Respiration is the process by which human beings and other living things obtain and use oxygen. Except for certain microorganisms, all living things require oxygen to live. Respiration also involves the elimination of carbon dioxide, a gas produced when cells use oxygen.
Respiration may be divided into two processes: (1) organismic respiration and (2) cellular respiration. Organismic respiration is the process by which animals take in oxygen from the environment and carry it to the cells of their tissues. Carbon dioxide is carried away from the cells and delivered to the environment. In cellular respiration, oxygen is used in chemical reactions within the cells. These reactions release energy and produce carbon dioxide and water as waste products.
Organisms carry out organismic respiration in various ways, depending on their size and environment. For example, single-celled organisms, such as diatoms and amebas, exchange oxygen and carbon dioxide directly with the environment through their cell membranes. In higher animals, however, each cell lacks direct contact with the environment. A system of specialized structures or organs is required to carry out organismic respiration in these animals.
This article deals chiefly with respiration in humans and other mammals. Respiration in other animals with lungs—such as birds, reptiles, and most adult amphibians—is carried out in similar ways. These animals all exchange gas with the environment by breathing.
Breathing
Structures of breathing.
The lungs are the organs of breathing. They are elastic structures in the chest cavity. Each lung contains millions of small air chambers called alveoli. A network of tiny blood vessels called capillaries lies within the walls of each alveolus.
Other important structures are the chest wall and the diaphragm. The chest wall includes the bones that form a protective cage around the chest cavity, the muscles associated with these bones, and the abdominal muscles. The diaphragm is a dome-shaped sheet of muscle that separates the chest cavity from the abdomen.
Gas enters and leaves the body through the nose and mouth. The pharynx (back of the nose and mouth), the larynx (voice box), and the trachea (windpipe) are the 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, gas is expelled from the lungs.
Inspiration occurs when the diaphragm and the muscles of the chest wall contract. This action lifts the ribs and makes the chest cavity longer and wider, causing the lungs to expand. The expansion of the lungs lowers the pressure in the alveoli, drawing fresh air into the lungs. Oxygen makes up about 20 percent of the volume of this fresh air. Almost all the rest of it is nitrogen. Only about 0.03 percent 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 percent and oxygen about 15 percent of this gas.
Oxygen and carbon dioxide are exchanged between the gas and the blood in the lungs 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 are capable of holding. This gas provides a reserve so exchange of oxygen and carbon dioxide can continue between breaths.
Control of breathing.
Breathing is regulated by the respiratory center, 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. The average rate in adult human beings is 12 to 15 breaths per minute. 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 center to adjust the rate and depth of breathing. When necessary, such as during exercise, muscles in the chest wall can be stimulated to speed up expiration. In this way, the respiratory center maintains normal levels of oxygen and acidity in the body.
Gas transport between the lungs and tissues
The circulatory system transports oxygen to body tissues and carries carbon dioxide away from them. Red blood cells play an essential role in this process. They contain hemoglobin, a molecule that can carry much oxygen. The cells 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 hemoglobin 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 hemoglobin 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. During the cycle, energy is captured and passed on to a 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.
Breathing 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 dissolved in the water moves 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, insects have a system of tiny air tubes called tracheae. These tubes carry air from the environment directly to all 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 most 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. Algae and many microbes also perform photosynthesis.
