Other structures important in breathing are the chest wall and the diaphragm. The chest wall includes the ribs--which form a protective cage around the chest cavity--and the muscles between the ribs. The diaphragm is a dome-shaped sheet of muscle that separates the chest cavity from the abdomen.
Air enters and leaves the body through the nose and the mouth. The pharynx (back of the nose and mouth), the larynx (voice box), and the trachea (windpipe) are the air passages that connect the nose and mouth with the lungs.
The process of breathing. Breathing consists of two acts, inspiration (breathing in) and expiration (breathing out). During inspiration, also called inhalation, air from the atmosphere is drawn into the lungs. During expiration, or exhalation, air is expelled from the lungs.
Inspiration occurs when the diaphragm and the muscles of the chest wall contract. This action makes the chest cavity longer and wider, causing the lungs to expand. The expansion of the lungs creates a slight vacuum in the alveoli, drawing fresh air into the lungs. Oxygen makes up about 20 per cent of the volume of this fresh air. Almost all the rest of it is nitrogen. Only about 0.03 per cent is carbon dioxide.
Expiration results when the diaphragm and other muscles relax, allowing the lungs to retract. This action causes the pressure of the gas in the alveoli to become greater than the atmospheric pressure. As a result, gas flows out of the lungs. Carbon dioxide makes up about 5 per cent and oxygen about 17 per cent of this gas.
Oxygen and carbon dioxide are exchanged between the lungs and the blood through the thin walls of capillaries in the alveoli. Blood entering these capillaries is low in oxygen and high in carbon dioxide. Oxygen that has been inhaled passes into the blood, while carbon dioxide moves from the blood into the alveoli.
Between breaths, when the respiratory system is "at rest," the lungs still contain almost half the gas they can hold. This gas provides a reserve so that the exchange of oxygen and carbon dioxide can continue between breaths.
Control of breathing. Breathing is regulated by the respiratory centre, groups of nerve cells in the brain stem. Every few seconds, these cells send bursts of impulses to the muscles involved in inspiration. These signals determine the rate and depth of breathing. Another group of special cells, called chemoreceptors, sense the oxygen and carbon dioxide levels in the blood and the acidity of cerebrospinal fluid surrounding the brain. Slight increases or decreases in carbon dioxide cause changes in the acidity of body fluids. These changes may affect various body functions. Chemoreceptors send signals to the respiratory centre to quicken or slow the rate of breathing. In this way, they help maintain normal levels of oxygen and acidity in the body.
Internal respiration
Internal respiration refers to the process by which oxygen is transported to body tissues and carbon dioxide is carried away from them. Red blood cells play an essential role in this process. They contain haemoglobin, a molecule that can carry large amounts of oxygen. They also contain an enzyme called carbonic anhydrase. This enzyme helps change carbon dioxide into bicarbonate ion, a form that is easily carried in blood.
Red blood cells pick up oxygen as they pass through the lungs. The heart then pumps this oxygen-rich blood through the arteries to capillaries in the body tissues. There, oxygen is released from the haemoglobin and passes through the capillary walls to the tissue cells. At the same time, carbon dioxide produced by the tissue cells enters the blood. Carbonic anhydrase in the red blood cells helps change most of the carbon dioxide to bicarbonate ions. Most of these bicarbonate ions move out of the red blood cells and are carried in blood plasma. The rest of the carbon dioxide entering the blood becomes associated with haemoglobin molecules or stays dissolved in plasma. When the blood reaches capillaries in the alveoli, these reactions reverse and the released carbon dioxide enters the gas in the alveoli.
Cellular respiration
Respiration in cells involves a series of chemical reactions that occur in the presence of oxygen. These reactions release energy from food substances and make it available so that the cells can function.
Cells can obtain some energy without oxygen by a chemical process called glycolysis. Glycolysis converts molecules of glucose (a simple sugar) into smaller molecules called pyruvic acid. This action releases energy, which is captured in a compound known as adenosine triphosphate (ATP). ATP is very important because it supplies energy to all cells. However, glycolysis produces only a small amount of ATP.
Cells require oxygen to obtain large amounts of ATP. When oxygen is present in a cell, pyruvic acid enters a series of chemical reactions called the Krebs cycle (see KREBS CYCLE). At various steps during the Krebs cycle, energy is captured and passed on to a second series of reactions called the electron transport chain. As a result of these reactions, carbon dioxide and water are formed and a great deal of energy is stored as ATP.
External respiration in animals without lungs
Many animals that live in water, including fish and shellfish, have gills for exchanging oxygen and carbon dioxide with their environment. When water comes in contact with the gills, oxygen moves easily from the water and passes through the thin membrane that separates the animal's blood from the water. At the same time, carbon dioxide moves from blood to water. Fish take in water through the mouth and force it out over the gills.
Other animals that lack lungs also have special ways of breathing. For example, earthworms breathe through their skin. They have a system of capillaries just beneath the skin. Oxygen and carbon dioxide are exchanged between air in the soil and the animal's blood. Insects have a system of tiny air tubes called tracheae for breathing. These tubes carry air from the environment directly to different parts of the body.
Some animals, such as amphibians, use more than one organ of respiration during their life. Frogs, for example, breathe through gills while they are tadpoles. Mature frogs breathe chiefly with lungs and also exchange gas with the environment through their skin.
Respiration in plants
In higher plants, oxygen and carbon dioxide move into and out of the roots and stems through the outer layers of cells. The majority of gas exchange in plants, however, takes place through small openings in the leaves called stomata.
Like animal cells, plant cells obtain energy through chemical reactions that break down glucose. Green plants also produce energy through a "reverse respiration" process called photosynthesis. In photosynthesis, the plant uses energy from light to make glucose. During this process, the plant takes in carbon dioxide from the environment and produces oxygen as a waste product. Certain bacteria also perform photosynthesis.
