Blood Vessels and Circulation
Chapter 20: Blood Vessels and Circulation
Blood Circulation
Principle Veins and Arteries
Internal Jugular Vein
Common Carotid Artery
Subclavian Vein
Subclavian Artery
Superior Vena Cava
Axillary Artery
Pulmonary Vein
Brachial Artery
Inferior Vena Cava
Hepatic Vein
Portal Vein
Radial Artery
Ulnar Artery
Superior Mesenteric Artery
Common Iliac Artery
Internal Iliac Artery
External Iliac Artery
Pulmonary Artery
Axillary Vein
Common Hepatic Artery
Cephalic Vein
Basilic Vein
Splenic Artery
Splenic Vein
Cubital Vein
Radial Vein
Renal Artery
Renal Vein
Abdominal Aorta
Median Vein of Forearm
Femoral Artery
Great Saphenous Vein
Femoral Vein
Classes of Blood Vessels
Arteries: Carry blood away from heart.
Arterioles: Smallest branches of arteries.
Capillaries: Smallest blood vessels; location of exchange between blood and interstitial fluid.
Venules: Collect blood from capillaries.
Veins: Return blood to heart.
Blood Vessels
The Structure of Vessel Walls
Walls have three layers:
Tunica intima
Tunica media
Tunica externa (adventitia)
Tunica adventitia: Contains connective tissue and external elastic lamina; smooth muscle cells.
Tunica media: Contains smooth muscle, collagen, and elastic tissue. Strengthens vessels and prevents blood pressure from rupturing them.
Tunica intima: Endothelium, internal elastic lamina.
The Vessel Wall
Tunica intima
Lines the blood vessel and is exposed to blood.
Endothelium: Simple squamous epithelium overlying a basement membrane and a loose connective tissue layer.
Acts as a selectively permeable barrier.
Secretes chemicals that stimulate dilation or constriction of the vessel.
Normally repels blood cells and platelets that may adhere to it and form a clot.
When surrounding tissue is inflamed, endothelial cells produce cell-adhesion molecules that induce leukocytes to adhere to the surface, causing them to congregate in tissues needing defensive actions.
Tunica media
Middle layer; consists of smooth muscle, collagen, and elastic tissue.
Strengthens vessels and prevents blood pressure from rupturing them.
Vasomotion: Changes in diameter of the blood vessel brought about by smooth muscle contraction.
The Tunica Externa
Outer layer; contains connective tissue sheath that anchors the vessel to adjacent tissues in arteries.
Contains collagen, elastic fibers.
In veins, contains elastic fibers, smooth muscle cells.
Vasa vasorum (“vessels of vessels”): Small arteries and veins in walls of large arteries and veins that supply cells of tunica media and tunica externa.
Walls of an Artery, a Capillary, and a Vein
Figure 20.1
Lumen: Central cavity of blood vessels.
Artery Structure: Composed of tunica intima (endothelium), tunica media (elastic laminae), and tunica externa (including vasa vasorum).
Capillary: Composed of an endothelial tube inside a thin basal lamina with no tunica media or externa.
Vein Structure: Similar to that of arteries but includes valves to prevent backflow.
Structure and Function of Arteries
Arteries branch into smaller vessels extending from the heart.
Decrease in lumen diameter and elastic fibers.
Increase in relative amount of smooth muscle.
Controlled by sympathetic division of the Autonomic Nervous System (ANS).
Three basic types of arteries:
Elastic arteries: Also called conducting arteries; large vessels (e.g., pulmonary trunk and aorta); tunica media has many elastic fibers and few muscle cells for even pulse force.
Muscular arteries: Also called distribution arteries; medium-sized arteries with a tunica media rich in muscle cells.
Arterioles: Small arteries with little or no tunica externa and thin or incomplete tunica media. Diameter changes occur with sympathetic or endocrine stimulation.
Comparison of Companion Vessels
Figure 20.3
Veins vs. Arteries:
Arteries: Have thicker tunica media, elastic lamina, and sinuses that drain into veins.
Valves in veins: Allow unidirectional blood flow back to the heart only, preventing backward flow.
Plaque: Shows typical deposits within artery walls due to atherosclerosis in provided diagrams.
