Blood vessels #4 - Capillaries

what is bulk flow

determines relative fluid volumes in the bloodstream and interstitial space (fluid compartments)

• this causes continuous mixing of fluid between plasma and interstitial fluid compartments and maintains interstitial environment

• arteriole to venule

filtration

movement of fluid and solutes out of the blood

• from plasma to interstitial fluid (inside cell)

• (across systemic capillaries)

absorption

movement of fluids and solutes into the blood

• from interstitial fluid to plasma (across systemic capillaries)

explain velocity of blood flow in the capillaries related to other vessels

• velocity in capillaries is slower for filtration and absorption

• the sam amount of blood must flow through the circulatory system per minute:

the arterioles move at a faster velocity because they have a larger diameter. Even though the capillaries are smaller, they make up a larger cross-sectional area. So, they carry the same amount of blood but in more vessels.

• volume of blood in per min mandates the same amount of volume out per minute

what happens to velocity as cross-sectional area goes up

as cross sectional area goes up, velocity goes down

• this is why velocity is so high in the aorta and arteries but goes down at the arterioles, capillaries, and venules (also because of diameter)

how are precapillary sphincters regulated

• extrinsic factors: sympathetic stimulation

• local factors: low O2, high CO2, K+, H+, NO

what is a thoroughfare channel

the channel in a capillary bed that goes straight from arteriole to postcapillary venule

• acts as a shunt

• this is used when not a lot of gas exchange is needed (high O2 and low CO2)

what is capillary filtration

removal of solutes as a solution flows across a porous membrane

4 types of transport for capillary filtration

1. diffusion through membrane

• lipid soluble substances (non-polar) ex: O2

2. movement through intercellular clefts

• water soluble substances (polar) ex: sugars, salts

3. movement through fenestrations

• water soluble substances (polar)

4. transport via vesicles or caveolae

• large substances via vascular transport ex; proteins, hormones, neurotransmitters

Hydrostatic Pressure (HP)

Pushes due to blood pressure

Osmotic Pressure (OP)

Sucks due to proteins like albumin

explain HPc and OPc at the arteriolar end

HPc is higher at the arteriolar end than the venule end because the blood pressure is higher due to the arteries being closer to the heart than the venules

• HPc = 35mmHg

• HPo = 26mmHg

this is an area of filtration where the bulk flow is out

*OP in the capillaries is alway in and never changes because there shouldn’t be much protein in the interstitial fluid.

• H2O is attracted to plasma proteins like albumin which are in the capillary, so flow is in (pulls/sucks)

explain HPif and OP if at the arteriolar end

HPif = 0 mmHg

• you don’t want fluid outside the vessel, that would lead to edema

• most of the excess fluid is removed by the lymphatic system

OPif = 1 mmHg

• very low protein content in interstitial fluid so the pull is not strong

*stays the same at venule end

explain HPc and OPc at the venule end

HPc is lower at venule end because the BP is lower at the venules so less is pushed out

• HPc = 17mmHh

• OPc = 26mm

this is an area of resorption because the osmotic pressure is higher that the hydrostatic pressure

*OP in the capillaries is alway in and never changes because there shouldn’t be much protein in the interstitial fluid.

Net Filtration Pressure (NFP)

NFP = (HPc + OPif) - (HPif + OPc)

arteriolar end: (35+1) - (0+26) = +10 mmHg (filtration/out flow)

venule end: (17+1) - (0+26) = -8 mmHg (reabsorption/in flow)

explain how blood moves during a NFP of zero

blood is still moving in and out, it’s just that they are even so there’s no net movement

explain what happens to residual fluid

ex: 24L/day out of capillaries and 20.4 back into capillaries

• residual goes to the lymphatic system for filtration and then is deposited back into blood