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Flashcards in Capillary function Deck (13):

The capillary bed

Flow of blood through capillaries depends upon:
Blood flow through arterioles
Patency of AV shunts
Patency of metarterioles
State of precapillary sphincters
Arteriole vasomotion


Capillary density

Increases with metabolic activity/need
Determines area available for gas/nutrient exchange
Determines inter-capillary spacing (distance blood-cell)


Diffusion and flow across capillary walls

Hydrostatic pressure- pressure due to fluid compression
Oncotic pressure- driving force of water moving from a dilute to a more concentrated environment due to osmosis
Diffusion os the main method for solutes to cross capillary walls, water can diffuse but no net flow as flow is bidirectional


Diffusion, filtration and pinocytosis definitions

Diffusion- passive movement down concentration gradients to and from capillaries
Filtration- movement of water to and from capillaries due to interaction between hydrostatic and oncotic pressures
Pinocytosis- movement of water to and from capillaries due to active transport across endothelial cells


Factors determining diffusion rate

Concentration gradient
Surface area
Permeability- ratio of pore size to size of solute molecule, pore density, path length, lipid solubility


Three types of solute based in physical properties

Lipid soluble e.g. oxygen
Small lipid-insoluble e.g. glucose
Large lipid-insoluble e.g. plasma roteins


Capillary exchange

Capillary membranes impair diffusion of lipid insoluble solutes
Continuous- found in muscle, lungs and CNS
Fenestrated- found in renal glomerulus and intestinal mucosa
Discontinuous- found in spleen, liver and bone marrow


Fluid homeostasis and capillary exchange

Capillary walls are a semi-permeable barrier
Water and electrolytes pass across capillary wall easier than plasma proteins
Therefore filtrate has a reduced protein content to capillary content


Starling principle of fluid exchange

Net intra-capillary pressure forces fluid out of capillaries
Net osmotic pressure of plasma proteins retained in capillaries opposes loss of fluid



In most beds there is net filtration
Net absorption only occurs in specialised cases where there is independent fluid input from the interstitium
Net fluid outflow from capillaries needs draining- lymphatics


Lymphatic circulation

Valves- one way flow
Intrinsic propulsion- rhythmic contractions of smooth muscle
Extrinsic propulsion- compression from tissue movements



Capillary filtration rate > lymphatic drainage = oedema
Causes: raised capillary pressure, reduced plasma oncotic pressure, increased capillary permeability, lymphatic insufficiency


Physiological regulation of exchange

Recruitment of more capillaries
Concentration gradients are steeper
Increased blood flow
Increased endothelial permeability