Structure and Function of Capillaries
Capillaries: Smallest vessels with thin walls; microscopic capillary networks permeate all active tissues.
Function: Location for all exchange functions of the cardiovascular system, where materials diffuse between blood and interstitial fluid.
Structure: Composed of an endothelial tube inside a thin basal lamina, lacking tunica media or externa. The diameter is similar to red blood cells.
Types of Capillaries
Continuous capillaries: Endothelial cells form a continuous lining. Tight junctions connect but do not form a complete seal; intercellular clefts are present.
Large particles (e.g., cells, proteins) cannot pass; smaller molecules (e.g., glucose) can pass.
Commonly found in muscle, skin, lungs, and CNS.
Fenestrated capillaries: Endothelial cells are continuous but contain pores (fenestrations) allowing smaller plasma proteins to move.
Commonly found in organs where fluid transport is critical (intestines, kidneys).
Sinusoids (discontinuous capillaries): Endothelial cells form an incomplete lining with large gaps and an absent basement membrane, permitting transport of large substances (formed elements, large proteins).
Found in bone marrow, spleen, and some endocrine glands.
Pathways
Simple pathway: One major artery delivers blood to an organ or region (e.g., splenic artery).
Arterial anastomosis: Two or more arteries converge to supply the same region (e.g., superior and inferior epigastric arteries).
Venous anastomosis: More common; two or more veins drain the same region (e.g., basilic, brachial, cephalic veins).
Arteriovenous anastomosis (shunt): Transports blood from artery directly to vein, allowing bypass to areas if hypothermic (in fingers, toes).
Portal system: Two capillary beds in sequence (e.g., hypothalamo-hypophyseal portal system).
Chapter 20: Blood Vessels and Circulation
Blood Circulation
Principle Veins and Arteries
Internal Jugular Vein
Common Carotid Artery
Subclavian Vein
Subclavian Artery
Superior Vena Cava
Axillary Artery
Pulmonary Vein
Brachial Artery
Inferior Vena Cava
Hepatic Vein
Portal Vein
Radial Artery
Ulnar Artery
Superior Mesenteric Artery
Common Iliac Artery
Internal Iliac Artery
External Iliac Artery
Pulmonary Artery
Axillary Vein
Common Hepatic Artery
Cephalic Vein
Basilic Vein
Splenic Artery
Splenic Vein
Cubital Vein
Radial Vein
Renal Artery
Renal Vein
Abdominal Aorta
Median Vein of Forearm
Femoral Artery
Great Saphenous Vein
Femoral Vein
Classes of Blood Vessels
Arteries: Carry blood away from the heart towards the capillaries. They are typically high-pressure vessels with thick, muscular walls to withstand the force of cardiac contractions.
Arterioles: Smallest branches of arteries, often too small to be seen with the naked eye. They play a critical role in regulating blood flow into capillary beds and are a primary site of resistance in the circulatory system.
Capillaries: The smallest and most numerous blood vessels, forming extensive networks within tissues. This is the primary site of exchange for nutrients, gases, hormones, and waste products between the blood and interstitial fluid.
Venules: Collect deoxygenated blood from capillary beds and merge to form larger veins. They have thin walls and low pressure.
Veins: Return blood to the heart. They are typically low-pressure vessels with thinner walls and larger lumens compared to arteries. Most veins, especially in the limbs, contain valves to prevent the backflow of blood.
Blood Vessels
The Structure of Vessel Walls
Walls typically have three layers (tunics), except for capillaries:
Tunica intima (interna)
Tunica media
Tunica externa (adventitia)
Capillaries, for efficient exchange, consist only of an endothelium and a thin basal lamina.
Tunica intima
The innermost layer of a blood vessel, directly lining the lumen and exposed to blood.
Composed of:
Endothelium: A simple squamous epithelium overlying a basement membrane and a loose connective tissue layer (subendothelial layer).
Acts as a selectively permeable barrier, regulating the passage of substances into and out of the blood.
Secretes various chemicals (e.g., nitric oxide, endothelin) that stimulate vasodilation or vasoconstriction.
Normally repels blood cells and platelets, preventing adhesion and clot formation in healthy vessels.
