Most animals have body systems specialized to obtain oxygen from the surroundings and convey it to all body parts, while usually getting rid of waste carbon dioxide at the same time. These are called respiratory systems. Typically, they link to a circulatory system. This is based on a circulating medium, usually a body fluid known as blood, which conveys the oxygen. Rather than carrying the oxygen in dissolved form, this fluid may contain substances specialized to attach to oxygen, termed respiratory pigments. The medium is kept flowing or circulating by some form of muscular pump, commonly called the heart.
External respiration is the interchange of the gases oxygen and carbon dioxide between the surroundings and the organism’s body, often called breathing. Internal respiration is the interchange of gases taking place inside the body, between the blood and tissues.
In simpler aquatic animals the respiratory gases, oxygen and carbon dioxide, diffuse in and out of tissues near the surface, and the medium carries them to and from more distant internal cells.
More complex animals have, as part of the respiratory system, specialized organs to increase the area of exposure between the circulating fluid to the external medium. These are often called gills in aquatic creatures and lungs in land animals.
In humans, as in most other vertebrates, the lungs are enclosed in the thorax or chest. The ribs support the body wall of the thorax, which has a domed base formed by the diaphragm muscle. The ribs slant downwards and forwards and have intercostal muscles between them. The diaphragm and intercostal muscles are the primary muscles of breathing or physical respiration.
When the ribs are raised by the action of the intercostal muscles, the volume of the thorax is increased. The volume of the thorax is also increased by downward contraction of the muscles of the diaphragm. Within the thorax, the lungs are held close to the body wall by atmospheric pressure. When the thorax expands, the lungs also expand and become filled with air drawn through the upper respiratory passages. This is breathing in or inhaling. Relaxation of the breathing muscles allows the springy, elastic tissues of the stretched lungs to return to their naturally contracted position, forcing air out of the lungs. This is breathing out or exhaling.
Up to 500 cu cm (30 cu in) of air are usually inhaled and exhaled with each breath. This is called tidal volume. About 3,300 cu cm (80 cu in) of additional air, called inspiratory reserve volume, can be inhaled on a forced inspiration and then exhaled; still another 1,000 cu cm, called expiratory reserve volume air, can be exhaled on a forced expiration. The sum of these three quantities is called the vital capacity. About 1,200 cu cm of air always remains in the lungs and cannot be exhaled; this volume is called the residual, or alveolar, air.
The human lungs are roughly pyramidal in shape, conforming to the shape of the thorax. They are not symmetrical. The right lung is larger and consists of three lobes. The left lung has two lobes and, near the medial (central) edge of the base, a scooped-out cardiac notch into which the heart fits. On the medial side of each lung is the root, by which the lung is attached to the mediastinum, or central partition of the chest. The root consists of pleural folds, bronchi (main airways), and pulmonary arteries and veins.
As the main airway or bronchus penetrates the substance of the lung, it divides and subdivides repeatedly until it ends in lobules, the structural and functional units of the lung. Accompanying the bronchus, the pulmonary arteries and veins divide at the same points. Nerves from the pulmonary plexus and lymphatic vessels are also distributed in the same manner. Within the lobule, the bronchiole divides into terminal bronchi, each thinner than a hair, which open into groups of pulmonary atria, or air spaces. Each of the atria opens in turn into a number of alveolar saccules, the walls of which are pouched out to form the numerous alveoli, or air sacs. There are about 350 million alveoli in each lung. The arterioles and venules of the lobules are connected by a dense network of capillaries that lie in the walls of the alveoli. This structure brings air and blood very close together over a huge surface area so that oxygen can be absorbed and carbon dioxide given off.
The principal nervous centre for controlling the rate and depth of physical respiration is in the respiratory section of the pons and medulla oblongata in the brain stem. The cells of this nucleus are sensitive to the acidity of the blood, which reflects the concentration of carbon dioxide in the blood plasma. When the acidity of the blood is high, usually caused by an excess of carbon dioxide, the respiratory centre stimulates the respiratory muscles to greater activity to “blow off” the excess carbon dioxide and take in more oxygen. When the carbon dioxide concentration is low, breathing is depressed (slower and/or shallower).
PHYSICAL RESPIRATION IN MORE COMPLEX ANIMALS .
Many aquatic animals carry on external respiration or breathing by means of gills, over which auxiliary respiratory mechanisms keep a constant current of fresh water flowing. The gills are branched to such an extent as to resemble feathers or plumes. In each branch fine blood vessels are subdivided so that the blood is separated from the water medium by two layers of cells, one being the wall of the fine blood vessel, or capillary, and the other being the epithelium of the gill. Oxygen readily diffuses in and carbon dioxide passes out. The extended surface produced by the branching enables large quantities of blood to be oxygenated in a short time.
In air-breathing creatures such as the earthworm, external respiration takes place through the capillaries in the skin. Amphibians like the frog respire through both skin and lungs. Insects breathe by means of air tubes, or tracheae, which open on the outside of the body and branch out through the tissues, carrying air to internal organs and structures. Reptiles and mammals respire solely by means of lungs. Birds have auxiliary air sacs in the body cavity and air spaces within certain bones, all of which connect with the lungs and act as aids to respiration.
The respiratory and circulatory systems of air-breathing animals can also adapt and modify for life in low-oxygen environments, such as high altitudes. For example, people living in the Andes at altitudes of 3,000 m (10,000 ft) or more have larger lungs, more highly branched capillary systems, and a faster heartbeat than people living at lower altitudes. Moreover, the blood of high-altitude dwellers contains 30 per cent more red cells than the blood of people living at sea level; they are therefore able to make efficient use of the available oxygen.
Aquatic mammals generally have large, complex systems of veins for the storage of blood. The blood volume of whales and seals is up to 50 per cent greater per kilogram of body weight than the blood volume of human beings, which enables them to supply their tissues with oxygenated blood for a long period without breathing. Whales may remain submerged from 15 minutes to more than an hour, depending on the species. The elephant seal may stay underwater for 30 minutes. When a seal begins an underwater dive, its heartbeat slows from 150 beats per minute to 10; the oxygen content of the arterial blood is 20 per cent. When the oxygen level drops to nearly 2 per cent, the seal must surface.