capillaries 2 - fluid exchange Flashcards
what is the importance of fluid exchange?
- Fluid exchange is important for normal physiological function, we need water for chemical reactions.
- Fluid re-absorption from tissues to blood can maintain circulation during hemorrhage.
- Abnormalities in fluid exchange can lead to oedema/tissue swelling.
describe fluid movement at the capillary wall
- Capillary wall is a semi-permeable membrane.
- Fluid moves across membrane into interstitial space due to blood flow which exerts a hydraulic pressure.
- 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.
- Fluid movement across capillary walls depends on the balance between hydraulic and oncotic pressures across the capillary wall.
what are the 4 pressures that determine filtrate rate ?
Osmotic pressures
pie p - plasma proteins
pie i - interstitial proteins
Hydrostatic pressures
Pc = Capillary blood pressure
Pi = Interstitial fluid pressure
Describe capillary permeability and reflection coefficients
Capillary permeability refers to how easily substances pass through capillary walls
The reflection coefficient measures how well a membrane blocks a substance:
σ = 1 → Completely blocked (e.g., proteins in continuous capillaries).
σ = 0 → Freely passes (e.g., water in all capillaries).
0 < σ < 1 → Partially blocked (some molecules pass).
Higher σ means less permeability to that substance.
describe starlings principle of fluid exchange and define the parameters involved
Starling’s Principle explains how fluid moves across capillary walls due to the balance of hydrostatic and oncotic pressures. It determines whether fluid leaves (filtration) or enters (reabsorption) the capillary.
Jv = Movement of fluid or ‘flux’
Lp = Hydraulic conductance of the endothelium – how leaky the endothelium is to fluid
σ = Reflection coefficient = fraction of the osmotic pressure that is exerted
A = Wall area
why does capillary filtration occur at rest?
- Even at rest, capillary filtration occurs because of the balance between hydrostatic and oncotic pressures. The main reasons include:
Capillary Hydrostatic Pressure (Pc):
* Blood pressure pushes fluid out of capillaries into tissues.
* While lower at rest than during exercise, it is still high enough to cause filtration, especially at the arterial end of capillaries.
Interstitial Oncotic Pressure (𝜋if):
* Some plasma proteins and solutes leak into the interstitial space, pulling fluid out of the capillaries.
Low Interstitial Hydrostatic Pressure (𝑃𝑖𝑓):
* Normally low, meaning there is little force pushing fluid back into the capillary.
Lymphatic Drainage:
* Excess filtered fluid is removed by the lymphatic system, preventing tissue swelling (edema).
describe the structure and function of lymphatic system
- Lymphatic circulation returns excess tissue fluid/solutes back to the cardio-vascular system. About 8 litres per day are filtered.
- Lymph vessels have valves and smooth muscle.
- Spontaneous contractions of the smooth muscle contributes to lymph flow.
- Surrounding skeletal muscle contractions & relaxation also contributes to lymph flow.
- Lymph also contains immune cells, especially at the lymph nodes
how does capillary pressure alter after fluid exchange ? give examples
Capillary pressure (hydrostatic pressure (𝑃𝑐)
* decreases as fluid moves out into the tissues. This happens because fluid loss reduces the pressure inside the capillary.
Examples:
Arterial to Venous End of Capillary
* At the arterial end: Higher capillary pressure (35mmHg) → More fluid pushed out (filtration).
* At the venous end: Lower capillary pressure (15mmHg) → Less fluid pushed out, and reabsorption occurs.
Dehydration
* Less blood volume → Lower capillary pressure → More fluid drawn back into capillaries to maintain blood volume.
Edema (Swelling)
* Increased capillary pressure (e.g., in heart failure) → Excess filtration → Fluid buildup in tissues.
what is the function and mechanism of solute exchange?
function - Nutrition of tissue.Hormone & drug delivery.
mechanism: Diffusion – individual molecules.
Concentration gradients across wall
obeys Fick’s law of diffusion
what is the function and mechanism of fluid exchange ?
function - Regulation of plasma and
Interstitial fluid volumes.
mechanism -
Bulk flow – all molecules together
Pressure gradients across wall
obeys Starling’s principle.
What are the different hydrostatic and oncotic pressures in a normal capillary bed – how do they change with hypovolemia?
Normal Hydrostatic and Oncotic Pressures in a Capillary Bed:
Capillary Hydrostatic Pressure (Pc):
* At the arterial end: ~35 mmHg (pushes fluid out of capillaries).
* At the venous end: ~15 mmHg (still pushing fluid out, but less than at the arterial end).
Plasma Oncotic Pressure (πp):
* Consistent throughout the capillary bed, around 25-28 mmHg (pulls fluid into the capillaries due to proteins like albumin).
Interstitial Hydrostatic Pressure (Pif) and Interstitial Oncotic Pressure (πif):
* Pif is usually low or negative, meaning it doesn’t push fluid into capillaries.
* πif is low, so it doesn’t pull much fluid out.
How These Pressures Change with Hypovolemia:
Decreased Capillary Hydrostatic Pressure (Pc):
* Hypovolemia (low blood volume) reduces overall blood pressure, which decreases Pc, leading to less fluid being pushed out of capillaries.
Increased Plasma Oncotic Pressure (πp):
* With less fluid in the blood, the concentration of proteins increases, which raises πp, helping pull more fluid back into the capillaries.
Potential Increase in Interstitial Hydrostatic Pressure (Pif):
* If tissues become overloaded with fluid (due to reduced reabsorption), Pif may rise, but this depends on the extent of fluid retention.
Do capillaries normally reabsorb or filter, and why?
- Capillaries normally filter fluid out at the arterial end and reabsorb it at the venous end.
- At the arterial end, capillary hydrostatic pressure (Pc) is higher, so fluid is pushed out into the tissues (filtration).
- At the venous end, oncotic pressure (πp) from plasma proteins is higher, so fluid is pulled back into the capillaries (reabsorption).
- This balance helps deliver nutrients and oxygen to tissues while also removing waste.
What is oedema, and how do changes in Pc , πP, and inflammation cause oedema?
8 Oedema is the swelling caused by excess fluid accumulating in the tissues. It often happens when the balance between fluid filtration and reabsorption is disrupted.
How Changes in Starling’s Forces Cause Oedema:
Increased Capillary Hydrostatic Pressure (Pc):
* Higher Pc means more fluid is pushed out of the capillaries into the tissues.
* This can happen in conditions like heart failure, where blood backs up in veins, increasing pressure.
Decreased Plasma Oncotic Pressure (πP):
* Lower πP means there are fewer proteins in the blood to pull fluid back into the capillaries.
* This can happen with malnutrition or liver disease, where protein levels are low.
Inflammation:
* Inflammation causes capillaries to become more permeable, allowing more fluid and proteins to leak out into tissues.
This can occur in conditions like infection or injury, leading to swelling.