When surrounding tissue is inflamed, endothelial cells produce cell-adhesion molecules (CAMS) that cause leukocytes to adhere to the surface, facilitating their migration into tissues needing defensive actions.
Internal Elastic Lamina: A layer of elastic fibers, present in arteries, that separates the tunica intima from the tunica media, allowing for distension and recoil.
Tunica media
The middle layer, and typically the thickest layer, especially in arteries.
Consists primarily of circularly arranged smooth muscle cells, collagen fibers, and elastic tissue.
Function:
Strengthens vessels and prevents blood pressure from rupturing them.
The smooth muscle cells are responsible for vasomotion: changes in the diameter of the blood vessel (vasoconstriction and vasodilation) brought about by their contraction or relaxation. This action regulates blood flow and pressure.
Contains External Elastic Lamina in larger arteries, which separates the tunica media from the tunica externa.
Innervated by the sympathetic nervous system, regulating vessel tone.
The Tunica Externa
The outermost layer, also known as the tunica adventitia.
Composed primarily of loose connective tissue (collagen and elastic fibers).
Function:
Anchors the vessel to adjacent tissues, providing structural support and limiting overexpansion.
Contains nerves, lymphatic vessels, and in large vessels, the vasa vasorum (“vessels of vessels”): small arteries and veins that supply oxygen and nutrients to the outer layers of the vessel walls themselves (tunics media and externa), which are too far from the lumen to be nourished by diffusion.
In veins, it is often the thickest layer and may also contain smooth muscle cells.
Walls of an Artery, a Capillary, and a Vein
Figure 20.1
Lumen: The central, blood-filled cavity of a blood vessel.
Artery Structure:
Composed of a relatively thick tunica intima (endothelium, internal elastic lamina), a very thick tunica media (smooth muscle, elastic laminae), and a tunica externa (including vasa vasorum in large arteries).
Arteries have smaller, more circular lumens than comparable veins, and their walls are robust to withstand high pulsatile pressure.
Capillary Structure:
Consists of only a single layer of endothelial cells and a thin basal lamina, making its wall extremely thin (a single cell thick).
This minimal barrier facilitates rapid exchange of substances between blood and surrounding tissues.
Lacks tunica media or externa.
Vein Structure:
Generally has a larger, often irregular, lumen and thinner walls compared to a companion artery.
The tunica media is much thinner and contains less smooth muscle and elastic tissue than arteries.
The tunica externa is often the thickest layer.
Includes valves (folds of the tunica intima) to prevent the backflow of blood, especially important in veins of the limbs and body wall where gravity can impede venous return.
Structure and Function of Arteries
Arteries form a branching network that carries oxygenated blood (except for the pulmonary artery) away from the heart, gradually decreasing in diameter as they extend into the periphery.
This branching system exhibits a programmed decrease in lumen diameter and elastic fibers.
Simultaneously, there is an increase in the relative amount of smooth muscle in their walls.
Arterial diameter and blood flow regulation are predominantly controlled by the sympathetic division of the Autonomic Nervous System (ANS), which stimulates smooth muscle contraction.
Three basic types of arteries:
Elastic arteries (Conducting arteries):
Largest arteries (e.g., pulmonary trunk, aorta, common carotid, subclavian, common iliac arteries).
Tunica media contains many elastic fibers and relatively few muscle cells, allowing them to stretch greatly during ventricular systole and recoil during diastole.
Their elastic recoil helps to maintain a continuous, even pulse force and blood flow by dampening pressure fluctuations, acting as a pressure reservoir.
Muscular arteries (Distribution arteries):
Medium-sized arteries (e.g., brachial, femoral, renal, splenic arteries) that distribute blood to specific organs and regions.
Have a very thick tunica media rich in smooth muscle cells and fewer elastic fibers compared to elastic arteries.
Capable of significant vasoconstriction and vasodilation to adjust blood flow to individual organs based on metabolic demand.
Arterioles (Resistance vessels):
Smallest arteries, leading into capillary beds.
Have little or no tunica externa and a thin or incomplete tunica media (mostly smooth muscle).
Their diameter changes significantly in response to sympathetic or endocrine stimulation, local tissue factors, and autoregulation.
