Cardiovascular System and Vessels and Blood Help
Vessels—Arteries, Capillaries, and Veins
The walls of blood vessels are composed of the following tunics (layers): the tunica interna, an inner layer of squamous epithelium, called endothelium, resting on a layer of connective tissue; the tunica media, a middle layer of smooth muscle fibers mixed with elastic fibers; and the tunica externa, an outer layer of connective tissue containing elastic and collagenous fibers. The tunica externa of the larger vessels is infiltrated with a system of tiny blood vessels called the vasa vasorum ("vessels of the vessels") that nourish the more external tissues of the blood vessel wall.
Principal Systemic Arteries
Arteries to the Head, Neck, and Upper Limbs
Three vessels branch off of the aortic arch, the brachiocepahlic trunk, the left common carotid a., and the left subclavian a. The brachiocephalic trunk branches into a right common carotid a. and a right subclavian a. Branches off these vessels supply the head, neck, and shoulder regions. The right and left subclavian arteries continue into the upper limb as the axillary a., which then becomes the brachial a. Branches off these vessels supply the upper limb.
Paired Arteries Off the Thoracic Aorta
Arteries From the Abdominal Aorta
Principal Systemic Veins
The major veins that return blood to the heart are the superior vena cava, returning blood from the head, neck and upper extremity, and inferior vena cava, carrying blood from the abdomen and lower extremity. The major veins are diagramed in Figure 16-2.
Blood pressure is the force per unit area exerted by the blood against the inner walls of blood vessels; it is due primarily to the action of the heart.
Factors that affect blood pressure:
- Heart rate: increased rate increases pressure
- Blood volume: increased volume increases pressure
- Peripheral resistance: decreased vessel diameter, increases resistance and thus pressure.
Normal blood pressure is about 120/80.
Arterial blood pressure is much greater than venous blood pressure due to ventricular contractions of the heart pulsating the blood into arteries and the recoiling of the arterial walls. Systolic and diastolic blood pressures are greater in systemic arteries (approx. 120/80) than in pulmonary arteries (approx. 30/15). Blood pressure decreases in arteries proportionate to their distance from the heart. Blood pressure decreases rapidly within the capillaries and is near zero where venous blood enters the heart (Figure 16-3).
Regulation of Blood Flow
Neural mechanism. Baroreceptors (sensory receptors that monitor blood pressure) in the walls of large vessels and the chambers of the heart detect a decrease in blood pressure. This initiates the following responses:
- Increased secretion of ADH from the pituitary. Under the action of ADH, the kidneys restore water to the blood, increasing blood volume.
- Sympathetic impulses sent to the heart, which increase heart rate.
- Sympathetic response of smooth muscle of vessels that changes vessel diameter and thus peripheral resistance.
Renal mechanism. A decrease in blood pressure in the kidneys activates the renin angiotensin system. The aldosterone produced alters the electrolyte balance and stimulates increased reabsorption of water by the kidneys, resulting in increased blood volume.
Practice problems for these concepts can be found at:
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