Chapter 38 Notes

The exchange of materials (whether nutrients, gasses or waste products) between an organism and its environment must take place across a moist cell membrane.

• The molecules must be dissolved in water in order to diffuse or be transported across the membrane.
• In protozoans, the entire external surface may be used for this exchange.
• The simple, multicellular animal (sponges, cnidarians) have body structures such that each cell is exposed to the surrounding waters.

Three-dimensional animals face the problem that some of their cells are isolated from the surrounding environment.

• These animals have specialized organs, where exchange with the environment occurs, coupled with special systems for the internal transport through body fluids to the cell.
• The association of specialized organs with an internal transport system not only reduces the distance over which molecules must diffuse to enter or leave a cell, but permits regulation of internal fluid composition.

I. Transport systems functionally connect body cells with the organs of exchange: an overview

Diffusion is too slow of a process to transport chemicals through the body of an animal.

· Time of diffusion is proportional to the square of the distance the chemical must travel.

· If a glucose molecule takes 1 second to diffuse 1 µm, it will take 100 seconds to diffuse 1 mm.

The presence of a circulatory system reduces the distance a substance must diffuse to enter or leave a cell.

• The distance is reduced because the circulatory system connects the aqueous environment of the cell with organs specialized for exchange.
• Molecules will diffuse or be transported into the blood which carries these molecules to the cells or to the exchange surface depending on whether the molecules are used (nutrients) or produced (wastes) by the cells.

à Oxygen diffuses from air in the lungs across the thin lung epithelium into the blood.

à This oxygenated blood is carried by the circulatory system to all parts of the body.

à As the blood passes through capillaries in the tissues, oxygen diffuses from the blood into the cells across the cells’ plasma membranes.

à Carbon dioxide is produced by the cells and moves in the opposite direction though the same system.

The circulatory system does more than move gases, it is a critical component to maintaining homeostasis of the body.

• The chemical and physical properties of the immediate surroundings of the cells can be controlled using the carrying capacity of the blood.

à The blood passes from the cells through organs (liver, kidneys) which regulate the nutrient and waste contend of the blood.

II. Most invertebrates have gastrovascular cavity or a circulatory system for internal transport

1. Gastrovascular Cavities

The cndarian body plan does not require a specialized internal trasport system. The body wall is only two cells thick and encloses a central gastrovascular cavity.

• The water intside the gastrovascular cavity is continuous with the surrounding water.
• The gastrovascular cavity functions in digestion and distribution of nutrients.
• Gastrodermal cells lining the cavity have direct access to the nutrients produced by digestion, however the structure of the bodyis such that nutrients only have a short distance to diffuse to the outer cell layer.

Planarians and other flatworms also have a gastrovascular cavity.

• The highly ramified structure of the cavity and the flattenened body shape ensure all cells are exposed to cavity contents.

A gastrovascular cavity cannot perform the necessary internal transport required by more eomplex animals, expecially if they are terrestrial.

• These animals have some type of circulatory system.

2. Open and Closed Circulatory Systems

Open and closed circulatory systems are alternative soulutions to moving materials efficiently through the bodies of animals.

• They usually function in combination with other organ systems which increases the efficiency of the overall body.

Insects, other arthropods, and mollusks have an open circulatory system.

Open circulatory system = Circulatory system in which hemolymph bathes internal organs directly while moving through sinuses. (See Campbell, Figure 38.2a)

• Hemolymph is body fluid which acts as both blood and interstital fluid.
• Circulation results from contractions of the dorsal vessel (heart) and bodymovements.
• Relaxation of the “heart” draws blood through the ostia (pores) inot the vessel.
• Chemical exchange between the hemolymph and cells occurs in the sinuses which are an interconnected system of spaces surrounding the organs.

A closed circulatory system is found in annelids, squids, octopuses, and vertebrates.

Closed circulatroy system= Circulatoryste n which blood is confined to vessels and distinct interstitial fluid is present.

• The heart (or hearts) pumps blood into large vessles.
• The major vessels branch into smaller vessels which supply blood to organs. (See Cabell, Figure 38.2b)
• Blood exchanges material with the interstital fluids which come into direct contact with the cells.

III. Diverse adaptations of a cardiovascular system have evolved n vertebrates

A cardiovascular system consists of a heart, blood vessels and blood.

