Capillaries II; Fluid Exchange Flashcards
Why is fluid exchange important?
Fluid exchange is important for normal physiological function, we need H2O for chemical reactions
Fluid re-absorption from tissues to blood can maintain circulation during haemorrhage
Abnormalities in fluid exchange can lead to oedema/tissue swelling
How does fluid move 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, which is ultimately the blood pressure
Large molecules (e.g. plasma proteins) cannot pass through membrane so they exert an osmotic pressure termed oncotic pressure which creates suction force to move fluid into capillary
Starling forces is a balance between two opposing forces:
Hydrostatic pressure, tends to favour filtration i.e. greater hydrostatic pressure in the capillary than the interstitial fluid
Osmotic pressure, which draws fluid back into the capillary
Fluid movement across capillary walls depends on the balance between hydraulic and oncotic pressures across the capillary
At a single capillary, these forces are usually not in balance; hydrostatic pressure dominates
The interstitial fluid that builds up is drained into the lymphatic system
What four pressures determine filtration rate?
Notice how the plasma proteins are of a greater concentration in the plasma than in the interstitial fluid, so that creates an inward-directed osmotic pressure gradient; forceful reabsorption
There are a few interstitial proteins present in the interstitial fluid but not the plasma
The net hydrostatic and osmotic pressure gradient can individually be calculated from the arrows’ values
Osmotic pressures:
pi p- plasma proteins (small)
pi i- interstitial proteins (big)
Hydrostatic pressures:
Pc- capillary blood pressure (big)
Pi- Interstitial fluid pressure (small)
What factors affect the net flow of fluid?
Jv= Lp A { (Pc - Pi) - sigma (pi p - pi i) }
Jv ∝ { Hydraulic pressure difference (Pc - Pi ) - Osmotic pressure difference (πp - πi) }
Jv= is the volume of fluid moved
Lp= hydraulic conductance of the endothelium- how leaky the endothelium is to fluid
s= reflection coefficient= fraction of the osmotic pressure that is exerted; if the membrane was entirely impermeable the value would be 1. If the coefficient is 0 that means the membrane is freely permeable. An in between value is semi-permeable to different extents
A= Wall area
Effective osmotic pressure = σ x potential osmotic pressure
s for plasma protein is 0.9 i.e. 10% plasma proteins are conducted across capillary wall into interstitial space
Starling’s forces normally favour filtration
How does the balance of Starling’s forces change along the capillary?
Well perfused capillaries filter along their entire length
The balance of starling forces change as we move along the capillary
As the blood continues through the narrow capillaries further pressure will be lost until it gets nearer to the venous end where the pressure is around 15
This will be opposed by the net osmotic pressure gradient
Due to the blood flow, enforcing blood through the capillaries, pressure is lost but the plasma osmotic pressure remains relatively constant because plasma proteins aren’t lost from the plasma
The outwards filtration force declines as you go down the capillary (but never go into the negative) so filter across the entire length
What is the role of lymphatic circulation and how is it adapted for that role?
Lymphatic circulation returns excess tissue fluid/solutes back in to the cardio-vascular system
Lymph vessels have valves and smooth muscle which aid in the direction of flow
Spontaneous contractions of the smooth muscle contributes to lymph flow
Surrounding skeletal muscle contractions/ relaxation also contributes to lymph flow
What does the overall control of extracellular fluid balance depend on?
Capillary filtration Capillary reabsorption Lymphatic system Starling's factors determine changes in fluid balance: Circulation Interstitial fluid Lymphatic system
What happens if a patient has low capillary pressure (e.g. from a haemorrhage)?
Low capillary pressure (Pc)- hypovolemia
Hypovolemia literally means low extracellular fluid volume but often refers to a low blood volume after for example a haemorrhage
So, you’ve lost a significant amount of blood volume:
Blood volume drops so venous return drops
Ultimately there will be less filling of the left ventricle (as there is less venous return)
There will be less stretch
And therefore, by the Frank Starling mechanism, there will be less contractile force generated in the ventricular contraction
This means in turn there will be less stroke volume
Stroke volume x heart rate gives you cardiac output and cardiac output in turn determines blood pressure
So since we start with a lower initial pressure value of 16, eventually the net pressure will decline into the negatives, favouring net reabsorption
This is good.
We have lost blood volume so in order to maintain blood volume, getting fluid back into circulation is like an automatic life supporting mechanism
What does increased Pc lead to? How do we check for such pathological conditions
Increased Pc- increased venous pressure (DVT, HF)
So by that logic an increase in capillary hydrostatic pressure should lead to an increase in filtration
This can occur in different pathological cases
We compress the superficial veins using a cuff
Changes in the arm volume are taken to represent swelling and this is taken to be due to accumulation of fluid in the interstitial space i.e. increased filtration
Why do we do the cuff thing?
Compressing the superficial veins impedes venous return
Flow through the capillary is driven by the hydrostatic pressure from arterial blood pressure
By the time to get to the venous end, the pressure has dropped significantly and the plasma is continuing to drain away into the venules
If you block that flow you will get blood accumulating at the venous end of the system, so that is going to generate a back pressure (opposite of previous graph) so a decline from 35 to 20 instead
This represents oedema
What is oedema?
Excess of fluid within the interstitial space
Imbalance between filtration, reabsorption, lymph function
Causes:
Increased capillary pressure (Pc)
Decreased plasma protein oncotic pressure (pP)
Inflammatory response- inflammation causes an increase in capillary permeability- increases Lp
If the membrane becomes leakier overall, meaning a decrease in the plasma osmotic pressure
Lymphatic problems
What are some clinical scenarios surrounding oedema and increased capillary pressure?
Clinical scenarios:
Dependent (gravitational) oedema- standing up for long periods
Deep venous thrombosis- an occlusion that blocks a deep vein impeding venous return to heart
Cardiac failure- as the heart weakens it loses the ability to generate sufficient arterial pressure to perfuse the system.
If the heart is less effective at maintaining cardiac output it seems inevitable that you get a backlof of blood at the venous end of the capillaries
E.g. Deep vein thrombosis (DVT) prevention of venous
Increase venous pressure causes ‘back-up’ of pressure leading to:
Increased PC across capillaries and increased filtration
FLOWnet = (Pc – Pi) – (πp – πi)
What causes oedema to occur from decreased plasma osmotic pressure (pi P)
Low protein oedema caused by:
Malnutrition/malabsorption: not enough protein intake to make plasma proteins, reduces pP
Nephrotic syndrome: urinary protein loss- replaced by liver production
Liver disease: not enough endogenous albumin produced
Generally:
How does inflammatory-mediated oedema occur?
Swelling is triggered by local chemical mediators of inflammation
Large increase in capillary permeability
Inflammation:
↑Lp, ↑protein permeability (↑ πi), ↓σ
chemicals/ insect bite/ nettle sting
infection/physical trauma/autoimmune disease
List some lymphatic problems.
Lymphatic obstruction:
Filariasis/elephantitis
Nematode infestation, larvae migrate to lymphatic system grow/mate/form nests- block lymph drainage
Lymphatic removal:
Lymphoedema
Caused by surgery to treat testicular cancer- removal of lymphatics
