Unit 2 Lecture 12: Capillary Filtration & Reabsorption Flashcards
What is capillary filtration? Capillary absorption?
Filtration: Fluid exiting the capillary
* Going from lumen into the interstital space to the organ
Absorption: Fluid entering the capillary
* Capillary is absorbing liquids
What is the primary function of capillary filtration/reabsorption?
Regulate the distribution of ECF between the plasma and interstitial space
According to this picture how does blood move from capillary to venules?
It needs a pressure gradient; beginning of the capillary has higher pressure than at the distal end of the capillary
Capillaries are the site of fluid exchange with the interstitial compartment. What are the four dictating pressures?
- Capillary hydrostatic pressure (Pc)
- Interstitial fluid pressure (Pif)
- Osmotic force (interstitial fluid proteins) (πIF)
- Osmotic force (plasma proteins)(πc)
Define hydrostatic pressure and osmotic pressure
Hydrostatic pressure: caused by fluid within a confined space
* ⇒ in capillary = blood pressure
Osmotic pressure: the force exerted by a dissolved substance to draw water towards it
What is capillary hydrostatic pressure? What is Interstitial fluid hydrostatic pressure?
- Capillary hydrostatic pressure (Pc) is essentially the pressure within the confined space that is moving out of the capillary (very big pressure)
- Interstitial fluid hydrostatic pressure (Pif) pressure of liquid in interstitial fluid; large space diffusing nutrients and gases to organs(pressure is very low)
Pif is a large space but the pressure is very minimal
REMEMBER: The hydrostatic pressures in and out of the capillary (interstitial space) & the osmotic pressure in and out of the capillary (interstitial space) are what dictate the overall movement of fluid going into the capillary or out of the capillary
Where does osmotic pressure (interstitial fluid proteins) point to? In the capillary or out the capillary?
Out the capillary as osmosis involves movement of water towards a dissolved substance which is actually trying to counteract the albumin causing the pressure to increase inside but there’s nothing equivalent in concentration to albumin outside the capillary hence why the concentration of interstitial fluid protein is low
Why is albumin a key contributor to osmotic pressure (plasma protein)?
Because albumin remains in the plasma, osmotic pressure does say that it is the force exerted by a substance to draw water towards it. The albumin draws fluid to the capillary causing a large inward pressure change because main function is to maintain osmotic force & move liquids from organs back into capillaries
Note: Osmotic pressure (plasma proteins) & Capillary hydrostatic pressure induce significant changes in pressure amongst the capillary
Which forces dictate capillary absorption and capillary filtration?
- Capillary absorption is dictated by osmotic pressure & Pif because the plasma protein pressure draws everything in
- Capillary filtration is dictated by hydrostatic pressure (Pc) & osmotic pressure (interstitial fluid proteins) as the liquid wants to move out from its confined high pressure space
What is the net filtration pressure formula?
(Pc)+(πIF)-(Pif)-(πc)
Note: +ve # is outward movement into the organ
-ve # is inward movement into the organ
Why does the pressure drop so much from the arterial end to the venous end in the capillaries
The capillaries must follow this pressure gradient to allow for gas and nutrient exchange to occur
What is the image showing?
- The pressure gradient of blood flow from 37mmHg to 17mmHg
- It shows which way the blood flows and it incorporates the factors that are in the net filtration formula
- Inward pressure is low during filtration but high after transition to indicate absorption
What does the transition point mean?
When the capillary goes from filtration to reabsorption
Note: When we are at the transition point, the movement of blood is 0mmHg
Why would filtration be more significant than reabsorption?
For example: when we vasodilate the upstream arterioles, we increase the pressure at the beginning of the capillary & the transition point is later on at the end of the capillary
Note: Net fluid movement outwards is highly favoured for the most part
* Favoured in exercise to get more O2 into the skeletal muscle
Why would absorption be favoured over filtration?
This happens when we have vasoconstriction of the upstream arterioles; not much fluid will leave the capillaries because of constricting and keeping everything close to the body so for the most part we have absorption while we have filtration at the beginning for a small length
What is something outside of exercise that can cause drop in pressure that is seen when absorption is favoured?
Decrease in Cardiac Output (CO); dehydration and sweating so we want to retain the water in our body
Remember: Reducing volume of liquid also drops the MAP as not much fluid is leaving the heart
The numbers on both sides should be equal but they’re not. What is the reasoning behind this?
- There must be a pressure gradient so one has to be greater than the other to have movement occuring
- ALSO…some of the excess liquid being reabsorbed goes to the initial lymphatic vessels
Note: There can be impairments when there is too much fluid in the lymphatic vessels affected drainage of the fluid
What is Edema?
Accumulation of fluid in the interstitial space (the connection between capillary and organ); where filtration and absorption occurs
What is elephantasis and what system does it affect and how?
This is a disease caused by mosquitoes which lay parasitic eggs in the lymphatic system. The eggs eventually hatch and cause blockage in the vessels so that drainage of the lymph nodes are not efficient and that causes build up
Facts about the lymphatic system
- Parallel to the venous circulation
- Open circulation
- One way valves; prevent backflow
Connection of capillary to lymphatic system is at initial lymphatics
Where is the lymph system present in? What are the functions of the lymphatic system?
Present in both systemic and pulmonary circulation collecting fluid to drain
- Fluid balance
- Immune surveillance
- Movement of immune cells
What does the bottom image show?
Some of the 7200L of blood is diverted to the lymph nodes (3L) & some of it is diverted to filtration (20L) and some of it is diverted to reabsorption (17L)
Note: Not all the fluid is reabsorbed so the excess fluid is sent to the lymphatic vessels to be drained out
What exactly are lymph vessels (What are they made of)?
Overlapping endothelial cells; intersitial fluid moving into or out of the lymph is dictated by the movement between endothelial cells
Note: Overlapping pattern is known as blind ending tube
How does interstitial fluid move into the lymph nodes even with their roof shingle pattern?
Fluid pressure outside vessel helps move interstitial fluid inside; pressure inside once the vessel is filled makes sure the edges stay closed
At the level of lymph capillaries how many layers of endothelial cell does it have?
One single layer (at the area of the intial lymphatics for a reference)
- Distal to that there are lymph vessels that are not single layer?
What makes up the lymph vessels distal to the lymph capillaries?
Smooth muscle makes up the distal lymph vessels; the proximal lymph capillaries such as initial lymphatics are made up of single layer endothelial cells
Note: Smooth muscle allows for contraction to move lymph fluid in forward direction
