A: tunica intima
B: tunica media
C: tunica adventitia
Label A-C and explain the function of A and B
B: throughfare channel
C: precapillary sphincter
A and B = vascular shunt, arteriovenous anastomoses, flow through capillary bed can be shut off and blood goes directly through from arteriole -> venule
What are the general features of capillaries?
Endothelial cells, connected by tight junctions, surrounded by BM (basal lamina)
What are the 3 types of capillary? Describe each.
1) Continuous: most common, least permeable, sealed endothelium, only allows small molecules to diffuse, 2 types: a) numerous transport vesicles (caveolae), primarily in skeletal muscle, lungs, skin and glands. b) few visicles, in CNS (BBB)
2) Fenestrated: mainly in exocrine glands, intestines, pancreas and glomeruli. small circular pores in endothelial cells, continuous BM, passage of H2O and salts, in tissues specialised for bulk exchange
3) Discontinuous/sinusoidal: in liver, spleen and bone marrow. highest permeability, gaps in cell junctions, larger endothelium opening allows RBC, WBC and serum proteins to pass, discontinuous BM, NO PINOCYTOTIC VESICLES.
A: Continuous capillary
B: Fenerstrated capillary
C: discontinuous/sinusoidal capillary
What are the 4 ways solutes can be exchanged across endothelium?
1) Diffusion (lipid soluble substances)
2) Paracellular transport (via intracellular spaces for small lipid-insoluble ions etc. e.g. kidney H2O and ion transport)
3) Via fenestrations (pores for water-soluble substances, macromolecules)
4) Via vesicles or caveolae (special pits that undergo endocytosis, for large substances)
What law determines solute diffusion across the endothelium?
Fick's law: amount moved = area x conc. gradient x diffusion coefficient
NB. small soluble molecules have higher diffusion coefficent
What determines fluid exchange across capillary?
Describe how the factors differ at different part of the capillary.
How do you calculate net filtration pressure? Give values and meanings for arteriole and venous ends.
1) hydrostatic pressure 2) oncotic pressure (conc of plasma proteins in capillary)
HP: capillary HP - filtration favoured at arteriolar end where HP greatest (35mmHg) and reabsorption at venous end where HP lowest (-17mmHg) (HPif usually 0)
OP: capillary OP from plasma proteins about 26mmHg, interstitual fluid OP - negligible
NET FILTRATION PRESSUE = (HPc- HPif) - (OPc - OPif)
So at artereriole end: NFP: 10mmHg = water forced out (filtration)
And at venous end: NFP: -8mmHg - water pulled in (reabsorption)
What is starling's hypothesis?
Fluid movement due to filtration across across capillary wall is due to balance between hydrostatic pressure and oncotic pressure.
NB: plasma proteins can't leave capillary -exert osmotic pressure that draws water in so fluid leaves capillary at arteriole end and returns at venous end
How much fluid moves capillaries -> interstitial fluid per day?
What happens to it?
Some reabsorbed, most drains to lymphatic system
Describe lymphatic capillaries.
Describe the flow of lymph from the capillary bed.
What can failure to maintain correct fluid distribution lead to?
Endothelium with large intercellular gaps surrounded by permeable BM, ends as blind sacs in tissues, one-way valves
Plasma flow through bed -> Net H2O from capillary to interstitial space -> net fluid into lymph capillaries then:
- Lymph returned to circulation via subclavian veins
- Water reabsorbed to circulation at lymph nodes
Oadema - fluid retention - increased volume in intersitial compartment - tissue swelling
What is systemic and peripheral oedema and how is it distinguished?
The amount of intersisital fluid is determined homeostatically. What 4 factors precipitate oedema?
Appears first in lower regions, leg venous pressure increases during prelonged standing. Distinguised by applying firm pressure to area (pitting oedema).
1) Increased capillary hydrostatic pressure e.g. if venous pressures increase by gravitational forces, in heart failure or with venous obstruction. Drives fluid to intersitial space.
2) Decreased plasma oncotic pressure e.g. hypoproteinemia during malnutrition/liver disease. Drives fluid to interstitual space. (-> kwashiorkor)
3) Increased capillary permeability - H2O flows easier and decrease in OP difference by allowing protein to leave vessel easily. E.g. vascular damage (burns, trauma)
4) Lymphatic obstruction e.g. fibrosis or filiariasis, tissue injury, inflammation of lymph vessels, elephantisis
What condition is this?
Kwashiorkor - severe malnutrition -> protein deficiency in blood and tissues -> decreased OPc, pitting oedema and water retention in gut.
How is oedema treated?
Diuretics - osmotic (increasesd H2O excretion) or loop ( increased Na+ excretion) (natriuresis)
What condition is this?