Lungs

There are two lungs, one on each side of the thorax. In childhood, they are pink, but for most adults they become grey. IN men and women who smoke or are exposed to highly polluted air, the lungs often turn black. Each lung is surrounded by a protective, double-membraned sack called a pleura. It allows the lungs to expand and contract inside the chest wall.

The respiratory system begins in the mouth and nose, where we take and exhale air. From there, it travels down the trachea, or windpipe, a long tube that divides into two smaller tubes, called bronchi, which enter the lungs. Once inside the lungs, the bronchi further divide into many smaller branches called bronchiolies, which lead to tiny grapelike air sacs called alveoli.

Most of the air we breathe is nitrogen (about 78%); the rest is oxygen (21%), with minute quantities of carbon dioxide (CO2), traces gases, and water vapor. Once the air is inside the alveoli, the oxygen separates from the other gases and diffuses along the walls of these tiny sacs. There it seeps through the alveoli into surrounding capillaries, and makes its way into the bloodstream. At the same time, carbon dioxide inside the blood seeps into the alveoli, from it is breathed out when we exhale.

Once oxygen is inside the blood, it binds with hemoglobin, the iron-rich part of the red blood cells. The blood is then pumped to the left side of the heart, and then on to tissues throughout the body.

Each quantity of oxygen-rich blood eventually reaches a part of the body where it surrenders its load of O2 and receives the carbon dioxide waste from the cells. Onward it flows, back to the right side of the heart, which pumps the carbon-dioxide-rich blood into the lungs and down into the tiny alveoli.

There the CO2 seeps past the walls of the capillaries and into the tiny alveoli, which expel the CO2 when we exhale. IN exchange for the CO2, the blood receives oxygen.

The act of breathing requires a well-coordinated effort among the muscles of the rib cage and the diaphragm, a dome-shaped muscle system located between the thorax and the abdomen. The diaphragm pulls downward, while muscles within the rib cage expand. This causes the lungs themselves to expand and open. During relaxed breathing, the downward movement of the diaphragm is about two-thirds of an inch, which only minimally opens the lungs. But during deep breathing the diaphragm will descend as much as three inches.

We tend to think of respiration as limited to breathing in and out, but the act of respiration includes the burning of fuel, which is the reason why we breathe at all. Once the oxygen reaches the cells, it is combined with blood sugar, or glucose, to cause tiny combustion reactions within the cells. These reactions cause the release of energy which the cells use to maintain life.

Through a very complex chain reaction, this combination of blood sugar and oxygen oxidizes within a particular part of the cell, called the mitochondria. From there, the energy is stored in a molecule called adenosine triphosphate, or ATP, which is found in living systems throughout nature. When energy is needed, ATP has the capacity to provide it to the cell.

As with other forms of combustion, carbon dioxide and heat are given off. This heat contributes to a normal body temperature.