They are the primary determinant of peripheral resistance and play a crucial role in regulating systemic blood pressure and directing blood flow to specific capillary beds.
Comparison of Companion Vessels
Figure 20.3
Veins vs. Arteries:
Arteries: Generally have thicker, more muscular, and elastic walls; a smaller, rounder lumen; and typically no valves (except at the heart's exit). They maintain their shape even when empty.
Veins: Have thinner, less muscular walls; a larger, often collapsed or irregular lumen; and possess valves (especially in the limbs) which are folds of the tunica intima.
Valves in veins allow unidirectional blood flow back to the heart only, preventing backward flow due to low pressure and gravity.
Valves are crucial for venous return, working with the skeletal muscle pump.
Plaque: Diagrams often illustrate typical deposits (composed of lipids, cholesterol, calcium, and cellular debris) within artery walls, characteristic of atherosclerosis (a specific type of arteriosclerosis).
Structure and Function of Capillaries
Capillaries: The smallest and most extensive of the blood vessels, forming microscopic capillary networks that permeate nearly all active tissues.
Function: The sole location for efficient exchange functions of the cardiovascular system. Here, vital materials like oxygen, nutrients, hormones, and waste products (carbon dioxide, metabolic byproducts) diffuse rapidly between blood and the surrounding interstitial fluid and cells.
Structure: Uniquely designed for exchange, composed of only an endothelial tube (one cell thick) inside a thin basal lamina. They lack tunica media or externa.
The diameter is often just wide enough to allow red blood cells to pass single file ().
Types of Capillaries
Continuous capillaries:
The most common type, characterized by endothelial cells forming a continuous, uninterrupted lining.
Tight junctions connect adjacent endothelial cells, limiting paracellular movement, but do not form a complete seal, as small intercellular clefts (gaps of about ) are present.
Large particles (e.g., cells, most proteins) cannot easily pass; smaller molecules (e.g., water, glucose, amino acids, ions) can readily pass through intercellular clefts or by transcytosis.
Commonly found in muscle, skin, lungs, and the central nervous system (where they form the basis of the blood-brain barrier, having particularly tight junctions and minimal clefts).
Fenestrated capillaries:
Endothelial cells are continuous but contain numerous small pores or fenestrations (windows, in diameter) that penetrate the endothelial cells.
These pores are typically covered by a thin glycoprotein diaphragm, making them more permeable than continuous capillaries.
Allow for rapid passage of fluids and relatively small plasma proteins.
Commonly found in organs where rapid fluid transport or absorption/filtration is critical, such as the small intestines (for nutrient absorption), kidneys (for filtrate formation), choroid plexuses (CSF production), and endocrine glands (for hormone secretion).
Sinusoids (discontinuous capillaries):
The least common and most permeable type of capillary.
Characterized by endothelial cells that form an incomplete lining with very large gaps (up to ) between cells, an absent or discontinuous basement membrane, and often larger lumens.
This loosely organized structure permits the transport of large substances, including formed elements (blood cells) and large plasma proteins.
Found in specific locations where extensive exchange or filtration of large particles is required: bone marrow (for entry of new blood cells into circulation), spleen (for removal of aged blood cells), liver (for processing of absorbed nutrients and removal of toxins), and some endocrine glands (e.g., adrenal cortex, anterior pituitary).
Pathways
Simple pathway: The most common circulatory route. One major artery delivers blood to an organ or region, which then branches into smaller arteries, arterioles, capillary beds for exchange, venules, and finally one major vein that drains blood away (e.g., splenic artery to spleen to splenic vein).
Arterial anastomosis: A convergence of two or more arteries that supply the same region. This provides an alternative route for blood flow (collateral circulation) if one artery is blocked or damaged, ensuring continued perfusion to the tissue (e.g., superior and inferior epigastric arteries supplying the abdominal wall; arterial anastomoses around joints).
Venous anastomosis: A much more common convergence compared to arterial anastomoses, where two or more veins drain the same region. This provides multiple routes for venous return, making venous blockage less problematic than arterial blockage (e.g., basilic, brachial, and cephalic veins in the arm; numerous anastomoses in the dorsal venous network of the hand and foot).