• The heart has of one or two atria, chambers that receive blood, and one or two ventricles, chambers that pump blood out.
• Arteries carry blood away from the heart to organs where they branch into smaller arterioles that five rise to microscopic capillaries (the site of chemical exchange between blood and that give rese to microscopic capillaries (the site of chemical exchange between blood and interstitial fluid).

à Capilaries have thin, porous walls and are usually arranged into networks called capillary beds that infiltrate each tissue.

• Capillaries rejoin to form venules which converge to form the veins that return blood to the heart.

An examination of vertebrate circulatory systems shows various adaptations have evolved within this taxon.

Amphebian- Fish have a 2-chambered heart with one atrium and one ventricle. (See Cambell, Figure 38.3a)

• Blood pumped form the ventricle goes to the gills; here oxygen diffuses into the gill capillaries and CO2 diffuses out; gill capillaries converge into artieries that carry blood to capillary beds in other organs; blood from the organs travel through veins to the atrium of the heart, then into the ventricle.
• Blood flows through two capillary beds during each complete circuit: one in the gills and a second in the organ systems (systemic capillaries).

à As blood flows through a capillary bed, blood pressure drops substantially.

· Blood flow to the tissues and back to the heart is aided by swimming motions.

Amphibians (Reptiles) have a 3 chambered heart with two atria and one ventricle. (See Cambell, Figure 38.3b)

· Blood flos through a pulmonary circuit (To the lungs and skin) and a systemic circuit (to all other organs) in a scheme called double circulation.

· Blood flow pattern: ventricle à lungs and skin to become oxygenated à left atrium à ventricle à all other organs à right artium.

àthe second passage through the ventricle ensures sufficient blood pressure for the systemic circulation.

· There is some mixing of oxygen-rich and oxygen-poor blood in the single ventricle although a ride present in the ventricle diverts most of the oxygenated blood to the systemic circuiut and most of the deoxygenated blood to the pulmonary circuit.

Most reptiles (excluding crocodilians) have a three chambered heart although the ventricle is partially fdivided.

· This reduces mixing of oxygenated and deoxygenated blood.

Birds and mammals have a four-chambered heart with two atria and two ventricles. (please note that crocodiles also have a four-chambered heart.)

· Double circulation is similar to that of amphibians except that oxygenated and deoxygenated blood does not mix due to the presence of two separate ventricles.

· The complete separation of oxygenated and eoxygenated blood increases the efficiency of oxygen delivery to the cells.

IV. Rhythmic pumping of the mammalian heart drives blood through pulmonary and systemic circuits

1. The heart: General from and function

The human heart is:

• Located beneath the sternum.
• Cone-shaped and about the size of a clenched first.
• Surrounded by a sac with a two-layered wall. (pericardium)
• Comprised mostly of cardiac muscle tissue.
• The two atria have thin walls and function as collection chambers for blood returning to the heart.
• The ventricles have thick, powerful walls that pump blood to the organs.

The heart chambers alternately contract and relax in a rhythmic cycle.

• A complete sequence of contraction and relaxation is the cardiac cycle.
• During systole, heart muscle contracts and the chambers pump blood.
• During diastole, the heart muscles relax and the chambers fill with blood.

There are four valves in the heart which prevent backflow of blood during systole: (See Campbell, Figure 38.4b)

• The valves consist of flaps of connective tissue.
• Atrioventricular valves are found between each atrium and ventricle and keep blood from flowing back to the atria during ventricular contraction.
• Semilunar valves are located where the aorta leaves the left ventricle and where the pulmonary artery leaves the right ventricle; these prevent blood from flowing back into the ventricles when they relax.
• A heart murmur is a defect in one or more of the valves that allosw backflow of blood. Serious defects are usually corrected by surgical replacement of the valve.

Heart Rate= The number of heartbeats per minute.

• Usualy measured by taking the pulse
• In humans, the average resting heat rate of a young adult is 60 beats per minute; rates vary depending on the individual’s activity level.
• Its is not uncommon for heart rates to change during the day.
• There is an inverse relationship between animal body size and pulse; elephants have a rate of 25 beats per minute while some shrews have 600 beats per minute.

Cardiac output = the volume of blood per minute that the left ventricle pumps into the systemic circuit; depends on heart rate and stroke volume.

• Stroke volume is the amount of blood pumped by the left ventricl each time it contracts. The average human stroke volume is about 75 ml per beat.
• The cartiac output of an average human is 5.25 L per minute.

