Chapter 31: Gas Exchange

Ventilation versus Perfusion

*Ventilation*: Involves bringing the respiratory medium (such as water or air) in contact with the respiratory surface (such as gills or lungs) through bulk flow. • Ventilation is important because the respiratory surface must exchange gases with the respiratory medium. *Perfusion* involves bringing a blood vessel in contact with a tissue such that blood circulates over the gas exchange surfaces. • In vertebrates, respiratory organs are highly perfused because oxygen travels from the respiratory organ to the rest of the body via blood vessels.

Mammal respiratory system

*lungs* consists of a dense net of capillaries just under the epithelium. • The mammal takes in air through the nose or mouth to the *nasal cavity* or *oral cavity*. • From there, the air goes to the *pharynx* (back of the throat) • The esophagus takes food to the stomach and the *trachea* takes air to the lungs. • The air enters the trachea at the *larynx* (voicebox). • The trachea then divides into two *bronchi* (one for each lung). • Air flows from the primary bronchi to the *terminal bronchioles* • At the end of the *respiratory bronchioles* are *alveolar ducts*, which contain *alveolar sacs* that are made up of *alveoli* • Gas exchange primarily occurs at the alveoli.

Diaphragm

A sheet of skeletal muscle on the bottom of the thoracic cavity- contracts, causing the flood of the diaphragm to drop and the ribcage to rise. • This creates negative pressure, which sucks air in. • To exhale, the diaphragm is relaxed and carbon dioxide goes out. • Humans use negative-pressure breaking. • Exhalation is normally passive, but it can also be active when you want to exhale a lot.

Gas exchange

Also known as respiratory exchange or respiration. It is the uptake of Oxygen from the environment and discharge of carbon dioxide to the environment. • Oxygen and carbon dioxide are therefore respiratory gasses • All oxygen transport in the body is passively diffused • The reaction of CO2 with water creates hydrogen atoms, reducing the pH of the blood and thus the body needs to maintain the blood's pH around 7.3 • Oxygen transport alternates between two process: bulk flow and diffusion.

Breathing in birds

Birds are considered to have the most efficient gas exchange mechanism • Birds evolved endothermy and the capacity to fly, which require a high efficient gas exchange mechanism. • Birds have long tubes (*parabronchi*) which use multiple accessory air sacs to keep air constantly moving though the lungs, allowing birds to extract more oxygen from the air than mammals, and it allows them to fly at higher altitudes without rest.

Why cant fish breathe out of water?

Even though air has more oxygen, when a fish is taken out of water, the lamellae stick together due to the surface tension of the small amount of remaining water that covers them. • This dramatically reduces their surface area and prevents the fish from acquiring enough oxygen to live. • They also cannot ventilate their respiratory organs in the air as they can in water.

Fick's law of diffusion

Expressed the rate of diffusion across a membrane in terms of all the factors that influence it. Q= DA x [(P1 - P2)/L] • Q is the rate of diffusion • D is a diffusional coefficient that varies based on the diffusing substances, medium and temperature. • A is the cross-sectional area of the membrane • P1 - P2 is the difference in partial pressure of gas between the two sides of the membrane • L is the distance over which the gas has to diffuse. • Diffusion rates will be greatest when A is large, (P1 - P2) is large, and L is small. • That is, when the membrane has a large cross-sectional area, when there is a large difference in the partial pressure of the gasses, and when the gas has to diffuse over a short distance.

Maximizing rate of gas diffusion

From flints law: 1. *Increasing the surface area of the respiratory surface (A)*: Respiratory organs have structures that maximize surface area, thus increasing the rate of diffusion. There area several types of respiratory organs. • External gills: extend from the body surface of some aquatic animals • Internal gills: which are protected from damage and predation by a shell or bony plate • Trachea: a system of air-filled tubes used by insects • Lungs: internal respiratory system 2. *Maximizing the partial pressure difference (P1 - P2)*: Ventilation and perfusion maximize the partial pressure difference. • Ventilation maintains a large partial pressure of oxygen outside the body, and perfusion ensures that the blood takes oxygen away as soon as it is taken in. • There is thus a constant flow of oxygen into the body 3. *Minimizing the diffusion path length (L)*: Gases are only able to diffuse across small distances, so the epithelial tissue in specialized respiratory structure are extremely thin 4. *Minimizing the diffusion coefficient (D)*: This is the aspect of Fick's equation that animals have the least control over because the diffusion coefficient is determine by the chemical nature of the respiratory medium and temperature.

