Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 263 of 367 Elastic Rebound As systolic pressure climbs, the arterial walls stretch, just as an extra puff of air expands a partially inflated balloon. This expansion allows the arterial system to accommodate some of the blood provided by ventricular systole. Arteries recoil and return to their original dimensions during diastole and the drop in blood pressure. This phenomenon is called elastic rebound. Some blood is forced back toward the left ventricle, closing the aortic valve and helping to drive additional blood into the coronary arteries. However, most of the push generated by elastic rebound forces blood toward the capillaries, which maintains blood flow through the arterial network while the left ventricle is in diastole. Muscular Compression Skeletal muscle contractions near a vein squeeze it, helping to push blood toward the heart. Blood travels in one direction because of valves located in small and medium-sized veins. When standing and walking, the cycles of contraction and relaxation that accompany normal movements assist venous return. If you stand at attention, with knees locked and leg muscles immobilized, that assistance is lost. The reduction in venous return then leads to a fall in cardiac output, which reduces the blood supply to the brain. This decline is sometimes enough to cause fainting, a temporary loss of consciousness. You would then collapse, but while you were in the horizontal position, both venous return and cardiac output would return to normal. The Respiratory Pump Inhalation causes the thoracic cavity to expand, reducing the pressure within the pleural cavities. This drop in pressure pulls air into the lungs. At the same time, blood is pulled into the inferior vena cava and right atrium from the smaller veins of the abdominal cavity and lower body. The effect on venous return through the superior vena cava is less pronounced due to the normal effect on gravity. With exhalation, the thoracic cavity decreases in size. Internal pressure then rises, forcing air out of the lungs and pushing venous blood into the right atrium. This mechanism is called the respiratory pump or thoracoabdominal pump. The importance of such pumping action increases during heavy exercise, when respirations are deep and frequent. Capillary Pressures and Capillary Exchange Capillary exchange plays such an important role in homeostasis. The most important processes that move materials across typical capillary walls are diffusion, filtration, and reabsorption. Diffusion Diffusion is the total movement of ions or molecules from an area where their concentration is higher to a place where their concentration is lower. A concentration gradient represents the difference between the high and low concentrations, and diffusion creates movement to eliminate that gradient. Diffusion occurs most rapidly when the distances involved are short, the concentration gradient is large, and the ions or molecules involved are small. Different substances diffuse across capillary walls by different routes: