What is the role of blood?

Blood is pumped around your body by your heart and through your blood vessels. It has many roles, including:

• carrying nutrients and oxygen throughout your body
• carrying white cells and antibodies to fight infection
• forming blood clots to stop bleeding
• carrying waste to the liver and kidneys
• to help regulate body temperature

What is blood made up of?

Blood is made up of cells and plasma. An average adult weighing 70kg has about 5 to 5.5 litres of blood in their body.

What are blood cells?

Most blood cells are made in the bone marrow. There are 3 main types of blood cells:

• red blood cells
• white blood cells
• platelets

Red blood cells

Red blood cells make up about 40 to 45% of your blood. They carry oxygen from the lungs to the rest of the body and return carbon dioxide so it can be exhaled.

Red blood cells have haemoglobin (Hb) — this is a protein that allows them to carry oxygen. Haemoglobin is also what makes red blood cells appear red.

White blood cells

White blood cells detect and fight infections or foreign materials that enter the body. They make up around 1% of your blood.

There are 5 main types of white blood cell — neutrophils, lymphocytes, monocytes, eosinophils and basophils. They work together to protect your body from illness.

Platelets

Platelets are small pieces of cells that help the blood clot. They are in the blood until there is an injury to a blood vessel. Then they clump together and help a clot form at the injury site to help stop you bleeding.

What is plasma?

Plasma is the straw-coloured fluid that helps carry the blood cells. It helps move nutrients, waste, proteins and hormones through your blood vessels.

Why do I need blood tests?

Your blood can give your health team useful information on how well your body is working and check for some diseases and health conditions. Here are some common blood tests:

• Full blood count — known as an FBC or CBC, measures the number of the different types of cells in your blood.
• Blood glucose test — measures the amount of glucose (sugar) in your blood and can be used to diagnose or monitor diabetes.
• Kidney function test — measures substances in your blood to see how well your kidneys are working.
• Liver function tests — measure substances in the blood that show how well your liver is working.

What are the different blood types?

Everyone has a particular blood type (blood group) which is inherited from their parents.

There are 2 systems that together make up your blood type:

• Blood type, such as A, B, AB, and O.
• Rhesus (Rh) type or group is positive or negative.

Doctors check a person’s blood type before a blood transfusion or organ transplant, to make sure it is compatible with the donor’s blood type. Blood type is also important if you’re pregnant. Your doctors will check your Rhesus type, to see if your blood is compatible with your baby’s blood.

Read more on blood types and the role of Rhesus groups.

What are blood vessels?

Blood vessels are the network of tubes through which blood is pumped around the body. Along with the heart and the blood, the blood vessels form the cardiovascular system.

There are 3 main types of blood vessels:

• Arteries — carry blood pumped away from the heart to the organs.
• Veins — return blood to the heart from the body organs.
• Capillaries — tiny vessels that connect arteries and veins. Their thin walls allow blood to come into close contact with body tissues to exchange oxygen, carbon dioxide, nutrients and waste products.

Blood leaves the heart in large arteries, then moves through smaller blood vessels to reach the capillaries in the tissues.

Oxygen and nutrients pass through the thin capillary walls into the body tissues, and waste products from the tissues pass into the capillary blood. The blood then leaves the capillaries and moves into veins that get larger as they get closer to the heart.

Once blood gets back to the heart it needs oxygen. It enters the right side of the heart and is pumped to the lungs where the carbon dioxide is removed and replaced with oxygen.

The walls of arteries are strong and elastic and can expand and contract to lower or increase blood pressure, according to your body’s needs. Blood pressure is the pressure of your blood on the walls of the arteries as your heart pumps blood around the body.

What are some diseases of blood and blood vessels?

Your blood and blood vessels can be affected by physical injury or disease.

There are many different problems that can happen with your blood. Here are some examples:

• Blood disorders — where the number of certain types of blood cells is either too high or too low, such as neutropenia, anaemia or leukopenia.
• Clotting disorders — where the blood doesn’t clot properly, leading to haemorrhage (bleeding), or where the blood clots too much, causing thrombosis.
• Cancers — affect blood cells, such as leukaemia.
• Haemochromatosis — an inherited disorder which leads to too much iron in the blood.
• Bloodstream infection — a severe infection when bacteria enter the bloodstream. It can also happen as a complication of a medical or surgical procedure.

Problems that may happen to blood vessels include:

• Atherosclerosis — where build-up of cholesterol in blood vessel walls leads to narrowing. When this happens in the blood vessels that supply the heart, it’s called coronary artery disease. Atherosclerosis can lead to heart attack and stroke.
• Peripheral vascular disease — involves problems with the arteries outside of the heart and brain (usually in the legs and feet). One of the causes is atherosclerosis.
• High blood pressure — over time, this can damage your arteries and kidneys.
• Raynaud phenomenon — a short-term constriction (tightening) of blood flow to the fingers and toes, causing them to become cold and appear blue or white.

Resources and support

Speak with your doctor about any concerns or if you have any questions about your blood or blood vessels. You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available 24 hours a day, 7 days a week.

Other resources with more information include the following:

• The Heart Foundation has information on your heart, how it works and coronary heart disease.
• The Leukaemia Foundation has blood cancer information, as well as general information on what is blood, bone marrow and how blood is made in the body.
• The Heart Research Institute (HRI) has information on cardiovascular disease, impact and risks.
• Pathology Tests Explained (PTEx) has detailed information about different blood tests your health team may recommend.
• Haemochromatosis Australia has information on inherited iron overload disorder.