle called a diaphragm.1872

This is a sheet of skeletal muscle that is located in the thorax.1881

So, you have the lungs, and then, just below that, it is a dome-shaped muscle. That is the diaphragm, and the diaphragm separates the body cavities.1887

Up here, above the diaphragm is the thorax, and then, below is the abdominal cavity.1902

Contraction of the diaphragm moves it downward, so it flattens out somewhat and the result... so, it moves downward1912

So, as you can imagine then, you have the lungs here, but then, the diaphragm drops down along with the...1921

The lungs expands as well. Did we draw that?1932

So, we have expansion, then, of the thoracic cavity, so the thoracic cavity becomes larger.1936

Right here, it states that air is pulled into the lungs by negative pressure.1947

And this negative pressure, meaning that the pressure in the thoracic cavity becomes lower than1951

the pressure outside in the atmosphere, is a result of the increase in size of the thoracic cavity.1958

So, increased size of the thoracic cavity is the result of the diaphragm contracting.1966

The diaphragm contracts and moves downward, and the rib muscles contract.1975

And when the rib muscles contract, the ribcage moves outward, so the ribcage expands. It moves outward.1995

The downward movement of the diaphragm combined with the outward movement in the ribcage results in a cavity that is larger than it originally was.2006

So, the thoracic cavity becomes larger. The result is negative pressure, and air is pulled into the body; and it is pulled into the lungs, as well.2016

As the thorax expands, the lungs expand and air rushes in.2024

In exhalation, the reverse process occurs, so this is during inhalation or inspiration of air. During expiration, the opposite occurs.2029

During expiration, the diaphragm relaxes. Rib muscles relax.2043

And we end up with a thoracic cavity that is decreased in size - decreased size thoracic cavity - and therefore, the air rushes out of the body.2057

Not every single last bit of air leaves the lungs during exhalation. There is a little bit of residual air left.2067

Alright, so, we talked about the process of breathing. Now, what controls the breathing? The breathing control center.2075

The breathing control center is located in the brain stem in a couple of structures called the medulla oblongata or just the medulla and the pons.2086

So, the medulla and pons are the breathing control center, and what the medulla does is it monitors the CO₂ level in the CSF.2098

CSF stands for cerebrospinal fluid. Cerebrospinal fluid is the fluid surrounding the brain.2111

Actually correction, it monitors pH. It monitors pH, which is closely related to the CO₂ level, OK?2120

So, it monitors the pH in the CSF, and the pH in CSF is reflective of the pH in the blood.2134

What happens in the blood is the blood goes to the cells and the tissues of the body, and then, it picks up CO₂.2142

And in the red blood cell, the CO₂ combines with water to form carbonic acid,2152

which dissociates into bicarbonate and hydrogen, so bicarbonate ion and hydrogen ions.2162

Therefore, if there is a lot of CO₂ in the blood, the blood will be a little bit more acidic because a lot of CO₂ is going to push this reaction.2172

It is a reversible reaction, but it is going to be pushed to the right if you are putting CO₂ into the system.2185

And the result is going to be an increase in hydrogen ion, and when hydrogen ions go up, pH goes down.2189

So, the pH of the blood is usually...a typical pH is 7.4, but when you are exercising a lot, then, the pH of the blood could drop a little bit.2196

During periods of activity, you are running, you are exercising, you need more oxygen.2208

And therefore, you need to breathe faster, and the way your body knows that you need more oxygen is you are generating CO₂.2214

By generating CO₂, you are going to be generating hydrogen ions, and the pH in your blood will drop.2223

When the pH in the blood drops, so decreased pH in the blood will be reflected in the CSF.2230

So, decreased pH in the blood triggers an increase in the rate and depth of breathing- increased breathing, rate and depth.2238

So, this is triggered. You are going to breath more quickly and more deeply.2252

And then, once pH rises back up to its normal level, then, the medulla will slow the rate back down.2257

Therefore, breathing, respiratory rate is primarily determined by carbon dioxide and not by oxygen.2264

The body, via looking at the pH, is actually monitoring the CO₂ level.2272

Now, oxygen does play a role.2277

The term for low oxygen is hypoxia, and there is a second set of mechanisms that can detect low oxygen.2280

They are actually oxygen sensor, for example, located in the arteries in your neck, in the carotid arteries.2291

So, if oxygen became concerningly low, that system would kick in and tell you to breathe.2297

But primarily, breathing is a function of the CO₂ level and not oxygen levels.2306

I mentioned the medulla, and the medulla sets the rate of breathing.2313

However, the pons modulates or modifies the breathing rhythm. It just makes sure that it is nice and even and smooth.2318

So, the breathing rate is set by the medulla, but it is adjusted or modulated. The rhythm of breathing is modulated by the pons.2326

OK, today we talked about the respiratory systems in different animals especially in humans.2336

