3.2.4- Tissue Fluid Flashcards
What is plasma and what is it largely composed of?
Plasma is a straw-coloured liquid that constitutes around 55 % of the blood
Plasma is largely composed of water (95 %) and because water is a good solvent many substances can dissolve in it, allowing them to be transported around the body
As blood passes through the capillaries what happens to the plasma?
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.
What is the difference between the composition of plasma and tissue fluid?
The composition of plasma and tissue fluid are very similar, although tissue fluid contains far fewer proteins
Proteins are too large to fit through gaps in the capillary walls and so remain in the blood
Where is tissue fluid around the body?
Tissue fluid bathes almost all the cells of the body that are outside the circulatory system
Exchange of substances between cells and the blood occurs via the tissue fluid
For example, carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then move into the capillary
What forces determine the volume of liquid that leaves the plasma to form tissue fluid?
Hydrostatic and oncotic pressure
Describe hydrostatic pressure.
This is the pressure exerted by a fluid, e.g. blood
The hydrostatic pressure in this example is the blood pressure, generated by the contraction of the heart muscle
Describe oncotic pressure.
This is the osmotic pressure exerted by plasma proteins within a blood vessel
Plasma proteins lower the water potential within the blood vessel, causing water to move into the blood vessel by osmosis
Describe what happens when blood is at the arterial end of the capillary.
When blood is at the arterial end of a capillary the hydrostatic pressure is great enough to force fluid out of the capillary
Proteins remain in the blood as they are too large to pass through the pores in the capillary wall
The increased protein content creates a water potential gradient (osmotic pressure) between the capillary and the tissue fluid
At the arterial end the hydrostatic pressure is greater than the osmotic pressure so the net movement of water is out of the capillaries into the tissue fluid
Describe what happens when blood is at the venous end of the capillary.
At the venous end of the capillary the hydrostatic pressure within the capillary is reduced due to increased distance from the heart and the slowing of blood flow as it passes through the capillaries
The water potential gradient between the capillary and the tissue fluid remains the same as at the arterial end
At the venous end the osmotic pressure is greater than the hydrostatic pressure and water begins to flow back into the capillary from the tissue fluid
Roughly 90 % of the fluid lost at the arterial end of the capillary is reabsorbed at the venous end
The other 10 % remains as tissue fluid and is eventually collected by lymph vessels and returned to the circulatory system
If blood pressure is high (hypertension) then the pressure at the arterial end is even greater
This pushes more fluid out of the capillary and fluid begins to accumulate around the tissues. This is called oedema
Describe the formation of lymph.
Some tissue fluid reenters the capillaries while some enters the lymph vessels
The lymph vessels are separate from the circulatory system
They have closed ends and large pores that allow large molecules to pass through
Larger molecules that are not able to pass through the capillary wall enter the lymphatic system as lymph
Small valves in the vessel walls are the entry point to the lymphatic system
The liquid moves along the larger vessels of this system by compression caused by body movement. Any backflow is prevented by valves
This is why people who have been sedentary on planes can experience swollen lower limbs
The lymph eventually reenters the bloodstream through veins located close to the heart
Any plasma proteins that have escaped from the blood are returned to the blood via the lymph capillaries
If plasma proteins were not removed from tissue fluid they could lower the water potential (of the tissue fluid) and prevent the reabsorption of water into the blood in the capillaries
After digestion lipids are transported from the intestines to the bloodstream by the lymph system
