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Blood and circulation

What makes up blood?

Blood transports materials and heat around the body and helps to protect against disease. It contains:

• plasma
• red blood cells
• white blood cells
• platelets

Plasma is a straw-coloured liquid that makes up just over half the volume of blood.

Plasma

• a straw-coloured liquid
• mainly water
• transports carbon dioxide, digested food, urea, hormones and heat

Red blood cells

• contain haemoglobin - a red protein that combines with oxygen
• they have no nucleus so they can contain more haemoglobin
• small and flexible so that they can fit through narrow blood vessels
• their job is to transport oxygen

White blood cells

• larger than red blood cells
• they have a nucleus
• can change shape
• defend the body by fighting germs that cause disease
• produce chemicals called antibodies - a protein that attacks germs and the poisons they make

Platelets

• involved in blood clotting and scab formation to stop bleeding if your skin is cut

The circulatory system consists of:

• a system of tubes (arteries, capillaries and veins)
• a pump (the heart)
• valves to ensure a one-way flow of blood

You have about 5.5 litres of blood in your body
That’s is the same amount of liquid as there is in these cola bottles!

Blood travels around the body in tubes called arteries, veins or capillaries

• Arteries carry blood away from the heart towards an organ
• Veins carry blood from an organ towards the heart.

An easy way to remember this is: Arteries Away or Vein In

Blood full of oxygen is called oxygenated blood. Blood with oxygen removed is called deoxygenated blood.

So, arteries carry oxygenated blood away from the heart, veins carry deoxygenated blood back to the heart

One important exception: unlike all other arteries and veins, the pulmonary artery (from the heart to the lungs) carries blood with oxygen removed and the pulmonary vein (from the lungs to the heart) carries blood full of oxygen.

Artery

• Carry oxygenated blood away from the heart
• Have thick muscular walls because the blood they contain is at high pressure
• Have small diameter passageways for blood to pass through
• Blood flows quickly

Vein

• Carry deoxygenated blood to the heart
• Have thin walls because they contain blood at low pressure
• Have larger diameter passageway for blood to pass through
• Blood flows more slowly
• Have valves to prevent low pressure blood flowing backwards

• Much smaller than arteries and veins
• Only one cell thick
• Substances such as oxygen, glucose and carbon dioxide can diffuse through them
• Blood is at very low pressure
• Where gas exchange takes place - oxygen passes through the thin capillary wall and into the tissues, while carbon dioxide passes from the tissues into the blood

This table lists the arteries and veins that are associated with the lungs, liver and kidneys.

The human circulatory system is a double circulatory system. It has two separate circuits and blood passes through the heart twice:

• the pulmonary circuit is between the heart and lungs
• the systemic circuit is between the heart and the other organs

Pulmonary circuit

The pulmonary circuit transports blood to the lungs. The blood is oxygenated there and then carried back to the heart. Exchange of oxygen and carbon dioxide happens in the lungs:

• carbon dioxide diffuses from the blood into the air in the alveoli
• oxygen diffuses from the air in the alveoli into the blood, and is absorbed by the red blood cells
• Reminder: Unlike all other arteries and veins, the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.

Systemic circuit

The systemic circuit transports blood around the rest of the body. It transports oxygen and nutrients from the heart to the body tissues and carries away deoxygenated blood containing carbon dioxide and other waste materials, which arrive back at the heart.

The heart

The heart is made of a special muscle called cardiac muscle which can tighten, or contract, regularly. It's job is to pump blood around your body. It is about the size of a clenched fist.

Unlike the rest of your muscles, your heart expands and contracts, non-stop, all day and all night for your entire life without a break. About 38 million times a year! Without getting tired!!

It pumps the blood which fills it up into arteries which carry the blood to the lungs or around the body.

• the right side pumps blood through the pulmonary circuit to the lungs
• the left side pumps blood through the systemic circuit to the rest of the body
• the left side has thicker walls because it needs to put the blood under higher pressure than the right side because this blood must be able to flow to the furthest parts of the body
• a septum separates the right and left sides

In general, blood flows into the heart from a vein, goes into an atrium, then a ventricle, and out through an artery.

The valves prevent the blood flowing backwards.

A heart beat

The heart is relaxed and blood flows into both sides from the veins.

The right and left atrium contract. Blood is forced into the right and left ventricles.

The ventricles quickly contract. The valves between each atrium and ventricle close. The blood is squeezed, at pressure, out of the heart into the arteries.

The heart relaxes and fills up with blood again… approximately 72 times every minute.

Test your knowledge.

The simplest way of measuring heart rate is to measure your pulse. As the heart beats, a pulse can be felt in locations where an artery passes over something solid, such as bone. Pulse locations include wrist, neck and upper arm.

The pulse rate is expressed in beats per minute.

To measure pulse rate

• gently press down on the inside of the wrist with your index and middle fingers
• when you have located the pulse start a stopwatch
• count the number of beats for 30 seconds
• multiply this number by two to give the number of beats in one minute
• record the number of beats per minute in a suitable table
• repeat twice more
• calculate the average number of beats per minute

A normal resting heart rate should be 60 – 100 beats per minute, but it can vary from minute to minute. The average is heart rate is 72 beats per minute. In healthy people, generally, the lower the resting pulse rate, the fitter they are.

Effect of exercise

During exercise, the muscle cells need more energy than usual. So, they need more oxygen and glucose, and they produce more waste carbon dioxide.

To meet this demand, the heart:

• beats more often – the heart rate increases
• beats more powerfully – pumping a greater volume of blood with each beat

The changes in heart rate are easily observed by measuring the pulse rate before and after exercise.

Coronary heart disease is the term that describes what happens when your heart's blood supply is blocked or interrupted. Coronary heart disease is a major cause of death both in Northern Ireland and worldwide.

The heart is a muscle and to work properly it needs its own supply of blood containing oxygen. Coronary arteries supply blood to the heart muscle. If these become blocked by fatty material and plaque, the result is coronary heart disease.

If a coronary artery is blocked, the blood supply to part of the heart muscle is cut off. That part of the heart cannot continue to contract, causing a heart attack.

Many factors increase the risk of developing Coronary heart disease.

• Smoking
• A poor diet that is high in cholesterol
• A lack of exercise

Healthy heart

To help prevent heart disease, you can:

• Eat healthy
• Exercise
• Stay a healthy weight
• Don’t smoke and avoid passive smoking
• Cut down on salt
• Cut down on sugar
• Limit saturated fat
• Fill up on fruit and veg
• Eat more fish

The most common symptom of coronary heart disease is chest pain.

A blood donation occurs when a person (the blood donor) voluntarily has blood drawn from them and used for transfusions.

Blood products

Blood products are produced from blood donated by blood donors.

They are the parts of blood that are given to a patient by transfusion. Patients rarely receive transfusions of whole blood in modern medicine.

Blood products include:

• red blood cells
• platelets
• plasma
• antibodies

The products are given to patients depending on their needs.

Blood products are screened for:

• infectious agents such as HIV
• blood group
• the presence of certain antibodies

Blood for transfusion must be compatible with that of the patient's blood, for instance, their blood group must match.

Before a transfusion, white blood cells are often removed to reduce the risk of infections or immune reactions.

Need for blood donors

More young people are needed to start giving blood to ensure there is enough blood in the future.Most people over the age of 16 can donate blood.

You can give blood if you:

• are fit and healthy
• weigh between 50kg and 158kg
• are aged from 17 to 66 (or 70 if you have given blood before)

The Northern Ireland Blood Transfusion Service is responsible for the collection, testing and distribution of over 55 000 blood donations each year from almost 800 donation sessions.