We’ve been talking about movement of blood a lot over the last few articles, so it’s time we looked at blood itself in a bit more detail. But don’t worry if like me you’re a little squeamish because there will be no pictures of actual blood today. We will take a brief look at all the individual components of blood, and most importantly look at the formation of tissue fluid in capillary beds.
Blood
There are four main components of blood: red blood cells, white blood cells, plasma and platelets.
- Red blood cells contain haemoglobin, which binds to oxygen and carries it around the body. They have no nucleus and have a biconcave disc shape.
- White blood cells are involved in the immune response (you’ll cover these in a lot more detail when you look at the immune system).
- Platelets are small fragments of cells that are important in blood clotting.
- Plasma is the straw-coloured liquid that contains everything else – water, dissolved substances, and large proteins such as antibodies.
The red blood cells and large proteins are too big to fit through the capillary walls. Normally white blood cells don’t pass through capillary walls either, but if there is an area of infection they can get out into the surrounding tissue (again we’ll look at this when we get to the immune response). However, water and small dissolved substances can pass through capillary walls, which is how tissue fluid is formed.
Formation of Tissue Fluid
Tissue fluid surrounds the cells in a tissue. It is mainly water and dissolved substances, but very few proteins because most are too large to fit through the capillary walls. Let’s take a look at how tissue fluid is formed in capillary beds.
A capillary bed is a network of capillaries in a tissue. Capillary beds provide a large surface area to deliver substances like oxygen and glucose to respiring cells. They also take away waste substances like carbon dioxide. Movement from the blood into tissue fluid happens by a clever system of pressure filtration.
At the arteriole end of the capillary bed, oxygenated blood arrives at quite high pressure. The hydrostatic pressure (the pressure of the fluid) is higher in the capillaries than in the tissue, so fluid is forced out through the capillary walls into the tissue. The hydrostatic pressure gradually decreases as we move through the capillary bed because some of the fluid has left. This partly explains why blood is at lower pressure in veins compared to arteries.
At the venule end, the blood has become deoxygenated because the oxygen has been delivered to the tissue. The fluid needs to get back into the blood otherwise the tissue would just constantly swell with fluid. There’s a different type of pressure going on at this end called oncotic pressure. This is all to do with osmosis. Remember that large plasma proteins can’t fit through the capillary walls, so there is a higher concentration of those in the blood compared to the tissue fluid. The concentration has become even higher at the venule end because some water has left the capillaries. That means that the water potential in the capillaries is lower than in the tissue, and water moves towards the area of lower water potential. Water is drawn back into the blood by osmosis.
Osmosis is not enough to get all of the lost fluid back into the capillaries, so any excess tissue fluid is drained away by the lymphatic system. Lymph vessels contain valves like veins, and drain into larger lymph vessels in the thorax which eventually drain back into the blood close to the heart. So the fluid still ends up back in the blood, it just takes an alternative pathway. The fluid in lymph vessels is called lymph, and also contains white blood cells and antibodies.
Summary
It’s quite difficult to get your head around formation of tissue fluid. Just remember that there are different types of pressure in action at each end of the capillary bed.
- Blood contains red blood cells, white blood cells, platelets and plasma.
- Capillary beds are where exchange happens in tissues.
- Hydrostatic pressure forces movement of fluid from the capillaries to the tissue at the arteriole end.
- Oncotic pressure (osmosis) draws fluid back into the capillaries from the tissue at the venule end.
- Excess tissue fluid drains into the lymphatic system.
