Chapter 23 External Respiration PDF

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Chapter 23: External Respiration Tunas represent one of the pinnacles of water breathing. Tunas are actually more mobile than any terrestrial predator. They require a respiratory system that can take up oxygen rapidly and a circulatory system that can deliver oxygen rapidly from the gills to the tissue. The tuna’s gills are exceptionally specialized for oxygen uptake and have eight times more surface area than that of a rainbow trout. The gill membranes are also exceptionally thin. Most fish drive water across their gills by a pumping cycle that is powered by their buccal and opercular muscles. Ram ventilation → a fish simply holds its mouth open while it swims powerfully forward, thereby “ramming” water into its buccal cavity and across its gills. In this way, the swimming muscles assume responsibility. Obligate ram ventilators → must swim continuously forward, or they suffocate.

Fundamental Concepts of External Respiration Gas exchange membrane or respiratory exchange membrane → a thin layer of tissue consisting typically of one or two simple epithelia, separates the internal tissues of the animal from the environmental medium. External respiration → the process by which oxygen is transported to the gas exchange membrane from the environmental medium and by which the carbon dioxide is transported away from the membrane into the environmental medium. Ventilation → bulk flow (convection) of air or water to and from the gas exchange membrane during breathing. Oxygen always crosses the gas exchange membrane by diffusion. The area and thickness of the gas exchange membrane play critical roles in oxygen acquisition. The rate of diffusion across a membrane increases as the thickness of the membrane decreases. Most of the body surface, the skin, is of very low permeability in mammals and tunas. A frog, by contrast, has a skin so permeable to gases that they are exchanged to a substantial extent across the skin. Lungs are not the only breathing organs. Gills → respiratory structures that are evaginated from the body and surrounded by the environmental medium. Lungs → respiratory structures that are invaginated into the body and contain the environmental medium. Lungs are adaptive for terrestrial life. Branchial → refers to structures or processes associated with gills. Pulmonary → refers to structures or processes associated with lungs. External gills → located on an exposed body surface and project directly into the surrounding environmental medium. Internal gills → are enclosed within a superficial body cavity. Usually requires an animal to use metabolic energy to ventilate them. Has its advantages nonetheless. The enclosing structures physically protect them and may help canalize the flow of water across the gills in ways that will enhance the efficiency and control of

breathing. Active ventilation → ventilation is active if the animal creates the ventilatory currents of air or water that flow to and from the gas exchange membrane. May be unidirectional, tidal, or nondirectional. Passive ventilation → ventilation is passive if environmental air or water currents directly or indirectly induce flow to and from the gas exchange membrane. Unidirectional → ventilation is unidirectional if air or water is pumped over the gas exchange membrane in a one-way path. Tidal (bidirectional) → ventilation is tidal if air or water alternately flows to and from the gas exchange membrane. Nondirectional → ventilation is nondirectional if air or water flows across the gas exchange membrane in many directions. Diffusion lungs → some lungs exchange gases with the environment entirely by diffusion and are termed diffusion lungs. Dual breather or bimodal breather → an animal that can breathe from either air or water.

Summary: Fundamental Concepts of External Respiration ●

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Oxygen always crosses the gas exchange membrane by diffusion. This means that oxygen enters an animal only if the oxygen partial pressure on the outside of the gas exchange membrane is higher than that on the inside. Breathing organs are categorized as gills if they are evaginated structures that project into the environmental medium. They are lungs if they are invaginated structures that contain the medium. Ventilation is the forced flow (convection) of the environmental medium to and from the gas exchange membrane. It is categorized as active if an animal generates the forces for flow using metabolic energy. Ventilation may be unidirectional, tidal, or nondirectional.

Principles of Gas Exchange by Active Ventilation Active ventilation is very common, and involves several specialized concepts that apply to a variety of animals. The rate of oxygen uptake by the breathing organ depends on (1) the volume flow of air or water per unit of time and (2) the amount of oxygen removed from each unit of volume. Rate of O2 uptake (mL O2/min) = Vmedium(CI-CE) Vmedium is the rate of flow (L/min) of the air or water through the breathing organ, CI is the oxygen concentration of the inhaled (inspired) medium (mL O2/L), and CE is the oxygen concentration of the exhaled (expired) medium. Oxygen utilization coefficient → expresses how thoroughly an animal is able to use the oxygen in the air or water it pums through its lungs or gills. Tuna are especially efficient in using the oxygen the water they drive over their gills. Rainbow trout use

33%, tuna use 50%-60%.

