The Human Circulatory System
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CIRCULATORY SYSTEM
The circulatory system contains several very different components, including the heart, a hollow muscular pump that stands at the operational center of the system that pumps liquid blood throughout the body through three types of flexible tubes, the blood vessels (Fig. 4.1). The arteries channel blood from the heart to all parts of the body needing service. Once there, the blood passes through narrow arteries and enters the capillaries, which are the narrowest blood vessels. Many substances and some blood cells pass into and out of the blood by moving through the thin porous capillary walls. The blood is then carried through the veins, which return the blood to the heart. The passage of blood through the ves- '" sels in a part of the body is called perfusion of that part. Some materials that are carried away from a region of the body do not pass into the blood but are collected by vessels called lymph capillaries (Fig. 4.2). The materials in these vessels make up a liquid called lymph, which is carried through the lymph vessels toward the heart. Along the way, the lymph passes through lymph nodes where harmful chemicals and microbes that might have entered it are removed. The lymph is finally added to the blood in the veins shortly before the blood enters the heart.
MAIN FUNCTIONS FOR HOMEOSTASIS Transportation
One main function of the circulatory system is transportation of materials within the body. Transportation helps maintain homeostasis by ensuring that the concentrations of substances surrounding body cells are kept at proper and fairly steady levels. Materials consumed by the cells are immediately replenished, and materials produced by the cells are swept away before their concentrations become too high. The flowing blood also transports useful materials from their point of entry into the body to the organs that need them. For example, oxygen from the lungs and nutrients from the digestive system are delivered to the muscles.
CIRCULATORY SYSTEM
The circulatory system contains several very different components, including the heart, a hollow muscular pump that stands at the operational center of the system that pumps liquid blood throughout the body through three types of flexible tubes, the blood vessels (Fig. 4.1). The arteries channel blood from the heart to all parts of the body needing service. Once there, the blood passes through narrow arteries and enters the capillaries, which are the narrowest blood vessels. Many substances and some blood cells pass into and out of the blood by moving through the thin porous capillary walls. The blood is then carried through the veins, which return the blood to the heart. The passage of blood through the ves- '" sels in a part of the body is called perfusion of that part. Some materials that are carried away from a region of the body do not pass into the blood but are collected by vessels called lymph capillaries (Fig. 4.2). The materials in these vessels make up a liquid called lymph, which is carried through the lymph vessels toward the heart. Along the way, the lymph passes through lymph nodes where harmful chemicals and microbes that might have entered it are removed. The lymph is finally added to the blood in the veins shortly before the blood enters the heart.
MAIN FUNCTIONS FOR HOMEOSTASIS Transportation
One main function of the circulatory system is transportation of materials within the body. Transportation helps maintain homeostasis by ensuring that the concentrations of substances surrounding body cells are kept at proper and fairly steady levels. Materials consumed by the cells are immediately replenished, and materials produced by the cells are swept away before their concentrations become too high. The flowing blood also transports useful materials from their point of entry into the body to the organs that need them. For example, oxygen from the lungs and nutrients from the digestive system are delivered to the muscles.