Tracheal systems

Gas exchange systems of branches tubes that infiltrate the body and carry oxygen directly to the cells of insects. • These are elaborate open ducts (called the trachea and tracheoles) that get air to the cells of insects. • The point where the trachea and tracheoles are connected to the outside environment is called the *spiracle* • No circulatory system is associated with the tracheal system. • The circulatory system in insects is for nutrient and metabolic waste transport, not gas exchange. • Insects do not have a single respiratory organ, but their respiratory structures are spread throughout their bodies. • Most insects do not engage in any ventilation at all • This system is not efficient for large animals because trachea have to extend to every cell in the body, which is why we don't have human sized bugs.

Diffusion of gas in the body

Gases always diffuse down their partial pressure gradients, from higher partial pressure to areas of lower partial pressure. • The partial pressure of oxygen in the blood is about .05 atm. • The partial pressure of carbon dioxide is .065 atm. If carbon dioxide's PP is greater in the tissues than in the blood, carbon dioxide diffuses into the blood.

Gills

Localized extension of the body that many aquatic animals use for gas exchange. • Gills are highly convoluted to maximize their surface area (exceeding the surface area of the body) • These organs are supported by *gill arches* lying between the mouth and the opercular flaps. • Fish suck water in through their mouths, then force the water over the gills. • Fish gills are made up of a number of different strands called *gill filaments* which are made up of individual "steps" called *lamellae* which contain capillaries. • Water flows opposite of blood in the capillaries so that oxygen can be exchanged in a countercurrent exchange

Breathing in mammals

Mammals are endotherms, requiring a lot of oxygen to maintain the levels of cellular respiration necessary for endotherm. • Unlike birds, which use unidirectional breathing, mammals use tidal air flow, in which air enters and exists via the same passageway. • Mammalian gas exchange mechanisms are highly complex, which elaborate lung structures that have feature that increase diffusion.

Tidal ventilation

Most air-breathing vertebrates use tidal ventilation, in which they breathe in and out by the same path.

Four major gases found in the air and their partial pressures

Oxygen: 20.95% Nitrogen: 78.08% Argon: .93% Carbon dioxide: .04%

Countercurrent exchange

Refers to the opposite flow of adjacent fluids that maximized transfer rates in fish gills. • As blood flows opposite of water, it becomes more and more oxygenated. • Countercurrent exchange allows fish with gills to extract a significant amount of oxygen in the water. • It is a common technique in animal physiology; it maximized the transfer of anything between two solutions, so long as the membrane separating the two solutions is permeable to it. • Countercurrent exchange could be used to maximize the transfer of temperature, oxygen, carbon dioxide, or any other gas.

Partial pressure

The percentage of a gas in the air times the air pressure • Partial pressure, not total pressure, determines the direction in which a gas will diffuse. • Partial pressure works on liquids too, as gas will dissolve in open liquid until they both have equal partial pressures. BUT oxygen only has 1/20 the concentration in water than in air, despite having equal partial pressures.

Mechanism in which birds process air

Requires two cycles of inhalation and exhalation. • This is why birds require multiple posterior and anterior air sacs and why the trachea of typical birds is longer and wider than that of similarly sized animals. • The bird inhales, and air from the environment flows through the trachea and into its posterior air sacs. • The bird exhales, which causes the air to come from the posterior air sac to the parabronchi of the lungs, where gas exchange takes place • The bird inhales, which moves the air from its lungs to its anterior air sacs. • The bird exhales, which moves the air from its anterior air sacs out of the body through the trachea.

Alveoli

Small sacs at the very end of the terminal respiratory bronchioles that increase the surface area of the lung and increase the surface area for absorption. • It is in the alveoli that the blood becomes oxygenated, so that it can deliver oxygen to the rest of the body. • It is also here that the blood unloads carbon dioxide, which will be exhales

Lungs

The invaginated respiratory surfaces of terrestrial vertebrates, land snails, and spiders that connect to the atmosphere by narrow tubes. • They are localized, so they can only transport oxygen to the rest of the body through a circulatory system • Lungs are dead-end sacs in most animals, with the exception of birds.

Surfactants

The water on the surface of the alveoli could cause them to stick together, leading to mammal lungs to collapse. • Pulmonary *surfactants* are secreted compounds that keep the surfaces of the lungs from sticking together, preventing collapse. • Without surfactants, lungs would collapse and likely die.

