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Blood and Blood Vessels


It is a liquid that flows through (or circulates) the body and transports substances. It comprises of a number of different cells and chemicals.

ComponentAbout it
Plasma This is a liquid that makes up the majority of blood. It's mainly water, but also has many different chemicals dissolved in it. These include waste products (like urea and carbon dioxide), salts, glucose, and hormones
Red blood cells These are specialised cells that transport oxygen around the body. They can do this because they contain haemoglobin which oxygen sticks to.

They have two specialisations, firstly they have no nucleus so there is plenty of space for haemoglobin; and their shape is a biconcave disk (see above).
White blood cells Please see the main article: Immune System
PlateletsThese are the fragments of other blood cells - small fragments of protein that clot blood. So if you have a cut for example, these would create a kind of a dam that stops blood leaving the blood vessels.

Blood Vessels

As you most likely know, blood is the most popular transport system in the body, carrying oxygen, hormones and sugars. However, there are different types of blood vessel that have different roles.

ArteryarteryHas a thick muscular wall that withstands pressure from heartbeat.Maintains blood pressure and smooth flow of blood to tissue.
CapillaryThin wall, and small vessel.Allows exchange of material from blood to tissue.
VeinveinWeaker wall with valves in.The valves prevent blood flowing backwards since it is at lower pressure.

Red Blood Cells and Haemoglobin

Red blood cells are very important cells. They are made in bone marrow and only last about 120 days before being destroyed. And crucially they contain haemoglobin and other chemicals that help it function.

Now more about haemoglobin. It is a protein that has got iron ions on it that pick up oxygen, and when these oxygen molecules are stuck onto it it is called oxyhaemoglobin. The relationship between oxygen and haemoglobin is represented in the oxygen dissociation curves.

oxygen dissociation curve

Put simply, the orange curve shows that, at first haemoglobin (Hb) is reluctant to pick up oxygen. But as the amount of O2 increases a few oxygen molecules attach to it, and this makes it much easier for the rest to attach. As the partial pressure of oxygen gets very high the curve levels off. This is because it already is almost fully saturated and getting more onto it is difficult.

The green line shows the approximate position of a curve for foetul haemoglobin it is in the position because their haemoglobin will pick up oxygen more easily than the mother's will. This means it will pick up the oxygen as the mother looses it. The foetul Hb is said to have a higher affinity for oxygen because it picks it up more easily.

To the right in purple is the curve in high CO2 concentrations, the Hb has a lower affinity to oxygen. The reason for this is known as the Bohr affect, and is because more carbon dioxide makes the blood more acidic.

Capillary and Lymphatic System

The capillary is a key part of the circulatory system, it allows for the exchange of substances being transported by the blood into the tissue fluid; and it is able to do this because it is permeable.

The tissue fluid is a liquid that surrounds all cells, substances like carbon dioxide and are exchanged through it. It has a similar composition to blood, except for larger proteins which are too big. A basic outline of the capillary is shown below along with the pressure changes through it.

outline of capillary and pressure changes along it

Blood enters the capillary through the capillary, at first hydrostatic (force of blood being pumped) pressure is very high, which forces fluid out of the and into the tissue fluid. However, as the blood travel through the capillary the pressure drops because volume is reducing. When it goes below the water potential, fluid drains back into the blood vessel.

A key point though is that, as the graph shows, most of the time the hydrostatic pressure is high so more fluid leaves the blood than goes back. This is where the lymph is important, becuase it removes excess fluid and carries it around the lymphatic system where it is later depositied into the blood stream again in the neck.