Iii

This squeezing moves the blood in the veins toward the heart because of one-way valves that prevent backflow.

Muscle relaxes:

Valve open

Valve closed

Muscle relaxes:

Valve open

Valve closed

Blood is propelled forward by muscle contractions and, in some body regions, by gravity.

Back pressure is due to contractions of atria, contractions of muscles, and, in some regions, by gravity.

Blood is propelled forward by muscle contractions and, in some body regions, by gravity.

Back pressure is due to contractions of atria, contractions of muscles, and, in some regions, by gravity.

49.13 One-Way Flow Veins have valves that prevent blood from flowing backward.

ing of these veins is highly desirable and can be aided by wearing support hose and periodically elevating the legs above the level of the heart.

When an animal walks or runs, its legs act as auxiliary vascular pumps, returning blood to the heart from the veins of the lower body. As a greater volume of blood is returned to the heart, the heart contracts more forcefully, and its pumping action is enhanced. This strengthening of the heartbeat is due to a property of cardiac muscle cells described by the Frank-Starling law: If the cells are stretched, as they are when the volume of returning blood increases, they contract more forcefully. The actions of breathing also help return venous blood to the heart. The ventilatory muscles create negative pressure that pulls air into the lungs (see Figure 48.11), and this negative pressure also pulls blood toward the chest, increasing venous return to the right atrium. In addition, some of the largest veins closest to the heart contain smooth muscle that contracts at the onset of exercise. This contraction can rapidly increase venous return and stimulate the heart in accord with the Frank-Starling law, thus increasing cardiac output.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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