We talked about gas exchange, ventilation and structures involved in respiration.2342

And now, we are going to do some practice questions starting out with example one.2347

Describe the tracheal system in insects. Include the terms spherical, trachea and tracheole in your discussion.2352

Well, in insects, air enters via openings in the body called sphericals.2360

It, then, enters tracheae, and these tracheae branch and become smaller and smaller and form tracheoles.2371

The tracheoles reach the cells of the body, and gas exchange occurs.2382

So, the respiratory system brings air directly to body tissues and cells.2390

Oxygen is picked up by the cells, so oxygen is picked up, O₂ delivered to the cells; and CO₂ is picked up.2399

The circulatory fluid, the hemolymph generally does not deliver the oxygen or pick up the CO₂.2415

It is delivered directly by the respiratory system.2423

Also, in some insects, there are air sacs. There may be air sacs at the end of the tracheoles to increase the amount of air that can be delivered.2426

Also, note that the larger branches of this system contain chitin for support, so there is some chitin in the tracheae.2439

How does the tracheal system differ from the respiratory system found in mammals? Well, obviously there are different structures that we talked about.2454

The respiratory system in mammals, we talked about structures such as the pharynx, the larynx, the bronchi, the alveolus.2464

But, a major difference - just a concept that you should understand - is that in the tracheal system, tracheal system delivers air directly to body cell.2474

So, oxygen is delivered directly by the respiratory system.2492

CO₂ is picked up directly by the respiratory system, whereas, in mammals oxygen is delivered by the blood, by the circulatory system.2497

And CO₂ is picked up by the blood.2510

The circulatory system does not act as a delivery route in insects for gas exchange.2517

Whereas, in mammals the circulatory system plays this intermediary role.2524

What role does the countercurrent exchange system play in gas exchange in fish?2531

Well, first, recall what a countercurrent exchange system is. It is fluids that are flowing near each other, but they are going in opposite directions.2536

So here, we have water enters the gills and flows in this direction.2547

Meanwhile, we have blood in a capillary nearby, and it is flowing in the opposite direction; and this is blood from the body that is deoxygenated.2556

So, this blood is going to have a very low O₂ saturation. We will just call it zero, and this is going to increase, let say 20, 40, 60, 80.2572

Meanwhile, the water entering the gills is going to start out with oxygen, let’s say, up at 100, and that is going to drop.2589

As the water passes near this blood, the oxygen is going to diffuse down its concentration gradient and enter the blood.2605

Meanwhile, CO₂ would diffuse the other way.2619

So, as the water passes through the gills, the O₂ concentration will drop.2622

However, at any given point, the O₂ concentration in the water is greater than the O₂ concentration in the blood.2628

So, all on this exchange system, gas exchange is occurring.2637

So, that is the role of a countercurrent exchange system in gas exchange in fish.2645

Example three: place the following structures in the order in which air travels after entering the respiratory system via the nasal cavities.2653

So, after entering the nasal cavities where the air is warmed, it is moistened and it is filtered.2662

The next step is going to be the pharynx, so the pharynx is going to be the next structure that the air will pass through.2670

From the pharynx, air will enter the larynx, so we did pharynx. We did larynx.2679

From there air, travels to the trachea, and that is going to branch off into the right and left bronchi; so it is going to enter a bronchus.2687

The bronchi, bronchus, if we are talking about one, is going to branch into smaller bronchioles.2703

And those are going to terminate in an alveolus, which is the site of gas exchange between the respiratory system in the blood.2711

So, this is the order that air travels through the respiratory system.2721

Example four: what role does the diaphragm play in ventilation?2733

Recall that the diaphragm is a sheet of muscle located in the thorax separating the thoracic cavity and the abdominal cavity.2737

When the diaphragm contracts, it moves downward. Therefore, the size of the thoracic cavity will be increased, so increased thorax size.2747

The result is going to be negative pressure in the thorax, and that pulls air in during inspiration. The opposite will occur when the diaphragm relaxes.2765

It is going to move up, and there is going to be a decrease in the size of the thorax. Air, then, will rush out of the lungs during expiration.2791

What role does pH have in the regulation of breathing by the medulla?2809

Well, the medulla monitors pH in the CSF, and the pH in the CSF mirrors the pH in the blood; so this is a measure of pH in the blood.2813

And in the blood, when CO₂ increases, the result is that pH decreases because recall CO₂ plus water form carbonic acid,2837

which associates to bicarbonate ion and hydrogen ion, so increased CO₂ translates the increase hydrogen ion, which is decreased pH.2858

Decreased pH leads the medulla to increase the rate and depth of breathing.2869

So, when the medulla senses that pH has dropped, there is an increase in the rate and depth of breathing.2881

That concludes this lecture on the respiratory system.2888

Thanks for visiting Educator.com.2892