Chapter 42: Gas Exchange & Circulation

Positive - raise pressure outside lung

Negative - lower pressure inside lung

Osmotic pressure is the tendency of a fluid to attract water - it's due to presence of large proteins and blood cells in capillary; these large substances act to draw water IN to the capillary.

. (It's kind of like osmolarity; water is drawn towards solutions with a higher osmotic pressure, just like it's drawn towards solutions with higher osmolarity). Osmotic pressure stays about the same in all parts of the capillary, at about 22 mmHg.

Hemocoel

A body cavity, present in arthropods and some mollusks, containing a pool of circulatory fluid (hemolymph) bathing the internal organs. Unlike a coelom, a hemocoel is not lined with mesodermally derived tissue.

Closed Circulatory System

A circulatory system in which the circulating fluid (blood) is confined to blood vessels and flows in a continuous circuit; found in vertebrates.

Many small/thin organisms don't have separate dedicated organ for gas exchange, they just diffuse gases across their body surface.

A lot of smaller aquatic invertebrates do this (worms, cnidarians; many of the same ones that lack circulatory systems). Amphibians also do a good bit of exchange across their skin, although most also have lungs.

Atherosclerosis is a disorder in which lipid deposits form plaques in the arteries. Like hypertension it's impacted by genetics and lifestyle (diet, lack of exercise, etc). These deposits clog the vessels, potentially forming dangerous blockages that can lead to a stroke or heart attack.

A stroke is a blockage of the arteries supplying blood to the brain. A heart attack occurs when coronary vessels supplying the heart muscle are blocked. In either of these cases, cells are deprived of oxygen and can die, leading to brain damage or loss of function for the heart, possibly ending in death if it's severe enough.

Blood

A type of connective tissue consisting of red blood cells and leukocytes suspended in a liquid extracellular matrix called plasma. Transports materials throughout the vertebrate body.

Blood returns to the heart via veins and is delivered to the atrium (receiving chamber). It then travels into the ventricle, where it is pumped out as the ventricle contracts. The blood then travels in a single loop (single circuit) back to the heart. It leaves the heart via arteries, and it's under fairly high pressure at this point since it just got pumped by the heart. It then travels through the respiratory system of the fish (gills) where it picks up oxygen and gets rid of CO2. Notice the blood changing color from blue to red to represent blood becoming oxygenated at the gill capillaries.

After it picks up oxygen, the blood travels though vessels which take it out to all of the other tissues in the body (the systemic capillaries), where it delivers that oxygen, and other things like nutrients. Since the oxygen is leaving the blood and entering the tissues, we see it turn blue again on the diagram, and from there it travels by veins back to the heart. (And then it repeats)

Lung ventilation is accomplished by sets of skeletal muscles that work to set up pressure gradients.

Air moves from high pressure to low pressure, so in order to get air into the lungs, the pressure there must be lower than the pressure outside them.

Both amphibians and reptile lungs are much less efficient than those of birds and mammals though. The reason for that is their metabolism.

Amphibs and reptiles are ectothermic, while birds and mammals are endothermic. That means the metabolic rates for amphibs and reptiles are much lower, so they don't have as great a need for fast gas exchange.

Blood doesn't leave the capillaries, but gases, nutrients and wastes can diffuse back and forth across the thin capillary wall.

And all of our body cells are in close proximity to capillaries so they can get access to these chemicals via diffusion. (Capillary walls are composed of simple squamous epithelial tissue, which you'll learn about in lab).

The flow of water across the gill lamellae is in the opposite direction from the flow of blood through the capillaries.

Any time there are two fluids that need to exchange materials, the exchange rate is more efficient if they move in the opposite direction, as process called countercurrent exchange.

LOOK AT PPT SLIDES This diagram shows the basics of gas exchange at both the lungs and the tissues. At the lungs, the PO2 in the alveoli is ~100 mm Hg, while the PCO2 is 40 mm Hg.

As the deoxygenated blood travels into the lung capillaries, it contains a PO2 of 40 mm Hg and a PCO2 of 46 mm Hg. So - oxygen diffuses from the alveolar air into the blood, while CO2 diffuses from the blood into the alveoli. As the oxygenated blood then leaves the lungs, it carries a PO2 of 100 mm Hg, while the PCO2 has dropped to 40 mm Hg. (matching the air in the alveoli).

These next two diagrams just show the flow of CO2 movement at the tissues and the lungs. LOOK AT PPT SLIDES

At the tissues, CO2 is moving into the blood, where it forms H+ and bicarb (catalyzed by carbonic anhydrase). The H+ binds to hemoglobin, while the bicarb is released to the plasma and is transported there.

Both the right and left sides of the heart contract and relax together. During atrial systole, the right and left atria are contracting, which sends blood down into the ventricles. This is followed by atrial diastole and ventricular systole. As the atria begin to relax, the ventricles begin to contract. That contraction squeezes the blood, generating pressure.

