Capillaries II - Fluid exchange

Question 1

Briefly describe the movement of fluid from the capillaries through the extravascular circulation

Answer 1

Capillary blood pressure causes fluid to filter across capillary wall, through the glycocalyx, to the interstitial fluid. From here fluid drains into lymphatic system which returns it back to the bloodstream.

Question 2

Why is fluid exchange important?

Answer 2

Important for normal physiological function - we need water for chemical reactions.

It happens on such a large scale – Entire plasma volume completes extracellular circulation in under one day

Needed to maintain blood volume and therefore blood pressure as well as interstitial fluid volume

Question 3

Why does fluid move from the capillary into the interstitial fluid?

Answer 3

Because capillary blood flow exerts a hydrostatic pressure forcing plasma to move into interstitial fluid

Question 4

Why does the plasma within the capillaries exert a hydrostatic pressure?

Answer 4

Because the flow of the plasma is incompressible - This means that the effects of pressure on the fluid’s density during flow is negligible. Fluid has constant density while flowing.

Question 5

Why does fluid from the interstitial fluid move back into the capillary?

Answer 5

Large molecules (e.g. plasma proteins, red blood cells and platelets) cannot pass through membrane so they exert an osmotic pressure termed oncotic pressure which creates suction force to move fluid into capillary.

Question 6

What is fluid movement across a capillary dependent on?

Answer 6

Balance between hydrostatic pressure, force for filtration, and oncotic pressure, force for re-absorption.

Question 7

What are the 4 starling pressures that affect fluid movement across a capillary?

Answer 7

Pc = Capillary blood pressure (Type of Hydrostatic pressure)
Pi = Interstitial fluid pressure (Type of Hydrostatic pressure)
π p - plasma colloid osmotic pressure (COP) (Type of Osmotic pressure)
π i - interstitial fluid COP (Type of Osmotic pressure)

Question 8

What are all 4 of these pressures relative to?

Answer 8

These pressures are measured relative to atmospheric pressure (760mmHg) so Pc = -10mmHg means a capillary blood pressure of 750mmHg

Question 9

What is the equation for starling’s principle of fluid exchange and what do each of the variables represent?

Answer 9

Jv = ∝ [( Pc - Pi ) - σ (π p - π i)]

Where: 
Jv  = Volume filtered per unit time 
Pc - Pi = Hydraulic pressure difference 
π p - π i = Osmotic pressure difference 
σ = Reflection coefficient

Question 10

What does this equation become when you account for the proportionality factor of the equation?

Answer 10

Jv = Lp A [( Pc - Pi ) - σ (π p - π i)]

A = Capillary surface area
hydraulic conductance of the endothelium, Lp = Hydraulic conductance of the endothelium

Question 11

What is the reflection coefficient (σ)?

Answer 11

It’s defined as the ratio of the effective osmotic pressure across an imperfect semi-permeable membrane to the full osmotic pressure across a perfect semi-permeable membrane.

Question 12

What is the equation for the reflection coefficient?

Answer 12

Equation for reflection coefficient: σ = ∆effective/ ∆ideal

Question 13

What is the reflection coefficient value for the capillary membrane?

Answer 13

σ for plasma protein is 0.9 e.g. 10% plasma proteins are conducted across capillary wall into interstitial space

Question 14

What is the reflection coefficient value for the capillary membrane?

Answer 14

σ for plasma protein is 0.9 e.g. 10% plasma proteins are conducted across capillary wall into interstitial space

Question 15

Under normal conditions does the balance of starling pressures favour filtration or reabsorption? Why is this the case?

Answer 15

Under normal conditions the balance of starling’s pressure favours filtration because the pressures that favour filtration, Pc and π i, outweigh the factors that favour reabsorption, π p.

Question 16

What happens to the excess filtered fluid in the interstitial fluid as a result of normal conditions favouring filtration in the capillary?

Answer 16

Excess filtered fluid is returned to the circulatory system via the lymphatic system.

Question 17

What does the lymphatic system do with the solutes/tissue fluid it collects from the interstitial fluid?

Answer 17

Returns excess tissue fluid/solutes back to the cardiovascular system

Question 18

Where do the lymphatic vessels drain back into the circulatory system?

Answer 18

In the neck veins - Right and left subclavian

Question 19

What mechanisms contribute to the flow of lymph through the lymphatic vessels?

Answer 19

Spontaneous contraction of the smooth muscle contributes to lymph flow.

Surrounding skeletal muscle contractions & relaxation also contributes to lymph flow.

