3.2.4 Tissue Fluid

Question 1

Tissue Fluid

Answer 1

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, any substances can dissolve in it, allowing them to be transported around the body
As blood passes through the 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
The composition of plasma and tissue fluid are very similar, although tissue fluid contains far fewer proteins
- proteins are far too large to fit through gaps in the capillary walls, and so remain in the blood
Tissue fluid bathes almost all of the cells in the body that are outside the circulatory system
Exchange of substances can occur between cells and the blood via the tissue fluid
- for example, carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and them move into the capillary

Question 2

Tissue Fluid Formation Forces

Answer 2

The volume of liquid that leaves the plasma to form tissue fluid depends on 2 opposing forces:
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
Oncotic pressure
- this is the osmotic pressure exerted by plasma proteins within a blood vessel
- plasma proteins lower the water potential within the blood vessels, causing water to move into the blood vessels by osmosis

Question 3

At the Arterial End

Answer 3

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

Question 4

At the Venous End

Answer 4

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 as is passes through the capillaries
The water potential gradient between the capillary and the tissue fluid remains thye 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
- roughy 90% of the fluid is lost at he arterial end of the capillary is reabsorbed at the venous end
- the other 10% remains as tissue fluid and is eventually collected by the lymph vessels and returned to the circulatory system
If the blood pressure is high (hypertension) the the pressure at the atrial end is even greater
- this pushed more fluid out of the capillary and fluid begins to accumulate around the tissues
- this is called oedema

Question 5

Formation of Lymphb

Answer 5

Some tissue fluid renters 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 re-enters 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 removes from 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 blood stream by the lymph system

Question 6

Valves in the Heart

Answer 6

Valves in the heart:
- open when the pressure of blood behind them is greater than the pressure in front of them
- close when the pressure of blood in the front is greater than the pressure behind them
Valves are important for keeping blood flowing forward in the right direction and stopping it flowing backwards
They are also important for maintaining the correct pressure in the chambers of thr heart
The right atrium and right ventricles are separated by the atrioventricular valve, which os otherwise known as the tricuspid valve
The right ventricle and the pulmonary artery are separated by the pulmonary valve
The left atrium and left ventricle are separated by the mitral valve, which is otherwise known as the bicuspid valve
The left ventricle and aorta are separated by the aortic valve
There are 2 blood vessels brining blood into the heart:
- the vena cava
- the pulmonary vein
There are 2 blood vessels taking blood away from the heart
- the pulmonary artery
- the aorta

Question 7

Coronary Arteries

Answer 7

The heart is a muscle and so requires its own blood supply for aerobic respiration
Then heart receives blood through arteries on its surface, called the coronary arteries
It’s important that the arteries remain clear of plaques, as this could lead to angina or a heart attack (myocardial infarction)