2. Control of the Heart

Cardiac muscle cells are myogenic (self-excitable) and can contract without imput from the nervous system. The tempo of contraction is controlled by the sinoatrial (SA) node, a specialized region of the heart, sometimes called the pacemaker.

· Located in the right atrium wall near the enterace of the superior vena cava.

· The SA node is composed of specialized muscle tissue which has characteristics of both muscle and nerve tissue.

· Contraction of the SA node initiates a wave of excitation that spreads rapidly from the node and causes the two atria to contract in unison.

· This wave of contraction will pass down the atria until it reaches the atriventricular (AV) node; a second mass of specialized muscle tissue located near the base of the wall separatind the atria.

· The impulse is delayed at the AV node for 0.1 second to ensure the atria are completely empty before the ventricles contract.

· The impulse is then carried by a mass of specialized muscle fibers to the tip of the ventricles; the impulse then travels through the Purkinje fibers upward through the ventricular walls.

· The impulses produce electrical currents as they pass through the cardiac muscle.

· An electrocardiogram (EKG) is the detection of these currents which are conducted through the body fluids to the body’s surface.

Although the SA node controls the rate of heartbeat, is infunced by several factors:

• Two antagonistic sets of nerves influence the heart rate; one speeds up contractions in the SA node, the other slows contractions.
• Hormones infuence the SA node wit some like epinephrine increasing the rate
• Other factors, including body temperature changes and exercise, also have an influence.

à exercise creates a greater demand for oxygen n the muscles and an increase in heart rate is an adaptation to meet the demand.

3. Blood Vessel Structure

The walls of arteries and veins have 3 layers:

• AN outer layer of connective tissue with elastic fibers that permits stretching and recoil of the vessel.
• A middle layer of smooth muscle and elastic fibers.
• An inner endothelium of simple squamous epithelium. (see Campbell, figure 38.7)
• The middle and outer layers of ateries are thicker than those in veins. Capillaries are comprised only of the endothelial lining which permits the exchange of chemicals with the interstitial fluids.

4. Blood Flow Velocity

There is a great difference in the speed at which blood flows through the various parts of the circulatory system. Blood travels about 30 cm/sec in the aorta and about 0.026 cm./sec in capillaries.

· The velocity decreases in accordance with the law of continuity which states tat a fluid will flow faster through narrow portions of a pipe than wider portions if the volume of flow remains constant.

· An artery fives rise to so many arterioles and then capillaries that the total diameter of vessels is much greater in capillary beds than in the artery, thus blood flows more slowly in capillaries.

· Resistance to blood flow is greater in the smaller vessels since the blood contacts more epithelial surface area.

· Blood flows faster as it enters the venules and veins since the cross-sectional area is decreased.

5. Blood Pressure

Fluids are driven through pipes by hydrostatic pressure which is the force exerted by fluids against the surfaces they contact.

Blood pressure = The hydrostatic force that blood exerts against a vessel wall.

• Pressure is greater in arteries than in veins and greatest during ventricular systole.

à This is the main force propelling blood from the heart through the vessels.

• Peripheral resistance results from impedance by arterioles; blood enters the arteries faster than it can leave.

à Thus, there is a pressure even during diastole, driving blood in capillaries continuously.

• Determined by cardiac output and degree of peripheral resistance.

à Stress may trigger neural and hormonal responses that cause the smooth muscles of vessel walls to contract, constricting blood vessels and increasing resistance.

• In veins pressure is near zero; blood returns to the heart by the action of skeletal muscles around the veins.

à Veins have valves that allow blood to flow only towards the heart.

• Breathing also helps return blood to the heart since the pressure change in the thoracic cavity during inhalation cause the vena cavae and large veins near the hear to expand and fill.

6. Blood Flow Though Capillary Beds

All tissues and organs receive a sufficent supply of blood even though only 5-10% of the capillaries are carrying lood at any one time; the supply is adequate due to the vast number of capillaries present in each tissue.

• Capillaries in the brain, heart, kidyneys, and liver usually carry a full load of blood.
• Two mechanisms regulate the distribution of blood in capillary beds. Both are controlled by nerve signals and hormones.
• One mechanism involves the contraction and relaxation of the smooth muscle layer in the walls of arterioles.

à Contraction of the muscle layer constricts the arteriole and reduces the blood flow from it into the capillary bed.

