The Blood Circulation System

Whether you are asleep, reading or playing football, blood keeps moving through your body without a pause. The heart pushes it into tubes called blood vessels. It travels all the way to your fingertips and toes, and then it comes back to the heart to be pushed out again. This never-ending movement of blood around the body is called circulation.

Three parts make up this system: the heart, the blood and the blood vessels. The heart works like a pump, the blood is what gets carried, and the vessels are the roads it travels along. Think of a water tank on a roof sending water through pipes to every tap in a house. The difference is that tap water leaves the pipes, while blood stays inside its vessels and returns to the heart in a closed loop.

People did not always know that blood travels around the body. The British doctor William Harvey proved in 1628 that blood circulates in our body.

Working together, the heart, the blood and the blood vessels do these jobs for the body:

• (a) It carries the oxygen we breathe into our lungs out to the tissues of the body, and brings carbon dioxide from the tissues back to the lungs.
• (b) After the digestive system has digested our food, it carries the absorbed nutrients to the cells that need them.
• (c) It takes unwanted waste to the kidneys so that the waste can be removed from the body.
• (d) It delivers the hormones, enzymes and minerals the body needs to its cells.
• (e) It helps control body temperature.

Blood is the red liquid you see when you cut your finger or graze your knee. Inside the body it always flows in one direction. To the naked eye it looks like one plain red liquid, but under a microscope it turns out to be made of several different parts.

A connective tissue is a tissue that links and supports the other parts of the body. Blood does this job as a liquid. By flowing everywhere, it connects every organ with every other organ. That is why blood is called a liquid connective tissue.

For example, how much of a 50 kg person's weight is blood? Seven percent of 50 is 3.5, and eight percent of 50 is 4. So about 3.5 to 4 kg of that person's weight is blood.

Although blood looks like a plain liquid, it contains both liquid and solid matter. The liquid part is called plasma, and the solid part is made of blood cells. Blood is 55 percent plasma and 45 percent blood cells. When a laboratory spins a sample of blood in a machine, the heavier cells sink to the bottom and the pale yellow plasma stays on top.

Inside the body the two parts do not separate. The blood cells float in the plasma and are carried along with it through the blood vessels. If a blood vessel is opened up, the three kinds of blood cells can be seen floating in the plasma like this:

Plasma is the liquid part of blood. It is 92 percent water. The rest is made up of nutrients, oxygen, metabolic waste and hormones, all dissolved in the water in the same way that sugar dissolves in tea. Metabolic waste means the leftover substances that cells produce while doing their work, which the body does not need.

There are three kinds of blood cells:

• (a) red blood cells
• (b) white blood cells
• (c) platelets

Red blood cells are also called erythrocytes. They are made in the marrow inside our bones, and billions of new ones are made in the body every day. Each one is round, 6 microns in diameter and 2 microns thick. A micron is one thousandth of a millimetre, so these cells can only be seen under a microscope. One cubic millilitre of blood holds about 4 to 5 million red blood cells.

A red blood cell has no nucleus, but it is flexible. It is round, flat, and dipped inwards on both sides, like a round flat sweet pressed in at the centre from above and below. This shape and this flexibility let red blood cells bend and slip easily through even the finest blood vessels, the capillaries.

Red blood cells contain haemoglobin, and it is haemoglobin that makes blood look red. Haemoglobin contains iron. It takes up oxygen and carries it to every tissue of the body.

In an adult, red blood cells are made by the bone marrow of the arm and leg bones, the backbone, the ribs, the sternum (breastbone) and the skull. A red blood cell lives for about 120 days. Dead red blood cells are collected in the spleen. Because old cells keep dying while the marrow keeps making new ones, the number of red cells in the blood stays steady.

When a person has anaemia, the iron in their haemoglobin also falls. With less iron there is less haemoglobin to carry oxygen, so less oxygen reaches the tissues. Anaemia is most likely in pregnant women, women who have recently given birth, and people who have had an operation.

