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The bronchi branch in the lungs into smaller and smaller bronchioles, ending in clusters of tiny air sacs called alveoli; there are 750 million alveoli in the lungs. The blood flows through the lungs in the pulmonary circulation. Through the thin membrane of the network of capillaries around the alveoli, the air and the blood exchange oxygen and carbon dioxide. The carbon dioxide molecules migrate from the erythrocytes in the capillaries through the porous membrane into the air in the alveoli, while the oxygen molecules cross from the air into the red blood cells.
The erythrocytes proceed through the circulatory system, carrying the oxygen in loose combination with hemoglobin and giving it up to the body cells that need it. In cellular respiration the blood cells release oxygen and pick up carbon dioxide. The lungs dispose of the carbon dioxide, left there by the red blood cells, in the process of breathing. With each breath, about one-sixth of the air in the lungs is exchanged for new air.
The chemical controls of breathing are mainly dependent on the level of carbon dioxide in the blood. The response is so sensitive that if the carbon dioxide level increases two-tenths of 1 per cent, the respiratory rate increases automatically to double the amount of air taken in, until the excess of carbon dioxide is eliminated. It is not lack of oxygen but excess of carbon dioxide that causes this instant and powerful reaction.
The carbon dioxide tension (Pco2), of arterial blood normally is 35 to 45 mm Hg. When the Pco2 increases, the respiratory centers are stimulated and breathing becomes more rapid; conversely, decrease of the Pco2 slows the rate of respiration. The Pco2 acts both directly on the respiratory centers and on the carotid and aortic bodies, chemoreceptors that are responsive to changes in blood Pco2, Po2, and pH (see also blood gas analysis).
On its way through the nasal passage, the cold air from outside is preheated by a large supply of blood, which gives off warmth through the thin mucous membrane that lines the respiratory tract. This same mucous lining is always moist, and dry air picks up moisture as it passes.
Dust, soot, and bacteria are filtered out by a barrier of cilia, tiny hairlike processes that line the passageways of the respiratory tract. The cilia trap not only foreign particles but also mucus produced by the respiratory passages themselves. Since the movement of the cilia is always toward the outside, they move the interfering matter upward, away from the delicate lung tissues, so that it can be expectorated or swallowed. Particles that are too large for the cilia to dispose of usually stimulate a sneeze or a cough, which forcibly expels them. Sneezing and coughing are reflex acts in response to stimulation of nerve endings in the respiratory passages. The stimulus for a cough comes from the air passages in the throat; for a sneeze, from those in the nose.
aer·o·bic res·pi·ra·tion(ār-ō'bik res'pir-ā'shŭn)
aerobic respirationa type of CELLULAR RESPIRATION that requires oxygen. GLUCOSE is broken down to release energy in a series of steps which can be grouped into three main stages:
A stable glucose molecule is first energized by the addition of a phosphate group from two ATP molecules (PHOSPHORYLATION) and then broken down to two molecules of three-carbon phosphoglyceraldehyde (PGAL) (glyceraldehyde 3 phosphate). Each PGAL molecule is oxidized by removal of two hydrogen atoms which are picked up by an NAD molecule. Since oxygen is present, NADH can undergo a MITOCHONDRIAL SHUNT to enter an ELECTRON TRANSPORT SYSTEM (ETS); See Fig. 16 . Four molecules of ATP are then synthesized in SUBSTRATE-LEVEL PHOSPHORYLATION over several steps, giving a net gain of two ATP molecules per molecule of glucose. Glycolysis is completed with the production of two molecules of three-carbon pyruvic acid (pyruvate) .
Stage 2 : oxidation and DECARBOXYLATION of pyruvic acid (pyruvate), which occurs in the MITOCHONDRIA in eukaryotes, to form two molecules of two-carbon ACETYLCOENZYME A (acetyl-CoA). CO2 is released in this process, together with two hydrogen atoms per pyruvic acid (pyruvate) molecule, which are picked up by NAD and passed down an ETS located on the inner membranes of mitochondrial cristae. Three molecules of ATP are produced per NADH, with oxygen acting as the final acceptor of hydrogen, producing water.
Stage 3: entry of acetyl-CoA into the KREBS CYCLE (TCA cycle). Each molecule of acetyl-CoA can turn the cycle once. As each glucose molecule is broken down to two acetyl-CoA molecules, the cycle will turn twice per glucose molecule, yielding 2 x 2 molecules of CO2 and 2x8 atoms of hydrogen. Six pairs of hydrogen atoms are picked up by NAD to produce 18 (6 × 3) molecules of ATP via the ETS. The remaining two pairs of hydrogen atoms are accepted by FAD molecules which move to the ETS to produce 4 (2 × 2) molecules of ATP. One molecule of ATP is produced directly by substrate-level phosphorylation for each turn of the cycle. For each molecule of glucose undergoing aerobic respiration the products are as shown in Fig. 16. The three stages are summarized in Fig. 17.
Note that the net production of ATP per molecule of glucose is 38 molecules since two were required at the start of glycolysis. Of these 38 ATP molecules only two (about 5%) are synthesized without oxygen (i.e. anaerobically); the other 36 are the product of aerobic respiration. Fats and proteins can also undergo aerobic respiration, entering the reactions at various stages; see ACETYLCOENZYME A for details.