à Relazation of the muscle layer dilates the arteriole and increses the blood flow into the capillary bed.

• The second mechanism involves the contraction and relaxation of precapillary sphincters,rings of smooth muscle located at the enterance to capillary beds.

à Contraction of these sphincters reduces blood flow into the capillary bed.

The diversion of blood from one area of the body to another changes the blood supply to capillary beds.

• After ingesting food, the digestive tract recieves a large supply of blood due to dilation of arterioles and the opening of precapillary sphincters associated with the system.
• During exercise, blood is diverted to the skeletal muscles.

7. Capillary Exchange

The exchange of materials between the blood and interstitial fluids that are in direct contact with the cells occurs across the thin walls of capillareis.

• The capillary wall is a single, “leaky” layer of flattened endothelial cells that overlap at their edges.
• Materials my cross these cells in vesicles (via endocytosis and exocytosis), by diffusion through the cell, or by bulk flow between the cells due to hydrostatic pressure.

Direction of fluid movement at any point along a capillary depends on the relative forces of hydrostatic pressure. And osmotic pressure. (See Cambell, Figure 38.11)

• Fluid flows out of a capillary at the upstream end near an arteriole and inot a capillary at the downstream end near a venule.
• About 85% of the fluid which leaves the blood at the arteriole end of the capillary, reenters from the interstitial fluid at the venous end.

à The remaining 15% of the fluid is eventually returned by lymphatic system.

V. The lymphatic system returns fluid to the blood and aids in body defense

Capillary walls leak fluid and some blood proteins which return to the blood via the lymphatic system. This is the 15% (4L/day) of fluid that does not re-enter the capillaries.

· The fluid, lymph, is similar in composisiton to interstial fluid.

· The lymph enters the system by diffusing into lymph cappillareis which intermingle with the blood capillaries.

· The lymphatic system drains into the dirculatory system at two locations near the shoulders.

· Lymph vessels have valves that prevent backflow and depend mainly on movement of skeletal muscles to squeeze the fluid along.

· Rhythmic contractions of vessel walls also help draw fluid into the lymphatic capillaries.

· Lymph nodes are specialized swellings along the system that filter the lymph and attack viruses and bacteria.

à This defense is conducted by specialized white blood cells inhabiting the lymph nodes.

à The nodes become swollen and tender during an infection due to rapid multiplication of the white blood cells present.

• Lymph capillaries penetrate small intestine villi and absorb fats, thus transporting them from the digestive system to the circulatory system.

IV. Blood is a connective tissue with cells suspended in plasma.

Vertebrate blood is connective tissue with several cell types suspended in a liquid matrix called plasma.

• The average human has 4 to 6 liters of whole blood (plasma + cellular elements).
• Cellular elements are the cells and cell fragments of the blood and represent about 45% of the blood volume.

A. Plasma

Water accounts for 90% of plasma which also contains electrolytes and plasma proteins.

Electrolytes = Inorganic salts in the form of dissolved ions that help maintain osmotic balance of the blood; some also help buffer the blood.

· Electrolyte balance in the blood is maintained by the kidneys.

Plasma proteins help buffer blood, help maintain the osmotic balance between blood and interstital fluids, and contribute to its viscosity.

· Some escort lipids through blood, some are immunoglobilns, some (fiberinogens) are clotting factors.

· Serum is blood plasma that has had the clotting factors removed.

Plasma also contains substances in trainsit though the body such as nutrients, metabolic wastes, respiratory gases, and hormones.

B. Cellular Elements

Erythrocytes = Red blood cells; biconacave discs that function in transport of oxygen.

• Each cubic millimeter of human blood contains 5-6 million erythrocytes.
• Lack nuclei and mitochondria; geverate ATP exclusively by anaerobic metabolism.
• Contain hemoglobin, an iron-containing protein that reversibly binds oxygen. About 250 million molecules of hemoglobin are in each erythrocyte.
• Erythrocyte production is controlled by a negative-feedback mechanism; if tissues are not receiving enough oxygen, the kidneys convert a plasma protein to the hormone erythropoietin, which stimulates production of erythrocytes in the bone marrow.

Leukocytes = White blood cells that function in defense and immunity.

• There are 5 types of leukocytes: basophils, eosinophils, neutrophilis, lymphocytes and monocytes.
• There are usually 5,000-10,000 leukocytes per cubic millimeter of blood although this number increases during an infection.
• Actually spend most of their time outside the circularoty system in the interstitial fluid and lymphatic system; large number are found in the lymph nodes.
• Lymphocytes become specialized during an infection and produce the body’s immune response.