At this point the AV valves shut, preventing the blood from returning to the atria and ensuring that it exits via the major arteries. When sufficient pressure is generated, the semilunar valves (pulmonary and aortic) open and blood is ejected from the heart into the pulmonary artery and aorta.

Air is first brought into the oral cavity by drawing it in through the nostrils as the floor of the mouth drops down. Once the mouth is filled with air, the animal closes its nostrils and raises the floor of its mouth. This decreases the volume in the mouth, raising the pressure of the air.

At this point the air pressure in the mouth is higher than the pressure in the lungs, so the air travels into the lungs along that pressure gradient (literally pushing air into the lungs).

Gas exchange in air has some major advantages - there are higher levels of oxygen in the air compared to the water, the air is less viscous so moves easier than water for ventilation.

BUT, one major downside is that thin gas exchange membranes dry out very quickly, so most terrestrial organisms undergo gas exchange with internal structures. As air is brought into the lungs, it is cleaned and humidified, so that the delicate lung tissues remain protected.

Each gill arch is composed of strong connective tissues and has extensions called gill filaments. Each slender gill filament has thin folds called lamellae and each of those folds contains a blood capillary network.

Because these folds of tissue are so thin, blood is VERY close to the surface, so its easy for gases to pass back and forth between the blood and the water. The extensive folds also means there's A LOT of surface area for exchange to take place. In addition to thin lamellae and huge surface, the direction of water flow and blood flow in the gill also improve efficiency.

For the blood vessels, there are three major types: arteries carry blood away from the heart; veins carry blood back to the heart, and capillaries are where exchange takes place between the blood and the tissues.

Because they are transport vessels, arteries and veins are relatively thick walled; oxygen and other components can't diffuse across them. Capillaries though are VERY thin (only a single cell layer).

So both of these gases are diffusing from higher to lower partial pressures. Oxygen diffuses into the blood at the lungs and then out of the blood at the tissues (delivering oxygen to our cells).

Carbon dioxide diffuses into the blood at the tissues (where it is being produced) and out of the blood at the lungs (where it's then exhaled).

Each of these disorders are pretty common in the U.S, where our diet is not great (too much fat, sugar, etc) and not enough exercise.

Cardiovascular diseases in general are the most common cause of death in this country (~25% of all deaths) and most are preventable!

The vessels are connected in a loop; the blood never comes directly in contact with organs, but it gets close enough for exchange via diffusion across the walls of the blood vessels.

Cephalopods, annelids and all of the vertebrate animals have this type of system.

If RBCs or hemoglobin values are low, anemia results. Anemia is any condition where the oxygen carrying capacity of the blood is below normal, and it can be caused by lots of things.

Certain nutritional deficiencies can prevent RBCs from being produced in adequate numbers, for instance iron deficiency. (Iron is a component of heme). Certain genetic disorders, like sickle cell, can lead to dysfunctional hemoglobin. Conditions like blood loss can also lead to anemia.

Blood Vessels • Arteries • Capillaries • Veins

Closed systems have different components and are different among different vertebrate animals.

The sequence of contraction and relaxation events of the heart that control this blood flow is called the cardiac cycle.

Contraction of the heart chambers is called systole, while relaxation is called diastole.

To recap - muscle movement creates volume changes; volume changes lead to pressure changes; pressure changes create air flow.

During inhalation, muscles contract, volume increases, pressure decreases, and air is pulled in because the pressure in the lungs is negative (less than) compared to the pressure outside the body. During exhalation, muscles relax, volume decreases, pressure increases, and air is pushed back out as the pressure in the lungs becomes greater than the pressure outside.

It takes two cycles of inhalation and exhalation to move one breath of air through the whole system. During the 1 st inhalation, air is drawn into the posterior air sacs. The 1st exhalation moves the air through the lungs.

During the 2nd inhalation, the air in the lungs is moved into the anterior air sacs as the next breath of air is drawn in to the posterior sacs. The initial breath is then pushed out during the 2nd exhale.

Hemoglobin has the important characteristic of being able to both bind and release oxygen easily. (Binds oxygen at the lungs; releases it at tissues).

Each hemoglobin protein contains four heme polypeptides, each with a heme group. An iron ion in the center of each heme group binds to the oxygen molecule. So each hemoglobin protein can carry four molecules of oxygen. Each RBC can contain millions of hemoglobin proteins, meaning that each RBC can carry up to billions of 22 molecules of O2! (And we have trillions of RBCs in our blood at any one time - that gives you an idea just how many molecules of oxygen we use!)

The opposite events occur during exhalation. The diaphragm relaxes, bringing it up; rib muscles relax, collapsing the chest cavity.

Each of these events decreases the volume of the chest cavity and lungs. Decreased volume means increased pressure. The air in the lungs is now at a higher pressure than the air outside the body, and air flows out.

