LEARNING OBJECTIVE: Identify the parts of the circulatory system, and recognize their major components and functions.

The circulatory system, also called the vascular system, consists of blood, heart, and blood vessels. The circulatory system is close circuited (i.e., there is no opening to external environment of the body). The function of this system is to move blood between the cells and the organs of the integumentary, digestive, respiratory, and urinary system that communicate with the external environment of the body. This function is facilitated by the heart pumping blood through blood vessels. The blood travels throughout the body transporting nutrients and wastes, and permitting the exchange of gases (carbon dioxide and oxygen).


Blood is fluid tissue composed of formed elements (i.e., cells) suspended in plasma. It is pumped by the heart through arteries, capillaries, and veins to all parts of the body. Total blood volume of the average adult is 5 to 6 liters.


Plasma is the liquid part of blood (fig. 1-3 1). Plasma constitutes 55 percent of whole blood (plasma and cells). It is a clear, slightly alkaline, straw-colored liquid consisting of about 92 percent water. The remainder is made up mainly of proteins. One of these proteins, fibrinogen, contributes to coagulation.

Blood Cells

The blood cells suspended in the plasma constitute 45 percent of whole blood. Its cells, which are formed mostly in red bone marrow, include red blood cells (RBCs) and white blood cells (WBCs). The blood also contains cellular fragments called blood platelets.55%


Figure 1-31.—Blood sample illustrating blood components.

When blood components are separated, the WBCs and platelets form a thin layer, called the buffy coat, between the layers of plasma and RBCs. These layers are illustrated in figure 1-31.

RED BLOOD CELLS.—Red blood cells, or erythrocytes, are small, biconcave, nonnucleated disks, formed in the red bone marrow (fig. 1-32). Blood of the average man contains 5 million red cells per cubic millimeter. Women have fewer red cells, 4.5 million per cubic millimeter. Emotional stress, strenuous exercise, high altitudes, and some diseases may cause an increase in the number of RBCs.

Figure 1-32.—A blood smear showing red blood cells, white blood cells, and platelets.

During the development of the red blood cell, a substance called hemoglobin is combined with it. Hemoglobin is the key of the red cell's ability to carry oxygen and carbon dioxide. Thus, the main function of erythrocytes is the transportation of respiratory gases. The red cells deliver oxygen to the body tissues, holding some oxygen in reserve for an emergency. Carbon dioxide is picked up by the same cells and discharged via the lungs.

The color of the red blood cell is determined by the hemoglobin content. Bright red (arterial) blood is due to the combination of oxygen and hemoglobin. Dark red (venous) blood is the result of hemoglobin combining with carbon dioxide.


Red blood cells live only about 100 to 120 days in the body. There are several reasons for their short life span. These delicate cells have to withstand constant knocking around as they are pumped into the arteries by the heart. These cells travel through blood vessels at high speed, bumping into other cells, bouncing off the walls of arteries and veins, and squeezing through narrow passages. They must adjust to continual pressure changes. The spleen is the “graveyard” where old, worn out cells are removed from the blood stream. Fragments of red blood cells are found in the spleen and other body tissues.

WHITE BLOOD CELLS.—White blood cells, or leukocytes, are almost colorless, nucleated cells originating in the bone marrow and in certain lymphoid tissues of the body (fig. 1-32). There is only one white cell to every 600 red cells. Normal WBC count is 6,000 to 8,000 per cubic millimeter, although the number of white cells may be 15,000 to 20,000 or higher during infection.

Leukocytes are important for the protection of the body against disease. Leukocytes can squeeze between the cells that form blood cell walls. This movement, called diapedesis, permits them to leave the blood stream through the capillary wall and attack pathogenic bacteria. They can travel anywhere in the body and are often named “the wandering cells.” They protect the body tissues by engulfing disease-bearing bacteria and foreign matter, a process called phagocytosis. When white cells are undermanned, more are produced, causing an increase in their number and a condition known as leukocytosis. Another way WBC's protect the body from disease is by producing bacteriolysins that dissolve the foreign bacteria. The secondary function ofWBCs is to aid in blood clotting.

BLOOD PLATELETS.—Blood platelets, or thrombocytes, are irregular- or oval-shaped discs in the blood that contain no nucleus, only cytoplasm (fig. 1-32). They are smaller than red blood cells and average about 250,000 per cubic millimeter of blood. Blood platelets play an important role in the process of blood coagulation, clumping together in the presence ofjagged, torn tissue.

