L5. Capillaries and Lymphatic vessels

Capillary structure

• single endothelial cell layer laid upon a basement membrane: diameter range= 4 - 10um

• Three types according to inter-endothelial cell distance: continuous, fenestrated and discontinuous

Starling principle → Capillary fluid exchange

Net filtration pressure

• Capillaries at the venous side have been considered as the only site where fluid absorption occurs.

• More recent measurement techniques show that filtration occurs along the entire length of capillaries (whilst excess filtrate is drained by lymphatics): this is true as long as hydrostatic capillary pressure is sufficient to keep NFP at a positive value.

• Overall, low Pc favours fluid absorption as it occurs in the intestinal mucosa and peritubular renal capillaries.

Skin capillaries at cardiac level (standing)

• Most sites wherein net inflow does not equal net outflow leave a residual amount of fluid in the interstitium.

• Lymphatic vessels converge into lymph nodes, involved in filtering and immune surveillance, thus recovering ‘spillover’ flow from the capillary network.

• Excess filtrate is then returned into the circulation via lymphatic vessel absorption and drainage onto the thoracic veins.

Lymphatic system

• Lymphatic capillaries contain a single layer of lymphatic endothelial cells (LECs), bearing large intercellular spaces and a discontinuous basement membrane.

• They drain onto collecting vessels endowed with smooth muscle, undirected valves and LECs

• Lymph is propelled towards the thoracic duct and veins through intermittent compression (skeletal muscle activity, intrathoracic and intra-abdominal pressure charges)

• Lymphatic capillary permeability allows the passage of fluid, macromolecules and cells

Primary tumours

Interstitial accumulation of fluid: Edema

Transcapillary solute exchange

• Permeability for a given molecule depends upon lipid solubility (hydrophobicity) and size (molecular weight)

• ‘‘The more lipophilic and small a molecule is, the higher its permeability will be.’’

• Solutes fall into one of the three categories:

  1. lipid soluble

  2. Small hydrophilic

  3. Large hydrophilic

• Transcapillary solute exchange occurs mostly via diffusion in favour of concentration gradients

• Notable exception is the brain, where continuous, impermeable capillaries utilize transporter-mediated facilitated diffusion (especially glucose): blood-brain barrier.

• Lipid soluble molecules diffuse through the endothelial cell membrane

• Lipid insoluble molecules (hydrophilic or water-soluble) require water-filled channels to allow transcapillary diffusion.

Transcapillary solute exchange diagram

• Lipophilic molecules via endothelial cell membranes

• Small hydrophilic molecules diffuse through aqueous channels and convective transport (solvent drag)

• Large hydrophilic molecules diffuse through vesicles and caveolar transcellular transport.

• Total capillary length in an adult human can reach 40,000km (4 × 10^7m).

Hydrophilic transcapillary transport via vesicles

Hydrophilic transcapillary transport via caveolae

Fibre matrix hypothesis: glycocalyx decreases the width of junctions and fenestrae

• Macromolecular meshwork of membrane glycolipids and glycoproteins

• Reduces the effective width of junctions and fenestrations

• Effective pore size is determined by steric hindrance, electrostatic interactions and interfibre spacing determines effective pore size.

Arteries

• Thick walled, muscular, high pressure (large elastic, windkessels)

• Regulate pressure and flow (small arteries and arterioles)

Vein

• Valves to prevent backflow, thing walls, collapsible

• Low pressure capacitance system

Capillaries

• Extremely thin walls

• Exchange vessels

• Maximised transmural diffusion

Transcytosis

The vesicular transport of macromolecules from one side of a cell to the other.

Endocytosis

A process where cells take in substances from outside the cell by engulfing them in a vesicle

Exocytosis

a process for moving large molecules out of the cell to the cell exterior

Pinocytosis

A form of endocytosis where the cell membrane folds inwards around the fluid and forms vesicles, and also breaks down particles