The peripheral circulatory system can be divided into the systemic and the pulmonary vessels. The peripheral circulatory system and the heart are regulated to maintain sufficient blood flow to tissues.

Functions of the Peripheral Circulation

The peripheral circulation functions to carry blood, exchange nutrients and gases, transport hormones, regulate blood pressure, and direct blood flow.

General Features of Blood Vessel Structure

• Blood is pumped from the heart through elastic arteries, muscular arteries, and arterioles to the capillaries.

• Blood returns to the heart from the capillaries through venules, small veins, and large veins.

• Except for capillaries and venules, blood vessels have three layers:
  The tunica intima consists of endothelium, basement membrane, and connective tissue.
  The tunica media, the middle layer, contains circular smooth muscle and elastic fibers.
  The outer tunica adventitia is connective tissue.

Arteries

• Large elastic arteries have many elastic fibers but little smooth muscle in their walls and carry blood from the heart to smaller arteries with little decrease in pressure.

• Muscular arteries have much smooth muscle and some elastic fibers and undergo vasodilation and vasoconstriction to control blood flow to different regions of the body.

• Arterioles are the smallest arteries and have smooth muscle cells and a few elastic fibers and undergo vasodilation and vasoconstriction to control blood flow to local areas.

Capillaries

• Capillaries consist of only endothelium and are surrounded by a basement membrane and loose connective tissue.

• Nutrient and waste product exchange is the principal function of capillaries.

• Blood is supplied to capillaries by arterioles. Precapillary sphincters regulate blood flow through capillary networks.

Veins

• Venules are endothelium surrounded by a basement membrane.

• Small veins are venules covered with a layer of smooth muscle.

• Medium-sized and large veins contain less smooth muscle and elastic fibers than arteries of the same size.

• Valves prevent the backflow of blood in the veins.

Aging of the Arteries

• Arteriosclerosis results from a loss of elasticity primarily in the aorta, large arteries, and coronary arteries.

• Atherosclerosis results from the deposition of plaques rich in cholesterol in the wall of blood vessels, reducing the diameter of the vessels.

Blood Vessels of the Pulmonary Circulation

• The pulmonary circulation moves blood to and from the lungs. The pulmonary trunk carries oxygen-poor blood from the heart to the lungs, and pulmonary veins carry oxygen-rich blood from the lungs to the left atrium of the heart.

Blood Vessels of the Systemic Circulation: Arteries

Aorta

• The aorta leaves the left ventricle to form the ascending aorta, aortic arch, and descending aorta, which consists of the thoracic and abdominal aorta.

Arteries of the Head and Neck

• The brachiocephalic, left common carotid, and left subclavian arteries branch from the aortic arch to supply the head and the upper limbs.

• The common carotid arteries and the vertebral arteries supply the head. The common carotid arteries divide to form the external carotids (which supply the face and mouth) and the internal carotids (which supply the brain).

Arteries of the Upper Limbs

• The subclavian artery continues as the axillary artery and then as the brachial artery, which branches to form the radial and ulnar arteries.

The Thoracic Aorta and Its Branches

• The thoracic aorta has visceral branches, which supply the thoracic organs, and parietal branches, which supply the thoracic wall.

The Abdominal Aorta and Its Branches

• The abdominal aorta has visceral branches, which supply the abdominal organs, and parietal branches, which supply the abdominal wall.

Arteries of the Pelvis

• Branches of the internal iliac arteries supply the pelvis.

Arteries of the Lower Limbs

• The common iliac arteries give rise to the external iliac arteries, and the external iliac artery continues as the femoral artery and then as the popliteal artery in the leg. The popliteal artery divides to form the anterior and posterior tibial arteries.

Blood Vessels of the Systemic Circulation: Veins

• The superior vena cava drains the head, neck, thorax, and upper limbs. The inferior vena cava drains the abdomen, pelvis, and lower limbs.

Veins of the Head and Neck

• The internal jugular veins drain the brain, anterior head, and anterior neck.