The circulatory system is a transport system to deliver things to/from cells.

Every cell needs things like oxygen, nutrients, ions, hormones, etc and needs to get rid of waste products like carbon dioxide. Those things enter and leave cells via diffusion and other types of membrane transport processes. Diffusion is only effective over short distances though.

The most efficient type of circulatory system is a two circuit pathway with a 4- chambered heart. That's what all birds and mammals have, as well as crocodilians.

In this heart, there are two atria and two ventricles.

LOOK AT PPT SLIDES These images show the gills in more detail. On the top left, you can see the overall gill structure; the operculum has been folded back to make the gills visible.

In the more magnified views in the other picture and diagrams, you can see the gill arches and gill filaments.

Decreases in CO2 do the opposite.

If respiratory rate is too high (hyperventilation, such as during panic attacks), too much CO2 is lost from the blood, not as much H+ are produced and pH rises. (Resulting in respiratory alkalosis). So CO2 levels are very important for acid/base balance.

The right ventricle pumps to lungs (pulmonary) and the left pumps to body (systemic). Since these chambers are entirely separate, they have been able to evolve different thickness and strengths. The left ventricle, which pumps blood to the whole body, is a stronger pump and can generate higher pressure than the right.

For example, in humans the pressure of the blood leaving the left ventricle during contraction is ~120 mmHg. Pressure leaving the right ventricle is only ~20mmHg.

Partial pressure is defined as the pressure of an individual gas within a mixture of gases.

For example, the partial pressure of oxygen at sea level is 160 mm Hg. That's because the total atmospheric pressure at sea level is 760 mm Hg, and oxygen makes up about 21% of the total gases in the air. (Nitrogen is the most abundant gas, at almost 80%). If we multiple 760 mm Hg by 0.21, we get 160 mm Hg, which is the partial pressure of oxygen alone. The partial pressure of oxygen in air decreases with elevation. At higher altitudes, the total air pressure is less, which means the PO2 is also lower. That's why it can be harder to breathe at high altitudes, and people can get altitude sickness. Oxygen levels are lower in water than in the air because oxygen doesn't dissolve well in water, so respiration is actually easier for terrestrial animals.

Aquatic vertebrates breathe via gills. Gills are outgrowths of the body surface or the pharynx (throat region).

For most verts, the gills form from the pharyngeal slits and arches that we discussed in the chordate section (one of the common features that all chordate embryos share).

Our last major topic is capillary exchange. Recall from our earlier definitions that capillaries are very thin walled vessels, and this is where all gases, nutrients, water, ions and wastes are exchanged between the blood and the tissues.

For the solutes, they diffuse back and forth across the capillaries according to their concentration gradients.

Our last topic is gas transport.

For vertebrates, as well as many invertebrates, oxygen is transported by hemoglobin. (Some inverts contain other molecules for O2 transport).

Mammalian heart is a four-chambered heart - two atria and two ventricles - that are just designated as right/left.

Four heart valves are also present to keep blood flowing the right direction. There are two atrioventricular (AV) valves and two semilunar valves.

The ventricles then begin to relax (diastole) and the semilunar valves shut again to ensure that the blood doesn't flow backwards from the arteries into the ventricles.

From that point on, the entire heart is relaxed, which allows it to fill up with blood again prior to the next cycle.

Respiratory systems take advantage of something called Boyle's Law, which state that pressure and volume are inversely related.

If you have an enclosed container, if you increase the volume of the container then air pressure will decrease. If the volume is decreased, the air pressure will increase.

Some of the other animals that lack circulatory systems include those with sac-like body plans, like cnidarians (jellyfish, etc) and acoelomates like flatworms.

In each of these animals, there are no complex tissues or organs, their body mass is relatively thin, and they all have high SA:V ratios, so the exchange rates between the animal and their environment is high

After blood has traveled through the systemic circuit, it returns to the right atrium of the heart via two major veins - the superior and inferior vena cavae. The blood is deoxygenated at this point because it has delivered that oxygen to the tissues. From the right atrium, the blood travels through the open AV valve into the right ventricle. The ventricle then pumps the blood out to the lungs via the pulmonary artery. Note the pulmonary valve (one of the semilunar valves) between the right ventricle and the pulmonary artery. The blood travels through the pulmonary artery and to the capillaries of the lungs, where the blood picks up oxygen and gets rid of CO2.

Fully oxygenated blood then returns to the heart via the pulmonary veins, which deliver it to the left atrium. From there it moves into the left ventricle (past the open left AV valve), which pumps it into the aorta. Note the last valve - the aortic valve - in between the left ventricle and aorta. The aorta is the largest artery of the body. It delivers that oxygenated blood into the systemic circuit and out to all of our tissues/organs. After passing through the systemic capillaries, it returns to the heart again via the vena cavae.