The O2 partial pressure in blood leaving a breathing organ depends on the spatial relation between the flow of the blood and the flow of the air or water The best single measure of the breathing organ’s effectiveness is measuring the oxygen partial pressure leaving the breathing organ. An animal with a high oxygen partial pressure in the blood leaving its breathing organ is particularly well adapted to maintain an oxygen partial pressure in the mitochondria that is sufficiently high for aerobic catabolism to proceed without being oxygen limited. Breathing organs with different designs exhibit differences in the blood oxygen partial pressure they can maintain. These differences depend on the spatial relation between the flow of blood and the flow of air or water. Tidally ventilated → are distinguished by the fact that the medium next to the gas exchange membrane is never fully fresh. Their breathing organs are never entirely emptied between breaths. Characteristically, the oxygen partial pressure of the blood leaving the breathing organ is below the oxygen partial pressure of the exhaled medium. Cocurrent gas exchange → the medium flow along the gas exchange membrane in the same direction as the blood. Resembles tidal ventilation. Countercurrent gas exchange → the medium and blood flow in opposite directions. When oxygen depleted afferent blood first reaches the gas exchange membrane, it initially meets medium that has already been substantially deoxygenated. Cross-current gas exchange → the blood flow breaks up into multiple streams, each of which undergoes exchange with the medium along just part of the path followed by the medium. Countercurrent exchange is superior to cross-current exchange, and cross-current exchange is superior to cocurrent or tidal exchange. (Not absolute because additional factors affect the way in which real breathing organs function in real animals.

Arterial CO2 partial pressures are much lower in water breathers than air breathers In water breathers, the partial pressure of carbon dioxide in the blood leaving the breathing organs is always similar to the carbon dioxide partial pressure in the ambient water, regardless of whether gas exchange is tidal, cocurrent, countercurrent, or cross-current. In air breathers the partial pressure of carbon dioxide in the blood leaving the breathing organs is usually far above the carbon dioxide partial pressure in the atmosphere. (The fundamental law of gas diffusion → gases diffuse in net fashion from areas of relatively high partial pressure to areas of relatively low partial pressure. This is true within gas mixtures, within aqueous solutions, and across gas-water interfaces.)

Summary: Principles of Gas Exchange by Active Ventilation ● ●

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The oxygen utilization coefficient during breathing is the percentage of the oxygen inhaled medium than an animal removes before exhaling the medium. The four major types of gas exchange that can occur during directional ventilation can be ranked in terms of their inherent ability to establish a high oxygen partial pressure in blood exiting the breathing organ. Countercurrent gas exchange ranks highest. Crosscurrent ranks second. Cocurrent and tidal ranks third. Air breathers tend to have much higher carbon dioxide partial pressures in their systemic arterial blood than water breathers. The difference arises during breathing and is principally a consequence of the differing physical and chemical properties of air and water.

Low O2: Detection and Response Most animals are fundamentally aerobic. Thus when the oxygen levels in an animal’s tissues becomes unusually low, the low oxygen levels typically mean trouble. Animals detect low levels and respond to them. The mechanisms involved in detection and response are elaborate. ● One scale of response to low oxygen is that of the whole body: the scale of organ systems. ● The second scale of response is intracellular, in cells throughout the body. Some species of animals are well adapted to living in environments where the ambient oxygen level is prone to fall to low levels. The two scales of responses to low oxygen levels interact with each other.

