Chapter 23 Summary

Summary

Fundamental Concepts of External Respiration

Oxygen always crosses the gas-exchange membrane by diffusion. This means that O₂ enters an animal only if the O₂ 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

The oxygen utilization coefficient during breathing is the percentage of the O₂ in inhaled medium that 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 O₂ partial pressure in blood exiting the breathing organ. Countercurrent gas exchange ranks highest. Cross-current gas exchange ranks second. Cocurrent and tidal gas exchange rank third.
Air breathers tend to have much higher CO₂ partial pressures in their systemic arterial blood than water breathers. This difference arises during breathing and is principally a consequence of the differing physical and chemical properties of air and water.

Introduction to Vertebrate Breathing

The gill surface area of most fish of 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 for 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, however, 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

The secondary lamellae are the principal sites of gas exchange in fish gills. Countercurrent gas exchange occurs in the lamellae.
Water flow across the gills is essentially unidirectional. It is driven by a buccal pressure pump and an opercular suction pump that act in an integrated rhythm, so that the buccal pump drives water across the gills when the opercular pump is being emptied of water and the opercular pump sucks water across the gills while the buccal pump is being refilled with water.
Some fish turn to ram ventilation when swimming fast enough. Others, such as tunas, are obligate ram ventilators and must swim all the time to avoid suffocation.
A lowered O₂ partial pressure in the blood is a more potent stimulus for increased ventilation in fish than an elevated CO₂ partial pressure.
Most of the 400 or so species of air-breathing fish have an air-breathing organ that is derived from the buccal cavity, opercular cavity, stomach, or intestines—or one that originates as an outpocketing of the foregut (e.g., swim bladder).

Breathing by Mammals

The lungs of mammals consist of dendritically branching airways that end blindly in small, thin-walled, well-vascularized outpocketings, the alveoli. The airways exhibit 23 levels of branching in the human adult lung, giving rise to 500 million alveoli. The airways in a mammalian lung are categorized as conducting airways, where little gas exchange with the blood occurs, and respiratory airways, where most gas exchange with the blood takes place.
Because of the blind-ended structure of the mammalian lung, the gas in the alveoli always has a substantially lower O₂ partial pressure and higher CO₂ partial pressure than atmospheric air.
Contraction of the diaphragm is a principal force for inhalation in mammals, especially large quadrupeds. External intercostal muscles may contribute to inhalation; internal intercostal muscles and abdominal muscles may contribute to exhalation. Inhalation occurs by suction as the lungs are expanded by contraction of inspiratory muscles. At rest, exhalation occurs passively by elastic rebound of the lungs to their relaxation volume when the inspiratory muscles relax.
The breathing rhythm in mammals originates in a central pattern generator in the pre-Bötzinger complex in the medulla of the brainstem.
The most potent chemosensory stimulus for increased ventilation in mammals is a rise in blood CO₂ partial pressure and/or H⁺ concentration, sensed in the medulla. The blood O₂ partial pressure, ordinarily a less influential factor in controlling ventilation, is sensed by the carotid bodies along the carotid arteries (humans) or by carotid and aortic bodies (certain other mammals). The control of ventilation during exercise involves stimuli generated in association with limb movement as well as chemosensory controls.
Pulmonary surfactant, a mix of lipids and proteins that affects surface tension, makes a critical contribution to maintaining the proper microscopic conformation of the lungs in all air-breathing vertebrates.

Breathing by Birds

The lungs of birds are relatively compact, rigid structures consisting mostly of numerous tubes, running in parallel, termed parabronchi. Fine air capillaries, extending radially from the lumen of each parabronchus, are the principal sites of gas exchange. Air sacs, which are nonrespiratory, are integral parts of the breathing system.
The lungs are ventilated by a bellows action generated by expansion and compression of the air sacs.
Airflow through the parabronchi of the paleopulmonal system (the major part of the lungs) is posterior to anterior during both inhalation and exhalation. Cross-current gas exchange occurs.

Breathing by Aquatic Invertebrates and Allied Groups

The gills of various groups of aquatic invertebrates are often independently evolved. Wide variation thus exists in both gill morphology and the mode of gill ventilation.
A single basic sort of breathing apparatus can undergo wide diversification within a single phyletic group. This general principle is illustrated by the molluscs, the great majority of which have breathing organs associated with the mantle and located in the mantle cavity. Whereas both aquatic snails and lamellibranchs employ ciliary ventilation, the gills are modest-sized leaflets in snails, but expansive sheets (used partly for feeding) in the lamellibranchs. Cephalopods, such as squids, ventilate their gills by muscular contraction. Most land snails lack gills and breathe with a lung derived from the mantle cavity.

Breathing by Insects and Other Tracheate Arthropods

Insects and many arachnids breathe using a tracheal system that connects to the atmosphere by way of spiracles on the body surface and ramifies throughout the body so that gas-filled tubes bring O₂ close to all cells.
The modes of gas exchange through the tracheal system include diffusion, conspicuous ventilation (such as abdominal pumping and autoventilation), and several forms of microscopic ventilation.
Aquatic insects may lack functional spiracles and breathe using superficial tracheal beds. Alternatively, they may have functional spiracles and breathe from the atmosphere, large bubbles, or plastrons.