Different types of vertebrates have different kinds of heart, but there is always at least one receiving chamber, called an atrium (or atria, pl.), and one pumping chamber, called a ventricle.

Heart and blood vessels; chambers

The numbers are showing the percentage of oxygen that is present in both fluids. If we look at the concurrent flow (on the right), we start off with 100% oxygenated water, and deoxygenated blood (0%). The partial pressure gradient (diffusion gradient) is very steep, and oxygen will quickly begin diffusing into the blood.

However, once the percentages reach 50/50, diffusion stops. So the blood has only been able to obtain 50% of the oxygen that was in the water.

With the rest of the lecture we're going to discuss gas exchange and transport. Gases are exchanged at the lungs (or gills) as well as at the tissues. Partial pressure gradients govern diffusion.

I mentioned partial pressures at the beginning of the respiratory lecture, but to review, the P.P. is the pressure exerted by an individual gas within a mixture of gases. At sea level, oxygen has a partial pressure in the atmosphere of 160 mm Hg. As I mentioned a few slides back, for mammals that pressure drops to ~100 mm Hg at the alveoli (due to tidal ventilation).

In bony fishes, ventilation is accomplished by several 8 muscles in the mouth and gill area, and by coordinated opening and closing of the mouth and operculum. In the diagram on the left, the mouth is open and the operculum is closed.

In this position, water is brought into the mouth. This is followed by closing the mouth and opening the operculum, which forces the water in the mouth to flow past the gill filaments as it exits via the gill slits. These movements constantly repeat, continually moving water in a single direction past the gills.

In order to work properly, the airways leading into the lungs need to be clear and wide open, the lung tissues need to remain elastic (easily stretching and recoiling during ventilation), and the we need to maintain surface area and maintain a thin gas exchange surface.

Infections & inflammation of the lungs, which can include pneumonia, tuberculosis, and bronchitis, lead to mucus production which can restrict airways, and inflammation which can produce edema (fluid build up) that thickens the alveoli and prevents proper gas exchange.

Movements of all gases in/out of the body occurs via diffusion; there is no active transport here.

Instead of diffusing along concentration gradients, gases diffuse along partial pressure gradients.

Gills can either be external or internal. External gills (on the right) have no secondary coverings; they are completely exposed to the outside environment (water), like those we see on the amphibians here.

Internal gills (on the left) are still outgrowths from the 7 body (they're not like our truly internal lungs), but they have a secondary covering that gives them some protection. This is the type that fishes have, and we'll look more specifically at the gills of bony fish to see how they work.

Blood is a type of connective tissues. As we've been discussing, it functions in transport - carrying oxygen, nutrients, hormones, wastes, water and ions throughout the body.

It also has several other functions. It contains components of the immune system that fight off pathogens, it helps control pH and osmotic balance, and it also helps regulate body temperature by transporting heat.

LOOK AT PPT SLIDES On this diagram, you can see the pressure in the arteries fluctuating between 120 and 80 (systolic and diastolic pressures), but it levels out to a steady pressure as it enters smaller arteries and eventually into capillaries.

It drops close to 0mm Hg in the largest veins as it returns to the heart.

Emphysema is a condition where the alveolar walls breakdown and fuse together.

It turns lots of little tiny alveolar sacs into fewer, larger sacs that reduces surface area - smoking is the major risk factor for this one, as well as for development of lung cancer.

When blood is being carried away from the heart in a vessel (leaving the heart), then it is an artery.

LOOK AT PPTS SLIDE Question: What type of blood vessel is shown at the arrow? a. Artery b. Vein c. Capillary In the vessel at the arrow, blood is being carried away from the heart (it's leaving the heart). That makes this vessel an artery. Students can get confused on these diagrams because this vessel is blue, which means it carries deoxygenated blood, and there's a misconception that arteries always carry oxygenated blood (represented in red). That's only the case for a portion of our circulatory system though, which we'll discuss in more detail shortly.

Gas exchange occurs across these surfaces via diffusion, so anything that maximizes diffusion rate will enhance respiration...

Larger surface areas provide a greater exchange surface, and many respiratory surfaces have folds or other modification to increase surface area. Most of these surfaces are also very thin so that the boundary between the air/water and the circulatory fluid is very small.

There are multiple types of gas exchange surfaces that different animals use.

Many organisms just exchange gas across body surface; no specialized structures.

In general, we think of these open circulatory systems as being less efficient than the type we have (which are called closed systems) and thus organisms that have them tend to be less active (they have lower metabolisms b/c their tissues can't receive oxygen and nutrients as quickly as ours).

Most molluscs have open circulatory systems (with the exception of the cephalopods), as well as arthropods. You can see the typical anatomy of these open circulatory systems in diagrams of the clam and grasshopper on PPT SLIDES .

In countercurrent flow, since the fluids are moving in the opposite direction, a favorable diffusion gradient is maintained across the entire surface of the gill.

