Material exchange at capillaries

Movement of biomolecules (nutrients and wastes) across thin capillary wall

From plasma to interstitial fluid (then into cells) and vise versa

Capillary density is related to the metabolic demands of the tissue

• Measured by the number of capillaries per unit of surface area

• Can be increased by growing new ones (angiogenesis)

Capillaries have the thinnest walls

• single layer of flattened endothelial cells that are “leaky”

• Supported by the basal lamina (basement membrane) of fibrous proteins

Two types of capillaries

• Continuous capillaries: have leaky junctions

• Fenestrated capillaries: have large pores

Types of transport across capillary endothelial cells

• Transcytosis

• Diffusion

• Water movement

Transcytosis

• a type of transcellular transport where macromolecules are captured in vesicles on one side of a cell, transported across the interior, and released on the opposite side

• For large proteins and other solutes too big to fit through fenestrations or junctions between cells

Diffusion for small molecules (gases, ions, glucose, amino acids, etc)

• through between-cell (paracellular) junctions and channels or fenestrations

• Direction of movement is determined by concentration or pressure gradients

Water moves by “bulk flow” and is driven by hydrostatic and osmotic pressures

• Filtration: water movement OUT OF capillaries

→ driven by capillary’s hydrostatic pressure

→ net filtration occurs at the arteriole end of a capillary bed

• Absorption: water movement INTO capillaries

→ driven by colloid osmotic pressure in the capillary

→ net absorption at the venous end of a capillary bed

The balance between hydrostatic pressure and osmotic pressure regulates bulk flow of fluid at a capillary!

The net pressure at any point determines the direction of fluid movement

Arteriole end

• hydrostatic pressure is higher and forces fluid out of capillary

Venous end

• osmotic pressure is higher and draws fluid back into capillary

Lymphatic system

• collects excess interstitial fluid and proteins and returns them to the circulatory sys

• picks up fat absorbed by the digestive sys and transfers it to CVS

• also serves as a filter for pathogens

Edema

Fluid accumulation in tissue

What causes edema?

• filtration rate is far greater than absorption rate

• inadequate drainage of lymph

Edema due to disruption of balance between filtration and absorption can occur when there is an…

• increase in hydrostatic pressure in vessels

• decrease in plasma protein concentration

→ what will this affect?

• increase in interstitial proteins

→ what will this do?