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
The tunica media, the middle layer, contains circular smooth muscle and elastic
The outer tunica adventitia is connective tissue.
- 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 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.
- 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
- The aorta leaves the left ventricle to form the ascending aorta, aortic
arch, and descending aorta, which consists of the thoracic and abdominal
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
- 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 is a measure of the force exerted by
blood against the blood vessel wall. Blood pressure moves blood through
- 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 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.
- Most exchange across the wall of the capillary is by
- 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
Nervous Regulation of Blood Vessels
- The vasomotor center (sympathetic division) controls
blood vessel diameter. Other brain areas can excite or inhibit the vasomotor
- Vasomotor tone is a state of partial contraction of
- 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.