Arteriovenous anastomosis (shunt): A direct connection or bypass that transports blood from an artery directly to a vein, completely bypassing an intervening capillary bed.
Located primarily in the fingers, toes, palms, and ears.
Function: Primarily involved in thermoregulation, allowing blood to bypass capillary beds in the skin to conserve heat in cold conditions, or to increase blood flow to the skin for heat dissipation in warm conditions.
Portal system: A unique circulatory pathway involving two capillary beds in sequence, connected by an intervening portal vein, instead of a direct artery-capillary-vein route.
Examples:
Hepatic portal system: Transports nutrient-rich, deoxygenated blood from the capillary beds of the digestive organs (stomach, intestines, pancreas, spleen) via the hepatic portal vein to a second capillary bed (sinusoids) in the liver before draining into the general circulation. This allows the liver to process absorbed substances before they reach the rest of the body.
Hypothalamo-hypophyseal portal system: Connects the hypothalamus to the anterior pituitary gland, transporting hypothalamic releasing and inhibiting hormones to regulate anterior pituitary function.
Chapter 20: Blood Vessels and Circulation
Blood Circulation
Principle Veins and Arteries
Internal Jugular Vein: Drains blood from the brain, face, and neck, converging with the subclavian vein to form the brachiocephalic vein.
Common Carotid Artery: Located in the neck, it branches into the internal carotid artery (supplying the brain) and the external carotid artery (supplying the face and neck).
Subclavian Vein: Drains blood from the upper limbs, superficial chest, and shoulders; it receives blood from the axillary vein and joins the internal jugular vein.
Subclavian Artery: Originates from the aorta (left) or brachiocephalic artery (right); supplies blood to the arms, chest wall, shoulders, back, and parts of the central nervous system.
Superior Vena Cava: A large vein that collects deoxygenated blood from the head, neck, upper limbs, and thorax (above the diaphragm) and delivers it to the right atrium of the heart.
Axillary Artery: A continuation of the subclavian artery, it supplies blood to the shoulder, lateral thoracic region, and upper arm, eventually becoming the brachial artery.
Pulmonary Vein: Carries oxygenated blood from the lungs back to the left atrium of the heart.
Brachial Artery: A continuation of the axillary artery, it runs through the upper arm and is a major pulse point; it branches into the radial and ulnar arteries in the forearm.
Inferior Vena Cava: The largest vein in the body, it collects deoxygenated blood from the abdomen, pelvis, and lower limbs (below the diaphragm) and delivers it to the right atrium of the heart.
Hepatic Vein: Drains deoxygenated blood from the liver directly into the inferior vena cava.
Portal Vein (Hepatic Portal Vein): Carries nutrient-rich, deoxygenated blood from the capillary beds of the digestive organs (stomach, intestines, pancreas, spleen) to the liver for processing.
Radial Artery: A primary artery of the forearm, branching from the brachial artery, supplying the lateral side of the forearm, wrist, and hand. It is commonly used to feel a person's pulse.
Ulnar Artery: A primary artery of the forearm, branching from the brachial artery, supplying the medial side of the forearm, wrist, and hand.
Superior Mesenteric Artery: Branches from the abdominal aorta and supplies arterial blood to the small intestine (jejunum, ileum), part of the large intestine (cecum, ascending colon, transverse colon), and pancreas.
Common Iliac Artery: Formed by the bifurcation of the abdominal aorta, it divides into the external iliac artery (to the lower limb) and the internal iliac artery (to the pelvis).
Internal Iliac Artery: Supplies blood to the pelvic organs (bladder, reproductive organs), pelvic wall, gluteal region, and medial thigh.
External Iliac Artery: Continues into the thigh as the femoral artery, supplying the lower limbs.
Pulmonary Artery: Carries deoxygenated blood from the right ventricle of the heart to the lungs.
Axillary Vein: Formed by the junction of the brachial and basilic veins, it drains the arm and shoulder area, becoming the subclavian vein.
Common Hepatic Artery: A major branch of the celiac trunk, it supplies blood to the liver, pyloric part of the stomach, and superior part of the duodenum.
Cephalic Vein: A large superficial vein of the upper limb that runs up the lateral side of the arm and drains into the axillary vein.
