Respiratory systems in animals

Animals generally meet their energy needs by oxidation of food, and the respiratory system supplies the oxygen necessary for cell metabolism while removing its waste product, carbon dioxide. Oxygen is available either dissolved in water or as a component of the air, and animals have evolved special organ structures to effectively obtain oxygen from their environment.

Organs of Gas Exchange

Single-cell and simple organisms, such as flatworms and protozoa, can obtain sufficient oxygen to meet their energy demands by simple diffusion through their body surface. Some amphibians utilize gas exchange through their skin to supplement their lung respiration, but generally, larger, more complex animals require specialized organ systems with a large surface area for gas exchange and a circulatory system for distribution of oxygen to each cell. The basic mechanism, however, for gas exchange between the environment and the blood and between the blood and cells is by diffusion. The three major types of gas exchange organs are the gill for water respiration, the lung for air and, in some special cases, water respiration, and the tracheas system of tubules for air respiration in insects.

Gills consist of several gill arches located in the operculum or gill cover on each side of the fish’s head. A gill arch contains two rows of gill filaments, and each filament has a row of parallel platelike structures on its surface called lamellae. The lamellae are everted structures that rise up from the filament surface and are only a fraction of a millimeter apart. Water flows between the lamellae, and oxygen diffuses from the water into the lamellar capillary blood. The lamellar blood flows in the opposite direction of the water flow and creates a countercurrent exchanger. The countercurrent maximizes the diffusion of oxygen into the lamellar capillary blood by maintaining a diffusion gradient over its entire length.

Lungs, in contrast to gills, are invaginations, where the surface turns in and forms a hollow or saclike structure. Lungs are typically divided into two functional areas: the conducting zone and the respiratory zone. The conducting zone branches from the trachea to the bronchioles and distributes air to the respiratory zone but is not involved in gas exchange. The respiratory zone comprises the majority of the lung and contains small respiratory bronchioles and ducts that lead to the primary gas exchange area, the alveolus. The alveoli vary from simple saclike structures in a pulmonate land snail to the complex alveolar wall structure of mammals. The alveolar wall is fifty micrometers thick, or about one-fiftieth the thickness of a sheet of paper, and is composed of epithelial cells covering the alveolar surface, an interstitial space, and the endothelial cells that make up the capillaries. This thin-wall structure allows for the diffusion of oxygen and carbon dioxide between the air and blood.

The insect tracheal respiratory system is unique because it is both the gas exchange and distribution system. Pairs of openings on the insect’s thorax and abdomen, called spiracles, regulate the movement of air in and out of a tubule system. The spiracles open and close in a pattern that allows unidirectional flow of air through the tubule system. The tubules branch and extend throughout the insect’s body and deliver oxygen to the cells independent of the circulatory system.

Air and Water Environments

Important aspects of the atmosphere for respiration are the barometric pressure and concentration of gases, temperature, and humidity. The atmospheric gases important to animals are oxygen, carbon dioxide, and nitrogen, and the atmosphere is a constant 20.95 percent oxygen, 0.03 percent carbon dioxide, and 79 percent nitrogen (plus other inert gases). The rate of diffusion of oxygen from the inspired air into the circulation depends on the partial pressure of the oxygen. The barometric pressure, however, decreases with increasing altitude, and this decreases the partial pressure of oxygen, which decreases the diffusion of oxygen into the blood. Thus, an animal’s difficulty in obtaining adequate oxygen at higher altitudes is related to the reduction in atmospheric pressure and not to a change in the percentage of oxygen in the atmosphere.

The temperature and amount of water vapor or humidity in the atmosphere are variable, and during inspiration, the inspired air is warmed to body temperature and saturated with water vapor (100 percent humidity). The heat and moisture come from the airways and can potentially cool and dehydrate an animal. Therefore, a minimal amount of air is inspired to prevent excess heat and water loss. However, heat-stressed animals will use this respiratory heat loss, or panting, to cool their bodies.

Water poses several challenges for respiration compared to air: a lower oxygen content, slower gas diffusion rate, higher viscosity, and greater weight. The amount of oxygen available in the water is thirty times less than that found in air. Thus, more water must flow over the gill surface for adequate oxygen delivery. The speed at which oxygen moves through the water is ten thousand times slower than oxygen moving through the air. Thus, the distance between the water and the gill surfaces can only be a fraction of a millimeter apart. In contrast, the lung gas exchange surfaces are a few millimeters apart.

Water’s greater viscosity and weight compared to air require more energy to move water over the gill surface. Water-breathing animals compensate for this by having a unidirectional flow through the gill. This avoids water being moved, stopped, and then moved again in the opposite direction, which works well for air, but would be very energy costly for the heavier, more viscous water.

The gill structure depends on water to support and separate the rows of lamellar structures. Thus, when a fish is exposed to air, the gill structure collapses on itself and greatly reduces the surface area available for oxygen diffusion. Thus, the fish will suffocate if not returned to the water.

Breathing Water and Breathing Air

Water can be moved through the gill lamellae by either opercular pumping or ram ventilation. Opercular pumping involves the movement of the mouth and opercular covering to create pressure gradients for unidirectional flow of water through the mouth, across the gill surface, and out the opercular covering (unidirectional flow). Ram ventilation takes advantage of the fish’s forward speed to flow water through the mouth and gill. Opercular pumping is used from rest to slow swimming speeds, and a fish switches over to ram ventilation when swimming at faster speeds.

