Capillaries

function of capillaries>

Exhange of materials between plasma and interstitial fluid (blood and tissues)

diffusion is the main mechanism by which ___ moves from blood to cells or vice versa

gases, nutrients, waste products

Structural features of capillaries that help facilitate diffusion of gases etc.

Distance traveled needs to be very short: capillary is very small, wall is thin

potential pathways for diffusion of gases

Across cell (plasma membrane), intracellualr cleft (fluid filled channel/pore)

Water soluble thru pore, lipid soluble thru plasma membrane

What maintains conc gradient of O2, CO2, Glucose, lactic acid?

Metabolism by cells: using up oxygen or producing CO2

3 types of capillaries: which lets proteins cross?

Continous, fenestrated, sinusoid: big holes

transcytosis

endo and exocytosis, too large for intercellular cleft

Bulk flow?

distribution of extracelllular fluid between capillary and interstitial space thru intercellular clefts

plasma vs interstitial fluid

plasma has high amount of protein, interstitial has very few

2 things that control thje movement of fluid across semipermeable memrbane: forces that contribute to bulk flow

Hydrostatic pressure of fluid

osmotic pressure of a fluid

Starling forces

forces that move protein free fluid across capillary wall:

hydrostatic pressure of cap and interstitial

osmotic pressure of plasma and interstitial

Net filtration pressure =?

P of Cap + osmotic force of inter - P of interstitial - osmotic force of capillary

fluid moving out of blood

fluid moving into blood

Filtration

absorption

arterial end of cap vs venous end

Aterial end favors filtration

venous end favors absorption

where does fluid that is not absorbed back into blood go?

lymphatics

decreased lymphatic flow =

edema elephantiasis

Examples of what can increase capillary hydrostatic pressure and thus cause?

edema

• More difficult to return blood to heart, blood pools in veins, increases venous pressure

  • prolonged standing

  • damage to valves in veins

• Fluid overload

  • Storehouse for blood : Extra Na+ and water in blood → blood volume and venous pressure too high

• Heart failure

  • Blood backs up in right atria and then systemic veins: increased venous pressure

edema from increased Pc on arterial end of capillary = injury or infection

histamine released that dilates arterioles and increases Pc on arteriolar side of capillary

• venous pressure stays same

• More filtration than absorption → edema

Edema from increased capillary permeability

Histamine: enables proteins to leak into interstitial space → increase in interstitial osmotic force

Bulk flow affect concentration of cyrstalloids (low MW solutes that can easily penetrate capillary pores/clefts) or volume of fluid>

Volume of fluid

what will cause edema from decreased capillary oncotic pressure?

liter of crystalloid → fluid shifts into interstitial space

___% of the blood volume is in the systemic venules and veins

% in arteries?

60%

15%

veins are capacitance vessels

how do veins act as blood resevoir?

1. Veins are very compliant (easily expand), increase in volume will cause very small increase in transmural pressure

   1. If more blood enters veins, veins expand with minimal elastic recoil and stores extra blood

2. when needed, venous return can change: constriction increases return

How venous constriction increases return?

Contraction of smooth muscle causes

• reduction in compliance

• Large increase in delta P (pressure in veins - pressure in right atrium)

• Small increase in resistance

cnstriction of aterioles causes vs constriction of veins?

arterioles = decrease flow by large increase in resistance

veins = increase flow by decreasing compliance and large increase in delta P

what causes smooth muscle contraction in veins?

SNS

when stand up, what prevents blood from pooling in veins of feet and causing faint?

SNS → venous smooth muscle contracts → venous constriction → increase in pressure gradient → sends blood to heart

Besides pressure gradient and smooth muscle contraction, what else helps move venous return back to heart?

skeletal muscle pump, respiratory pumps, valves

(skeletal msucle relaxed + valves shut, then contracts and open valves)