Blood Coagulation

To protect the body from excessive blood loss, blood has its own power to coagulate, or clot. If blood components and linings of vessels are normal,circulating blood will not clot. Once blood escapes from its vessels, however, a chemical reaction begins that causes it to become solid. Initially a blood clot is a fluid, but soon it becomes thick and then sets into a soft jelly that quickly becomes firm enough to act as a plug. This plug is the result of a swift, sure mechanism that changes one of the soluble blood proteins, fibrinogen, into an insoluble protein, fibrin, whenever injury occurs.

Other necessary elements for blood clotting are calcium salts; a substance called prothrombin, which is formed in the liver; blood platelets; and various factors necessary for the completion of the successive steps in the coagulation process. Once the fibrin plug is formed, it quickly enmeshes red and white blood cells and draws them tightly together. Blood serum, a yellowish clear liquid, is squeezed out of the clot as the mass shrinks. Formation of the clot closes the wound, preventing blood loss. A clot also serves as a network for the growth of new tissues in the process of healing. Normal clotting time is 3 to 5 minutes, but if any of the substances necessary for clotting are absent, severe bleeding will occur.

Hemophilia is an inherited disease characterized by delayed clotting of the blood and consequent difficulty in controlling hemorrhage. Hemophiliacs can bleed to death as a result of minor wounds.


The heart is a hollow, muscular organ, somewhat larger than the closed fist, located anteriorly in the chest and to the left of the midline. It is shaped like a cone, its base directed upward and to the right, the apex down and to the left. Lying obliquely in the chest, much of the base of the heart is immediately posterior to the sternum.

Heart Composition

The heart is enclosed in a membranous sac, the pericardium. The smooth surfaces of the heart and pericardium are lubricated by a serous secretion called pericardial fluid. The inner surface of the heart is lined with a delicate serous membrane, the endocardium, similar to and continuous with that of the inner lining of blood vessels.

The interior of the heart (fig. 1-33) is divided into two parts by a wall called the interventricular septum. In each half is an upper chamber, the atrium, which receives blood from the veins, and a lower chamber, the ventricle, which receives blood from the atrium and pumps it out into the arteries. The openings between the chambers on each side of the heart are separated by flaps of tissue that act as valves to prevent backward flow of blood. The valve on the right has three flaps, or cusps, and is called the tricuspid valve. The valve on the left has two flaps and is called the mitral, or bicuspid, valve. The outlets of the ventricles are supplied with similar valves. In the right ventricle, the pulmonary valve is at the origin of the pulmonary artery. In the left ventricle, the aortic valve is at the origin of the aorta. See figure 1-33 for valve locations.

Figure 1-33.—Frontal view of the heart—arrows indicate blood flow.

The heart muscle, the myocardium, is striated like the skeletal muscles of the body, but involuntary in action, like the smooth muscles. The walls of the atria are thin with relatively little muscle fiber because the blood flows from the atria to the ventricles under low pressure. However, the walls of the ventricles, which comprise the bulk of the heart, are thick and muscular. The wall of the left ventricle is considerably thicker than that of the right, because more force is required to pump the blood into distant or outlying locations of the circulatory system than into the lungs located only a short distance from the heart.

Heart Functions

The heart acts as four interrelated pumps. The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava. It pumps the deoxygenated blood through the tricuspid valve to the right ventricle. The right ventricle pumps the blood past the pulmonary valve through the pulmonary artery to the lungs, where it is oxygenated. The left atrium receives the oxygenated blood from the lungs through four pulmonary veins and pumps it to the left ventricle past the mitral valve. The left ventricle pumps the blood to all areas of the body via the aortic valve and the aorta.

The heart's constant contracting and relaxing forces blood into the arteries. Each contraction is followed by limited relaxation or dilation. Cardiac muscle never completely relaxes: It always maintains a degree of tone. Contraction of the heart is called systole or “the period of work.” Relaxation of the heart is called diastole or “the period of rest.” A complete cardiac cycle is the time from onset of one contraction, or heart beat, to the onset of the next.

