HISTOLOGY OF CIRCULATORY SYSTEM III

MEDIUM SIZED, MUSCULAR ARTERIES T. INTIMA Subendothelial layer thin Prominent internal elastic lamina T. MEDIA Smooth muscles- 40 layers Elastic fibers T. ADVENTITIA Connective tissue, vasa vasorum, lymphatics

SMALL ARTERIES & ARTERIOLES <0.5mm lumen Thin subendothelial connective tissue Int. elastic lamina absent in small arterioles T. Media: smooth muscle layer Adventitia is thin Hypertension

CAPILLARIES Diameter : 5-7 µm Single layer of endothelium; permit metabolic exchange Function in groups- Capillary Beds Richness conforms to metabolic activity of tissue Structure and pulsatile flow of blood through capillaries and thoroughfare channel.

TYPES Continuous Fenestrated Sinusoidal

CONTINUOUS CAPILLARIES No fenestrations, complete basal lamina Pinocytic vesicles Location: Muscles, nervous tissue, exocrine glands

FENESTRATED CAPILLARIES Transcellular openings (80nm) Limited by cell membrane Covered by diaphragm ( thinner than cell membrane, proteoglycans) In endocrine glands Kidneys etc

SINUSOIDAL CAPILLARIES Diameter 30-40µm Endothelial cells discontinuous layer Fenestrations without diaphragm Incomplete basal lamina Macrophages Found in liver, hematopoietic organs

PERICYTES Cells at periphery of continuous capillaries enclosed in own basal lamina Mesenchymal origin Actin, myosin– contractile properties Role in repair process Diabetic Microangiopathy

POST CAPILLARY VENULE Diameter 0.5-1mm Endothelium + thin subendothelial connective tissue Media– pericytes MUSCULAR VEINS Thin subendothelial layer, may be absent Media: smooth muscles + reticular fibers valves

Inflammation & Edema

LARGE VEINS Well developed intima Thin media Very thick adventitia Further three layers Smooth muscles

LYMPH VASCULAR SYSTEM LYMPH CAPILLARIES: Single layer of endothelium Incomplete basal lamina LYMPH VESSELS: Structure similar to veins, thinner walls LARGE DUCTS: Similar to veins, smooth muscles

Medical Application Metastasis Lymph edema