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The primary function of respiration is performed by the lungs and their associated tissues. Air must be breathed in through the mouth and nose through the larynx (voice box) into the main airway, the trachea (windpipe). Inside the chest, the trachea branches into the two main airways called bronchi, which in turn successively branch many times into small bronchi called bronchioles. These airways end in very small sacs called alveoli. These alveoli have a very thin membrane separating the air space from the blood in the capillaries. Oxygen (O2) diffuses through the alveolar membrane across the capillary membrane and into the blood to be taken to all the tissues of the body. Tissues excrete carbon dioxide (CO2) into the blood, which is carried back to the lungs. Carbon dioxide diffuses from the blood into the alveoli and is carried back through the airways and out of the lungs with the exhaled air. The mouth and nose humidify dry air to ensure that the linings of the lower airways do not dry out. The main airway divides to supply the left and right lungs. These large airways are cylindrical. Their circular shape is maintained by C-shaped cartilage in the walls. The stiff walls prevent collapse of the airways and the loss of gases through the walls of these “conducting” airways. The airways branch repeatedly into smaller airways. As the airways become smaller, they have less cartilage, until, in the very smallest airways, the cartilage is absent. These thin airways, which are called respiratory bronchioles, have alveoli budding from their walls. Gases may diffuse through the walls of these airways. These bronchioles become alveolar ducts and then erupt into lobular sacs of alveoli. There are about 300 million alveoli in an individual’s lungs, which provide about 70 square meters of extremely thin membrane through which most gas exchange occurs.

The primary goal of the cardiovascular system is to provide blood flow, carrying oxygen and other nutrients to all tissues to meet their requirements. The cardiovascular system performs this function by maintaining a blood pressure high enough to push sufficient blood flow throughout the body, especially the vital organs. To keep the blood pressure up, the heart must pump sufficient amounts of blood even when the demand for increased blood flow to some tissues occurs. The blood vessels also play an important role in maintaining blood pressure. The heart and the blood vessels work in a coordinated manner to maintain blood pressure and blood flow.

There are nine main systems in the human body—the nervous, cardiovascular, respiratory, gastrointestinal, renal, endocrine, reproductive, thermoregulatory, and musculoskeletal systems. These systems are essential to sustain life, and they work together to perform their functions efficiently. All the other systems need the to operate or coordinate their functions. The first six of these systems are discussed in this article.