Basilic Vein: A large superficial vein of the upper limb that runs up the medial side of the arm and joins the brachial vein to form the axillary vein.
Splenic Artery: A major branch of the celiac trunk, it supplies arterial blood to the spleen, parts of the stomach, and the pancreas.
Splenic Vein: Drains deoxygenated blood from the spleen, parts of the stomach, and pancreas, eventually joining the superior mesenteric vein to form the hepatic portal vein.
Cubital Vein (Median Cubital Vein): A superficial vein that connects the cephalic and basilic veins in the cubital fossa (bend of the elbow); it is a common site for venipuncture (blood draws).
Radial Vein: A deep vein of the forearm that runs alongside the radial artery, draining into the brachial vein.
Renal Artery: Branches directly from the abdominal aorta, supplying oxygenated blood to the kidneys.
Renal Vein: Drains deoxygenated blood from the kidneys directly into the inferior vena cava.
Abdominal Aorta: The continuation of the thoracic aorta below the diaphragm; it gives rise to arteries supplying the abdominal wall, viscera, pelvis, and lower limbs.
Median Vein of Forearm: A superficial vein located in the central anterior aspect of the forearm, typically draining into the basilic or median cubital vein.
Femoral Artery: The main arterial supply to the thigh and leg, continuing from the external iliac artery, palpable in the groin.
Great Saphenous Vein: The longest superficial vein in the body, running along the medial aspect of the leg and thigh, draining into the femoral vein in the groin.
Femoral Vein: A deep vein in the thigh, parallel to the femoral artery, receiving blood from the great saphenous vein and ultimately becoming the external iliac vein.
Classes of Blood Vessels
Arteries: Carry blood away from the heart towards the capillaries. They are typically high-pressure vessels with thick, muscular walls to withstand the force of cardiac contractions.
Arterioles: Smallest branches of arteries, often too small to be seen with the naked eye. They play a critical role in regulating blood flow into capillary beds and are a primary site of resistance in the circulatory system.
Capillaries: The smallest and most numerous blood vessels, forming extensive networks within tissues. This is the primary site of exchange for nutrients, gases, hormones, and waste products between the blood and interstitial fluid.
Venules: Collect deoxygenated blood from capillary beds and merge to form larger veins. They have thin walls and low pressure.
Veins: Return blood to the heart. They are typically low-pressure vessels with thinner walls and larger lumens compared to arteries. Most veins, especially in the limbs, contain valves to prevent the backflow of blood.
Blood Vessels
The Structure of Vessel Walls
Walls typically have three layers (tunics), except for capillaries:
Tunica intima (interna)
Tunica media
Tunica externa (adventitia)
Capillaries, for efficient exchange, consist only of an endothelium and a thin basal lamina.
Tunica intima
The innermost layer of a blood vessel, directly lining the lumen and exposed to blood.
Composed of:
Endothelium: A simple squamous epithelium overlying a basement membrane and a loose connective tissue layer (subendothelial layer).
Acts as a selectively permeable barrier, regulating the passage of substances into and out of the blood.
Secretes various chemicals (e.g., nitric oxide, endothelin) that stimulate vasodilation or vasoconstriction.
Normally repels blood cells and platelets, preventing adhesion and clot formation in healthy vessels.
When surrounding tissue is inflamed, endothelial cells produce cell-adhesion molecules (CAMS) that cause leukocytes to adhere to the surface, facilitating their migration into tissues needing defensive actions.
Internal Elastic Lamina: A layer of elastic fibers, present in arteries, that separates the tunica intima from the tunica media, allowing for distension and recoil.
Tunica media
The middle layer, and typically the thickest layer, especially in arteries.
Consists primarily of circularly arranged smooth muscle cells, collagen fibers, and elastic tissue.
Function:
Strengthens vessels and prevents blood pressure from rupturing them.
The smooth muscle cells are responsible for vasomotion: changes in the diameter of the blood vessel (vasoconstriction and vasodilation) brought about by their contraction or relaxation. This action regulates blood flow and pressure.
Contains External Elastic Lamina in larger arteries, which separates the tunica media from the tunica externa.
Innervated by the sympathetic nervous system, regulating vessel tone.