Question 20

What are the 3 main functions of the lymphatic system?

Answer 20

Preservation of fluid balance – Maintains extracellular fluid balance by draining excess interstitial fluid back into circulatory system

Nutritional function – Intestinal lymph vessels, Lacteals, take up and transport chylomicrons to the venous circulation.

Defence function – Lymph nodes phagocytose foreign particulates. Bacterial and viral antigens activate nodal lymphocytes which are then released into the efferent lymph.

Question 21

What are the initial lymphatic vessels called?

Answer 21

lymphatic capillaries

Question 22

How do the lymphatic capillaries form a network around surrounding tissues?

Answer 22

They are anchored to the surrounding tissues by the anchoring filaments which are made from protein fibrillin.

Question 23

What is the name of the vessels that the lymphatic capillaries feed into?

Answer 23

Collecting vessels

Question 24

What vessels do the collecting vessels then feed into?c

Answer 24

Afferent lymphatic vessels

Question 25

What structure do the collecting vessels and the afferent lymphatic vessels contain and what does this structure do?

Answer 25

Both contain luminal semilunar valves which direct the lymph centrally within the vessel.

Question 26

What structure do the afferent lymphatic vessels feed into?

Answer 26

Lymph nodes

Question 27

What is a lymph node?

Answer 27

A mass of lymphocytes and phagocytic cells surrounded by sinuses in which the lymph flows through

Question 28

What is a lymph sinus?

Answer 28

Endothelial tubes with gaps in them to allow lymphocytes to join the lymph.

Question 29

Once the lymph flows through the lymph nodes what vessels does it end up in?

Answer 29

Efferent lymphatic vessels

Question 30

At what point does the lymphatic vessels drain back into the circulatory system?

Answer 30

They feed into the lymphatic trunks which feed into the lymphatic ducts which in turn feed into the left and right subclavian veins.

Question 31

What is hypovolemia?

Answer 31

A decrease in volume of blood plasma

Question 32

What can cause hypovolemia to be induced?

Answer 32

Haemorrhage

Question 33

What are the effects of a loss of blood volume?

Answer 33

Reduces venous return and reduces preload and therefore cardiac output – Starling’s law

Question 34

What effect does a reduced cardiac output have on arterial blood pressure?

Answer 34

Reduced arterial blood pressure as BP = CO X TPR

Question 35

What effect does reduced arterial blood pressure have on capillary blood pressure?

Answer 35

Causes a drop in the capillary blood pressure (Pc) beginning at arterial end of capillary

Question 36

What effect does the loss of blood volume have on plasma osmotic pressure? Explain why it has this effect

Answer 36

No plasma osmotic pressure (COP) stays the same because despite the loss of plasma protein within blood the concentration of plasma proteins stays the same.

Question 37

What does the decrease in capillary blood pressure mean for the 4 starling forces?

Answer 37

Means the starling forces no longer favour filtration but favour reabsorption.

Question 38

What are the effects of the starling forces favouring reabsorption over filtration?

Answer 38

Increases blood volume
Increases Central venous pressure – due to increased venous return
Increases Cardiac Output
Rises Blood pressure

Overall this helps to combat the effects caused by being in a state of hypovolemia

Question 39

What is another way that the effects of blood volume loss can be combated?

Answer 39

Sympathetic nerve-induced vasoconstriction of pre-capillary arterioles.

Question 40

How does a high capillary blood pressure lead to an Oedema?

Answer 40

High capillary BP means the starling forces favour filtration thus resulting in an increase in capillary filtration rate relative to reabsorption.

Question 41

What is an Oedema?

Answer 41

Excess of fluid within the interstitial space due to an Imbalance between filtration and reabsorption.

Question 42

Give examples of things that can cause an Oedema to form?

Answer 42

Increased capillary pressure (Pc)

Decreased plasma protein oncotic pressure (πP)

Inflammatory response – Breakdown of the capillary barrier due to increased size of fenestrations and glycocalyx disruption causes Hydraulic conductance (Lp) and protein permeability (Pprotein) to increase

Lymphatic problems

Question 43

What must the plasma protein concentration be lower than for a low protein oedema to form?

Answer 43

30 g/L

Question 44

What are some ways in which a low protein oedema can form?

Answer 44

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 45

How does inflammation lead to the development of an oedema?

Answer 45

Inflammation leads to Increased Lp, Increased protein permeability (↑ πi), and decreased protein reflection coefficient ↓σ
All this increases filtration leading to development of an oedema