Capillaries: Structure, Exchange Functions & Physiologic Regulation

Capillaries are the smallest blood vessels (5–10 µm diameter), composed of a single layer of endothelial cells with basement membrane. They form vast networks (~10 billion) for exchange.

Structural Types

Type	Endothelium	Location	Permeability
Continuous	Tight junctions, no fenestrations	Muscle, skin, lung, BBB	Low (small molecules only)
Fenestrated	Pores (60–80 nm) with diaphragm	Kidney glomeruli, endocrine glands, intestinal mucosa	High (small proteins)
Sinusoidal (Discontinuous)	Large gaps, no basement membrane	Liver, spleen, bone marrow	Very high (cells, large proteins)

Core Exchange Functions

1. Nutrient delivery: Glucose, amino acids, fatty acids → tissues.
2. Oxygen delivery: Hb‑bound O₂ diffuses down gradient.
3. Waste removal: CO₂, urea, lactate → blood.
4. Fluid balance (Starling forces): Hydrostatic pressure out; oncotic pressure in. Net filtration at arteriolar end, reabsorption at venular end. Lymphatics return excess.
5. Immune trafficking: Leukocyte margination, adhesion, transmigration (diapedesis) at post‑capillary venules.
6. Thermoregulation: Cutaneous capillary dilation → heat loss; constriction → conservation.

Regulation

• Precapillary sphincters & metarterioles: Local metabolic control (O₂, CO₂, pH, adenosine, K⁺).
• Autonomic: Sympathetic → constriction (α1); limited parasympathetic.
• Endothelial mediators: NO (vasodilator), endothelin‑1 (constrictor), prostacyclin.

Clinical Relevance

• Capillary leak syndrome (sepsis, burns, anaphylaxis) → edema, hypovolemia.
• Diabetic microangiopathy: basement membrane thickening, pericyte loss → retinopathy, nephropathy, neuropathy.
• Hypertensive remodeling: rarefaction (↓ density) → ↑ peripheral resistance.