Platelets = Fragments of cells 2 to 3 µm in diameter.

• Originate as pinched-off cytoplasmic fragments of large cells in the bone marrow.
• Lack nuclei.
• Function in blood clotting.

C. The formation of blood cells and platelets

The cellular elements of the blood must be replaced as they wear out.

· The average erythrocyte circulates in the blood for 3-4 months before being destryed by phagocytic cells in the liver and spleen.

· The components are usually recycled with new molecules being constructed from the macromolecule components of the old cells.

The multipotent stem cells give rise to all three of the cellular elements.

• These are found in the red marrow of bones, especially the ribs, vertebrae, breastbone, and pelvis.
• Form in the early embryo and are renewed by mitosis.
• Produce a number of new blood cells equivalent to the number of dying cells.

Using DNA technology to correct genetic defects in multipotent stem cells may provide a treatment for such diseases as leukemia and sickle-cell anemia.

D. Blood clotting

A clot form when platelets clump together to form a temporary plug and release clotting factors (some are also released from damaged cells) that initiate a complex reaction resulting in conversion of inactive fibrinogen to active fibrin. (see Campbel, Figure 38.14)

· Fibrin aggregates into threads that form the clot.

· Anticlotting factors normally prevent spontaneous clotting in the absence of injury.

VI. Cardiovasculas diseases are the leading cause of death in the United States and many other developed nations.

Diseases of the heart and blood vessels are referred to as cardiovascular diseases.

• Account for more than 50% of all deaths in the United States
• May culminate in a heart attack or stroke.

Heart attack = Death of the cardiac muscle resulting from blockage of arteries in the brain.

Stroke = Death of nervous tissue in the brain often resulting from blockage of arteries in the brain.

A thrombus is often associated with a heart attack or stroke.

Thrombus = A blood clot that blocks a key blood vessel.

• If it blocks the coronary arteries, a heart attack occurs.
• An embolus is a moving clot.
• If the thrombus or embolus blocks an artery in the brain, a stroke results.

Arteries may gradually become impaired by atherosclerosis.

Atherosclerosis = Chronic cardiovascular disease characterized plaques that develop on the inner walls of arteris and narrow the bore of the vessels.

• Decreases blood flow though the vessles

• Increase the risk of clot formation and heat attack.

Arteriosclerosis = Degenerative condition of arteries (from of atherosclerosis) in which plaques become hardened by calcium deposits.

Angina pectoris = Chest pains that occur when the heart recieves insufficient oxygen due to the oxygen due to the build up of plaques in the arteries.

• An indicator that the heart is not receiving sufficient oxygen.

Hypertension = High blood pressure; promotes atherosclerosis.

• Increases risk of heart attack and stroke.

Smoking, lack of exercise and a diet rich in animal fats correlate with increased risk of cardiovascular disease.

Abnormally high concentrations of LDL’s (low density lipoproteins) in the blood correlate atherosclerosis; HDL’s (high density lipoproteins) acctually reduce deposition of cholesterol in arterial plaques.

• Exercise tends to increase HDL concentration.
• Smoking increases the LDL to HDL.

VIII. Gas exchange supplies oxygen for cellular respiration and disposes of carbon dioxide: an overview

Circulatory systems transport oxygen and carbon deoxide between respiratory organs and other parts of the body.

A. Gas Exchange and the Respiratory Medium

Gas exchange = The movement of O₂ and CO₂ between the animal and its environment.

• Supports cellular respiration by supplying O₂ and removing CO₂.

The respiratory medium (source of oxygen) for terrestrial animals is the air, while it is water for aquatic animals.

• Air is 21% oxygen while the amount of dissolved oxygen in water varies due to temperature, solute concentrations, and other factors.

Respatory surface = Portion of the animal surface where gas exchange with the respiratory medium occurs. Oxygen diffuses in; carbon dioxide diffuses out.

• O₂ and CO₂ can only diffuse though membranes if they are first dissolved in the water that coats the respiratory surface.
• This surface must be large enough to provide O₂ and expel CO₂ for the entire body.

B. Respiratory Surfaces: Form and Function

A variety of respiratory surfaces have evolved that are adaptive for organism size and environment.

