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Blood Vessels Lecture: Capillary Exchange and Hydrostatic and Osmotic / Oncotic Pressure

from 5-22-12


How capillaries exchange materials between blood inside the capillary and the fluid just outside the capillary well, outside the cell

(( Blood Flow through capillaries and Capillary Dynamics

Fluid Movements: Bulk Flow

Hydrostatic Pressure

Colloid Osmotic Pressure

Hydrostatic-Osmotic Pressure Interactions

Blood flow through the capillaries and “capillary dynamics” – ch. 19, p. 738 of the 7th edition.))

Every living cell in the body is two (2) cell diameters of a capillary

  1. how nutrients get out to the cells
  2. how wastes can be picked up

19.15 P. 709 (8th edition) – Summary of control of arteriolar smooth muscle in the systemic circulation. (figure 19.14 in 7th edition; page 736). 

DON’T memorize this diagram.

Shows “extrinsic” mechanisms that control smooth muscle activity.

a) If smooth muscle contracts, the diameter increases and resistance goes up

b) If it relaxes, diameter increases and resistance goes down.

In the diagram, you can see that

a)    nerves can control whether the vessel or contracts and

b)   hormones can control whether vessel dilates or contracts

green constrict


three  hormones that make it constrict

one that makes it relax

sympathetic nerves – make it constrict

nerves built into vessel itself

When cells are actively metabolizing; they are creating metabolic by-products; most of them make the vessel

GREEN – dilate/ relax.

A few make it contract/ restrict.

The cells of the vessel themselves, can create by-products, that can make the cell contract or relax.

And neuro endocrine can make it constrict or dilate.

Just know that there are hormones that make it constrict or dilate and nerves can too.

a)    constricts – resistance goes up

b)   dilate – resistance goes down

Fenestrated capillaries:

a)    openings

Focus on, of the four methods, # 2 and # 3. (1 and 4 have less volume being moved).

# 2 — Fenestra

# 3 – Intercellular Clefts

Most of the materials moved through the cell, waste through the cells; move through the fenestra and intercellular clefts.

Page 719.  Figure 19.17: Fluid Flow at capillaries. (8th edition); In 7th edition: Figure 19.16, Fluid flows at capillaries; page 740.

Capillary Exchange is a result of a driving force (it is not passive)

It is an Active process, that is dependent on a driving force.

1)   something drives them out

2)   something drives them back in

One arteriole is shown in this diagram and so is one  Capillary, in this diagram, is just one (not a whole capillary bed). This is done for simplicity of explanation.

-across capillary

-exchange occurs

-arterial – into the exchange vessel

-back in the middle

+supply and drainage.


Hydrostatic and Oncotic (osmotic) Pressure

A balance between two specific pressures

1)   Hydrostatic Pressure – “blood pressure”

  1. The important version of blood pressure in the body is MAP (mean arterial pressure).

2)   Oncotic Pressure

  1. Is oncotic is a sub-set of osmotic pressure?

Hydrostatic – force fluid into a closed container; and it goes up.

If it goes high enough the container will burst. “Static”, just putting fluid into a closed container.


Blood Pressure – the pressure the fluid puts on the walls.


Osmotic pressure: is pressure due to solutes in the fluid. Solutes flow down their concentration gradient.


For water,the concentration flows UP the concentration gradient from low to high.


ONCOTIC pressure is the component of the pressure due just to dissolved components, PROTEINS (not carbohydrates, amino acids, minerals).


Why ignore the proteins – Look at the fenestra, which are small openings. A.A.s and Carbohydrates could go through any one of those openings. The amount of those nutrients inside or outside could go out through any one of those openings.


Proteins are too big to fit through one of these openings.


**caution- temporary info**


IN THEORY, no proteins can escape from the blood. Only carbohydrates, and amino acids and minerals……


PROTEINS are too big to move about and therefore….


They are trapped in the capillaries. Inside the blood there will be lots of proteins, because they can’t move about.  They can’t get out.

High proteins in the plasma Low concentration of proteins in the interstitial fluids. If proteins could flow, they would go out—through the interstitial fluids. # of proteins in the fluid outside will be different than the # of proteins inside. Inside the blood, lots of proteins, bc they can’t get out. Outside, there will be few or none.

High Conc. Of proteins in the plasma.

Low Conc. Out in the interstitial fluid.

Higher gradient of proteins.

If proteins could flow out , they would go out into the interstitial fluid. 

Water is going to move out into the interstitial fluid.

If the mountain won’t go to mohammed, then mohammed must go to the mountain.

The arteriolar end and

Venular end.

Hydrostatic pressure – P of the water pushing against walls – opening; such as fenestra or interstitial cleft; water will squirt out through the opening b/c of interstitial pressure

e.g. Balloon; water filled; poke it; — a little geyser forms; All of the fenestra and intercellular clefts; pinholes; geysers shooting out; caused by hydrostatic pressure.

Arterial end– Net hydrostatic pressure is 35 Torrs. 35 mmHg)– pushing WATER and any solute small enough to go through the openings.

–small solutes out

–water out

+++(making “geysers” out; water and any solute small eough to go out

n  nutrients are carried out to the cell

There are proteins trapped in there, that are trying to pull water back in , back through the same openings.

Pushing – hydrostatic

Pulling – osmotic

So there is a tension between a pushing force, that is. If Hydrostatic is strongest pressure – pushing out – then the net movement will be out (to the capillary beds). Pulling force – if Osmotic Pressure is strongest – net movement will be in (from the capillary beds into the arterial supply and venous return). NET movement will be determined by the strongest pressure.

Pushing h20 and any solute….

Q: At the Arterial  end, (supply-end)which force is strongest?

A: Outward is strongest, and therefore

Q: At the arterial end, which direction will [fluid and nutrients] will be carried out?

A: fluid and nutrients will be carried OUT at the arterial end of blood supply

Q: At the venous end, which pressure or force is greatest?

A: Osmotic.

Q: At the venous end, the net movement will be

A:  in. At the venous end of blood circulation, the net movement of fluid and nutients will be IN.

 closing words

At the arterial end, which is the supply end…fluid and nutrients move out. Those nutrients go to the target cells. The target cells meanwhile, have metabolized nutrients, from the cycle before, creating wastes, which they deliver out, into the interstitial fluid, and  Interstitial fluid at the venous end, it flows in with all the wastes.

So, fluid and nutrients out—at the arterial end.  Fluid and wastes back in, at the venous end. Whether fluid moves in or out, depends on the value of those forces. Wherever hydrostatic force is greatest, movement will be out.Wherever oncotic force is greatest, movement will be in.





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