95. Capillary Circulation – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Capillaries are the principal exchange vessels of the body. They facilitate the transfer of oxygen, carbon dioxide, nutrients, water, hormones, and metabolic waste products between blood and tissues. Microcirculation extends from arterioles to venules and is essential for normal cellular function. Most body cells lie close to capillaries, except the cornea and lens, which are avascular.
• Capillaries consist of a single layer of endothelial cells resting on a basement membrane and lack a tunica media, making them highly permeable. They arise from arterioles or metarterioles, and many originate behind precapillary sphincters, which regulate blood flow according to local metabolic needs. Thoroughfare channels connect metarterioles directly to venules and allow blood to bypass capillaries at rest.
• Types of capillaries:
  • Continuous capillaries: Least permeable; found in skeletal muscle, lungs, connective tissue, and the blood–brain barrier.
  • Fenestrated capillaries: Have pores that increase permeability; found in intestinal villi, endocrine glands, renal glomeruli, choroid plexus, and ciliary processes.
  • Sinusoidal (discontinuous) capillaries: Highly permeable with large gaps; found in the liver, spleen, and bone marrow.
• Capillary blood flow is slow (0.03–0.07 cm/s) because of the large total cross-sectional area, allowing efficient exchange. Red blood cells pass through capillaries in single file. Capillaries have no direct autonomic innervation; their blood flow is controlled mainly by arterioles and precapillary sphincters in response to local metabolites. Pericytes (Rouget cells) help maintain capillary stability, permeability, and angiogenesis.
• Exchange across capillary walls occurs by diffusion, vesicular transport (transcytosis), and filtration. Diffusion is the most important mechanism for gases and small solutes, whereas transcytosis transports large lipid-insoluble molecules.
• Capillary filtration is governed by Starling forces:
  • Forces promoting filtration: Capillary hydrostatic pressure and interstitial oncotic pressure.
  • Forces opposing filtration: Plasma oncotic pressure (mainly due to albumin) and interstitial hydrostatic pressure.
  • Normally, filtration predominates at the arteriolar end, while limited reabsorption occurs toward the venular end. Excess interstitial fluid is returned by the lymphatic system.
• Edema is the abnormal accumulation of excess fluid in the interstitial space. It develops when filtration exceeds lymphatic and venous drainage. Major causes include:
  • Increased capillary hydrostatic pressure (e.g., heart failure, venous obstruction).
  • Decreased plasma oncotic pressure (e.g., liver disease, nephrotic syndrome, malnutrition, burns).
  • Increased capillary permeability (e.g., inflammation, histamine, bradykinin).
  • Impaired lymphatic drainage, producing lymphedema, which is typically non-pitting.
• Arteriovenous anastomoses (AV shunts) are direct connections between arterioles and venules that bypass capillaries. They are abundant in the fingers, toes, palms, and ear lobes, receive dense sympathetic innervation, and play an important role in thermoregulation.