9) Capillaries 2

Question 1

What is filtration and reabsorption?

Answer 1

• Filtration: Movement of fluid out of the capillaries to interstitial fluid
• Reabsorption: Movement of fluid back into the capillaries from interstitial fluid

Question 2

Why is fluid exchange important?

Answer 2

• It is important for normal physiological functions as we need water for chemical reactions

Question 3

How do we maintain circulation during haemorrhage?

Answer 3

• Fluid is re-absorbed from the tissue into the blood

Question 4

What can abnormalities in fluid exchange lead to?

Answer 4

• Oedema or tissue swelling

Question 5

How is fluid moved into interstitial space?

Answer 5

• Blood flowing through the capillaries exerts a hydrostatic pressure/ blood pressure called hydraulic pressure
• The hydrostatic pressure of the blood is greater than the hydrostatic pressure of the interstitial fluid
• This creates a pressure gradient which moves fluid down the pressure gradient.
• This pressure moves fluid from the blood (high pressure) to the interstitial fluid (low pressure)

Question 6

How is fluid moved back into the capillary?

Answer 6

• Larger molecules (e.g. plasma proteins) are unable to pass through the membrane
• These molecules exert an osmotic pressure called oncotic pressure
• This creates suction forces which move fluid into the capillary

Question 7

What does Starling’s forces/ fluid movement across the capillaries depend on?

Answer 7

• The balance between hydraulic and oncotic pressure across the capillary wall

Question 8

In which direction is there a bulk flow of fluid at capillaries?

Answer 8

• There is a bulk flow of fluid out of the capillaries
• This is because oncotic and hydraulic pressures are normally not balanced
• Hydraulic pressure is normally higher causing more fluid to leave the capillaries

Question 9

What are the four pressures that determine filtration rate?

Answer 9

• Osmotic pressure of plasma proteins (favours filtration) and is lower
• Osmotic pressure of interstitial proteins (favours reabsorption) and is higher
• Hydrostatic pressure of capillary blood (favours reabsorption) and is higher
• Hydrostatic pressure of interstitial fluid (favours filtration) and is lower

Question 10

What is the equation for Starling’s principle of fluid exchange?

Answer 10

Jv= Lp x A{ (Hydraulic pressure difference) - Reflection coefficient(Osmotic pressure difference) }

Jv: volume of fluid moved
Lp: Hydraulic conductance of the endothelium (how leaky it is)
A: Wall area
Reflection coefficient: fraction of osmotic pressure that is exerted
(If reflection coefficient = 1 then membrane is entirely impermeable to a substance. If reflection coefficient = 0 then membrane is entirely permeable to a substance)

Question 11

What is the equation for effective osmotic pressure?

Answer 11

• Effective osmotic pressure = Reflection coefficient x potential osmotic pressure
  (This equation shows what fraction of the osmotic pressure is exerted on the membrane)

Question 12

How does Starling’s forces change as we go down the capillary?

Answer 12

• As we progress down the capillary (from arteriole to venous end) the capillary hydrostatic pressure decreases
• However on the other hand plasma osmotic pressure stays the same as plasma proteins are not lost from the capillaries so plasma protein concentration remains the same

Question 13

Where does filtration occur in well perfused capillaries?

Answer 13

• Along their entire length

Question 14

What does the lymphatic circulation do?

Answer 14

• They return excess tissue/solutes back to the CVS

Question 15

How is lymph flow maintained in lymphatic vessels?

Answer 15

• Lymph vessels have valves and smooth muscles
• Contractions of the smooth muscles contribute to lymph flow
• Surrounding skeletal muscle contraction also contribute to lymph flow
• Valves prevent backflow

Question 16

What does the control of extracellular fluid balance depend on?

Answer 16

• Capillary filtration
• Capillary reabsorption
• Lymphatic system

Question 17

At what places does Starling’s law determine fluid balance?

Answer 17

• In circulation
• In interstitial fluid
• In lymphatic system

Question 18

How does change in hydrostatic pressure affect change in favourability of filtration and reabsorption?