To avoid anaemia, a person should eat more of these foods:

• green vegetables
• fruits
• iron-rich foods

White blood cells are bigger than red blood cells. Together, the white blood cells of the blood are called leucocytes. Each white blood cell has a nucleus in its middle. They measure from 8 to 20 micrometres across, and there are 4,000 to 11,000 of them in each cubic millilitre of blood. Like red blood cells, they are made in the bone marrow.

The chief work of white blood cells is to protect the body from disease. When germs get into the body from outside, white blood cells destroy them, working as antibodies and as the body's defence against disease. They also help clear away things the body does not need. Because they fight germs, white blood cells can be thought of as the body's soldiers.

Platelets are the smallest blood cells, only 2 to 4 microns across. There are 150,000 to 400,000 platelets in each cubic millilitre of blood.

Platelets carry several substances that blood needs in order to clot. When you are hurt and start bleeding, platelets work to stop the bleeding. If you fall and graze your knee while playing, it bleeds at first, but within a few minutes the blood thickens into a clot and stops flowing. That is the platelets at work.

The main work of blood is to take oxygen from the lungs to the cells, and to take the carbon dioxide made in the cells back to the lungs. Blood also carries nutrients to every cell and helps the body get rid of waste. Besides these, blood:

• (a) keeps body temperature controlled and balanced
• (b) carries hormones to different parts of the body
• (c) protects the body from diseases
• (d) keeps the tissues moist
• (e) helps balance the amount of fluid and salt in the body
• (f) carries different kinds of nutrients to the tissues

Everyone's blood looks alike, but it is not all of one kind. Human blood is divided into four main groups: A, B, AB and O.

Every blood group is either Rhesus (Rh) positive or Rh negative. Splitting each of the four groups in this way gives eight subgroups:

Most people's blood is Rh positive. People with Rh negative blood are very few in Nepal. Each person has blood of one group only.

Someone who has lost a lot of blood through illness or an accident may need blood from another person. But not just any blood will do. For a transfusion, the blood group and the Rh factor must match. Blood can be given like this:

Because a person with group AB can take blood from any group, group AB is called the universal recipient. A person with group O needs group O blood when they themselves need blood, but they can give blood to people of every other group, so group O is called the universal donor.

When blood is given, the blood group is not the only thing to check. The Rh factor, positive or negative, must be checked too. So a person with group A+ who needs blood should receive A+ blood. A person who is A+ should not be given A- blood, and a person who is A- should not be given A+ blood. The Rh must match on both sides.

So whenever one person's blood is transfused into another person, both the blood group and the Rh, positive or negative, must match. That is why, before a patient is given someone else's blood, the blood group and Rh factor are cross-matched. Cross-matching means testing the blood of both the giver and the receiver to make sure they match before any blood is given.

Example: a patient's blood group is B negative (B-). Which blood can this patient be given?

• Step 1: Look at the group. The patient is group B. By the table, group B can receive group B and group O blood.
• Step 2: Look at the Rh. The patient is Rh negative, so the donor must also be Rh negative.
• Step 3: Put the two together. B- or O- blood fits.
• Step 4: Cross-match the blood of the donor and the patient before the blood is given.

Not everyone can give blood. Certain conditions are set so that giving blood does not harm the donor. Healthy women and men who meet all of these conditions can donate blood:

• has completed 17 years of age and has not yet reached 60
• weighs more than 45 kg
• has a haemoglobin level of 12 to 16 grams
• is a healthy woman or man

Even a donor who meets these conditions cannot give blood again and again. The body needs time to make new blood, so there must be a gap between donations:

A year has 12 months. Twelve divided by 3 is 4, so a man can donate 4 times a year. Twelve divided by 4 is 3, so a woman can donate 3 times a year.

The blood we donate goes to patients whose lives are in danger: people hurt in accidents, people having operations, people who have lost a lot of blood. If a little of our blood gives a patient a new life, that is the greatest good deed and the greatest service in the world. That is why giving blood brings the deepest satisfaction a person can feel.