Most of the oxygen is removed from the water and enters the blood.

For humans, the average heart rate is around 70 or 75 bpm (beats per minute), which means that this whole cycle takes a little less than a second to complete.

Note the sound waves on the diagram too - this represents the sounds that are made as the heart valves shut, creating a characteristic "lub dub" sound. The first sound is the AV valves shutting, and the second is the semilunar valves.

LOOK AT PPT SLIDES These diagrams compare countercurrent flow (which is what is actually seen in fish gills) with a hypothetical concurrent flow.

On the left, we have water and blood flow in the opposite direction and on the right, both water and blood are moving in the same direction.

So on the arterial end of the capillary, blood pressure is greater than osmotic pressure, and the net pressure (the balance between the two) tends to push fluid OUT.

On the venous end, blood pressure has dropped below osmotic pressure, so the net pressure tends to drawn fluid back IN.

This chemical reaction shows how CO2 becomes bicarb. When CO2 enters the blood, an enzyme called carbonic anhydrase catalyzes this first reaction, where CO2 combines with water to form carbonic acid.

Once that carbonic acid forms, most of it dissociates (breaks apart) into two ions: H+ (hydrogen ions) and HCO3- (bicarbonate). The majority of the H+ is buffered by hemoglobin and other proteins in the blood, but the bicarb is released to the plasma. Carbonic anhydrase is primarily found in RBCs, so that's where most of these reactions are taking place.

Respiration means gas exchange, and we're particularly talking about oxygen and carbon dioxide.

Organisms exchange these gases with either the air or water, via a variety of different organs, including lungs, gills and the skin (body surface).

For example, very small animals like hydras and certain roundworms are so tiny that the diffusion distance between their outer environment and each body cell is relatively short.

Oxygen and other chemicals in the water can move via simple diffusion across their entire body in a short period of time. So there's no need for a dedicated organ system for circulating these things.

The circulatory system is also involved, since it transports these two gases.

Oxygen comes in at the respiratory surface (lungs, gills, or skin), is picked up by the blood (hemoglobin in RBCs) and then delivered to the tissues at capillaries. CO2 makes the reverse trip.

Blood is composed of both a liquid portion, called plasma, as well as solid formed elements.

Plasma is mostly water, with lots of dissolved solutes, including proteins, salts/ions, nutrients, gases, wastes and hormones.

• Amphibians - positive pressure ventilation - Air is pushed into lungs as mouth/throat contract

Positive pressure ventilation of frog lungs

The contraction of heart chambers generates pressure, which pushes the blood out of the chamber and generates flow.

Relaxation of the chambers allows pressure to drop so that the heart can fill up with blood again.

LOOK AT PPT SLIDES This diagram gives a comparison of different vertebrate lungs. Amphibians have the simplest lung structure; basically smooth sacs. They don't need highly efficient lungs because they can accomplish a large percentage of their gas exchange across their skin surface.

Reptile lungs are a little more complex, with some folding to increase surface area. Since reptiles have keratinized skin, body surface exchange cannot occur.

Recall Boyle's Law - pressure and volume are inversely related.

So as the volume of the lungs increases, the air pressure in the lungs decreases. That sets up a favorable gradient for air to move into the lungs because the lung pressure at that point is lower than the atmospheric pressure surrounding the body.

In the systemic capillaries, oxygen and CO2 are being exchanged between the blood and the tissues of the body. Most tissues have a PO2 of 40 mm Hg and a PCO2 of 46 mm Hg (at rest; it changes during exercise).

So the partial pressure gradients are favorable for oxygen to diffuse from the blood into the tissues and for CO2 to diffuse from the tissues into the blood.

Blood flow is dependent on a pressure gradient; fluids always move from high pressure to low pressure. If we think about the way blood flows, it moves out of the heart and into arteries, then through capillaries, then through veins where it returns to the heart.

So the pressure is highest at the arteries, lower in capillaries, and lowest in veins. It continually declines as it moves away from the heart and gets farther from the source of pressure.

With this system the fluid loses a lot of pressure when it travels through gill capillaries (because it spreads out from one big vessel into lots of tiny capillaries).

So while its better than an open circulatory system, it's a lower pressure/lower flow system compared to next ones we'll discuss. Fishes don't have to fight gravity, and most are ectothermic with lower metabolic rates, so this system works well enough for them.

For larger organisms, SA:V ratios are smaller and tissues/organs are thicker and more complex. In these cases direct diffusion from environment is not going to reach every cell in the body.

So, as animals got larger, circulatory systems evolved. And there are different types - not all are like ours - but in general each type contains three components: 1) the circulatory fluid which carries the dissolved gases, nutrients, etc,; 2) a pump/pumps, which generates pressure to get the fluid moving; and 3) the vessels which carry the fluid around the body.

