Blood vessels and tissue fluidAQA A-Level Biology: Revision notes
Section 1
Arteries, arterioles and veins
Each type of vessel has a structure that suits its function.
Arteries carry blood away from the heart at high pressure. Their walls are thick and contain collagen (to withstand pressure), elastic fibres (which stretch as the heart pumps and recoil, evening out the pulses of pressure) and some smooth muscle. The lumen is narrow and the endothelium is smooth.
Arterioles are smaller vessels leading to the capillaries. Their walls have more smooth muscle and less elastic tissue. When the muscle contracts the lumen narrows (vasoconstriction) and less blood enters the capillary bed; when it relaxes the lumen widens (vasodilation) and flow increases. This controls the distribution of blood, for example to the muscles during exercise.
Veins carry blood back to the heart at low pressure. They have thin walls with little elastic tissue or muscle, a wide lumen with little resistance to flow, and valves that prevent backflow. Contraction of skeletal muscles squeezes the veins and helps to push the blood along.
Elastic tissue does not push blood along. It stretches and recoils to even out pressure. Valves, not elastic tissue, prevent backflow.
Section 2
Capillaries and capillary beds
Capillaries are the exchange vessels. Their walls are a single layer of flattened endothelial cells, so the diffusion distance is very short. The lumen is so narrow that red blood cells are squeezed through, which brings them close to the wall and slows the flow to allow time for exchange. There are gaps between the cells through which fluid and small molecules can leave.
Capillaries form dense capillary beds in tissues, giving a very large total surface area and cross-sectional area, which further slows blood flow. Substances such as oxygen and glucose pass out of the blood and carbon dioxide and waste pass in.
Section 3
Formation of tissue fluid
Tissue fluid is the fluid that surrounds the cells. It forms from blood plasma.
At the arteriolar end of a capillary bed, the hydrostatic pressure of the blood (produced by the heart) is high. It is greater than the opposing force from the plasma proteins, so a net outward force pushes fluid out through the gaps in the capillary walls. Water, glucose, amino acids, oxygen and ions leave. Plasma proteins and red blood cells are too large and stay in the blood.
Tissue fluid therefore has a similar composition to plasma but with much less protein. Cells exchange substances with it: they take in oxygen and nutrients and release carbon dioxide and waste.
Section 4
Return of tissue fluid
At the venous end the hydrostatic pressure has fallen because fluid has left the capillary. The plasma proteins remaining in the blood give it a lower water potential than the tissue fluid, so water moves back into the capillary by osmosis.
Not all tissue fluid returns this way: the rest drains into lymph vessels and is returned to the blood near the heart.
Worked example: hydrostatic pressure 4.0 kPa at the arteriolar end and 2.0 kPa at the venous end, against 3.3 kPa from plasma proteins. At the arteriolar end the net force is 4.0 − 3.3 = 0.7 kPa outwards; at the venous end it is 3.3 − 2.0 = 1.3 kPa inwards.
If the plasma protein concentration is low (for example in liver disease), less water is drawn back by osmosis, so fluid builds up and causes swelling (oedema).
In explanations of fluid movement, always name BOTH pressures (hydrostatic and the osmotic effect of plasma proteins) and say which is greater.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Blood vessels and tissue fluid
- A student examined transverse sections of an artery and of the vein running alongside it in a mammal, using a light microscope. The artery had a thicker wall and a narrower lumen than the vein.Explain how the structure of a vein is adapted to its function.2 marks
- During exercise, blood flow to the skeletal muscles increases while blood flow to the digestive system decreases. This is controlled by the arterioles that supply the capillary beds in these organs.Blood flows more slowly in the capillaries than in the arterioles. Explain why this is an advantage.2 marks
- In a capillary in the leg of a healthy person, the hydrostatic pressure of the blood is 4.0 kPa at the arteriolar end and falls to 2.0 kPa at the venous end. The plasma proteins exert an opposing force of 3.3 kPa throughout the capillary, which draws water back into the blood by osmosis. A patient with severe liver disease makes too little albumin, a plasma protein, and has swollen ankles caused by excess tissue fluid.Use the data to explain the movement of fluid at the arteriolar end and at the venous end of the capillary in a healthy person.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).