Lungs Structure and Function
The lung is a branching system of tubes and air sacs. Air enters the nose and mouth and is directed in the throat to the trachea which carries the air in the chest. The trachea splits into two major bronchi, one for each lung. The bronchi branch into smaller and smaller tubes that end ending in air sacks (alveoli) where the gas exchanges occur. Adult lungs contain about 600 million alveoli, air-filled sacs that are surrounded by capillaries. Oxygen in the inhaled air enters the capillaries in alveoli and attaches to hemoglobin molecules. At the same time, carbon dioxide leaves the capillaries and enters the alveoli. The carbon dioxide leaves the lungs when you exhale.
The unborn infant is suspended in fluid and does not need lungs; the mother’s placenta exchanges oxygen and carbon dioxide. The fetal lungs are filled with amniotic fluid which is expelled during birth. After birth, the newborn baby inhales air for the first time. Deep breathing begins about 30 seconds after birth, and if all goes well, respiration continues at a rate of about 20 to 30 breaths per minute for the duration of life. Lungs are not yet fully mature at birth and breathing problems are the number one health problem in infants and young children.
Breathing is continuous and dynamic interaction of respiratory muscles, monitors and regulators in the brain. The brain receives information from multiple receptors such as blood chemoreceptors, receptors in the lung, and muscle receptors. A breathing center in the brainstem is a vital computer that decides how fast and deep breathing should be and sends signals to respiratory muscles.
Breathing is accomplished by expansion and contraction of the chest. Chest movement is just visible with quiet breathing at rest. Breathing accelerates with exertion and chest movement becomes more obvious as exertion increases. A runner, after an 800 meter competition, has a heaving chest and cannot speak until his maximal breathing effort subsides.
A person suffering an asthmatic attack stops other activities and concentrates all his or her energy in getting enough air exchange in the obstructed lungs. The small tubes in the lung can constrict and block air flow, especially the flow of air leaving the lungs.
Inspiration is achieved by the contraction of the diaphragm muscles and expansion of the chest. Expiration occurs when the diaphragm and the chest all relaxes reducing the volume inside the chest. Expiration can be forced by the contraction of abdominal muscles that increase intra-abdominal pressure, forcing the diaphragm up like a piston.
Blood levels of oxygen and carbon dioxide are monitored to determine the effectiveness of breathing. Low blood oxygen (hypoxemia) and high blood carbon dioxide (hypercapnia) increase the breathing effort.
Chemoreceptors in the carotid arteries, aortic arch, and brainstem (medulla) activate the inspiratory drive. High carbon dioxide is the strongest stimulus for increased respiratory effort. Low oxygen is also a stimulus to increase respiration.
Changes in intrathoracic pressure influence blood flood and heart action. Venous blood flows from the body to the right side of heart. Negative pressure in the chest promotes venous return. Positive pressure impedes venous return. The right ventricle pumps venous blood with low oxygen into the lungs to receive more oxygen.
The lungs are the only organs that receive arterial blood with low oxygen concentration. Oxygenated blood leaves the lungs via the pulmonary veins, returns to the left atrium of the heart to be pumped to the remainder of the body by the left ventricle.