Negative pressure breathing

• A breathing system used by mammals in which air is pulled into the lungs. • the filling of the lungs is usually active (involves the contraction of a muscle), while the emptying of lungs is passive (involves the relaxation of a muscle)

Ventilation

• Because oxygen concentration is low in water, fish have to increase contact between their respiratory medium and the respiratory surface. • They do so through *ventilation*, a process of bringing the respiratory medium in contact with the respiratory surface. • Crayfish and lobsters perform ventilation by pushing a current of water over their gills with paddle-like appendages. • Muscles and clams use cilia to move water over their gill. • Fish, on the other hand, have a current of water that flows unidirectionally through their bodies, over the gills, and out the opercular flaps. Fish spend a lot of energy ventilating their gills.

Fish gill components

• Fish gills are made up of a number of different strands called *gill filaments* which are made up of individual "steps" called *lamellae* which contain capillaries. • Water flows opposite of blood int he capillaries so that oxygen can be exchanged in a countercurrent exchange • A

Simple diffusion of gasses in animals

• Not all animals use a complex system to transport oxygen through the body • Some animals, like sponges, flatworms, and fish larvae, are so small that the surface area of theirs skin is enough to meet their gas transfer requirements • Higher organisms develop complex systems for transporting oxygen, but still rely some on simple diffusion.

Oxygen transport

• Oxygen transport alternates between two process: bulk flow and diffusion. • Bulk flow: In humans, oxygen is brought in via air. The muscles contract, generative a vacuum that sucks air in. • Diffusion: Once air is int he body, it has to diffuse across the surface of respiratory organs, into the blood • Bulk flow: The hear pumps oxygenated blood to the rest of the body • Diffusion: Once this blood reaches the capillaries of target tissue, the oxygen diffuses out of the red blood cell and into the cells that need oxygen.

Residual volume (RV)

• The lungs always have some amount of dead space, containing "stale air" that is low in oxygen content but remains in the lungs and cannot be expelled. • This stale air is important because it keeps the inside walls of the lungs from sticking to one another. • The fact that "new air" mixes with "old air" means that the partial pressure of oxygen at the respiratory exchange surface will be lower than the partial pressure of oxygen in the environment. • This makes gas exchange less efficient in animals that use tidal ventilation than in other animals, such as fish.

Efficiency of mammal lungs in terms of Fick's diffusion equation

• The lungs are surrounded by thin capillaries, reducing L: the capillaries surrounding the alveoli are extremely thin (less than two micrometers each), which promotes diffusion of oxygen and carbon dioxide • The alveoli have a large cumulative surface area, increasing A: Humans have more than 300 million alveoli which combine to form a surface area of about 70 square meters

Efficiency of birds has exchange mechanisms in terms of Fick's equation

• They minimize L, the distance over which the gas has to diffuse: Birds have among the thinnest of all respiratory surface. The epithelial walls of bird are up to 1/10 as thick as the mammalian epithelial walls • They maximize A, the cross-sectional area of the membrane: Parabronchi are long tubes with multiple accessory air sacs to increase surface are • They maximize P1-P2, the partial pressure of difference: Unidirectional air flow allows birds to constantly move freshly oxygenated air out, which maximized the partial pressure difference and promotes diffusion. Countercurrent flow of air and blood also maximizes the rate of diffusion.

Why we can't breathe water

• Water is denser than air: At 1 atm and 20 C, water is more than 800 times denser than air • Water is more viscous than air: Water is about 30 to 100 times more viscous than air, depending on the temperature • Oxygen is more soluble in air than in water: At equal partial pressures and temperatures, water contains only about 1/20th of the oxygen that air contains • Oxygen diffuses faster in air than in water: Oxygen diffuses about 8,000 to 200,000 times fast in air than in water. • Aquatic animals have to be efficient in breathing because extracting O2 from the water is energy costly. • A liter of air has 40x more oxygen than water. • Thus, aquatic animals with the same metabolic rate as land animals must move 40 times as much of the respiratory medium. • Because of how viscous water is, moving water over a fish's gills is more difficult.

Unidirectional air flow

• Where mammals use tidal ventilation, birds have unidirectional air flow. • *unidirectional air flow* is made possible by several air sacs. • The air sacs do not facilitate gas exchange, rather, gas exchange occurs at the parabronchi which are surrounded by blood capillaries. • Unidirectional air flow means that all of the air that comes into contact with the paracronchi is oxygenated.