Counter current is more efficient. A pressure gradient exists along the entire surface of the lamella, and the blood is able to remove most of the oxygen from the water as it passes through.

So....fish gills are very efficient! They have a very thin respiratory surface (the blood is in very close proximity to the water), they have a huge surface area (all those folds [lamellae]) and they have countercurrent exchange which ensure that the blood receives the maximum amount of oxygen from the water.

Carbon dioxide transport is a bit different that oxygen.

Some CO2 is dissolved in the blood, and hemoglobin actually transports a portion of it as well. Most of the CO2 though (~70%) is transported as bicarbonate ions in the plasma.

Insects have air tubes called tracheae - network of tubes with spiracles that control air flow - very efficient.

Terrestrial vertebrates have lungs. Lungs are invaginations of the body adapted for breathing air. Because they are internal, the tissues are protected from drying out.

Even though arthropods have open systems, many of them are some of the most active animals and can have high metabolic rates.

That because their respiratory system has a network of tubes that delivers oxygen to their tissues, so they don't rely on the circulatory fluid for that.

The net pressures on each end don't balance equally though, so more fluid leaves the blood than re-enters.

That excess fluid is taken up by a different system called the lymphatic system; it enters lymphatic capillaries where it flows through lymph nodes on way back to bloodstream - important in immunity and defense. It helps cleanse the fluid and gives the immune system a better chance of catching pathogens.

Reptiles, birds and mammals ventilate the lungs via negative pressure.

That means air is pulled into the lungs as the lung volume increases.

These organisms also tend to be ones that either live in the water or live in pretty humid environments. That's because their skin is thin.

That's great for gas exchange, but it also makes them vulnerable to drying out (If skin is permeable to gases, then it's also permeable to water). This is true of gas exchange surfaces in general - if they 6 are thin enough to allow gas exchange, they also allows water to pass and can dry out easily (that's why terrestrial verts have internal lungs).

Hypertension (high blood pressure) is a very common disorder. It's impacted by lots of different factors, including genetics, diet and other lifestyle factors.

The "old" guidelines defined hypertension as a blood pressure reading of 140/90 or greater, but that was revised in 2017 to anything greater than 130/80. Hypertension is dangerous because it makes the heart work harder than it should (it has to pump against a higher pressure), so it weakens the heart over time, as well as weakening the blood vessels.

At the lungs, the reverse occurs. Bicarb moves back into RBCs and carbonic anhydrase catalyzes the same reaction in reverse, forming CO2 and water

The CO2 then diffuses out of the blood and into the lungs where it can be exhaled.

Gas Exchange

• Gases diffuse along partial pressure gradients - Partial pressure = pressure of individual gas within a mixture

Blood pressure is the pressure of the blood on the vessel walls (essentially water pressure) and is always higher within the capillary than in the tissue fluid - it forces fluid OUT of the capillary.

The blood pressure declines as the blood moves through the capillary though; it's about 35 mmHg on the arterial end and only about 15 mmHg on the venous end.

Since the fluid is not always contained in vessels, and instead gets dumped out into a large cavity, this is a low pressure, low flow type of system.

The circulatory fluid is being pumped, but it's not moving very quickly.

Hemolymph

The circulatory fluid of animals with open circulatory systems (e.g.,insects), in which the fluid is not confined to blood vessels.

The circulatory system picks up oxygen from the lungs/gills/etc (where it's being exchanged with the environment) and then carry it all over the body so that each cell has what it needs (mammalian lungs as example - all of our gas exchange is taking place in the chest, the blood then picks up that oxygen and carries it everywhere).

The circulatory system doesn't just facilitate exchange with respiratory surfaces; but also carries things from/to the digestive system, excretory system, etc. It's our piping system that allows essential chemicals to travel to all tissues/organs.

In contrast to open system, closed circulatory systems can support more active lifestyles because they deliver oxygen and nutrients to tissues more quickly. (These are more efficient systems)

The fluid of these systems is called blood, and that blood remains completely enclosed within blood vessels (it doesn't spill out into a large cavity like in the open system). Since the blood in enclosed in smaller spaces, these systems maintain a higher pressure and higher flow rate.

The picture shows blood that has been spun down in a centrifuge.

The formed elements pack at the bottom and the plasma stays on top. In humans, the formed elements make up about 45% of the total; the majority of that are the red blood cells.

For the clam, there is a single heart that pumps the fluid into vessels which open up into the hemocoel cavity. Other vessels then collect the fluid and deliver it back to the heart.

The grasshopper is similar but it has multiple pumps (hearts).

The simplest vertebrate circulatory system is a one-circuit system, which is the type most fishes have.

The heart of these animals has only two chambers: one atrium and one ventricle.

Endotherm lungs are very efficient organs.