The Tunica Externa
The outermost layer, also known as the tunica adventitia.
Composed primarily of loose connective tissue (collagen and elastic fibers).
Function:
Anchors the vessel to adjacent tissues, providing structural support and limiting overexpansion.
Contains nerves, lymphatic vessels, and in large vessels, the vasa vasorum (“vessels of vessels”): small arteries and veins that supply oxygen and nutrients to the outer layers of the vessel walls themselves (tunics media and externa), which are too far from the lumen to be nourished by diffusion.
In veins, it is often the thickest layer and may also contain smooth muscle cells.
Walls of an Artery, a Capillary, and a Vein
Figure 20.1
Lumen: The central, blood-filled cavity of a blood vessel.
Artery Structure:
Composed of a relatively thick tunica intima (endothelium, internal elastic lamina), a very thick tunica media (smooth muscle, elastic laminae), and a tunica externa (including vasa vasorum in large arteries).
Arteries have smaller, more circular lumens than comparable veins, and their walls are robust to withstand high pulsatile pressure.
Capillary Structure:
Consists of only a single layer of endothelial cells and a thin basal lamina, making its wall extremely thin (a single cell thick).
This minimal barrier facilitates rapid exchange of substances between blood and surrounding tissues.
Lacks tunica media or externa.
Vein Structure:
Generally has a larger, often irregular, lumen and thinner walls compared to a companion artery.
The tunica media is much thinner and contains less smooth muscle and elastic tissue than arteries.
The tunica externa is often the thickest layer.
Includes valves (folds of the tunica intima) to prevent the backflow of blood, especially important in veins of the limbs and body wall where gravity can impede venous return.
Structure and Function of Arteries
Arteries form a branching network that carries oxygenated blood (except for the pulmonary artery) away from the heart, gradually decreasing in diameter as they extend into the periphery.
This branching system exhibits a programmed decrease in lumen diameter and elastic fibers.
Simultaneously, there is an increase in the relative amount of smooth muscle in their walls.
Arterial diameter and blood flow regulation are predominantly controlled by the sympathetic division of the Autonomic Nervous System (ANS), which stimulates smooth muscle contraction.
Three basic types of arteries:
Elastic arteries (Conducting arteries):
Largest arteries (e.g., pulmonary trunk, aorta, common carotid, subclavian, common iliac arteries).
Tunica media contains many elastic fibers and relatively few muscle cells, allowing them to stretch greatly during ventricular systole and recoil during diastole.
Their elastic recoil helps to maintain a continuous, even pulse force and blood flow by dampening pressure fluctuations, acting as a pressure reservoir.
Muscular arteries (Distribution arteries):
Medium-sized arteries (e.g., brachial, femoral, renal, splenic arteries) that distribute blood to specific organs and regions.
Have a very thick tunica media rich in smooth muscle cells and fewer elastic fibers compared to elastic arteries.
Capable of significant vasoconstriction and vasodilation to adjust blood flow to individual organs based on metabolic demand.
Arterioles (Resistance vessels):
Smallest arteries, leading into capillary beds.
Have little or no tunica externa and a thin or incomplete tunica media (mostly smooth muscle).
Their diameter changes significantly in response to sympathetic or endocrine stimulation, local tissue factors, and autoregulation.
They are the primary determinant of peripheral resistance and play a crucial role in regulating systemic blood pressure and directing blood flow to specific capillary beds.
Comparison of Companion Vessels
Figure 20.3
Veins vs. Arteries:
Arteries: Generally have thicker, more muscular, and elastic walls; a smaller, rounder lumen; and typically no valves (except at the heart's exit). They maintain their shape even when empty.
Veins: Have thinner, less muscular walls; a larger, often collapsed or irregular lumen; and possess valves (especially in the limbs) which are folds of the tunica intima.
Valves in veins allow unidirectional blood flow back to the heart only, preventing backward flow due to low pressure and gravity.
Valves are crucial for venous return, working with the skeletal muscle pump.
Plaque: Diagrams often illustrate typical deposits (composed of lipids, cholesterol, calcium, and cellular debris) within artery walls, characteristic of atherosclerosis (a specific type of arteriosclerosis).
