(3) Structural & Functional Differences of Blood Vessels - Capillaries

Capillary structure

• Single layer of endothelial cells (no smooth muscle).

• Extremely thin walls → optimal for diffusion.

• Very small lumen (just large enough for RBCs).

• No elastic tissue.

• Very large total cross-sectional area.

Capillary function

• Site of gas diffusion (O₂, CO₂).

• Nutrient delivery (glucose, amino acids).

• Waste removal.

• Water movement (Starling forces).

Key hemodynamic features of capillaries

• Slowest velocity of all vessels.
  → Slow velocity = time for nutrient/gas exchange.

• Moderate hydrostatic pressure (not too high to rupture).

What controls flow in capillaries?

Flow controlled by precapillary sphincters and arteriole tone.

Capillary network organization (microcirculation)

capillaries form dense interconnected networks called capillary beds or microvascular beds, which are organized for maximum exchange efficiency

Metarterioles

serve as a transitional channel between arterioles and true capillaries

Thoroughfare channel

direct route to venules when capillary exchange is minimized

Precapillary sphincters

rings of smooth muscle at the entrance to each capillary

When are precapillary spincters open?

increased perfusion

When are precapillary sphincters closed?

blood bypasses through thoroughfare channel

What do precapillary sphincters regulate?

• which tissues receive blood

• how much of the capillary network is active

• total surface area for exchange

Continuous capillary structure

• tight endothelial junctions

• continuous basement membrane

• least permeable

• pinocytotic vesicles

• perictyes on outer wall regulate stability and repair

Continuous capillary locations

• skin

• muscle

• CNS

Continuous capillary function

• controlled exchange

• restricts passage of most proteins and cells

• CNS subtype forms blood-brain barrier with astrocyte foot processes

Fenestrated capillary structure

• endothelial cells contain pores with thin diaphragms

• continuous basement membrane

Fenestrated capillary locations

• kidneys

• endocrine glands

• intestinal mucosa

Fenestrated capillary function

• rapid exchange of fluids and small molecules

• essential for filtration, secretion, and absorption

Sinusoidal (discontinuous) capillary structure

• large gaps between endothelial cells

• incomplete basement membrane

• very large lumen

• allows passage of large proteins and cells

Sinusoidal capillary locations

• liver

• spleen

• bone marrow

Sinusoidal capillary function

• Exchange of large substances (albumin, clotting factors)

• Removal of old RBCs (spleen)

• Release of new blood cells (bone marrow)

• Detoxification (liver)

Endothelial cells support of capillary function

perform critical regulatory tasks

• Release nitric oxide (NO) (vasodilation)

• Release endothelin (vasoconstriction)

Pericytes support of capillary function

contractile cells wrap around capillaries, crucial in forming blood-brain barrier

Diffusion of capillaries

Driven by concentration gradients.
Examples:

• O₂ from blood → tissues

• CO₂ from tissues → blood

Filtration & absorption of capillaries

Driven by hydrostatic & oncotic pressures (Starling forces).
Controls:

• Fluid balance

• Edema formation

• Nutrient delivery

Transocytosis of capillaries

Vesicles transport material across endothelial cells.

• Used for large or lipid-insoluble substances

• Example: insulin crossing endothelium

Paracellular transport of capillaries

movement through small gaps between cells (continuous), pores (fenestrated), or large gaps (sinusoidal)

Metabolic autoregulation of capillary perfusion

Tissues that are active (high CO₂, low O₂, more H⁺, more adenosine) trigger:

• Precapillary sphincter relaxation

• Increased blood flow

• Local vasodilation

Myogenic mechanism of capillary perfusion

Vessels respond to stretch:

• Increased pressure → vasoconstriction

• Decreased pressure → vasodilation

Helps stabilize capillary pressure.

Neural control of capillary perfusion

Sympathetic tone regulates arterioles and metarterioles, indirectly affecting capillaries.

Hormonal control of capillary perfusion

• Angiotensin II → constriction (↓ capillary flow)

• ANP → dilation (↑ capillary flow)

• Epinephrine → organ-specific effects

Tissues with high capillary density

• Heart

• Skeletal muscle (during training increases)

• Kidneys

• Endocrine glands

High density capillary function

supports high metabolic activity

Low density capillary tissues

• tendons

• ligaments

Glomerular capillaries

• in the kidney

• fenestrated capillaries with high pressure

• specialized for filtration

Hepatic sinusoid capillary beds

• Discontinuous capillaries

• Allow large proteins & cells to move

• Macrophage-rich (Kupffer cells)

Blood-brain barrier capillary bed

• Most restrictive continuous capillaries

• Tight junctions, pericytes, astrocyte end-feet

• Only lipid-soluble substances pass easily

Intestinal capillary beds

• Fenestrated

• Specialized for nutrient absorption

Lymphatic capillary

excess interstitial fluid created by capillary filtration enters to prevent:

• edema

• protein accumulation

• impaired tissue oxygenation

lymph returns fluid back to venous circulation