Blood Vessel Structures and Types

Blood Vessel Types

• Arteries: Distribute blood to tissues.
• Capillaries: Permit exchange between blood and tissues.
• Veins: Return blood to the heart.

Blood Vessel Structure

• Tunica Intima: Innermost layer composed of endothelium, basement membrane, and loose connective tissue.
  • Contains the internal elastic lamina in arteries.
• Tunica Media: Middle layer consisting of smooth muscle cells, elastic fibers, and collagen.
  • Contains the external elastic lamina in arteries.
• Tunica Adventitia: Outermost layer made of collagen and elastic fibers.

Arteries

• Large Elastic Arteries:
  • Thick intima, numerous elastic laminae in the media, and an underdeveloped adventitia.
  • Examples: Aorta and its major branches.
• Muscular Arteries:
  • Prominent internal elastic lamina, multiple layers of smooth muscle in the media, and a well-developed adventitia.
• Arterioles:
  • Less than 0.5 mm in diameter.
  • Thin subendothelial layer, absent elastic laminae, and a thin adventitia.

Functions of Arteries

• Elastic recoil of vessels.
• Ensure blood flow during diastole.
• Control blood flow to large areas (e.g., femoral artery to the leg).
• Control blood flow into capillary beds.
• Affect peripheral resistance (increased resistance leads to increased blood pressure).

Capillaries

• Single layer of endothelial cells forming a tube.
• Diameter: 5-10 µm
• Length: 50 µm
• Represent over 90% of all blood vessels.
• Slow blood flow facilitates exchange of water, solutes, and macromolecules between blood and tissues.

Types of Capillaries

• Continuous (Type I): Found in muscle, lung, CNS, skin, connective tissue, and exocrine glands.
  • Contains pinocytotic vesicles for fluid and macromolecule transport.
  • Blood-brain barrier is formed by continuous capillaries with few pinocytotic vesicles.
• Fenestrated (Type II): Found in intestinal mucosa, endocrine glands, renal glomerulus, and pancreas.
  • Pores (30-50 nm) in the endothelium, closed by a thin diaphragm except in renal glomeruli.
• Sinusoidal (Type III): Found in liver and hemopoietic organs (e.g., bone marrow and spleen).
  • Tortuous path, larger lumen (30-40 µm), discontinuous layer of endothelial cells with wide gaps.
  • Contains macrophages associated with endothelial cells.
  • Incomplete or discontinuous basal lamina.

Pericytes

• Cells of mesenchymal origin.
• Long cytoplasmic processes partially surrounding the endothelial layer of capillaries and postcapillary venules.
• Contractile function.
• Differentiate into new blood vessels after tissue injury.

Microcirculation

• Pre-capillary sphincters: Control blood flow into capillaries.
• Thoroughfare channels: Provide a route for blood to enter the venous system without traversing the capillary bed.

Veins

• Venules: Similar to capillaries, but with a thin media and adventita.
• Small and medium-sized veins: Have a subendothelial layer in the intima, a small amount of elastic tissue and smooth muscle in the media, and a thicker adventitia.
• Large veins: Have well-developed layers, with the adventitia being the most developed.
  • Contain valves to ensure unidirectional blood flow.

Functions of Veins

• Contribute to venous return and cardiac output through sympathetic innervation of smooth muscle.
• Venules act as sites for leukocyte migration into connective tissue.

The Heart

• Endocardium: Innermost layer.
• Myocardium: Middle layer, composed of cardiac muscle.
• Epicardium: Outermost layer.

Cardiac Muscle

• Striated due to ordered arrangement of actin and myosin.
• Involuntary.
• Cells have a single, central nucleus and are branched.
• Connected by intercalated discs.

Conducting System

• Purkinje fibers: Specialized conducting myofibers with one or two central nuclei and sparse myofibrils.
• Play a major role in electrical conduction and impulse spread to the ventricular muscle.

Heart Valves and Fibrous Skeleton

• Fibrous skeleton: Dense connective tissue that anchors valves and surrounds the atrioventricular canals.
• Heart valves: Composed of dense connective tissue covered with a thin layer of endothelium.

Lymphatic Vascular System

• Lymphatic capillaries: Thin, closed-ended vessels with a single layer of endothelium and an incomplete basal lamina.
• Larger lymphatics: Similar to veins, but with thinner walls and more numerous internal valves.

Explore the types and structures of blood vessels, including arteries, capillaries, and veins. This quiz covers the layers of blood vessels and their functions in the circulatory system. Test your knowledge on large elastic arteries, muscular arteries, and arterioles.