• Protozoa and other unicellular organisms exchange gases over their entire surface area.
• Animals such as sponges, cndarians and flatworms have body structures such that the plasma membrane of each body cell contacts the outside environment and can function in exchange.
• In animals with a more three-dimensional body plan, most of the body cells are isolated from the environment and the respiratory surfaces are generally thin, moist epithelium with a right blood supply.

à Usually only a single cell layer seperates the respiratory medium from the blood or capillaries.

Animals that are relatively small or have a shape 9lon, thin) that results in a high surface to volume ratio may use their outer skin as a respiratory organ.

· Earthworms have moist skin which overlays a dense network of capillaries.

· Gases diffuse across the entire surface (O₂ in; CO₂ out)and into the circulatory system.

· Earthworms and other animals (some amphibians) that use their skin for gas exchange must live in water or damp places in order to keep the exchange surface moist.

Most other animals lack sufficient body surface area to exchange gases for the entire body. These animals possess a region of the body surface that is extensively branched or folded, thus providing a large enough respiratory surface area for gas exchange.

• In most aquatic animals, external gills are present and in direct contact with the water.
• terrestrial animals have internal respiratory surfaces that open to the atmosphere through narrow tubes.

à Air lacks the supportive density and moisturizing qualities of water.

à Lungs and insect tracheae are two variations.

IX. Gills are respiratory adaptations of most aquatic animals

Gills are outfoldings of the body surface specialized for gas exchange. (See Cambell, Figure 38.17)

• In some invertebrates (i.e. echinoderms), gills have simple shapes and are distributed over the entire body.
• In other invertebrates (i.e. annelids), gills may be flap-like and extend from each body segemt or be clustered at one end and be long and feathery.
• Other animals (mollusks, fishes) have gills that are localized on a body region where the surface is finely subdivided to provide a large amount of surface area.
• The gills must be efficient since although water keeps the respiratory surface wet, it has a lower oxygen concentration than air.

Ventilation = Any method of increasing the flow of the respiratory medium over the respiratory surface; brings in a fresh supply of O₂ and removes CO₂.

Due to the density and low oxygen concentration of water, fish must expand a large amount of energy to ventilate water.

• Fish have a unique arrangement of blood vessels in their gills, which maximizes O₂ uptake from H₂O.
• Called countercurrent exchange, blood flows opposite to the direction in which water passes over gills, maintaining a constant concentration gradient for oxygen between the blood and the water passing over the gill surface. (See Campbell, Figures 38.18 and 25.19)

X. Tracheae are respiratory adaptations of insects

Air has several advantages over water as a respiratory medium:

• A higher oxygen concentration.
• Oxygen and carbon dioxide diffuse faster though air than water.
• Respiratory surfaces do not have to be ventilated as thoroughly.

The major disadvantage is that respiratory surfaces are continually desiccated.

• the evolution of respiratory surfaces within the body (tracheae, lungs0 helped solve this problem.

Tracheae are tiny are tubes that branch thoughout the insect body; air enters the system through pores called spiracles and diffuses thought the small branches which extend to the surface of nearly every cell.

• SOme small insects rely on diffusion alone to move O₂ into the system and CO₂ out; others use rythmic body movements for ventilation.
• Cells are exposed directly to the respiratory medium so insects do not use their circulatory systems to transport O₂ and CO₂.

à This is a moajor reason why the open circulatory system works so well in insects.

XI. Lungs are the respiratory adaptation of most terrestrial vertebrates

Lungs are highly vascularized invaginations of the body surface that are restricted to one location.

· The circulatory system must transport oxygen from the lungs to the rest of the body.

· Land snails use an internal mantle as a lung.

· Spiders possess booklungs.

· Various degrees of lung development are found in terrestrial vertebrates: frogs have simple balloonlike lungs with limited surface area; mammals have highly subdivided lungs with a large surace area.

A. Form and FUnction of Mammalian Respiratory Systems

Located in the thoracic cavity, mammalian lungs are enclosed in a sac consisting of two layer held together by the surface tension of fluid etween the layers.

• Air entering the nostrils is filtered by hairs, warmed and moisteded.
• the air then travels through the parynx, then through the glottis, and into the larynx (which possesses vocal cords and functions as a voice box).
• the flow then enters the cartilage-lined trachea that forks into two bronchi which further branch into finer bronchioles that dead-end in alveoli.
• Alveoli are lined with a thin layer of epithelium which serves as the respiratory surface.
• Oxygen dissolves in the moist film covering the epithelium and diffuses across to the capillaries covering each alveolus; carbon dioxide moves into the opposite direction by diffusion.