The mammalian lung is divided into millions of tiny sacs called alveoli, which are surrounded by blood capillaries. The number of these sacs dramatically increases the surface area of the lungs. The structure of the alveoli is also VERY thin. The walls of the alveoli (as well as the walls of the blood capillaries that surround them) are composed of simple squamous epithelium, which is the thinnest tissue type animals have.

The heart is the pump of the circulatory system. It is there to generate pressure, which creates blood flow.

The more pressure that can be generated, the faster the blood will flow and the more efficiently oxygen and other chemicals can be transported. Hearts are composed of cardiac muscle and they contract and relax at regular intervals, which is controlled by pacemakers and the nervous system.

Asthma is a condition is which the bronchioles (airways) become inflamed and constrict.

The narrows the airways and makes it harder to ventilate the lungs.

Respiratory Systems

The other half of Ch. 42 covers respiration.

Because inhaled and exhaled air occupy the same passageways, "fresh" inhaled air mixes with the remains of the "used" exhaled air as it comes in. That means that the air that hits the respiratory surfaces (the alveoli, for mammals) has less oxygen than it does outside the body.

The partial pressure of oxygen in the atmosphere is 160 mmHg. By the time it hits our alveoli it has dropped to ~100 mmHg; that's because it's mixing with that previous exhaled air that has already been through gas exchange.

Virtually all animal cells produce ATP via aerobic cellular respiration. That is why we breathe. We have to get oxygen into our body and to our cells so that they can accomplish cellular respiration (do you remember specifically what oxygen is used for in this process???...it's the terminal electron acceptor for the electron transport chain).

The process of cellular respiration also produces carbon dioxide as a waste product (during pyruvate processing and the citric acid cycle); respiratory systems help us eliminate that CO2 from the body.

Amphibians and most reptiles have this system, along with a 3- chambered heart: two atria and a single ventricle.

The right atrium receives blood from the systemic circuit, while the left atrium receives blood from the pulmonary circuit. The single ventricle pumps to both circuits, so each circuit has about the same level of pressure and flow.

The AV valves are situated in between the atrium and ventricle on each side of the heart. They prevent blood from backing up into the atria during ventricular contraction.

The semilunar valves are found in between the ventricles and the major arteries. They prevent blood from backing up into the ventricles after it's been pumped out.

Circulation

The system responsible for moving oxygen, carbon dioxide, hormones, nutrients, wastes, and other materials within the animal body.

This is a big advantage for these animals.

The systemic circuit is now a much higher pressure than what can be generated with a three chambered heart, which means much higher blood flow rates and more efficient oxygen and nutrient delivery. Birds and mammals are both endotherms (maintain steady, high body temp) which means their metabolic rates are very high. Having a four-chambered heart with high systemic pressure allows us to "feed" that high metabolism. Note **Make sure you know the pathway of blood flow through these systems; we'll cover the 4-chambered heart in more detail in the next part of this lecture.

The pulmonary circuit carries blood to the respiratory capillaries (lungs), and then takes it back to the heart again. That means the heart has a chance to re-pump it and reestablish pressure.

The systemic circuit then carries it to everywhere else, and then returns it to the heart. So the blood travels though the heart twice, and gets pumped twice, maintaining a higher pressure and flow.

As the heart contracts and relaxes, it generates blood pressure, which is what ensures proper blood flow. In the major arteries, the pressure fluctuates in response to contraction and relaxation of the ventricles.

The systolic pressure is the higher number, and results from ventricular systole (contraction). Diastolic pressure is lower, and represents the pressure within the vessels during diastole (relaxation). The typical systemic blood pressure values for humans is 120/80 (systolic/diastolic).

LOOK AT PPT SLIDES This picture on the right shows just how thin this barrier is.

The walls of the alveolus and capillary are in the middle. The top of the picture shows the air inside the alveolus, and the bottom shows a portion of a red blood cell that is moving through the capillary. So oxygen and CO2 have a miniscule distance to cross via diffusion.

Rather than being subdivided into tiny sacs like mammals, bird lungs have millions of very tiny, thin-walled tubes called parabronchi that are surrounded by blood vessels.

The way they ventilate their lungs is more efficient than in mammals, and is similar to the countercurrent flow of fish.

Leukemia is a cancer of the WBCs.

There are multiple types depending on the type of WBC involved, but one of the symptoms is immunosuppression.

Open circulatory system example of PPT SLIDES. The fluid of this system - hemolymph - is not completely contained within the vessels of the system. There are a series of open-ended vessels that carry the fluid throughout body where it's "dumped" into the open body cavity called a hemocoel, where the fluid directly bathes the organs.

There are one or more pumps (hearts) that push fluid to generate pressure and send it out into the hemocoel. Once it enters that open cavity though, it loses pressure and slowly makes it way back to into the pump, which keeps the fluid circulating.

For most multicellular animals of any significant size, diffusion is not sufficient to exchange those materials with the outside environment.

