The Circulatory System: Structure, Blood Vessels and the Heart

UNIT 3: THE CIRCULATORY SYSTEM

This system carries nutrients and oxygen to all the tissues of the body, removes the waste products of metabolism, carries hormones from endocrine glands to their target organs and equalizes body temperature. (B.2.) It includes the heart, the blood and lymph vessels, in addition to the blood, lymph and tissue (extracellular) fluid.

Blood Vessels

Arteries carry blood from the heart to the tissues, whereas veins carry blood from the tissues to the heart.

Capillaries, the finest vessels, run through the tissues. Their endothelial wall consists of a single layer of cells to permit the exchange of gases, nutrients and wastes by simple diffusion.

Arteries are successively coated by fibrous, smooth, elastic and endothelial tissues that make (B.3) them thicker, tougher and very resistant to internal pressures, allowing at the same time expansion and contraction with each heartbeat. The aorta, the largest artery, is about an inch in diameter as it leaves the heart and its wall is about 1/8 inch thick.

Veins, on the other hand, have thinner walls; however, they are supplied with valves along their length to prevent the back-flow of the blood.

Tissue fluid surrounds and bathes the cells of our body, so that blood does not actually have direct contact with the cells.

The Heart

This vital organ is a powerful muscular structure located in the chest directly behind the breast bone. Its walls are made of cardiac muscle tissue held together by strands of connective tissue. (B.6) Enclosing the heart we find a tough, connective tissue sac. Internally, this organ is lined by a layer of smooth, thin, flattened cells, which prevents blood from clotting. Thus, any injury or disease roughening it may cause the formation of a thrombus within the heart or the vessels.

As you all know, the heart is divided into four chambers: the upper right and left atria and the lower right and left ventricles. The latter, having much thicker walls, pump the blood out of the heart to the lungs and around the body. Besides this, the heart is supplied with valves that close automatically, preventing blood from flowing in the wrong direction.

In spite of having so much blood, heart tissue is too thick to receive nourishment directly; instead, (B.4) it has a circulation of its own, branching off the aorta.

Beating is an inherent capacity of the heart, since embryonic development and throughout life. At the normal rate, each complete beat takes about 0.85 seconds. It has been demonstrated that the sinoatrial node initiates the heartbeat and regulates its rate of contraction; thus, it has been called the “pacemaker”. It may well be noted here that ordinary muscles convey impulses at a rate of 5 m/sec., whereas nodal tissue conveys impulses 10 times faster.