Cardiac Cycle

The cardiac cycle is coordinated by specialized tissues that initiate and distribute electrical (cardiac) impulses (fig. 1-34). The contractions of the heart are stimulated and maintained by the sinoatrial (SA) node, commonly called the pacemaker of the heart. The SA node is an elongated mass of specialized muscle tissue located in the upper part of the right atrium. The SAnode sets off cardiac impulses, causing both atria to contract simultaneously. The normal heart rate, or number of contractions, is about 70 to 80 beats per minute.

This same cardiac impulse continues to travel to another group of specialized tissue called the atrioventricular (AV) node. The AV node is located in the floor of the right atrium near the septum that separates the atria. The cardiac impulse to the AV node is slowed down by junctional fibers. The junctional fibers conduct the cardiac impulse to the AV node; however, these fibers are very small in diameter, causing the impulse to be delayed. This slow arrival of the impulse to the AV node allows time for the atria to empty and the ventricles to fill with blood.

Once the cardiac impulse reaches the far side of the AV node, it quickly passes through a group of large fibers which make up the AV bundle (also called the bundle of His). The AV bundle starts at the upper part of the interventricular septum and divides into right and left branches. About halfway down the interventricular septum, the right and left branchesterminate into Purkinje fibers. The Purkinje fibers spread from the interventricular septum into the papillary muscles, which project inward from the ventricular walls. As the cardiac impulse passes through the Purkinje fibers, these fibers in turn stimulate the cardiac muscle of the ventricles. This stimulation of the cardiac muscles causes the walls of the ventricles to contract with a twisting motion. This action squeezes the blood out of the ventricular chambers and forces it into the arteries. This is the conclusion of one cardiac cycle.

Blood Pressure

Blood pressure is the pressure the blood exerts on the walls of the arteries. The highest pressure is called systolic pressure, because it is caused when the heart is in systole, or contraction. A certain amount of blood pressure is maintained in the arteries even when the heart is relaxed. This pressure is the diastolic pressure, because it is present during diastole, or relaxation of the heart. The difference between systolic and diastolic pressure is known as pulse pressure.

Normal blood pressure can vary considerably with an individual's age, weight, and general condition. For young adults, the systolic pressure is normally between 120 and 150 mm of mercury, and the diastolic pressure is normally between 70 and 90 mm of mercury. On average, women have lower blood pressure than men.

Figure 1-34.—Cardiac cycle.


Blood vessels form a closed circuit of tubes that transport blood between the heart and body cells. The several types of blood vessels include arteries, arterioles, capillaries, venules, and veins.

Blood Vessel Classifications

The blood vessels of the body fall into three classifications:

  1. Arteries and arterioles—distributors
  2. Capillaries—exchangers
  3. Veins and venules—collectors Arteries and Arterioles

Arteries are elastic tubes constructed to withstand high pressure. They carry blood away from the heart to all parts of the body. The smallest branches of the arteries are called arterioles. The walls of arteries and arterioles consist of layers of endothelium, smooth muscle, and connective tissue. The smooth muscles of arteries and arterioles constrict and dilate in response to electrical impulses received from the autonomic nervous system.


At the end of the arterioles is a system of minute vessels that vary in structure, but which are spoken of collectively as capillaries. It is from these capillaries that the tissues of the body are fed. There are approximately 60,000 miles of capillaries in the body. As the blood passes through the capillaries, it releases oxygen and nutritive substances to the tissues and takes up various waste products to be carried away by venules. Venules continue from capillaries and merge to form veins.

Veins and Venules

Veins and venules form the venous system. The venous system is comprised of vessels that collect blood from the capillaries and carry it back to the heart. Veins begin as tiny venules formed from the capillaries. Joining together as tiny rivulets, veins connect and form a small stream. The force of muscles contracting adjacent to veins aids in the forward propulsion of blood on its return to the heart. Valves, spaced frequently along the larger veins, prevent the backflow of blood. The walls of veins are similar toarteries, but are thinner and contain less muscle and elastic tissue.

Arterial System

Arterial circulation is responsible for taking freshly oxygenated blood from the heart to the cells of the body (fig. 1-35). To take this oxygenated blood from the heart to the entire body, the arterial system begins with the contraction of blood from the left ventricle into the aorta and its branches.

AORTA.—The aorta, largest artery in the body, is a large tube-like structure arising from the left ventricle of the heart. It arches upward over the left lung and then down along the spinal column through the thorax and the abdomen, where it divides and sends arteries down both legs (fig. 1-3 5).