There has to be a way to circulate those chemicals effectively in the body in order for them to reach all cells. That's the main function of circulatory system.

Bird ventilation is more efficient due to the presence of structures called air sacs that surround the lungs.

These air sacs allow air to flow in a single direction through the respiratory passages and lung tissue, so that "fresh" air and "used" air don't mix.

Formed elements include red blood cells, white blood cells and platelets.

These are all produced in blood forming tissues which are located in our bone marrow.

If we compare and contrast the circulatory systems of different animals, we actually see that some animals don't have circulatory systems (there are no organs dedicated to this function).

These are animals that are small and simple enough to be able to get everything they need through direct diffusion with environment or their gastrovascular cavity (gut).

The last component of the formed elements are platelets.

These are cellular fragments that participate in blood clot formation. When a vessel is injured/torn, circulating platelets bind to the site of the injury, stimulating a chain reaction that quickly brings in more platelets via positive feedback. This platelet plug temporarily seals the blood vessel until a more permanent clot can be formed.

Red blood cells are also called erythrocytes (which literally means "red cell"), and they are the most numerous blood cells.

These are packed with the protein hemoglobin, which transports oxygen. Hemoglobin contains a pigment called heme, which gives it the red color.

White blood cells are also called leukocytes. (literally meaning "white cell").

These cells are part of the immune system and function to defend the body against pathogens, clean up cellular debris, and control inflammation.

Internal gills form from the pharyngeal slits and arches during development. If we look closely at the diagrams, you can see that there are openings in the gills where water moves though (slits) and then gill filaments that arise from the arches.

These gills are considered internal because they are covered by a bony flap called the operculum, which protects the delicate gill filaments. Since they are not directly exposed to water like external gills though, they must be ventilated. Ventilation is the active process of moving water or air through the respiratory surfaces (gills or lungs), and is controlled by skeletal muscle movements.

Many aquatic organisms (inverts and verts) have gills - these are folded, thin projections from body that can be either external or internal.

They are structures adapted for gas exchange in water.

Weak/faulty valves produce a distinctive sound called a heart murmur as the blood rushes past the valve.

They can damage the heart over time by making the heart work too hard.

This reaction is also important b/c it tells us that CO2 concentrations are directly related to acid production. Significant increases in CO2 can overwhelm the buffering capacity of hemoglobin and release too many H+, decreasing blood pH.

This can occur during hypoventilation, when the breathing rate is too low and not enough CO2 is being exhaled. (Resulting in respiratory acidosis).

In mammals and reptiles, the air moving into and out of the lungs follows the same route. (Same passageways leading in and out).

This is called tidal ventilation.

To accomplish inhalation (bringing air into the lungs), the first thing that occurs in contraction of the diaphragm and rib muscles. The diaphragm is a large skeletal muscle situated in between the thoracic and abdominal cavities. When it contracts, it flattens out and moves downward.

This movements, in addition to contraction of the rib muscles that elevate the ribs, causes the volume of the chest cavity to increase. As the chest cavity volume increases, lung volume increases.

The air sacs prevent the inhaled and exhaled air from mixing too much, so the lungs are exposed to higher partial pressures of oxygen than in mammals.

This one-way flow also allows birds to have a similar countercurrent exchange of air and blood flow that fish have. (Technically it's a cross-current exchange for birds, but the concept is similar). Air is flowing one way and blood is flowing another, so it allows their blood to pick up more oxygen from the air than the mammalian system does.

Water is also exchanged (fluid exchange) and that's what we're going to discuss in more detail.

This process is controlled by two forces - blood pressure and osmotic pressure - and these forces oppose each other.

Circulation

Transport

1. Ventilation 2. Gas exchange 3. Circulation 4. Cellular Respiration

Ventilation with environment and gas exchange are part fo the respiratory system process. Circulation is part of the circulatory system process.

Tetrapod systems are more efficient because the we don't have the advantage of being surrounded by water, and the blood has to fight gravity more in order to move.

We have two circuits instead of one: the pulmonary circuit and the systemic circuit.

This diagram depicts the major events of the cardiac cycle. LOOK AT PPTS SLIDES

We'll start at the top with atrial systole

This diagram shows how the valves work. LOOK AT PPT SLIDES When they are open, blood is able to flow from one chamber to the next. When they are closed, the flaps of the valve form a seal and prevent the blood from backing up the wrong way.

With the AV valves, they allow blood to flow from atrium to ventricle, but prevent blood from moving from the ventricles back into the atria, ensuring that the blood continues to move in only one direction.

Question: Blood pressure is lowest in the... a) Vessels that carry blood away from the heart b) Vessels that return blood to the heart c) Vessels in which exchange occurs between the blood and tissue

b) Vessels that return blood to the heart

Open Circulatory System

systems where blood, rather than being sealed tight in arteries and veins, suffuses the body and may be directly open to the environment at places such as the digestive tract.