Tissue Fluid And Lymph Flashcards
What is tissue fluid
Tissue fluid is the fluid that surrounds all the cells in the body and it is formed in the blood
How is tissue fluid formed
The junctions between the single cells that make up the capillary walls are not tight. As a result the capillary walls are very permeable to everything in the blood expect the erythrocytes and large plasma proteins. So in the capillary beds a fluid that is basically plasma without the plasma proteins and the red blood cells moves out of the capillaries and bathes the individual cells of the body. This is tissue fluid
What % of the tissue fluid is back inside the blood vessels after the blood leaves the capillary beds
90%
Why does only 90% of the tissue fluid go back inside the blood vessel
There are two factors. Water potential: the plasma proteins and albumin exert an osmotic effect. They give the blood a water potential of -3.3kPa. The tissue fluid surrounding the capillaries has a water potential of about -1.3kPa. Water moves from an area of high water potential to an area of lower water potential so the tendency is for water to move into the blood by osmosis. This pressure is called oncotic pressure. There is also hydrostatic pressure which is the residual pressure from the heartbeat that is still present as the blood enters the arterial end of the capillary beds which tends to force fluid out through the leaky capillary walls. The balance between oncotic and hydrostatic pressure determines whether tissue fluid moves out of or into the capillaries.
What takes place at the arterial end of the capillaries
As the blood flows from the arterioles into the capillary bed the hydrostatic pressure tending to force fluid out of the capillary is relatively high 3.3kPa. The oncotic pressure is fairly constant so remains at 2kPa. The hydrostatic pressure forcing water out is higher than the oncotic pressure moving water in so fluid is squeezed out of the capillary and fills the spaces around all of the cells. This is the tissue fluid. It contains all the dissolved substances present in the blood apart from large plasma proteins and erythrocytes
What happens at the venous end of the capillaries
The blood moves steadily through the capillary system and as it does so the balance of forces between the hydrostatic pressure and oncotic pressure changes. The hydrostatic pressure falls to around 1kPa because the pressure from the pulse is completely lost and the fluid moves out of the capillaries to form tissue fluid so the volume of blood in the capillaries is lowered. However oncotic pressure is still 2kPa so the pressure exerted by water moving into the capillaries by osmosis is greater than the hydrostatic pressure forcing fluid out so water returns to the capillaries by the time blood enters the venules most but not all of the tissue fluid has returned to the blood vessels.
Where does the remaining 10% of the tissue fluid return to
The tissue fluid drains into blind ended tubes called lymph capillaries and becomes lymph
What is the lymph
The lymph is the fluid that travels in the lymphatic system
What is the structure of lymph
Lymph capillaries join up to form larger vessels. They have one way valves that prevent the lymph flowing backwards and lymph is moved through the vessels by the contraction of the body muscles as we move about. The lymph is returned to the blood in the neck area where it joins the left and right subclavian veins
What are the lymph glands
They are the glands in the lymph system that contain lymphocytes that make antibodies. They are found at intervals along the lymph vessels. The antibodies are emptied into the blood with the lymph at the subclavian vein. The lymph glands also remove bacteria and other pathogens to be taken in and destroyed by phagocytes
