Capillary function

Question 1

The capillary bed

Answer 1

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

Question 2

Capillary density

Answer 2

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

Question 3

Diffusion and flow across capillary walls

Answer 3

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

Question 4

Diffusion, filtration and pinocytosis definitions

Answer 4

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

Question 5

Factors determining diffusion rate

Answer 5

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

Question 6

Three types of solute based in physical properties

Answer 6

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

Question 7

Capillary exchange

Answer 7

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

Question 8

Fluid homeostasis and capillary exchange

Answer 8

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

Question 9

Starling principle of fluid exchange

Answer 9

Net intra-capillary pressure forces fluid out of capillaries

Net osmotic pressure of plasma proteins retained in capillaries opposes loss of fluid

Question 10

Lymphatics

Answer 10

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

Question 11

Lymphatic circulation

Answer 11

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

Question 12

Oedema

Answer 12

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

Question 13

Physiological regulation of exchange

Answer 13

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