lec 9 - capillaries and veins

capillary vs vein structure

capillaries: very thin wall and small lumen - can be so small only a RBC can fit through - good for exchange

venules/veins: as vessels get bigger the wall structure get more complex

primary function of capillaries

exchange of nutrients and metabolic end products

thin walled - endothelial cells with no smooth muscle

hgihly branched network (high CSA)

blood velocity in capillaries

velocity = flow/total CSA

so when you increase area have decreased velocity (this is what we see in the capillaries)

large CSA and low velocity help enable fluid exchange

capillary circulation 

link between arterial and venous circulation 

several routes between arterioles and venules - controlled by precapillary sphincters and metarterioles (help redirect blood to the capillary bed which is needed the most) 

pathway blood takes is variable dependant on metabolic demand 

starlings forces

hydrostatic pressure: capillary and interstitial

osmotic pressure (colloid): capillary and interstitial

starlings forces equation

balance = Pc + πIF - PIF - πC

PC+πIF is out 

Pif - πC is in 

balance of fluid exchange depends on

location

in the arterioles get net filtration, in the venules get net reabsorption 

fluid exchange at arteriole end of capillary 

high Pc (because higher blood pressure at arterioles end than in venule end of capillary), everything else low

get a positive number e.g. ~10mmHg 

so water moves out - net filtration 

fluid exchange at venule end of capillary

lower pressure at this end (Pc)

get a negative number e.g. -10mmHg

water moves in - net reabsorption 

role of arteriole resistance in capillary hydrostatic pressure

small changes in the Pc alter the balance of fluid exchange in/out of capillaries

drop in BP occurs over the arterioles

vasodilation: decreased resistance, small drop in BP, increase Pc = fluid out

vasoconstriction: increased resistance, large drop in BP, decrease in Pc = fluid in in

what is a major determinant pf Pc

the resistance of the small arterioles upstream

net filtration

collection of fluid in the interstitial spaces

e.g. in the kidneys 

net (re)absorption

movement of fluid into the blood

e.g. in lungs

fluid exchange - lymphaticsystem

drain excess interstitial fluid into the systemic circulation

damaged lymphatics - poor draining of excess fluid from ISF = lymphedema 

functions of venules and veins 

collect blood from capillaries and take it back to the heart 

veins have thinner walls than arteries and are low pressure and low resistance 

~60% of blood volume within venous system and ~40% is in veins 

veins capacitance 

high capacitance vessels - blood volume reservoir 

veins pressure vs volume

high compliance

initially small increase in pressure = large increase in volume

plataeu - large increase in pressure = small increase in volume (near max volume)

arteries are compliant but less so than veins

determinants of venous pressure

total blood volume (~60% in venous circuit) - increasing total blood volume increases venous blood pressure

hormonal and paracrine venodilators and venoconstrictors can decrease/increase venous pressure

venoconstriction and flow

venoconstriction increases flow

unlike vasoconstriction which reduces flow

venous pressure and sumpathetic nervous system

noradrenaline binds to a1-adrenergic receptors → causes venoconstriction → increased flow

venous pressure and blood pressure

venous pressure determines venous return

venoconstriction = increased VR = increased EDV = increased SV = increased CO 

venous pressure - role of posture - vertical (standing) -

different heights from ground = different gravitational forces = blood pools in lower limbs 

venous pressure increases with distance below the heart 

have low venous pressure (eg-35) above the heart and high venous pressure at the feet (eg105) when standing with the heart being 1

venous pressure - role of posture - horizontal (laying down)

same height from ground = same gravitational forces

venous pressure similar throughout the body 

vein valves

preventing venous pooling

unidirectional blood flow towards the heart counteracts gravity

skeletal muscle pump

venous pooling counteracted by muscle pump

thermoregulation changes blood distribution - cutaneous veinsin extremities dilate to heat, also makes it harder for blood to get back to heart so can be dangerous for long time 

important factors in venous return

valves

skeletal muscle pump

respiratory pump 

respiratory pump - venous return inhalation 

diaphragm decreased intrathoracic pressure, increased intrabdominal pressure 

pulls blood in thoracic vena cava, compresses abdominal vena cava 

facilitates venous return 

respiratory pump - venous return exhalation

diagram relaxes → increased intrathoracic pressure, decrease intra-abdominal pressure → valves prevent backflow of blood 

negative pressure created by deep inhalation aids blood flow in vena cava → important during exercise