Capillaries II

Question 1

What is the importance of fluid exchange?

Answer 1

• Fluid exchange is important for normal physiological function, we need H2O for chemical reactions.

Question 2

Give a simple reason as to why we have fluid re-absorption?

Answer 2

• Fluid re-absorption from tissues to blood can maintain circulation during haemorrhage.

Question 3

If fluid filtration is damage what are the consequences?

Answer 3

• Abnormalities in fluid exchange can lead to oedema/tissue swelling. This is in the interstitial (between the basement membrane) space.

Question 4

Describe the structures that surround the capillary in a body cell

Answer 4

On image

Question 5

How does fluid move out of the capillaries and where does it move to?

Answer 5

• Capillary wall is a semi-permeable membrane.

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

Question 6

What do large molecules create?

Answer 6

• Large molecules (eg. plasma proteins) cannot pass through membrane so they exert an osmotic pressure termed oncotic pressure which creates suction force to move fluid into capillary.

Question 7

What does fluid movement across capillaries depend on?

Answer 7

• Fluid movement across capillary walls depends on the balance between hydraulic and oncotic pressures across the capillary wall.

Question 8

Define oncotic and hydraulic/ hydrostatic pressure and why is one greater than the other?

Answer 8

Oncotic = reabsorption
Hydrostatic or Hydraulic = filtration
Hydrostatic pressure > oncotic pressure gradient – the essence of starlings law of capillary exchange

Question 9

What four pressures determine filtration rate?

Answer 9

On image

Question 10

Describe Starlings law for fluid exchange

Answer 10

On image

Question 11

Starlings forces normally favour filtration

What factors promote filtration?

What factors promote reabsorption?

Answer 11

Capillary blood pressure and interstitial proteins

Plasma proteins

Question 12

What develops if filtration is found in excess?

Answer 12

Oedema

Question 13

How is excess fluid returned to the circulation?

Answer 13

• Excess fluid is returned to the circulation via lymphatic system.

Question 14

Describe the pressure from the arteriole end to the venous end

Answer 14

• This diagram shows at the arteriole end there is a pressure of around 35mmHg. Continuing along the capillary pressure is lost. We have a declining gradient of hydrostatic pressure.

Question 15

Does the osmotic pressure change?

Answer 15

• The osmotic pressure does not change and would give a horizontal line

Question 16

Describe the structure of lymph vessels?

Answer 16

• Lymph vessels have valves and smooth muscle

Question 17

What is the purpose of contractions of the lymph vessels

Answer 17

• Spontaneous contractions of the smooth muscle contribute to lymph flow.
• Surrounding skeletal muscle contractions/relaxation also contributes to lymph flow.

Question 18

Overall control of extracellular fluid balance depends on:

Answer 18

• Capillary filtration
• Capillary reabsorption
• Lymphatic system

Question 19

Starling’s factors determine changes in fluid balance:

Answer 19

• Circulation
• Interstitial fluid
• Lymphatic system

Question 20

What is low capillary pressure called?

Answer 20

Hypovolemia

Question 21

Describe the premise of hypovolemia (6)

Answer 21

• This is what happens when we lose blood. You lose a large amount of blood.
• The blood volume decreases and venous return decreases.
• There is less preload due to less blood returning the heart. So cardiac output drops.
• Therefore, the pressure at the arteriole end is going to be much lower to start with and dropping to a much lower pressure.
• In terms of the osmotic pressure, there is no change as the concentration of cells in the blood remains the same.
• Therefore, the net balance of forces is now in favour of reabsorption. This is good as we need to maintain blood volume, so we withdraw fluid from tissue, to compensate for blood loss.

Question 22

What is oedema?

Answer 22

• Excess of fluid within the interstitial space.

* Imbalance between filtration, reabsorption, lymph function.

Question 23

What are the causes of oedema?

Answer 23

• Increased capillary pressure (Pc)
• Decreased plasma protein oncotic pressure (πP)
• Inflammatory response
• Lymphatic problems

Question 24

Give some scenarios of increased capillary pressure

Answer 24

• Dependent (gravitational) oedema – standing up for long periods
• Deep venous thrombosis
• Cardiac failure
• eg. Deep vein thrombosis (DVT) prevention of venous return
• Increases venous pressure causes ‘back-up’ of pressure leading to…
• Increased PC across capillaries and increased filtration

Question 25

Describe a decrease in plasma protein oncotic pressure

Answer 25

Low protein oedema…

Malnutrition: Low protein intake to produce plasma proteins

Nephrotic syndrome: urinary protein loss - replaced by liver function

Liver disease: not enough endogenous albumin produced

Reduced plasma protein concentration = reduced plasma oncotic pressure - the greater influence of Pc and Pie I = fluid efflux from capillaries into interstitial fluid = oedema.

Question 26

Describe inflammatory mediated oedema

Answer 26

Swelling is triggered by a rise in local chemical mediators of inflammation

The large increase in capillary permeability

Rate of bulk movement increases, protein permeability increases, decrease in reflection coefficient

e.g chemicals, insect bite, nettles, infection, autoimmune disease.