Structure and Function of Capillaries
Capillaries: The smallest and most extensive of the blood vessels, forming microscopic capillary networks that permeate nearly all active tissues.
Function: The sole location for efficient exchange functions of the cardiovascular system. Here, vital materials like oxygen, nutrients, hormones, and waste products (carbon dioxide, metabolic byproducts) diffuse rapidly between blood and the surrounding interstitial fluid and cells.
Structure: Uniquely designed for exchange, composed of only an endothelial tube (one cell thick) inside a thin basal lamina. They lack tunica media or externa.
The diameter is often just wide enough to allow red blood cells to pass single file ().
Types of Capillaries
Continuous capillaries:
The most common type, characterized by endothelial cells forming a continuous, uninterrupted lining.
Tight junctions connect adjacent endothelial cells, limiting paracellular movement, but do not form a complete seal, as small intercellular clefts (gaps of about ) are present.
Large particles (e.g., cells, most proteins) cannot easily pass; smaller molecules (e.g., water, glucose, amino acids, ions) can readily pass through intercellular clefts or by transcytosis.
Commonly found in muscle, skin, lungs, and the central nervous system (where they form the basis of the blood-brain barrier, having particularly tight junctions and minimal clefts).
Fenestrated capillaries:
Endothelial cells are continuous but contain numerous small pores or fenestrations (windows, in diameter) that penetrate the endothelial cells.
These pores are typically covered by a thin glycoprotein diaphragm, making them more permeable than continuous capillaries.
Allow for rapid passage of fluids and relatively small plasma proteins.
Commonly found in organs where rapid fluid transport or absorption/filtration is critical, such as the small intestines (for nutrient absorption), kidneys (for filtrate formation), choroid plexuses (CSF production), and endocrine glands (for hormone secretion).
Sinusoids (discontinuous capillaries):
The least common and most permeable type of capillary.
Characterized by endothelial cells that form an incomplete lining with very large gaps (up to ) between cells, an absent or discontinuous basement membrane, and often larger lumens.
This loosely organized structure permits the transport of large substances, including formed elements (blood cells) and large plasma proteins.
Found in specific locations where extensive exchange or filtration of large particles is required: bone marrow (for entry of new blood cells into circulation), spleen (for removal of aged blood cells), liver (for processing of absorbed nutrients and removal of toxins), and some endocrine glands (e.g., adrenal cortex, anterior pituitary).
Pathways
Simple pathway: The most common circulatory route. One major artery delivers blood to an organ or region, which then branches into smaller arteries, arterioles, capillary beds for exchange, venules, and finally one major vein that drains blood away (e.g., splenic artery to spleen to splenic vein).
Arterial anastomosis: A convergence of two or more arteries that supply the same region. This provides an alternative route for blood flow (collateral circulation) if one artery is blocked or damaged, ensuring continued perfusion to the tissue (e.g., superior and inferior epigastric arteries supplying the abdominal wall; arterial anastomoses around joints).
Venous anastomosis: A much more common convergence compared to arterial anastomoses, where two or more veins drain the same region. This provides multiple routes for venous return, making venous blockage less problematic than arterial blockage (e.g., basilic, brachial, and cephalic veins in the arm; numerous anastomoses in the dorsal venous network of the hand and foot).
Arteriovenous anastomosis (shunt): A direct connection or bypass that transports blood from an artery directly to a vein, completely bypassing an intervening capillary bed.
Located primarily in the fingers, toes, palms, and ears.
Function: Primarily involved in thermoregulation, allowing blood to bypass capillary beds in the skin to conserve heat in cold conditions, or to increase blood flow to the skin for heat dissipation in warm conditions.
Portal system: A unique circulatory pathway involving two capillary beds in sequence, connected by an intervening portal vein, instead of a direct artery-capillary-vein route.
Examples:
Hepatic portal system: Transports nutrient-rich, deoxygenated blood from the capillary beds of the digestive organs (stomach, intestines, pancreas, spleen) via the hepatic portal vein to a second capillary bed (sinusoids) in the liver before draining into the general circulation. This allows the liver to process absorbed substances before they reach the rest of the body.
Hypothalamo-hypophyseal portal system: Connects the hypothalamus to