Capillaries II Flashcards
what is fluid exchange important for?
normal physiological function
-we need water for chemical reactions
what is the importance of fluid re-absorption from tissues to blood?
can be used to maintain circulation during haemorrhage
what can abnormalities in fluid exchange lead to?
oedema/tissue swelling
eg. patients with lymphoedema due to the removal of lymph nodes or damage to the lymph nodes, due to radiotherapy for testicular cancer
where does fluid across the membrane to, and why does it move?
Fluid moves across the membrane into interstitial space due to blood flow which exerts a hydraulic pressure
which molecules cannot pass through the membrane?
Large molecules (e.g. plasma proteins)
what force do these large molecules exert and what is the importance of this?
an osmotic pressure termed oncotic pressure
o The oncotic pressure creates a suction force to move fluid into the capillary
what does fluid movement across capillary walls depend on?
depends on the balance between hydraulic and oncotic pressures across the capillary wall.
what are the 4 pressures that determine filtration rate and which direction do they act in?
Hydrostatic: capillary BP (out of capillary) interstitial fluid pressure (into capillary)
Osmotic: plasma proteins (into capillary) interstitial proteins (out of capillary)
are there fewer interstitial proteins or plasma proteins?
interstitial proteins
Starling’s Principle of Fluid Exchange
look at equation in notes
movement of fluid is the hydraulic pressure difference minus the osmotic pressure difference
Net balance of Starling’s forces in most capillaries is favours filtration
starling’s principle of fluid exchange equation
Jv = Lp x A x (the hydraulic pressure difference minus the osmotic pressure difference)
- In normal conditions, Lp and A are constant. Lp is how leaky the endothelium is to fluid, A is the wall area
- You can change the area (A) by changing the recruitment of capillaries
- You can change Lp by making the capillaries more or less leaky
What happens to the excess filtered fluid?
It is returned to the circulation via the lymphatic system- specialised vessels that collect tissue fluid/interstitial fluid and drain it back into circulating volume (plasma)
where do well perfused capillaries filter?
filter along their entire length
how does the pressure differ as you go from the arterial to venous end?
- the pressure drops as you go from arterial to venous end, so the filtration is less by the time you get to the venous end
- there is a declining gradient of hydrostatic pressure as you move from the arterial to venous end
what happens to the fluid that leaves the capillary and isn’t absorbed?
becomes part of the interstitial fluid and later is returned to the blood plasma via the lymphatic system
what does the lymphatic circulation do?
returns excess tissue fluid/solutes back to the CVS
-drains into the vena cava where all the lymphatic vessels converge
what do lymph vessels have?
valves and smooth muscle
how is smooth muscle involved in lymph flow?
spontaneous contractions of the smooth muscle contributes to lymph flow
-surrounding skeletal muscle contractions and relaxation also contributes to lymph flow
what does lymph also contain?
immune cells, especially at the lymph nodes
the overall control of extracellular fluid balance depends on:
- Capillary filtration
- Capillary reabsorption
- Lymphatic system
starling’s factors determine changes in fluid balance in:
- Circulation
- Interstitial fluid
- Lymphatic system
Organisation of The Lymphatic System
look at picture in notes
what happens when you go from the arterial to the venous end?
A bit of filtration to begin with but then reabsorption
The osmotic pressure is now able to overcome the hydraulic pressure
Useful self correcting mechanism during low blood pressure more fluid absorbed back into the circulation increasing blood volume
why is there a reduction in (downstream) capillary pressure?
Sympathetic nerve-induced vasoconstriction of pre-capillary arterioles
Drop in CO leads to drop in BP
(BP = CO x TPR)
so Pc is reduced
how does an increased Pc come about and what does it cause?
- causes oedema (as in DVT)
- get blood backing up at the venous end of the system, venous return to the heart is opposed, and pooling of blood at the venous end increases pressure here
- affects the hydrostatic pressure gradient
what is oedema?
- Excess of fluid within interstitial space
- Imbalance between filtration, reabsorption, lymph function
causes of oedema?
- Increased capillary pressure (Pc) (eg. increase in pressure at the venous end raising the pressure slightly)
- Decreased plasma protein oncotic pressure due to reduced plasma protein concentration (πP) (eg. don’t have enough albumin, which decreases the force with which fluid is sucked back in, leading to excess filtration)
- Inflammatory response
- Lymphatic problems
give an example of when you get increased capillary pressure:
Deep Vein Thrombosis (DVT) – prevention of venous return
- Increases venous pressure causes “back-up” of pressure leading to:
o Increased Pc across capillaries and increased filtration
low protein oedema can be caused by 3 things:
- Malnutrition/malabsorption: not enough protein intake to make plasma proteins
- Nephrotic syndrome: urinary protein loss, replaced by liver production
- Liver disease: not enough endogenous albumin produced
what is Inflammatory-Mediated Oedema
- Swelling is triggered by local chemical mediators of inflammation
- Large increase in capillary permeability
what is inflammation caused by?
- Increased LP (hydraulic conductance)
- Increased protein permeability (reducing the plasma osmotic pressure)
- Chemicals/Insect Bite/Nettle Sting
- Infection/Physical Trauma/Autoimmune disease
give an example of an organism that causes lymphatic obstruction:
- Filariasis/Elephantitis
o Nematode infestation
o Larvae migrate to lymphatic system
o They can grow/mate/form nests and thus, block lymph drainage
what is the term for lymphatic removal and what occurs?
- Lymphoedema
o Causes by surgery to treat testicular cancer
o Removal of lymphatics
what are the two main functions of capillaries and their corresponding mechanisms:
1) Solute exchange
Nutrition of tissue, hormone & drug delivery.
Diffusion – individual molecules.
Concentration gradients across wall
obeys Fick’s law of diffusion.
2) Fluid exchange
Regulation of plasma and Interstitial fluid volumes
Bulk flow – all molecules together
Pressure gradients across wall
obeys Starling’s principle.
