Premed · Premed · Anatomy Physiology 2
Lecture 5: Blood Vessels and Circulation
Anatomy and Physiology II
Learning Objectives
By the end of this lecture, students will be able to:
- Compare the structure and function of arteries, capillaries, and veins
- Distinguish between elastic arteries, muscular arteries, and arterioles
- Describe the structure of capillary beds and mechanisms of capillary exchange
- Explain the role of veins as capacitance vessels and mechanisms aiding venous return
- Trace the major systemic and pulmonary circulatory routes
- Describe the special circulations: hepatic portal, cerebral, and fetal
Lecture Content
I. General Structure of Blood Vessel Walls
Most blood vessels, with the exception of capillaries, are built from three concentric layers called tunics.
Tunica Intima (Tunica Interna)
The tunica intima is the innermost layer, consisting of a simple squamous endothelium resting on a basement membrane. In arteries, an internal elastic lamina, a sheet of elastin, marks the boundary with the next layer. The endothelium is far more than a passive lining: it actively produces nitric oxide (a vasodilator), endothelin (a vasoconstrictor), and various anti-thrombotic substances that help maintain smooth blood flow.
Tunica Media
The tunica media is the middle layer and is typically the thickest layer in arteries. It is composed of smooth muscle cells interwoven with elastic fibers. The smooth muscle is controlled by sympathetic vasomotor nerve fibers, which regulate vessel diameter and thereby influence blood flow and blood pressure. An external elastic lamina may be present at the outer boundary of this layer.
Tunica Externa (Tunica Adventitia)
The outermost layer, the tunica externa, consists of collagen fibers that protect the vessel and anchor it to surrounding structures. In larger vessels, this layer contains the vasa vasorum, small blood vessels that supply the vessel wall itself with oxygen and nutrients, and the nervi vasorum, nerve fibers that regulate vascular tone.
<image>A comparative cross-sectional diagram of an artery, vein, and capillary side by side. Panel A: A muscular artery showing a thick tunica media with prominent smooth muscle layers, internal and external elastic laminae, thin tunica intima with endothelium, and tunica externa with vasa vasorum. Panel B: A companion vein of similar diameter showing a thinner tunica media, thicker tunica externa relative to the artery, a wider lumen, and a venous valve (inset showing valve leaflets in open and closed positions). Panel C: A capillary showing only a single layer of endothelial cells and basement membrane, with a red blood cell passing through for scale. All three vessels are drawn to the same scale to emphasize relative size differences.</image>
II. Types of Arteries
Elastic (Conducting) Arteries
The largest arteries in the body, including the aorta, common carotids, subclavians, common iliacs, and pulmonary trunk, are classified as elastic arteries. Their tunica media is rich in elastic fibers, which allow these vessels to act as pressure reservoirs. During ventricular systole, they stretch to accommodate the surge of blood, and during diastole, they recoil to maintain relatively continuous blood flow. This elastic dampening smooths the pulsatile pressure generated by the heart.
Muscular (Distributing) Arteries
Medium-sized arteries such as the brachial, femoral, renal, and mesenteric arteries contain proportionally more smooth muscle and less elastin in their tunica media. This gives them a greater capacity for vasoconstriction and vasodilation, making them well suited for distributing blood to specific organs and body regions.
Arterioles
Arterioles are the smallest arteries, with diameters ranging from 10 to 300 micrometers and a tunica media consisting of only 1 to 2 layers of smooth muscle. Despite their small size, arterioles are the major site of peripheral resistance and are often called "resistance vessels." By adjusting their diameter, they control blood flow into capillary beds. This regulation occurs through local metabolic signals, neural input, and hormonal mechanisms. Metarterioles, short vessels that connect arterioles directly to capillary beds, possess precapillary sphincters that can open or close individual capillaries.