Answer 18

• A decrease in capillary hydrostatic pressure would favour an increase in reabsorption
• An increase in capillary hydrostatic pressure would favour an increase in filtration

Question 19

What is hypovolemia?

Answer 19

• When we have low hydrostatic pressure in the capillary (e.g. blood loss)

Question 20

How does Starling’s forces help to counteract hypovolemia?

Answer 20

• During hypovolemia the hydrostatic pressure of the blood is lower than usual
• Initially Starling’s forces favours the filtration of fluid.
• However as we move down the capillary the pressure within the capillary decreases until it is lower than the interstitial fluid hydrostatic pressure.
• Now Starling’s forces favours the reabsorption of fluid till the end of the capillary
• This maintains the blood volume lost which increases venous return and increases cardiac output

Question 21

How do nerves reduce capillary pressure?

Answer 21

• Sympathetic nerve-induced vasoconstriction of pre-capillary arterioles leads to a drop in downstream capillary pressure

Question 22

How do we get an increased pressure in the blood stream?

Answer 22

• A blockage in the veins can lead to the flow of blood stopping which increases volume of blood
• This results in increased blood pressure

Question 23

What is oedema?

Answer 23

• Build of excess fluid within the interstitial space

- Can result from an imbalance of Starling’s forces (filtration, reabsorption and lymph functions)

Question 24

What are the different ways in which oedema caused?

Answer 24

• Increased capillary pressure: Filtration will be favoured more than reabsorption which means there will be a build up of interstitial fluid
• Decreased plasma protein: This causes decreased oncotic pressure so favourability of reabsorption decreases leading to a build up of interstitial fluid
• Inflammatory response: They increase membrane permeability which means the membrane is more leaky allowing more filtration to occur. Furthermore some plasma proteins may also be lost in the process so decreases plasma protein oncotic pressure which means less reabsorption takes place causing a build up of interstitial fluid.
• Lymphatic problems: Hinders the circulation of excess interstitial fluid back into the blood stream. This means that the interstitial fluid builds up.

Question 25

What are the different clinical ways in which capillary pressure is increased?

Answer 25

• Gravitational Oedema: Results from standing for long periods of time causing gravity to pull down blood which slightly impairs venous return back to the heart. This causes a slight build up of blood in the veins at the feet leading to an increase in venous pressure
• Deep vein thrombosis: A blockage in the veins which impedes venous return to he heart causing a build up of blood in the veins and increases pressure in the capillary
• Cardiac failure: As the heart weakens, it loses its ability to produce sufficient arterial pressures to pump blood around the body. This causes a shift in the volume of venous blood which increases pressure at the venous end of the capillaries and eventually hydrostatic pressure within the capillaries

Question 26

What are the different clinical ways in which plasma osmotic pressure is decreased?

Answer 26

• Malnutrition: Not enough protein intake means not enough plasma protein is made so concentration of plasma proteins decreases so less plasma osmotic pressure.
• Nephrotic syndrome: High amount of protein lost through urine which reduces concentration of plasma protein and so lowers plasma osmotic pressure. However it can be replaced by liver production.
• Liver disease: Reduces the amount of albumin (plasma protein) formed which decreases plasma protein concentration and hence decreases plasma osmotic pressure

Question 27

How does inflammatory-mediated oedema occur?

Answer 27

• Swelling is triggered by local chemical mediators of inflammation
• This causes a large increase in capillary permeability
• This means they are more leaky and so a greater amount of filtration occurs
• Large gaps in the membranes also means that some plasma proteins may be lost which reduces plasma oncotic pressure and increases interstitial fluid oncotic pressure
• These effects reduces the forces of reabsorption and contribute to the swelling

Question 28

How does lymphatic problems cause oedemas?

Answer 28

• Blockage or removal of lymph nodes can prevent recirculation of interstitial fluid back into the CVS
• This means the excess interstitial fluid builds up causing oedemas