KEY BRANCHES OF THE AORTA.—Key arterial branches of the aorta are the coronary, innominate (brachiocephalic), left common carotid, and left subclavian. The coronary arteries are branches of what is called the ascending aorta. The coronary arteries supply the heart with blood. There are three large arteries that arise from the aorta as it arches over the left lung. First is the innominate artery, which divides into the right subclavian artery to supply the right arm, and the right common carotid to supply the right side of the head. The second branch is the left common carotid, which supplies the left side of the head. The third branch is the left subclavian, which supplies the left arm.

ARTERIES OF THE HEAD, NECK, AND BRAIN.—The carotid arteries divide into internal and external branches, the external supplying the muscle and skin of the face and the internal supplying the brain and the eyes.

ARTERIES OF THE UPPER EXTREM­ITIES.—The subclavian arteries are so named because they run underneath the clavicle. They supply the upper extremities, branching off to the back, chest, neck, and brain through the spinal column (fig. 1-35).

The large artery going to the arm is called the axillary. The axillary artery becomes the brachial artery as it travels down the arm and divides into the ulnar and radial arteries. The radial artery is the artery at the wrist that you feel when you take the pulse of your patient (fig. 1-35).

Figure 1-35.—Principal vessels of the arterial system.

ARTERIES OF THE ABDOMEN.—In the abdomen, the aorta gives off branches to the abdominal viscera, including the stomach, liver, spleen, kidneys,and intestines. The aorta later divides into the left and right common iliacs, which supply the lower extremities (fig. 1-35).

ARTERIES OF THE LOWER EXTREM­ITIES.—The left and right common iliacs, upon entering the thigh, become the femoral artery. At the knee, this same vessel is named the popliteal artery (fig. 1-35).

Venous System

Venous circulation is responsible for returning the blood to the heart after exchanges of gases, nutrients, and wastes have occurred between the blood and body cells (fig. 1-36). To return this blood to the heart for reoxygenation, the venous system begins with the merging of capillaries into venules, venules into small veins, and small veins into larger veins. The blood vessel paths of the venous system are difficult to follow, unlike the arterial system. However, the larger veins are commonly located parallel to the course taken by their counterpart in the arterial system. For instance, the renal vein parallels the renal artery, the common iliac vein parallels the common iliac artery, and so forth.

Figure 1-36.—Principal vessels of the venous system.

THREE PRINCIPAL VENOUS SYSTEMS.— The three principal venous systems in the body are the pulmonary, portal, and systemic.

VEINS OF THE HEAD, NECK, AND BRAIN.—The superficial veins of the head unite to form the external jugular veins. The external jugular veins drain blood from the scalp, face, and neck, and finally empty into the subclavian veins.

The veins draining the brain and internal facial structures are the internal jugular veins. These combine with the subclavian veins to form the innominate veins, which empty into the superior vena cava (fig. 1-36).

VEINS OF THE UPPER EXTREM­ITIES.—The veins of the upper extremities begin at the hand and extend upward. A vein of great interest to you is the median cubital, which crosses the anterior surface of the elbow. It is the vein most commonly used for venipuncture. Also found in this area are the basiic and cephalic veins, which extend from the midarm to the shoulder.

The deep veins of the upper arm unite to form the axillary vein, which unites with the superficial veins to form the subclavian vein. This vein later unites with other veins to form the innominate and eventually, after union with still more veins, the superior vena cava (fig. 1-36).

VEINS OF THE ABDOMEN AND THO­RACIC REGION.—The veins from the abdominal organs, with the exception of those of the portal system, empty directly or indirectly into the inferior vena cava, while those of the thoracic region eventually empty into the superior vena cava (fig. 1-36).

VEINS OF THE LOWER EXTREMITIES.—In the lower extremities (fig. 1-36), a similar system drains the superficial areas. The great saphenous vein originates on the inner aspect of the foot and extends up the inside of the leg and thigh to join the femoral vein in the upper thigh. The great saphenous vein is used for intravenous injections at the ankle.

The veins from the lower extremities unite to form the femoral vein in the thigh, which becomes the external iliac vein in the groin. Higher in this region, external iliac unites the internal iliac (hypogastric) vein from the lower pelvic region to form the common iliac veins. The right and left common iliac veins unite to form the inferior vena cava.