• The external jugular veins drain the posterior head and posterior neck.

Veins of the Upper Limbs

• The deep veins are the brachial, axillary, and subclavian; the superficial veins are the basilic, cephalic, and median cubital.

Veins of the Thorax

• The left and right brachiocephalic veins and the azygos veins return blood to the superior vena cava.

Veins of the Abdomen and Pelvis

• Posterior abdominal wall veins join the azygos veins.

• Veins from the kidneys, adrenal glands, and gonads directly enter the inferior vena cava.

• Veins from the stomach, intestines, spleen, and pancreas connect with the hepatic portal vein, which transports blood to the liver for processing. The hepatic veins from the liver join the inferior vena cava.

Veins of the Lower Limbs

• The deep veins course with the deep arteries and have similar names.

• The superficial veins are the small and great saphenous veins.

The Physiology of Circulation

Blood Pressure

• Blood pressure is a measure of the force exerted by blood against the blood vessel wall. Blood pressure moves blood through vessels.

• Blood pressure can be measured by listening for Korotkoff sounds produced as blood flows through arteries partially constricted by a blood pressure cuff.

Pressure and Resistance

• Blood pressure fluctuates between 120 mm Hg (systolic) and 80 mm Hg (diastolic) in the aorta. If constriction of blood vessels occurs, resistance to blood flow increases, and blood flow decreases.

Pulse Pressure

• Pulse pressure is the difference between systolic and diastolic pressure. Pulse pressure increases when stroke volume increases.

• A pulse can be detected when large arteries are near the surface of the body.

Capillary Exchange

• Most exchange across the wall of the capillary is by diffusion.

• Blood pressure, capillary permeability, and osmosis affect movement of fluid across the wall of the capillaries. There is a net movement of fluid from the blood into the tissues. The fluid gained by the tissues is removed by the lymphatic system.

Local Control of Blood Vessels

• Blood flow through a tissue is usually proportional to the metabolic needs of the tissue and is controlled by the precapillary sphincters.

Nervous Regulation of Blood Vessels

• The vasomotor center (sympathetic division) controls blood vessel diameter. Other brain areas can excite or inhibit the vasomotor center.

• Vasomotor tone is a state of partial contraction of blood vessels.

• The nervous system is responsible for routing the flow of blood, except in the capillaries and precapillary sphincters, and is responsible for maintaining blood pressure.

Regulation of Arterial Pressure

• Mean arterial pressure (MAP) is proportional to cardiac output times the peripheral resistance.

Baroreceptor Reflexes

• Baroreceptors are sensory receptors that are sensitive to stretch.
  
  Baroreceptors are located in the carotid sinuses and the aortic arch.

  The baroreceptor reflex changes peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure.

Chemoreceptor Reflexes

• Chemoreceptors are sensitive to changes in blood oxygen, carbon dioxide, and pH.
  Chemoreceptors are located in the carotid bodies and the aortic bodies. The chemoreceptor reflex increases peripheral resistance in response to low oxygen levels.

Hormonal Mechanisms

• Epinephrine released from the adrenal medulla as a result of sympathetic stimulation increases heart rate, stroke volume, and vasoconstriction.

• Renin is released by the kidneys in response to low blood pressure. Renin promotes the production of angiotensin, which, when activated, causes vasoconstriction and an increase in aldosterone secretion. Aldosterone reduces urine output. Angiotensin can also cause vasoconstriction.

• ADH released from the posterior pituitary causes vasoconstriction and reduces urine output.

• Atrial natriuretic hormone is released from the heart when atrial blood pressure increases. It stimulates an increase in urine production, causing a decrease in blood volume and blood pressure.

Short-Term and Long-Term Regulation

• The baroreceptor mechanisms are most important in short-term regulation of blood pressure.

• Hormonal mechanisms, such as the renin-angiotensin- aldosterone system and atrial natriuretic hormone, are more important in long-term regulation of blood pressure.