The Circulatory System
Lesson 8, Page 1 of 12

Overview of Lesson 8

The Circulatory System—Part I
Anatomy and Physiology

The circulatory system is responsible for the transport of water and dissolved materials throughout the body, including oxygen, carbon dioxide, nutrients, and waste. The circulatory system transports oxygen from the lungs and nutrients from the digestive tract to every cell in the body, allowing for the continuation of cell metabolism. The circulatory system also transports the waste products of cell metabolism to the lungs and kidneys where they can be expelled from the body. Without this important function toxic substances would quickly build up in the body.

Anatomy of the Circulatory System

The human circulatory system is organized into two major circulations. Each has its own pump with both pumps being incorporated into a single organ—the heart. The two sides of the human heart are separated by partitions, the interatrial septum and the interventricular septum. Both septa are complete so that the two sides are anatomically and functionally separate pumping units. The right side of the heart pumps blood through the pulmonary circulation (the lungs) while the left side of the heart pumps blood through the systemic circulation (the body).

The Heart

The human heart is a specialized, four-chambered muscle that maintains the blood flow in the circulatory system. It lies immediately behind the sternum, or breastbone, and between the lungs. The apex, or bottom of the heart, is tilted to the left side. At rest, the heart pumps about 59 cc (2 oz) of blood per beat and 5 l (5 qt) per minute. During exercise it pumps 120-220 cc (4-7.3 oz) of blood per beat and 20-30 l (21-32 qt) per minute. The adult human heart is about the size of a fist and weighs about 250-350 gm (9 oz).

The human heart begins beating early in fetal life and continues regular beating throughout the life span of the individual. If the heart stops beating for more than 3 or 4 minutes permanent brain damage may occur. Blood flow to the heart muscle itself also depends on the continued beating of the heart and if this flow is stopped for more than a few minutes, as in a heart attack, the heart muscle may be damaged to such a great extent that it may be irreversibly stopped.

The heart is made up of two muscle masses. One of these forms the two atria (the upper chambers) of the heart, and the other forms the two ventricles (the lower chambers). Both atria contract or relax at the same time, as do both ventricles.

An electrical impulse called an action potential is generated at regular intervals in a specialized region of the right atrium called the sinoauricular (or sinoatrial, or SA) node. Since the two atria form a single muscular unit, the action potential will spread over the atria. A fraction of a second later, having been triggered by the action potential, the atrial muscle contracts.

The ventricles form a single muscle mass separate from the atria. When the atrial action potential reaches the juncture of the atria and the ventricles, the atrioventricular or AV node (another specialized region for conduction) conducts the impulse. After a slight delay, the impulse is passed by way of yet another bundle of muscle fibers (the Bundle of His and the Purkinje system.) Contraction of the ventricle quickly follows the onset of its action potential. From this pattern it can be seen that both atria will contract simultaneously and that both ventricles will contract simultaneously, with a brief delay between the contraction of the two parts of the heart.

The electrical stimulus that leads to contraction of the heart muscle thus originates in the heart itself, in the sinoatrial node (SA node), which is also known as the heart's pacemaker. This node, which lies just in front of the opening of the superior vena cava, measures no more than a few millimeters. It consists of heart cells that emit regular impulses. Because of this spontaneous discharge of the sinoatrial node, the heart muscle is automated. A completely isolated heart can contract on its own as long as its metabolic processes remain intact.

The rate at which the cells of the SA node discharge is externally influenced through the autonomic nervous system, which sends nerve branches to the heart. Through their stimulatory and inhibitory influences they determine the resultant heart rate. In adults at rest this is between 60 and 74 beats a minute. In infants and young children it may be between 100 and 120 beats a minute. Tension, exertion, or fever may cause the rate of the heart to vary between 55 and 200 beats a minute.

The heart's pacemaker is the. . .

Sinoatrial (SA) Node.
Autonomic Nervous System.
Atrioventricular (AV) node.

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