Blood Vessel Structure

Blood Vessel Structure

• A blood vessel is composed of three layers: tunica intima, tunica media, and tunica adventitia.
• Tunica intima consists of a single layer of endothelial cells and a subendothelial layer of loose connective tissue.
• Endothelial cells are polygonal, elongated in the direction of blood flow, and bulged into the lumen.
• They have a mesenchymal origin, contain desmin and vimentin, and lie on the basal lamina.

Endothelial Cells

• Endothelial cells have numerous complexes of miosin and actin, allowing for contraction and relaxation.
• They have Weibel-Palade bodies, which store and release von Willebrand factor and P-selectin.
• They have microvilli, which connect with leukocytes.

Capillaries

• There are three types of capillaries: continuous, fenestrated, and discontinuous sinusoidal.
• Continuous capillaries have no fenestrae, a well-developed basal lamina, and are found in muscle tissue, exocrine glands, and blood-brain barrier.
• Fenestrated capillaries have several circular transcellular openings in endothelial cells and are found in endocrine glands, intestine, and renal glomerulus.
• Sinusoidal capillaries have no basal lamina or a discontinuous one, and are found in liver, spleen, and bone marrow.

Pericytes

• Pericytes are support cells for capillaries and small venules.
• They have their own basal lamina, are of mesenchymal origin, and are located along the outside of capillaries and small venules.
• They possess a small Golgi complex, mitochondria, rER, microtubules, and proteins related to contraction.

Functions of Pericytes

• Not specified in the text.

Capillary Bed

• A capillary bed is an interconnected network of vessels running through tissues.
• It consists of metarterioles, capillaries, and venules.
• Precapillary sphincters regulate the flow of blood to tissues.

Veins

• Veins are formed by the convergence of capillaries into a system of channels.
• They become larger in diameter as they approach the heart.
• They are higher in diameter than arteries and are rather elongated or oval than round in shape.
• Adventitia is the best-developed layer.

Postcapillary Venules

• They are 0.1 to 0.5 mm in diameter.
• They have a thin wall and are collapsed.
• They have tunica intima, endothelium, and a thin subendothelial layer, and pericytes.

Small or Medium-Sized Veins

• They have a muscular wall, are 1-9 mm in diameter, and are collapsed.
• Intima is lined with endothelium, with a thin or no subendothelial layer.
• Media has small bundles of smooth muscle cells intermixed with reticular fibers.
• Adventitia is well-developed, with collagen fibers, fibroblasts, nerves, and vasa vasorum.

Large Veins

• They are big venous trunks, close to the heart.
• Intima is well-developed.
• Media is thin, with few layers of smooth muscle cells, and an abundance of collagenous tissue.
• Adventitia is the thickest, with longitudinal layers of smooth muscle cells.

Heart

• The heart is a muscular organ that contracts rhythmically.
• It produces atrial natriuretic factor.
• The structure of the wall consists of endocardium, myocardium, and epicardium.

Heart Structure

• The cardiac fibrous skeleton is a dense connective tissue that forms a central region of the heart.
• It is a site of origin and insertion of cardiac muscle cells.

Endocardium

• It is homologous with the t.intima of blood vessels.
• It consists of a single layer of squamous endothelial cells, a thin subendothelial layer of loose connective tissue, a muscular-elastic layer, and the subendocardial layer.

Myocardium

• It is the thickest layer, homologous with the t.media of blood vessels.
• It consists of cardiac muscle arranged in layers and grouped into three populations.

Epicardium

• It is the serous covering of the heart and forms the visceral layer of the pericardium.
• It is homologous with the t.adventitia of blood vessels.
• It is covered externally by mesothelium.

Lymphatic Vascular System

• It returns the extracellular liquid (lymph) to the bloodstream.
• The lymph circulates in one direction only.
• Lymphatic vessels are found in all organs, are blind-ended, and consist of a single layer of endothelium.
• They have numerous internal valves and no fenestrations in their endothelial cells.

Artery Structure

• Arteries consist of three tunics: tunica intima, tunica media, and tunica adventitia
• Tunica intima is lined with endothelium
• Tunica media is composed of elastic laminae and vascular smooth muscle cells
• Tunica adventitia is made up of connective tissue, vasa vasorum, and nerves

Elastic Arteries

• The largest arteries
• Function: transport blood with oxygen and nutrients to tissues and organs
• Intima: lined with endothelium, subendothelial layer, collagen and elastic fibers, fibroblasts, and internal elastic lamina
• Media: concentric arranged, perforated elastic laminae (30-75), vascular smooth muscle cells, reticular and collagen fibers, proteoglycans, and glycoproteins
• Adventitia: connective tissue, well developed, nerves, and vasa vasorum
• Staining: resorcine-fuchsin and orcein

Muscular Arteries

• Medium-sized arteries
• Control the affluence of blood to organs by contracting or relaxing smooth muscle cells in the media
• Intima: lined with endothelium, subendothelial layer, and internal elastic lamina is prominent, thicker in older people
• Media: concentric layers of helically arranged smooth muscle cells, some elastic fibers, reticular fibers, proteoglycans, and glycoproteins
• Adventitia: fibroblasts, type I collagen fibers, longitudinally oriented elastic fibers, nerves, and adipose tissue
• Function: regulate blood pressure and flow to organs

Arterioles

• Narrow lumen (less than 0.5 mm in diameter)

• Intima: lined with endothelium, subendothelial layer is very thin, no internal elastic lamina in very small arterioles

• Media: 1 or 2 circularly arranged layers of smooth muscle cells, no external elastic lamina

• Adventitia: very thin

• Function: regulate blood flow to capillaries

Capillaries

• Single layer of endothelial cells resting on a basal lamina
• Permit metabolic exchange between blood and surrounding tissues