III. Capillaries
Capillaries are the smallest blood vessels in the body, with diameters of only 8 to 10 micrometers, barely wide enough for a single red blood cell to squeeze through. Their walls consist solely of a single layer of endothelium resting on a basement membrane, making them ideally structured for exchange. It is at the capillary level that gases, nutrients, and waste products are exchanged between the blood and the surrounding tissues. The body contains an estimated 10 billion capillaries with a combined surface area of approximately 600 square meters.
Types of Capillaries
Continuous capillaries are the most common type, found in the skin, muscles, lungs, and brain. Their endothelial cells are joined by tight junctions with small intercellular clefts that allow limited passage of small molecules. In the brain, these tight junctions are particularly restrictive, forming the blood-brain barrier. Fenestrated capillaries have small pores (fenestrations) in their endothelial cells and are found wherever rapid exchange or filtration is needed, including the kidneys, intestinal villi, endocrine glands, and choroid plexus. Sinusoidal capillaries (sinusoids) are the most permeable type, featuring large, irregular shapes, wide gaps between endothelial cells, and an incomplete basement membrane. Found in the liver, spleen, bone marrow, and adrenal medulla, sinusoids allow the passage of large molecules and even blood cells.
Capillary Beds and Blood Flow
A capillary bed is a network of capillaries connecting an arteriole to a venule. A thoroughfare channel (metarteriole) provides a direct connection from arteriole to venule, bypassing the true capillaries. Precapillary sphincters, rings of smooth muscle at the entrance to each true capillary, regulate blood flow into the bed based on local metabolic conditions. When the sphincters are open, blood flows through the capillary bed and the tissue is fully perfused. When they are closed, blood bypasses the capillary bed entirely through the thoroughfare channel.
IV. Capillary Exchange Mechanisms
Diffusion
Diffusion is the most important mechanism for the exchange of gases, nutrients, and wastes across capillary walls. Substances move down their concentration gradients through endothelial cells, intercellular clefts, or fenestrations. Lipid-soluble substances such as oxygen and carbon dioxide diffuse directly through endothelial cell membranes, while water-soluble substances such as glucose, amino acids, and ions pass through intercellular clefts or fenestrations.
Transcytosis
Transcytosis involves vesicular transport of large molecules such as proteins and hormones across endothelial cells. Though it occurs, it is relatively slow and quantitatively minor compared to diffusion.
Bulk Flow (Filtration and Reabsorption)
Bulk flow is the movement of fluid driven by pressure gradients and is governed by the Starling forces. Two opposing pressures determine the net direction of fluid movement. Hydrostatic pressure (HP) is the fluid pressure that pushes fluid out of the capillary. Capillary hydrostatic pressure (blood pressure) is approximately 35 mmHg at the arteriolar end and drops to about 17 mmHg at the venular end, while interstitial fluid hydrostatic pressure is approximately 0 mmHg. Colloid osmotic pressure (oncotic pressure, OP) is exerted by plasma proteins, primarily albumin, and pulls fluid into the capillary. Capillary oncotic pressure is approximately 26 mmHg, while interstitial fluid oncotic pressure is about 1 mmHg.
The net filtration pressure (NFP) is calculated as (HPcap - HPif) - (OPcap - OPif). At the arteriolar end, the NFP is positive, meaning there is net filtration as fluid moves out of the capillary into the interstitial space. At the venular end, the NFP is negative, meaning there is net reabsorption as fluid is drawn back into the capillary. Approximately 85% of the fluid filtered at the arteriolar end is reabsorbed at the venular end. The remaining 15% is collected by lymphatic capillaries and returned to the circulation.
<image>A diagram of capillary exchange and Starling forces. Panel A: A capillary bed showing an arteriole on the left, true capillaries in the middle with precapillary sphincters, a thoroughfare channel, and a venule on the right. Panel B: An enlarged single capillary showing filtration at the arteriolar end (arrows pointing outward, with hydrostatic pressure of 35 mmHg exceeding oncotic pressure of 26 mmHg giving a net filtration pressure of +10 mmHg) and reabsorption at the venular end (arrows pointing inward, with hydrostatic pressure of 17 mmHg being less than oncotic pressure of 26 mmHg giving a net filtration pressure of -8 mmHg). The interstitial space shows fluid and a lymphatic capillary collecting excess filtrate. All pressures are numerically labeled.</image>
V. Veins and Venules
Venules
The smallest venules, called postcapillary venules, are very porous and serve as the primary site where white blood cells emigrate from the blood into tissues during inflammation (diapedesis). Larger venules gradually acquire a thin tunica media.
