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GE 345: Week 3

Pulmonary Circulation

Updated by Tracey 23 July 02

Pulmonary vessels have larger diameters than their systemic counterparts. This allows the pulmonary arteries to accommodate 2/3 of the stroke volume output of the right ventricle with each beat.

During heavy activity blood flow through the lungs can increase by seven times. Extra flow is accommodated by: increasing the number of open capillaries; and distending the capillaries, increasing flow rate through each capillary. These changes reduce pulmonary vascular resistance enough that pulomary arterial pressure rises very little.

Outward forces are slightly higher in the pulmonary capillaries than inward, which causes a slight flow of fluid from the capillaries into the interstitial spaces. Extra fluid is pumped back to the circulation through the pulmonary lymphatic system.

Gas Transport

Diffusion of oxygen from the alveoli into the pulmonary blood and carbon dioxide in the opposite direction occurs after ventilation with fresh air. Kinetic motion of the molecules causes diffusion, as molecules move from areas of higher concentration to areas of lower concentration.

Kinetic motion of gases creates pressure. The total pressure a mixture of gases exerts is equal to the sum of the pressures of each individual gas. The diffusion rate for each gas is directly proportional to the individual gas' partial pressure. The air we breathe consists of about 79% nitrogen and 21% oxgen. At sea level, total air pressure is about 760 mm Hg. 79% (600mm Hg) of the pressure is from nitrogen, while 21% (160 mm Hg) is cause by oxygen.

Since cells use oxygen and produce carbon dioxide, oxygen tends to diffuse from the blood to the cells, and carbon dioxide diffuses from the cells to the blood.

3% of oxygen transported to the tissues is dissolved in the blood. The other 97% is transported by hemoglobin. When O₂ pressure is high, as in the pulmonary capillaries, oxygen binds with hemoglobin. When O₂ pressure is low, as in the tissue capillaries, oxygen is released from the hemoglobin.

During strenuous activity, three times as much oxygen is tranpsorted than normal. In well-trained endurance athletes, cardiac output can increase up to seven times normal values, yielding a 20-fold increase in oxygen transport to the tissues.

Carbon monoxide combines with hemoglobin at the same site as oxygen, and with greater tenacity. Therefore, it displaces oxygen on the hemoglobin. Treatment is usually pure oxygen, which can displace the carbon monoxide more successfully by creating high alveolar pressure of O₂

About 7% of CO₂ is transported in dissolved form. 70% is combined with water to form hydrogen and bicarbonate ions, of which the hydrogen joins with hemoglobin, which acts as a buffer. 15-25% of carbon dioxide is transported in direct combination with hemoglobin and plasma proteins.