For air breathers, inspiration (inflating the lungs) can be accomplished by either positive-pressure or negative-pressure breathing. Positive-pressure breathing requires air pressure to inflate the lungs, which is similar to inflating a balloon or tire with compressed air. The pressure is considered positive because it is greater than atmospheric pressure. For example, frogs use positive-pressure breathing by closing their mouths and then elevating the floor of the mouth. This compresses and pressurizes the air and forces it into the lungs. The elastic lung tissue is stretched like an inflated balloon by the increased volume. The process of the air moving out of the lung is called expiration; when the frog relaxes and opens its mouth, the lung elastic recoil forces the air out, similar to a balloon deflating.

With negative-pressure breathing, the lung is pulled open by contraction of the diaphragm. The pressure becomes negative (below atmospheric pressure), and air flows into the lung until it equalizes with the atmospheric pressure. If additional inflation is required, such as during exercise, accessory inspiratory muscles lift the ribs to inflate the lungs further. Expiration is accomplished by the relaxation of the inspiratory muscles, and the lung elastic recoil increases airway pressure and air flows out of the lung.

Inspiration is always an active process, whereas expiration results from the passive elastic recoil of the lung tissue. However, active expiration is possible by contracting muscles that pull the ribs down and by using abdominal muscles to push the diaphragm farther into the thoracic (chest) cavity.

Setting Breathing Rate

In water-breathing animals, such as fish and lobsters, the level of oxygen sets the ventilation rate (volume of water moved through the gill per minute) such that as oxygen content in the water decreases, the frequency of breathing movements increases. During fast swimming, fish using ram ventilation regulate the mouth opening so that the amount of water flowing over the gills just meets tissue oxygen demand. A wider mouth opening than is necessary increases the fish’s frictional drag through the water and thus decreases the energy efficiency. Carbon dioxide is highly soluble in water and easily diffuses from water-breathing animals. Thus, blood carbon dioxide levels in water-breathing animals are very low and not used to regulate respiration rate.

In air-breathing animals, the blood levels of carbon dioxide and oxygen regulate the ventilation rate (air volume moved in and out of the lungs per minute). Carbon dioxide quickly diffuses from the small capillaries in the brain circulation into the fluid surrounding the brain cells (cerebral spinal fluid). Here the carbon dioxide reacts with water and forms carbonic acid. The hydrogen ions released from the carbonic acid stimulate chemoreceptor cells that, in turn, stimulate the respiratory center in the medulla, located in the brain stem. Higher concentrations of carbon dioxide increase the hydrogen ion concentration and thus increase ventilation rate. Air-breathing animals primarily regulate ventilation rate by carbon dioxide produced from metabolism and not low blood oxygen levels.

However, oxygen can regulate ventilation in animals at high altitudes. Oxygen partial pressure is sensed by chemoreceptors in the aorta and the carotid artery. These peripheral chemoreceptors sense the partial pressure of oxygen in the blood plasma, and as the partial pressure of oxygen in the air decreases, such as with altitude, the partial pressure of oxygen in blood also decreases. This increases ventilation, which then compensates for the lower oxygen partial pressure. In addition to low oxygen partial pressure, the peripheral chemoreceptors are stimulated by blood acidosis. For example, lactic acid released from skeletal muscles during strenuous exercise stimulates the ventilation rate in animals and humans.

Principal Terms

Alveolus: The thin-walled, saclike lung structure where gas exchange takes place

Chemoreceptor: Specialized nervous tissue that senses changes in pH (hydrogen ions) and oxygen

Countercurrent Exchanger: The process where a medium (air or water) flowing in one direction over a tissue surface encounters blood flowing through the tissue in the opposite direction; this improves the gas diffusion by maintaining a concentration gradient

Diffusion: The process by which gas molecules move from a higher to a lower concentration through a medium or across a permeable barrier; the rate at which gases cross a barrier is increased by the surface area, and gas concentration gradient is decreased by the thickness of the barrier; gas solubility determines the amount that crosses the barrier

Gill: An evaginated organ structure where the membrane wall turns out and forms an elevated, protruding structure; typically used for water respiration

Lung: An invaginated organ structure where the membrane wall turns in and forms a pouch or saclike structure

Bibliography

Fish, F. E. “Biomechanics and Energetics in Aquatic and Semiaquatic Mammals: Platypus to Whale.” Physiological and Biochemical Zoology, vol. 73, no. 6, 2000, pp. 683–698. doi: 10.1086/318108.

Pough, F., H. Heiser, J. B. Heiser, and W. N. McFarland. Vertebrate Life. 11th ed. Oxford University Press, 2023.

Schmidt-Nielson, Knut. Animal Physiology: Adaptation and Environment. 5th ed. Cambridge UP, 2010.

Tonozzi, Caroline. "The Respiratory System in Animals." MSD Veterinary Manual, Nov. 2022, www.msdvetmanual.com/respiratory-system/respiratory-system-introduction/the-respiratory-system-in-animals. Accessed 2 July 2023.

Weibel, Ewald R. The Pathway for Oxygen. Harvard UP, 1984.

Willmer, Pat, Graham Stone, and Ian Johnston. Environmental Physiology of Animals. 2nd ed. Blackwell Science, 2017.

Withers, P. C. Comparative Animal Physiology. 2nd ed. Saunders, 2001.