Veins
Compared to their arterial counterparts, veins have thinner walls and larger lumens. In veins, the tunica externa is the thickest layer. The venous system operates at low pressures, typically 15 mmHg or less, and contains about 60 to 65% of the total blood volume, earning veins the designation of "capacitance vessels" or "blood reservoirs." Many veins, particularly those in the limbs where gravity opposes flow, contain venous valves, folds of the tunica intima that prevent the backflow of blood.
Mechanisms Aiding Venous Return
Because the venous system is a low-pressure system, several mechanisms assist in returning blood to the heart. The skeletal muscle pump operates when contracting muscles compress surrounding veins, pushing blood toward the heart. The respiratory pump exploits pressure changes during breathing: inhalation decreases thoracic pressure and increases abdominal pressure, creating a pressure gradient that drives venous blood toward the heart. Venoconstriction, mediated by sympathetic stimulation, reduces venous volume and increases venous return. Venous valves prevent retrograde flow between contractions. Finally, cardiac suction during ventricular relaxation creates a slight negative pressure in the atria that helps draw blood in.
VI. Circulatory Routes
Pulmonary Circulation
The pulmonary circuit carries blood from the right ventricle through the pulmonary trunk, into the right and left pulmonary arteries, through the pulmonary capillaries where gas exchange occurs, and back through the pulmonary veins to the left atrium. It is a low-pressure circuit, operating at approximately 25/8 mmHg.
Systemic Circulation
The systemic circuit carries oxygenated blood from the left ventricle through the aorta, into systemic arteries and arterioles, through capillary beds, and back through venules and veins to the superior and inferior venae cavae, which return blood to the right atrium. It is a high-pressure circuit, operating at approximately 120/80 mmHg.
Major Arterial Branches of the Aorta
The ascending aorta gives rise to the right and left coronary arteries. The aortic arch branches into the brachiocephalic trunk, left common carotid artery, and left subclavian artery. The descending thoracic aorta supplies branches including the bronchial, esophageal, and intercostal arteries. The abdominal aorta gives off the celiac trunk, superior mesenteric artery, renal arteries, gonadal arteries, inferior mesenteric artery, and the common iliac arteries.
Special Circulations
The hepatic portal system is a unique arrangement in which venous blood from the gastrointestinal tract, spleen, and pancreas drains into the hepatic portal vein, which carries nutrient-rich blood to the liver sinusoids. The liver processes nutrients and detoxifies substances before the blood exits via the hepatic veins into the inferior vena cava. This constitutes a portal system because blood passes through two capillary beds before returning to the heart.
The Circle of Willis (cerebral arterial circle) is an anastomosis at the base of the brain that connects the internal carotid and vertebral-basilar arterial systems. Formed by the anterior communicating artery, anterior cerebral arteries, internal carotid arteries, posterior communicating arteries, and posterior cerebral arteries, it provides collateral blood supply that protects the brain if one feeding artery becomes occluded.
Fetal circulation features several unique structures that bypass the non-functional fetal lungs, since gas exchange occurs at the placenta. The umbilical vein carries oxygenated blood from the placenta to the fetus. The ductus venosus shunts a portion of this blood past the liver. The foramen ovale, an opening between the right and left atria, and the ductus arteriosus, which connects the pulmonary trunk to the aorta, both serve to divert blood away from the pulmonary circuit. Umbilical arteries carry deoxygenated blood from the fetus back to the placenta. At birth, as the newborn begins breathing, these shunts close and the circulatory pattern transitions to the adult arrangement.
