Blood is made of cells and plasma. There are three types of blood cells – red cells, white cells and platelets. All are made in the marrow found in many bones.
Red blood cells deliver oxygen from the lungs to the rest of the body, and carry waste products to be released by the lungs or the kidneys. Red blood cells contain haemoglobin, which is the protein that binds and releases oxygen.
Platelets are small cells that help the blood clot.
Plasma is the clearish fluid that carries the cells. It also carries the nutrients from our diet such as sugars, fats, proteins, vitamins and minerals.
As well as carrying cells, nutrients, oxygen and waste, blood also helps to regulate body temperature.
What are blood vessels?
Blood vessels carry blood around the body. The three main types of blood vessels are:
- arteries that carry blood pumped from the heart – these are the largest and strongest
- veins that return blood to the heart
- capillaries, which are tiny vessels that connect arteries and veins, and allow blood to come into close contact with tissues for the oxygen, carbon dioxide, food and waste.
Blood leaves the heart in large arteries, then moves through progressively smaller ones to the capillaries in tissues. It leaves in veins that get larger as they get closer to the heart.
The arteries can expand and contract to lower or increase blood pressure, according to your needs.
Diseases of blood and blood vessels
The blood can be affected by trauma or diseases in other parts of the body leading to anaemia, in which the supply of oxygen to tissues is too low, or polycythaemia, in which there are too many red blood cells. Also, there are cancers of blood cells like leukaemia and lymphoma.
Last reviewed: July 2015