Capillaries II

Question 1

what is fluid exchange important for?

Answer 1

normal physiological function

-we need water for chemical reactions

Question 2

what is the importance of fluid re-absorption from tissues to blood?

Answer 2

can be used to maintain circulation during haemorrhage

Question 3

what can abnormalities in fluid exchange lead to?

Answer 3

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

Question 4

where does fluid across the membrane to, and why does it move?

Answer 4

Fluid moves across the membrane into interstitial space due to blood flow which exerts a hydraulic pressure

Question 5

which molecules cannot pass through the membrane?

Answer 5

Large molecules (e.g. plasma proteins)

Question 6

what force do these large molecules exert and what is the importance of this?

Answer 6

an osmotic pressure termed oncotic pressure

o The oncotic pressure creates a suction force to move fluid into the capillary

Question 7

what does fluid movement across capillary walls depend on?

Answer 7

depends on the balance between hydraulic and oncotic pressures across the capillary wall.

Question 8

what are the 4 pressures that determine filtration rate and which direction do they act in?

Answer 8

Hydrostatic:
capillary BP (out of capillary)
interstitial fluid pressure (into capillary)

Osmotic:
plasma proteins (into capillary)
interstitial proteins (out of capillary)

Question 9

are there fewer interstitial proteins or plasma proteins?

Answer 9

interstitial proteins

Question 10

Starling’s Principle of Fluid Exchange

look at equation in notes

Answer 10

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

Question 11

starling’s principle of fluid exchange equation

Answer 11

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

Question 12

What happens to the excess filtered fluid?

Answer 12

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)

Question 13

where do well perfused capillaries filter?

Answer 13

filter along their entire length

Question 14

how does the pressure differ as you go from the arterial to venous end?

Answer 14

• 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

Question 15

what happens to the fluid that leaves the capillary and isn’t absorbed?

Answer 15

becomes part of the interstitial fluid and later is returned to the blood plasma via the lymphatic system

Question 16

what does the lymphatic circulation do?

Answer 16

returns excess tissue fluid/solutes back to the CVS

-drains into the vena cava where all the lymphatic vessels converge

Question 17

what do lymph vessels have?

Answer 17

valves and smooth muscle

Question 18

how is smooth muscle involved in lymph flow?

Answer 18

spontaneous contractions of the smooth muscle contributes to lymph flow
-surrounding skeletal muscle contractions and relaxation also contributes to lymph flow

Question 19

what does lymph also contain?

Answer 19

immune cells, especially at the lymph nodes

Question 20

the overall control of extracellular fluid balance depends on:

Answer 20

• Capillary filtration
• Capillary reabsorption
• Lymphatic system

Question 21

starling’s factors determine changes in fluid balance in:

Answer 21

• Circulation
• Interstitial fluid
• Lymphatic system

Question 22

Organisation of The Lymphatic System

Answer 22

look at picture in notes

Question 23

what happens when you go from the arterial to the venous end?

Answer 23

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

Question 24

why is there a reduction in (downstream) capillary pressure?

Answer 24

Sympathetic nerve-induced vasoconstriction of pre-capillary arterioles

Drop in CO leads to drop in BP
(BP = CO x TPR)
so Pc is reduced

Question 25

how does an increased Pc come about and what does it cause?

Answer 25

• 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

Question 26

what is oedema?

Answer 26

• Excess of fluid within interstitial space

- Imbalance between filtration, reabsorption, lymph function

Question 27

causes of oedema?

Answer 27

• 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

Question 28

give an example of when you get increased capillary pressure:

Answer 28

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

Question 29

low protein oedema can be caused by 3 things:

Answer 29

• 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

Question 30

what is Inflammatory-Mediated Oedema

Answer 30

• Swelling is triggered by local chemical mediators of inflammation
• Large increase in capillary permeability

Question 31

what is inflammation caused by?

Answer 31

• Increased LP (hydraulic conductance)
• Increased protein permeability (reducing the plasma osmotic pressure)
• Chemicals/Insect Bite/Nettle Sting
• Infection/Physical Trauma/Autoimmune disease

Question 32

give an example of an organism that causes lymphatic obstruction:

Answer 32

• Filariasis/Elephantitis
  o Nematode infestation
  o Larvae migrate to lymphatic system
  o They can grow/mate/form nests and thus, block lymph drainage

Question 33

what is the term for lymphatic removal and what occurs?

Answer 33

• Lymphoedema
  o Causes by surgery to treat testicular cancer
  o Removal of lymphatics

Question 34

what are the two main functions of capillaries and their corresponding mechanisms:

Answer 34

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.