1.6 (PSIO 202)

Hemodynamics: Capillary Exchange

How do oxygen and nutrients move out of the blood?

by crossing the capillary wall and entering the ISF

How do CO2 and metabolic waste products move in the blood?

opposite direction of oxygen and nutrients

How big is the average capillary?

\~1 mm long and \~8um lumen diameter

What is the average blood flow velocity of capillaries?

\~0.1 cm/sec (VERY SLOW)

How are density and metabolic activity related in capillaries?

density of capillaries in a given tissue is proportional to the tissue’s metabolic activity

What is a capillary bed?

a network of capillaries where exchange of materials with the tissue cells can occur

When is diffusion efficient?

when diffusion distances are short

What do intercellular clefts let cross the capillary wall?

water and water-soluble “solutes”

What can pass through endothelial cell membranes?

some small molecules and gasses

What can pass through fenestrations?

large molecules (can pass through very easily)

What are metarterioles?

an anastomosis (shunt) from arteriole to venule, supplying the local tissue capillary bed

What is a precapillary sphincter?

a smooth muscle ring/cuff that regulates capillary blood flow from metarteriole; regulated by ANS and local tissue metabolite build up

What are continuous capillaries?

least permeable, tight junctions between adjacent endothelial cells, intercellular clefts allow passage of small molecules (like H2O, O2, and CO2)

Where are continuous capillaries located?

lungs, skeletal muscle and connective tissues

What are fenestrated capillaries?

pores (fenestrations; “little window”) passing through endothelial calls allow for increased permeability and high rate of exchange of small molecules and fluid

Where are the fenestrated capillaries located?

kidneys, endocrine glands, small intestine

What are sinusoidal capillaries?

bigger fenestrations, few tight junctions, wide intercellular clefts allow for exchange of large molecules (or even whole cells)

Where are sinusoidal capillaries located?

liver, spleen, bone marrow and pituitary gland

What is diffusion?

movement of molecules or ions from high to low concentration until equilibrium is reached (PASSIVE PROCESS)

What is bulk flow?

movement of a fluid from a region of higher pressure to one of lower pressure until equilibrium (PASSIVE PROCESS)

What is transcytosis?

substances enter pinocyotic vesicles, move through endothelial cells via endocytosis then exocytosis (ACTIVE PROCESS)

What forces favor movement of fluid out of the capillary space into the ISF?

blood hydrostatic pressure (BHP) and interstitial fluid osmotic pressure (IFOP)

How does blood hydrostatic pressure (BHP) work in the capillaries?

pushes fluid out through the capillary pores

What is interstitial fluid osmotic pressure (IFOP)?

‘pulls’ fluid out of the capillary via osmosis; compared to the BHP (this pressure is very small; BHP>>IFOP)

What are the bulk inward forces into the capillary?

blood colloid osmotic pressure (BCOP) and interstitial fluid hydrostatic pressure (IFHP)

What is blood colloid osmotic pressure (BCOP)?

differences in protein concentration between plasma and ISF tends to pull water from the ISF into the capillary

What is interstitial fluid hydrostatic pressure (IFHP)?

pressure exerted by interstitial fluid, but typically is very small (BCOP>>IFHP)

What does IFOP stand for?

interstitial fluid osmotic pressure

What does BHP stand for?

blood hydrostatic pressure

What does IFHP stand for?

interstitial fluid hydrostatic pressure

What does BCOP stand for?

blood colloid osmotic pressure

What does NFP stand for?

net filtration pressure

Albumin (does/ does not) move freely through normal capillary pores.

does not

Water and smaller molecules (do/ do not) move freely through normal capillary pores

do

What 2 things help maintain BCOP by attracting water into the intravascular (capillary) space?

Na+ and albumin

What is the net filtration pressure?

the net movement of fluid driven by the difference between the inward and outward pressures

How can you find the net filtration pressure?

(BHP + IFOP) - (BCOP+IFHP) ; outward filtration - inward reabsorption

What are the MAJOR forces that determine fluid movement across the capillary wall?

BHP and BCOP

What does filtration in the capillaries result in?

a constant flow of fluid that washes over the tissue cells at the arterial end of the capillary, carrying nutrients and oxygen with it

What does reabsorption in the capillaries result in?

a return of fluid to the capillary at the venous end, thereby depositing wastes into the venous system

What is the arterial end net filtration pressure in a capillary?

\+10 mmHg

What is the venous end net filtration pressure in a capillary?

\-7 mmHg

Why does fluid exit a capillary?

since capillary hydrostatic pressure is greater than blood colloidal osmotic pressure

What is the capillary hydrostatic pressure at the arterial end of a capillary?

35 mmHg

What is the blood colloidal osmotic pressure at the arterial end of a capillary?

25 mmHg

What is the capillary hydrostatic pressure at the venous end of a capillary?

18 mmHg

What is the blood colloidal osmotic pressure at the venous end of a capillary?

25 mmHg

What vessels absorb the excess fluid that exits the capillaries and returns it to the circulatory system?

lymphatic vessels