B. Ventilating the Lungs

Vertebrates ventilate lungs by breathing (alternate inhalation and the exhalation of air).

• Maintains a maximum O₂ concentration and minimum concentrationin the alveoli.

Frogs ventilate the lungs y positive pressure breathing. Air is pushed down the windpipe into lungs.

• Air is pulled into the mouth by lowering the floor region; this enlarges the oral cavity.
• The nostrils and mouth are closed and the floor of the mouth is raised, forcing air down the trachea.
• Air is exhaled by elastic recoli of lungs and by compression of the lungs by the muscular body wall.

Mammals ventilate their lungs by negative pressure breathing. During inhalation, air is pulled inot the lugs by the negative pressure created as the thoracic cavity enlarges by two possible mechanisms:

• When a mammal is at rest, most of the shallow inhalation results from concentration of the diaphragm.

à the diaphragm is a dome-shaped, thin sheet of muscle that forms the bottom wall of the thoracic cavity. When it contracts, it pushes downward towards the abdomen, enlarging the thoracic cavity.

à this lowers the air pressure in the lungs below atmospheric pressure and causes inhalation.

• Action of the rib muscles in increasing lung volume is important during vigorous excerceise.

à Contraction of the rib muscles pulls the ribs upwards, which expands the rib cage.

à As the thoracic cavity enlarges, the lungs also expand, since the surface tension of the fluid between the layers of the lung sac causes the lungs to follow.

à Lung volume increases, resulting in negative pressure within the alveoli, causing air to rush in.

Exhalation occurs when the diaphragm and/or the rib muscles relax, decreasing the volume of the thoracic cavity.

The amount of air inhaled and exhaled depends upon size, activity level and state of health.

• Tidal volume is the volume of air an animal inhales and exhales with each breath during normal quiet breating. Averages about 500 ml in humans.
• vital capacity is the maximum air volume that can be inhaled and exhaled during forced breathing. Averages 3400 - 4800 ml in college-age females and males, respectively.
• Residual volume is the amount ot air that remains in the lungs even afer forced exhalation.

Birds have a more complex process for ventilation:

• Besides lungs, birds have 8 or 9 air sacs in their abdomen, neck and wings that serve to trim the density of the body and act as sinks for the heat dissipation by metabolism of flight muscles.
• The air sacs do not exchange gases, they serve as bellwos to keep the air moving.
• The lungs also have tiny channels called parabronchi; air flow thought the entire systems, lungs and air sacs, in only one direction reguardless of whether it is inhaling or exhaling.
• the continuous flow of air though the parabronchi provides a constant supply of oxygen to the blood whether the bird is inhaling or exhaling.

C. The Control of Breathing

Breathing is an automatic action, we inhaler when nerves in the breathing control centers of the medulla oblongata and pons send impulses to the rib muscles or diaphragm, stimulating the muscles to contract.

• This occurs about 10 - 14 times per minute and the degree of lung expansion is controlled by a negative-feedback mechanism involving stretch receptors in the lungs.
• The medulla's control center also monitors blood and cerebrospinal fluid pH, which drops as blood CO₂ concentrations increase. When it senses a drop in pH (increased CO₂ level), the tempo and depth of breathing are increased and the excess CO₂ is removed in exhaled air.
• O₂ concentration in the blood only effects the breathing control centers when it becomes severely low.

à O₂ sensors in the aorta and carotids send signals to the breathing control centers and the centers respond by increasing the breathing rate.

The breathing control centers thus respond to a variety of neutral and chemical signals.

• the response is an adjustment in the rate and depth of breathing to meet the demands of the body.
• The control is effective since it is coordinated with control of the circulatory system.

D. Loading and Unloading of Oxygen and Carbon Dioxide

Whether a gas enters or leaves the blood depends on the concentration gradient of gases which measured as differences in partial presure (e.g. P_O2 = partial presure of oxygen). Oxygen comprises about 21% of the atmosphere and carbon dioxide about 0.03%.

