Anatomy & Physiology

Anatomy & Physiology Study Guide response to local changes in the composition of interstitial fluid that accompany neural activity. When you read, write, speak, or walk, specific regions of your brain become active. Blood flow to those regions increases almost instantaneously, ensuring that the active neurons will continue to receive the oxygen and nutrients they require. The brain receives arterial blood through four arteries. An interruption of flow in any one of these vessels will not significantly reduce blood flow to the brain as a whole because these vessels form an anastomosis. However, a plaque or a blood clot may still block a small artery, and weakened arteries may rupture. Such incidents temporarily or permanently shut off blood flow to a localized area of the brain, damaging or killing the dependent neurons. Signs and symptoms of a cerebrovascular accident (CVA), commonly called a stroke, appear. Blood Flow to the Heart The coronary arteries rise at the base of the ascending aorta, where systemic pressures are highest. Each time the heart contracts, it squeezes the coronary vessels; thus, blood flow is reduced. In the left ventricle, systolic pressures are high enough that blood can flow into the myocardium only during diastole; over this period, elastic rebound helps drive blood along the coronary vessels. Normal cardiac muscle cells can tolerate these brief circulatory interruptions because they have substantial oxygen reserves. When you are resting, coronary blood flow is 250 mL/min. When the stress on your heart increases, local factors, such as reduced O2 levels and lactic acid production, dilate the coronary vessels and increase blood flow. Epinephrine released during sympathetic stimulation promotes the vasodilation of coronary vessels while increasing the strength of cardiac contractions and heart rate. As a result, coronary blood flow increases while vasoconstriction occurs in other tissues. Blood Flow to the Lungs The lungs contain roughly 300 million alveoli, delicate epithelial pockets where gas exchange occurs. Every alveolus is enclosed by an expansive capillary network. Blood flow through the lungs is regulated primarily by local responses to oxygen levels within individual alveoli. When an alveolus contains oxygen in abundance, the associated vessels dilate, and blood flow increases, promoting the absorption of oxygen from the alveolar air. When the oxygen content of the air is very low, the vessels constrict, and blood is shunted to alveoli that still contain significant levels of oxygen. This mechanism maximizes the efficiency of the respiratory system because the circulation of blood through the capillaries of an alveolus has no benefit unless that alveolus contains oxygen. Blood pressure in pulmonary capillaries (average: 10 mm Hg) is lower than that in systemic capillaries. The BCOP (25 mm Hg) is the same as elsewhere in the bloodstream. As a result, reabsorption exceeds filtration in pulmonary capillaries. Fluid moves continuously into the pulmonary capillaries across the alveolar surfaces, thereby preventing a buildup of fluid in the alveoli that could interfere with the diffusion of respiratory gases. If the blood pressure in pulmonary capillaries rises above 25 mm Hg, fluid enters the alveoli, causing pulmonary edema.

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