Capillaries: Fluid movement Flashcards
Why is fluid exchange important?
What can fluid exchange abnormalities lead to?
- For chemical reactions to occur.
- Oedema - during a drop in BP (e.g. haemorrhage, dehydration), the blood volume, and interstitial/cell volume need to be controlled.
What is Hydraulic pressure?
What is Oncotic pressure?
What does fluid movement depend on?
LOOK AT DIAGRAM!
- Fluid moves across membrane into interstitial space due to the blood flow, and it exerts this pressure during its movement.
- Larger molecules can’t pass membrane, so they exert an osmotic pressure, called oncotic pressure - this pressure creates a suction force to bring fluid back into capillary.
- The balance between the hydraulic and oncotic pressures.
The old Starling’s principle of fluid exchange:
What is the direction of movement dominated by?
What was the relationship between net filtration and the pressures?
What does this principle tell us?
LOOK AT DIAGRAM!
- Pc (capillary bp) and πp (osmotic pressure of plasma proteins)
- Net filtration is proportional to the Hydraulic pressure difference – Osmotic pressure difference: (Pc - Pi) - (πp-πi).
• Damage to endothelium makes it more leaky - plasma proteins can now pass through and affect osmotic pressure. - The Pc decreases from the arterial to venous end, but the πp doesn’t change along the capillary:
• When Pc > πp = Filtration
• When Pc < πp = Reabsorption
What is wrong with this principle?
- Fluid filtration actually occurs along the WHOLE length of capillaries, and reabsorption doesn’t occur.
- The interstitial osmotic pressure isn’t as small as it’s made seem - πp = πi
- Glycocalyx isn’t taken into account, but is vital in fluid exchange.
• By the old principle, increasing πp and reabsorption by giving colloid fluids (IV with large proteins) should increase blood volume, but it doesn’t.
The revised Starling’s principle of fluid exchange: ARTERIAL end:
What is the role of glycocalyx in the capillary?
What does the stream of fluid filtration cause?
How does this affect the osmotic pressure difference used to work out net filtration?
LOOK AT DIAGRAM!
- Acts as a barrier, so plasma proteins can’t pass through the intracellular spaces - can only pass using the vesicle (cavaeole) system.
- Fluid moving into the interstitial space moves the plasma proteins away from the endothelium and into the πi - this creates a low πg (Subglycocalyx region).
- Its now πp - πg (instead of πp - πi).
The revised Starling’s principle of fluid exchange: VENOUS end:
How does Pc change at the venous end and what does it cause?
How does this affect the osmotic pressure difference?
What will the gradient change affect filtration at the venous end?
LOOK AT DIAGRAM!
- Pc is lower at this end, which causes the πi to move into πg = ↑πg.
- It will decrease the osmotic gradient (πp - πg) gradient
- It promotes filtration, even with a low Pc.
How does Hypovolaemia affect Pc?
How will this affect the amount of filtration and reabsorption?
What’s the importance of this response?
- During a ↓BV, for compensation, CO/SV will be decreased = ↓BP. Also, there’ll be sympathetic-mediated vasoconstriction = ↓↓BP = ↓Pc.
- πp > Pc, so VERY little filtration occurs with LOTS of reabsorption - ↑BV again.
- It supports the CVP, ↑CO and BP again = better end-organ perfusion.
What occurs to some of the interstitial fluid during exchange?
What the the lymph vessels’s key features and their roles?
- Goes into the lymphatic system, which returns the excess fluid back into the blood.
- Valves and smooth muscle, which contract spontaneously to cause lymph flow. Also, contractions skeletal muscles cause lymph flow.
What controls ECF balance?
What can occur if this is imbalanced?
- Capillary filtration, Capillary reabsorption, and the Lymphatic system
- Excess filtration will lead to Oedema
What can cause an increase in Pc?
How can an increase in Pc (hydraulic capillary pressure) cause oedema, using DVT as an example?
- Standing up for long periods, DVT, Heart failure
- Venous return is prevented, so it ↑Venous pressure, which will lead to “back up” of pressure. This will ↑Pc across the capillary = ↑Filtration = Gravitational Oedema
What can decrease πp?
How can a decrease in πp (plasma protein osmotic pressure) cause oedema?
- Malnutrition, Malabsorption, Liver failure, Nephrotic syndrome
- ↓Plasma [protein] = ↓πp. Pc becomes much greater than πp = ↑Filtration = Low Protein Oedema
What can cause inflammation?
How does inflammation lead to oedema?
- Infection, Trauma, Autoimmune disease
- It ↑Capillary permeability = ↑Filtration = Oedema
What 2 lymphatic problems can cause oedema?
Lymphatic obstruction (e.g. larvae block vessels) and Lymphatic removal (surgery for testicular cancer). The continuous filtration leads to oedema.
