Unit 20

Pre-text assignments:

1 Learn the key words and phrases:

blood vessels, arteries, arterioles, capillaries, venules, veins, extensions, squamous epithelial cells, semipermeable membranes

2 Match the blood vessel or heart condition with its description:

Blood Vessels

The blood vessels are organs of the cardiovascular system, and they form a closed circuit of tubes that carries blood from the heart to the body cells and back again. These vessels include arteries, arterioles, capillaries, venules, and veins. The arteries and arterioles conduct blood away from the ventricles of the heart and lead to the capillaries. The capillaries function to exchange substances between the blood and the body cells, and the venules and veins return blood from the capillaries to the atria.

Arteries are strong, elastic vessels that are adapted for carrying the blood away from the heart under relatively high pressure. These vessels subdivide into progressively thinner tubes and eventually give rise to fine branches called arterioles. Capillaries are the smallest blood vessels. They form the

connections between the smallest arterioles and the smallest venules. Capillaries are essentially extensions of the inner linings of these larger vessels, in that their walls consist of endothelium – a single layer of squamous epithelial cells. These thin walls form the

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semipermeable membranes through which substances in the blood are exchanged for substances in the tissue fluid surrounding body cells.

The distribution of blood in the various capillary pathways is regulated mainly by the smooth muscles that encircle the capillary entrances.

Venules are the microscopic vessels that continue from the capillaries and merge to form veins. The veins, which carry blood back to the atria, follow pathways that roughly parallel those of the arteries.

The walls of veins are similar to those of arteries in that way they are composed of three distinct layers. However, the middle layer of the venous wall is poorly developed. Consequently, veins have thinner walls that contain less smooth muscle and less elastic tissue than those of comparable arteries.

Blood pressure is the force exerted by the blood against

the inner walls of the blood vessels. Although such a force occurs throughout the vascular system, the term blood pressure most commonly refers to systemic arterial pressure.

The arterial blood pressure rises and falls in a pattern corresponding to the phases of the cardiac cycle. That is, when the ventricles contract (ventricular systole), their walls squeeze the blood inside their chambers and force it into the pulmonary trunk and aorta. As a result, the pressures in these arteries rise sharply. The maximum pressure achieved during ventricular contraction is called the systolic pressure. When the ventricles relax (ventricular diastole), the arterial pressure drops, and the lowest pressure that remains in the arteries before the next ventricular contraction is termed the diastolic pressure.

The surge of blood entering the arterial system during a ventricular contraction causes the elastic walls of the arteries to swell, but the pressure drops almost immediately as the contraction is completed, and the arterial walls recoil. This alternate expanding and recoiling of an arterial wall can be felt as a pulse in an artery that runs close to the surface.

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Post-text assignments:

1 Answer the questions using the topical unit:

1 Distinguish between systolic and diastolic blood pressure.

2 What cardiac event is responsible for the systolic pressure? For the diastolic pressure?

3 What causes a pulse in an artery?

2 Respond to the given questions:

1Based on your understanding of the way capillary blood flow is regulated, do you think it is wiser to rest or to exercise following a heavy meal? Give a reason for your answer.

2If a patient develops a blood clot in the femoral vein of the left leg, and a portion of the clot breaks loose, where is the blood flow likely to carry the embolus? What symptoms is this condition likely to produce?

3When a person strains to lift a heavy object, intrathoracic pressure is increased. What do you think will happen to the rate of venous blood returning to the heart during such lifting? Why?

4Why is a ventricular fibrillation more likely to be life threatening than an atrial fibrillation?

composed of longitudinally arranged thick collagen fibres which merge with the surrounding connective tissue. (vein or aorta).

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