7 The Respiratory System
Trace air through the respiratory tract, explain how breathing and gas exchange work, and follow oxygen and carbon dioxide as blood transports them between lungs and tissues.
Airflow through the respiratory tract
The respiratory system brings oxygen into the body and removes carbon dioxide, a waste product of cellular metabolism. Its functions depend on coordinated airflow, gas exchange between air and blood, and transport of gases by the cardiovascular system.
Air travels through the nasal cavity or mouth, pharynx, larynx, trachea, bronchi, and progressively smaller bronchioles. The nasal passages warm, moisten, and filter incoming air. In much of the conducting tract, mucus traps particles and cilia move the mucus toward the throat, where it can be swallowed or expelled.
The two functional zones
The carries air from the nose and mouth through the terminal bronchioles. It conditions and distributes air but does not exchange gases with blood.
The begins with the respiratory bronchioles and includes the alveolar ducts and , the microscopic air sacs where gas exchange occurs.
Lungs and pleural membranes
The paired lungs sit in the chest on either side of the heart. The right lung has three lobes, while the left has two, leaving room for the heart.
Each lung is covered by a thin visceral pleura, and the chest wall is lined by a parietal pleura. A small amount of fluid between these membranes reduces friction and helps the lungs follow chest-wall movements.
How breathing moves air
is the movement of air between the atmosphere and the . Air flows down pressure gradients, from higher pressure toward lower pressure.
During quiet inhalation, the diaphragm contracts and moves downward while the external intercostal muscles lift the ribs. This expands the chest cavity and lungs, lowering pressure inside the lungs below atmospheric pressure so air flows in. During quiet exhalation, these muscles relax. Elastic recoil reduces lung volume and raises pressure inside the lungs, pushing air out. Exhalation becomes active during forceful breathing, when additional muscles help compress the chest.
During exercise, breathing becomes faster and deeper. This increases ventilation, helping bring more fresh air to the while removing more carbon dioxide.
Gas exchange in lungs and tissues
Gas exchange occurs by : each gas moves from an area of higher toward an area of lower . In the lungs, the —the thin barrier formed by alveolar and capillary walls—separates air from blood. Oxygen diffuses from into pulmonary capillary blood, while carbon dioxide diffuses from blood into and is exhaled.
At body tissues, the gradients are reversed. Oxygen leaves the blood and diffuses into cells, while carbon dioxide produced by cells diffuses into the blood. Gas exchange in the lungs is called ; exchange between blood and tissues is called .
The roles of ventilation and
Effective lung gas exchange requires both ventilation, meaning air reaches the , and , meaning blood flows through nearby capillaries. If an alveolus receives little fresh air, gas exchange there is limited even when blood flow is adequate.
Transport of respiratory gases
Blood carries oxygen from the lungs to tissues and carbon dioxide from tissues back to the lungs.
Oxygen transport: Oxygen is carried mainly bound reversibly to , a protein in red blood cells. A small amount is dissolved in plasma. In lung capillaries, oxygen binds to ; in tissues, it dissociates and diffuses into cells. Active tissues tend to have higher carbon dioxide and acidity, conditions that encourage to release oxygen.
Carbon dioxide transport: Carbon dioxide travels in three forms: mostly as in plasma, with smaller amounts bound to and dissolved in plasma. In red blood cells, much of the carbon dioxide is converted reversibly into bicarbonate and hydrogen ions. In the lungs, the reaction reverses, allowing carbon dioxide to form again and diffuse into the for exhalation.