Introduction to Vertebrate Breathing When we think of the sequence from fish to mammals and birds, caution is always called for thinking of it as an evolutionary sequence. Comparisons among today's animals is nonetheless often provide revealing insights to trends that occurred during evolution. One property of breathing organs that is of enormous importance is the total surface area of the gas exchange membrane. The lines for amphibians and reptiles other than birds are similar. Moreover, the lines for most fish are similar. These similarities tell us two things: ● The total lung area of a nonavian reptile is roughly similar to the total lung area of an amphibian of the same body size. ● The total gill area of a fish of particular body size is roughly similar to the total lung area of an amphibian or reptile of that size, suggesting that when vertebrates emerged onto land, there was not immediately much of a change in the area of gas exchange surface in their breathing organs. Mammals and birds are similar to each other. They exhibit a dramatic step upward in the area of gas exchange surface. They evolved independently, probably in association with their evolution of homeothermy. The fact that they have large gas exchange surface areas does not mean that they have large lungs compared with reptiles or amphibians. Their lungs are extraordinarily densely filled with branching and rebranching of airways. In nonavian reptiles and amphibians the lungs typically have parts that are simply like balloons.

The thickness of the barrier between the blood and the environmental medium also shows significant evolutionary trends in the major vertebrate groups. Mammals tend to have a thinner sheet of tissue between blood and air than lizards, crocodilians, or other nonavian reptiles. Tuna have a similar thickness to mammals. Many amphibians that lack lungs have an epidermis that is vascularized and 100% of gas exchange occur across the skin. Groups of vertebrates that are well defended against water loss through their skin tend to exhibit little exchange of gases through their skin. The control of the active ventilation of the gills or lungs is the final subject that deserves mention in introducing vertebrate breathing. The rhythmic muscle contractions that are responsible for inhalation and exhalation movements are of central importance. Rhythmogenesis → sets of neurons that initiate rhythmic outputs of nerve impulses. Continuous breathing → each breath is followed promptly by another breath in a regular, uninterrupted rhythm. Intermittent breathing or periodic breathing → breathing in which breaths or sets of breaths are regularly interrupted by extended periods of apnea, periods of no breathing. The central pattern generator for breathing in all vertebrates are believed to be located in the brainstem: in the medulla, and sometimes other associated parts, of the brain.

Summary: Introduction to Vertebrate Breathing ●

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The gill surface area of most fish for a given body size is similar to the lung surface area of amphibians and nonavian reptiles of the same size. Compared with the latter groups, mammals and birds have much more lung surface area, helping to meet their far higher needs of gas exchange. The barrier between the blood and the air or water in the breathing organs is notably thin in mammals and thinnest in birds. The skin can account for 25% or more of gas exchange in some fish, turtles, and other nonavian reptiles, and up to 100% in some amphibians. The skin is a minor contributor to gas exchange in mammals and birds. The breathing muscles of vertebrates are skeletal muscles activated by motor-neuron impulses. The breathing rhythm originates in a central pattern generator in the brainstem.

Breathing by Fish Many fish start life as a tiny larvae that breathe only by diffusion across their body surfaces. As a young fish grows, its body surface becomes too thick for diffusion to suffice. Gill slits → lateral pharyngeal openings, gills are arrayed across these lateral openings, and a protective external flap, the operculum, covers the set of gills on each side. There are four gill arches that run dorsoventrally between the gill slits on each side of the head. The arches, which are reinforced with skeletal elements, provide strong supports for the gills proper. Each gill arch bears two rows of gill filaments.

Each gill filament bears a series of folds, called secondary lamellae on its lower and upper surfaces. The lamellae run perpendicular to the long axis of the filament and number 1040 per millimeter of filament length on each side. The secondary lamellae are the principal sites for gas exchange. Water flows along the surfaces of the secondary lamellae from the buccal side to the opercular side. Conversely, blood within the secondary lamellae flows in the opposite direction (countercurrent gas exchange).

Gill ventilation is usually driven by buccal-opercular pumping Buccal pressure pump → develops postive pressure in the buccal cavity and thus forces water from the buccal cavity through the gill array in to the opercular cavity. Opercular suction pump → develops negative pressure in the opercular cavity and thus sucks water from the buccal cavity into the opercular cavity. THE BUCCAL PRESSURE PUMP The stage is set for the buccal pressure pump to operate when a fish fills its buccal cavity with water by depressing the floor of the cavity while holding its mouth open. The mouth is then closed, and the buccal pump enters its positive-pressure phase. The fish raises the floor of the buccal cavity and water from the buccal cavity flows through the gills into the opercular cavity. THE OPERCULAR SUCTION PUMP...