• The partial pressure of a gas is the proportion of the total atmospheric pressure (760 mm) contributed by the gas; P_O2 = 160 mm Hg (0.21 x 760) and P_CO2 = 0.23 mm Hg.
• gases always diffuse from areas of high partial pressure to thouse of low partial pressure.
• Blood arriving at the lungs from the systemic circulation has a lower P_O2 and a higher P_CO2 than air in the alveoli.

à Thus, the blood exchanges gases with air in the alveoli and the P_O2 of the blood increases while the P_CO2 decreases.

• In systemic capillaries gradients of partial presure favor diffusion of oxygen out of the blood and diffusion of CO₂ into it, since cellular respiration rapidly depletes interstitial fluid of O₂ and adds CO₂. (see Campbell, Figure 38.25)

E. Respiratory Pigments and Oxygen Transport

Oxygen is carried by respiratory pigments in the blood of most animals since oxygen is not very soluble in water.

• The pigments are proteins which contain metal atoms; the metal atoms are responsible for the color of the protein.

In arthropods and mollusks, hemocyanin is the O₂ carrying pigment.

• The oxygen-binding component is copper and results in a blue color.
• It is dissolved directly in plasma, not confined to cells.

Hemoglobin is the oxgen-transporting pigment in almost all vertebrates.

• Consists of 4 subunits, each containing a heme group that bonds O₂; an iron atom is at the center of each heme group and actually binds to oxygen.
• The binding of oxygen to hemoglobin is reversible. (Binding occurs in the lungs; release occurs in the tissues.)
• Binding of O₂ to one subunit induces a shape change that increases the affinity of the other 3 subunits for oxygen (cooperativity).
• The unloading of oxygen from the heme group results in a conformational change that stimulates unloading from the other three.
• the cooperative nature of this mechanism is evident in the dissociation curve depicted in Figure 38.26.
• Bohr shift is the lowering of hemoglobins' affinity for oxygenupon a drop in pH. (see Campbell, Figure 38.26b) This occurs in active tissues due to the enterance of CO₂ into the blood.

F. Carbon Dioxide Transport

Hemoglobin not only transports oxygen but also helps the blood transport carbon dioxide and assists in buffering the blood against harful pH changes.

Carbon dioxide is transported by the blood in three forms:

• Dissolved CO₂ in the plasma (7%).
• Bound to the amino goups of hemoglobin (23%).
• As bicarbonate ions in the blood (70%).

Carbon dioxide from cells diffuses into the blood plasma and then into erythrocytes; in the erythrocytes, carbonic anhydrase catalyzes a reversible reaction wherin CO₂ is converted into bicarbonate.

• The CO₂ reacts with water to form carbonic acid.
• The carbonic acid quickly dissociates to bicarbonate and hydrogen ions.
• Bicarbonate then diffuses out of the erythrocyte and into the blood plasma.
• The hydrogen ions attach to hemoglobin and other proteins which results in only a slight change in the pH.
• The process is reversed in the lungs.

G. Adaptations of Diving mammals

Diving mammals such as seals, dolphins, and whales have special adaptations which allow them to make long underwater dives.

Weddell seals, which make dives to 200-500 m depths for 20 minutes or more, have been extensively studies.

• These mammals can store large amounts of oxygen, about twice as much per kilogram body weight as humans.

à 70% of the oxygen load is found in the blood (51% in humans) and 5% in the lungs (36% in humans).

à They also possess a higher myoglobin (oxygen storing pigment) concentration in their muscles.

• The high oxygen load in the blood may be due to the higher blood volume in these seals (twice the amount of blood per kilogram of body weight as humans).
• The seals also have a very large spleen which can store about 24 L of blood.

à the spleen probably contracts after a dive and forces more erythrocytes loaded with oxygen into the blood.

• The high concentration of myoglobin permits these seals to store about 25% of their oxygen in muscles (compared to 13% in humans).

These adaptations provide diving mammals with a large oxygen reservoir at the beginning of a dive, but they also have several adaptations to conserve oxygen during the dive.

• A diving reflex slows the pulse, and oxygen consumption declines as the cardiac output slows.
• Most of the blood is routed to the brain, spinal cord, eyes, adrenal glands, and placenta due to regulatory mechanisms that reduce blood flow to the muscles.
• The reduced blood flow to the muscles results in a depletion of the myoglobin oxygen reserves after about 20 minutes, the muscles then shift to anaerobic production of ATP.

The special adaptations found in diving mammals emphasizes that amimals can respond to environmental pressures in the short term though physiological adaptation as well as in the long term by natural selection.