Unit 2 Lecture 12: Capillary Filtration & Reabsorption

Question 1

What is capillary filtration? Capillary absorption?

Answer 1

Filtration: Fluid exiting the capillary
* Going from lumen into the interstital space to the organ
Absorption: Fluid entering the capillary
* Capillary is absorbing liquids

Liquid in filtration moves through cell-to-cell junctions

Question 2

What is the primary function of capillary filtration/reabsorption?

Answer 2

Regulate the distribution of ECF between the plasma and interstitial space

Question 3

According to this picture how does blood move from capillary to venules?

Answer 3

It needs a pressure gradient; beginning of the capillary has higher pressure than at the distal end of the capillary

Question 4

Capillaries are the site of fluid exchange with the interstitial compartment. What are the four dictating pressures?

Answer 4

1. Capillary hydrostatic pressure (Pc)
2. Interstitial fluid pressure (Pif)
3. Osmotic force (interstitial fluid proteins) (πIF)
4. Osmotic force (plasma proteins)(πc)

Question 5

Define hydrostatic pressure and osmotic pressure

Answer 5

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

Question 6

What is capillary hydrostatic pressure? What is Interstitial fluid hydrostatic pressure?

Answer 6

• 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

Question 7

Where does osmotic pressure (interstitial fluid proteins) point to? In the capillary or out the capillary?

Answer 7

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

πIF<πc

Question 8

Why is albumin a key contributor to osmotic pressure (plasma protein)?

Answer 8

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

Question 9

Which forces dictate capillary absorption and capillary filtration?

Answer 9

• 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

Question 10

What is the net filtration pressure formula?

Answer 10

(Pc)+(πIF)-(Pif)-(πc)

Note: +ve # is outward movement into the organ
-ve # is inward movement into the organ

Question 11

Why does the pressure drop so much from the arterial end to the venous end in the capillaries

Answer 11

The capillaries must follow this pressure gradient to allow for gas and nutrient exchange to occur

Question 12

What is the image showing?

Answer 12

• 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

Question 13

What does the transition point mean?

Answer 13

When the capillary goes from filtration to reabsorption

Note: When we are at the transition point, the movement of blood is 0mmHg

Question 14

Why would filtration be more significant than reabsorption?

Answer 14

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

Question 15

Why would absorption be favoured over filtration?

Answer 15

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

Question 16

What is something outside of exercise that can cause drop in pressure that is seen when absorption is favoured?

Answer 16

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

Question 17

The numbers on both sides should be equal but they’re not. What is the reasoning behind this?

Answer 17

• 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

Question 18

What is Edema?

Answer 18

Accumulation of fluid in the interstitial space (the connection between capillary and organ); where filtration and absorption occurs

Question 19

What is elephantasis and what system does it affect and how?

Answer 19

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

Question 20

Facts about the lymphatic system

Answer 20

1. Parallel to the venous circulation
2. Open circulation
3. One way valves; prevent backflow

Connection of capillary to lymphatic system is at initial lymphatics

Question 21

Where is the lymph system present in? What are the functions of the lymphatic system?

Answer 21

Present in both systemic and pulmonary circulation collecting fluid to drain

1. Fluid balance
2. Immune surveillance
3. Movement of immune cells

Question 22

What does the bottom image show?

Answer 22

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

Question 23

What exactly are lymph vessels (What are they made of)?

Answer 23

Overlapping endothelial cells; intersitial fluid moving into or out of the lymph is dictated by the movement between endothelial cells

Interstitial fluid moves between endothelial cells and then into the lymph vessel

Note: Overlapping pattern is known as blind ending tube

Question 24

How does interstitial fluid move into the lymph nodes even with their roof shingle pattern?

Answer 24

Fluid pressure outside vessel helps move interstitial fluid inside; pressure inside once the vessel is filled makes sure the edges stay closed

Question 25

At the level of lymph capillaries how many layers of endothelial cell does it have?

Answer 25

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?

Question 26

What makes up the lymph vessels distal to the lymph capillaries?

Answer 26

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