4.3 - Tissue Fluid and Lymph Flashcards
What is tissue fluid?
- is the watery fluid that surrounds individual cells and creates their environment
- it is formed from blood plasma at the arteriole end of the capillaries
it is made from substances that leave blood plasma - it contains: glucose, amino acids, ions and oxygen
- unlike blood, tissue fluid does not contain blood cells or big proteins as they are too large to be pushed through the capillary walls
- in capillary beds substances move out of the capillaries into the tissue fluid by ‘pressure filtration’
What is the purpose of tissue fluid?
- is how materials are exchanged between blood and cells
- cells take in oxygen and nutrients like amino acids and glucose from the tissue fluid and release metabolic waste into the tissue fluid.
- essential for the efficient exchange of materials
How is tissue fluid formed?
as blood passes through capillaries some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body - this results in the formation of tissue fluid. (high hydrostatic pressure forces the fluid out of the capillaries at the arteriole end)
What does the formation of tissue fluid depend on?
the volume of liquid that leaves that plasma to form tissue fluid depends on two opposing forces:
1) Hydrostatic pressure
This is the pressure exerted by a fluid eg. blood. High hydrostatic pressure develops in the arteriole end of the capillary due to the heart-pumping
2) Oncotic pressure
This is the osmotic pressure exerted by plasma proteins within a blood vessel
It is the tendency for water to move into the capillaries.
(plasma proteins lower the water potential within the blood vessel, causing water to move into the blood vessel by osmosis)
high hydrostatic pressure than oncotic pressure means that the fluid is forced out of the capillaries
when oncotic pressure is greater than hydrostatic pressure fluid moves back into the capillaries
Why does the pressure change from the arteriole end to the venous end?
The hydrostatic pressure is high at the arteriole end due to the heart pumping and the oncotic pressure is less than the hydrostatic. At the venous end, the oncotic pressure remains but is greater than the hydrostatic pressure due to the loss of fluid and increase in plasma proteins. This lowers the water potential so the fluid will move into the capillaries. This pressure and fluid movement change is needed as we need constant movements of substances and substances like O2 in this fluid need to be replenished.
What are the stages of tissue fluid formation?
1) Blood in the arteriolar end of the capillary has high hydrostatic pressure, meaning it has a high water potential
2) Because it has a lot of water, it also has lots of dissolved ions and small molecules like oxygen, glucose and amino acids
3) These molecules, including water, are small enough to be forced out of the capillary lining because of this pressure.
4) This is called ‘ultrafiltration’ as only small molecules are forced out of the capillaries. Cells and proteins remain in the blood as they are too large to cross the membrane.
5) By the time blood arrives at the venous end of the capillary, the blood has lost some water and ions, so has a reduced hydrostatic pressure.
6) Due to fluid loss, and an increasing concentration of plasma proteins (which don’t leave capillaries as they are too large), the water potential at the venule end of the capillary is lower than the water potential in the tissue fluid.
7) This means that some water re-enters the capillaries from the tissue fluid at the venule end by osmosis.
8) 90% re-enters the capillaries at the venule end of the capillary bed. 10% is extra fluid and will get returned to the blood through the lymphatic system
Explain the formation of lymph
1) Some tissue fluid that is not reabsorbed by the capillaries at the venule end, enters the lymph vessels. Once inside, this fluid is called ‘lymph’
2) The lymph vessels are separate from the circulatory system as they have closed ends and large pores that allow large molecules to pass through.
3) Larger molecules that are not able to pass through the capillary wall enter the lymphatic system as lymph as small valves in the vessel walls are the entry point to the lymphatic system
4) The liquid moves along the larger vessels of this system by compression caused by body movements. Valves prevent any backflow.
5) The lymph eventually reenters the circulatory system in the chest cavity in the subclavian veins.
Purpose of lymph
- If any plasma proteins that have escaped from the blood are returned to the blood via the lymph capillaries as if any plasma proteins were not removed from the tissue fluid they could lower the water potential of the tissue fluid and prevent the reabsorption of water into the blood in capillaries.
- after digestion, lipids are transported from the intestines to the bloodstream by the lymph system
Movement of the lymph
3 main ways:
1) Hydrostatic pressure of the tissue fluid leaving the capillaries
2) Contraction of body muscles squeezes the lymph vessels. Valves ensure the fluid inside them moves away from the tissues towards the heart.
3) Enlargement of the thorax during breathing reduced the pressure in the thorax, drawing lymph into this region, away from the tissues.
Compare and contrast the transport of fluid in a vein with its transport in a lymph vessel
Similarities:
- both have low pressure
- both have valves
- both use muscle squeezing to move fluid
Differences:
- faster flow in the vein
- heart causes mass flow in the vein
- flow to the heart in the vein but to glands in the lymph
High blood pressure leads to an accumulation of tissue fluid. Explain how?
High blood pressure = high hydrostatic pressure
This increases outward pressure from the arterial end of the capillary and reduces inward pressure at the venule end of the capillary
so more tissue fluid is formed and less tissue fluid is reabsorbed, which causes the accumulation.
This accumulation causes swelling. Can cause oedema.
