Lungs and respiration

The air around us provides our body tissues with the oxygen they need for oxidizing nutrients derived from food to provide energy for the processes of living. Respiration is the process by which the body gets that oxygen and expels waste carbon dioxide. External respiration involves air flow in and out of lungs. From the lungs, blood supplies tissue cells with the oxygen they need for the energy-releasing chemical reactions collectively called internal respiration.

The respiratory system within the lungs—illustrated here by a model made from a resin cast—consists of a closely interlinked system of air passages, arteries, and veins. Deoxygenated blood, shown in blue, is pumped from the heart through a treelike arrangement of pulmonary arteries and arterioles until it flows through capillaries in and around the alveoli, which are the terminations of the bronchial “tree,” shown in white. Oxygen is absorbed and carbon dioxide is released before the blood, now oxygenated and shown in red, returns through a similar arrangement of pulmonary veins to the heart. The model is seen from the front so the left side of the chest appears on the right. The heart lies in the left side of the chest, and to accommodate it, the two-lobed left lung is smaller than the three-lobed right lung.

Parts of the respiratory system

Some tiny, primitive animals allow oxygen to reach all their cells by diffusing it in and out through openings in the body wall. A human’s much larger body needs an active pump to supply oxygen to the lungs where the gas can be absorbed into the blood for transportation throughout the body. This system consists basically of two bellows (the lungs) that fill with air, then empty, as muscles make them open and close.

Seen from the outside, lungs appear as two roughly pyramidal spongy structures that almost fill the conical thoracic (chest) cavity, walled by ribs and backbone, with the diaphragm a sheet of muscle as the floor. The left lung is smaller than the right, to allow room for the heart. The left lung is divided into two main lobes, the right into three; each lobe is subdivided into segments. Both lungs and the cavity in which they lie are lined by the pleura a thin, smooth membrane that secretes a lubricating fluid that prevents friction between lungs and ribcage.

The lungs and passages supplying them with air form complex structures that parallel the trunk and branches of an inverted tree. This bronchial tree’s internal surface provides an area for gaseous exchange more than 40 times larger than the body’s surface.

The “tree trunk” is the windpipe or trachea—a wide, flexible tube stiffened by up to 20 broad bands of cartilage and roughly 4.5 inches (11 centimeters) in length. Some tracheal bands can be felt at the front of the throat. Inhaled air reaches the top of the trachea via the nasal cavity or mouth and the pharynx—all of which are lined with mucous membranes that warm the air and moisten it. From the pharynx, air passes through a slim aperture, the glottis. When swallowing, this is closed automatically by a flap (the epiglottis) that prevents food from entering the lungs. From the glottis, air flows down through the larynx (the voice box), the position of which can be identified by the tracheal cartilage called the Adam’s apple, at the top of the trachea below the chin. Changing the length and tension of the vocal cords—elastic bands of tissue stretched across the top of the larynx-changes vocal pitch.

At its lower end, the trachea divides into two hollow branches known as bronchi. Each branch supplies air to one lung. In each lung, the bronchus divides into subbranches. The smallest subdivisions of a bronchus are bronchioles, less than one millimeter in diameter and walled by smooth muscle. The bronchial tree’s outermost “twigs” are more than 250,000 tiny respiratory bronchioles about half a millimeter in diameter. Sprouting from these are some 6 million “leaves”: tiny alveoli that resemble minute, clustered, air sacs. It is here that gaseous exchange takes place as you breathe in and out.

Breathing is accomplished by lowering the diaphragm and raising the ribcage, to expand the lungs and breathe in, then by relaxing the diaphragm and lowering the ribcage to breathe out

Breathing in, breathing out

When you breathe in, the muscles of the dome-shaped diaphragm contract. This pulls the dome down, thereby lowering the floor of the chest cavity. Meanwhile, the ribs’ intercostal muscles contract, pulling the ribs up and out. These actions expand the chest cavity, creating a vacuum that causes air to flow into the lungs as they expand.

When you breathe out, the muscles of the diaphragm relax, and it rises in the chest cavity. The intercostal muscles also relax, and the elasticity of the chest wall causes the ribs to sink down and inwards, expelling air from the lungs. When breathing out forcefully, muscles of the abdomen contract to increase the pressure, which pushes the diaphragm upwards and forces air from the lungs.

Alveoliz the air sacs of the bronchial “tree,” allow air to come into close contact with blood in capillaries, so that gas exchange can take place.

At rest you may breathe in as little as .25 pint (57 centiliters) of air with each breath. During exercise, however, when the body’s need for oxygen increases, each breath may draw in as much as 2 pints (4.5 liters).

Normally, about one-third of breathed-in air gets no farther than the bronchi or bronchioles. The rest fills alveoli. Oxygen molecules are absorbed by fluid lining each alveolus’s thin wall, then pass to blood cells in a thinwalled capillary part of a mesh that surrounds each alveolus and carries deoxygenated blood from the pulmonary artery. Oxygen molecules attach to the hemoglobin molecules of red blood cells. Meanwhile, carbon dioxide molecules, dissolved in the blood plasma, pass out from the capillaries into the alveolus. Expired air also contains water vapor from the moisture lining alveolar walls.

How fast we breathe depends on the carbon dioxide level in the bloodstream. The higher the level the more acid the blood. Cells in the brainstem and elsewhere monitor changes in the acid level and, as it rises, stimulate the breathing reflex.

Lungs are composed of spongy tissue in which are thousands of tiny branches of veins, arteries, and bronchioles. Surrounding each lung is a thin membrane, the visceral pleura, which permits the lungs to move smoothly within the pleural cavity—the space occupied by the lungs in the chest.

Respiratory defenses

The body has various means of preventing and combating disease or injury in the delicate respiratory system. Mucous membranes warm and moisten inspired air to prevent it from chilling or drying the bronchioles and alveoli. Cough and sneeze reflexes expel food or other particles that find their way into the respiratory passages and the bronchial tree. Inside alveoli, cells called phagocytes swallow bacteria and dust, and tiny whiplike cilia projecting from the inner walls of the bronchial tree lash these foreign bodies up toward the trachea in blobs of mucus, which are coughed up as “phlegm.”