What Is Blood?
Blood Composition, Function And Types Explained

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red blood cells picture

What Is Blood?


What Is Blood?
Interesting Facts
What Does Blood Do?
What Is Blood Made Of?
What Are Blood Types?
What Is The Rhesus Factor?
The Cultural Significance of Blood

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What Is Blood?

Quick Summary: Blood is a liquid that circulates in the veins and arteries of body. It carries oxygen and food to the cells in the body and takes away carbon dioxide and other waste products. It fights infection, keeps you warm and distributes chemicals.

Blood is the red fluid that circulates through the veins and arteries of your body. It is pumped by the heart to the lungs and other tissues in the body and returned to the heart again for further pumping (this is known as the circulatory system). Almost half the volume of blood consists of blood cells - that is red blood cells (erythrocytes) which carry oxygen and white blood cells (leukocytes) that defend the body against infection; and platelets (thrombocytes) that help the blood clot. The rest of the fluid is made up of plasma, a watery, yellowish fluid that contains sugar, protein, salts, fats and minerals. The average adult has 4 to 5 liters of blood whereas a newborn baby has only 300 milliliters. Blood is alkaline (pH 7.4).

Interesting Facts

• Oxygen turns blood bright red when you bleed (when it is exposed to oxygen in the air). When it circulates in your veins it is reddy brown in color.
• Most people’s blood belongs to 1 of 4 blood types: A, O, B and AB. O is the most common.
• Blood is either Rhesus positive (Rh+) or Rhesus negative (Rh-). If your blood is Rh+ and your group is A, you are said to be A positive. If your blood is Rh- and your group is O, you are O negative.
• When you receive blood from another person, it is called a transfusion. Your body will only accept blood from certain groups that match yours. Blood transfusions are given to people who have lost too much blood because of an operation or injury. It is also given to replace diseased blood.
• During a 24-hour period an adult human heart pumps 36,000 liters of blood through 20,000 km of blood vessels.
• A bruise is caused by dead blood cells. When capillaries burst, the blood cells leak out into surrounding tissue and die off. The various colors in a bruise show the different stages of the cells' breakdown and finally the body gets rid of them and normal skin color returns.

What Does Blood Do?

• Transports oxygen, nutrients, hormones and enzymes around the body.
• Transports carbon dioxide and waste materials from the body to the organs of excretion (kidneys, liver, lungs).
• Helps fight infection (with leucocytes and antibodies).
• Prevents the loss of body fluids after accidents by clotting.
• Regulates body temperature.

What Is Blood Made Of?

types of blood cells

A: Plasma

Plasma makes up 55 percent of blood volume. It is a slightly thick, straw-colored fluid. It is mostly water (90 to 92 percent) and the rest is plasma proteins (albumin, globulin, fibrinogen and prothrombin).
Plasma helps to transport the following essential substances around the body:
• Mineral salts: Sodium chloride, commonly known as table salt, sodium carbonate and the salts of potassium, magnesium, phosphorus, calcium, iron, copper, iodine - which help nerve conduction and ensure that tissue cells keep the right acid balance.
• Nutrients: Amino acids, fatty acids, glucose, glycerol, vitamins. Most of these come from digested food and are absorbed (by the plasma proteins in blood) from the intestines to be used by cell tissues for energy, repair and cell reproduction.
• Waste: Waste products, for example urea, are transported to the liver for breakdown, and then to the kidneys to be excreted as urine.
• Hormones: Chemical messengers produced by the endocrine glands. Plasma transports them to various organs and their job is then to change or influence that organ's activity or behavior.
• Enzymes: The chemical catalysts in the body. They produce or speed up chemical changes in other substances but remain unchanged themselves.
• Gases: Oxygen (O2) and carbon dioxide (CO2) are dissolved in plasma.
• Antibodies: The body's protectors. These complex proteins are produced by lymphocytes in response to the presence of antigens, such as viruses and bacteria, in the body. The cardiovascular system (the system that pumps blood around the body) works closely with the lymphatic system in protecting the body against disease.

B: Cells

There are three types of blood cells:

Erythrocytes (also known as Red Blood Cells/ Red Corpuscles)
Structure: Button shaped, small biconcave cells with no nucleus or other organelles.
Function: Contain the protein hemoglobin which allows them to transport oxygen to cells in the body (when hemoglobin carries oxygen it is referred to as oxyhemoglobin). Iron and vitamin B12 are required for this process.
General characteristics:
• Approximately 5,000,000 red blood cells per milliliter of blood.
• Produced in red bone marrow.
• Life span of about 120 days.
• Broken down in the spleen and then the liver (where any spare iron is retrieved and recycled).
• Oxyhemoglobin gives blood its characteristic red color.

Leucocytes (also known as White Blood Cells/White Corpuscles)
Structure: Large irregularly shaped cells which contain a nucleus.
Function: To protect the body from infection.
General characteristics:
• Approximately 8,000 leucocytes per milliliter of blood in a healthy body.
• Number can increase rapidly when infection is present.
• Produced in bone marrow.
• Life span varies from hours to years depending on type and usage.
• Can pass through capillary walls into tissues.

Thrombocytes (also known as Platelets)
Structure: Small, fragile cell fragments, contain mitochondria but no nucleus.
Function: Contain various elements that are responsible for blood clotting.
General characteristics:
• Approximately 250,000 per milliliter of blood.
• Produced in bone marrow. When more platelets are required the liver produces a hormone called thrombopoietin that stimulates the bone marrow to produce large cells called megakaryocytes. These cells are then broken up into hundreds of cell pieces, these fragments are platelets.
• Life span approximately 10 days. Destroyed by the spleen.

What Are Blood Types?

In 1902 an Austrian physician named Karl Landsteiner began studying why some patients died as a result of blood transfusions. He discovered the existence of different human blood types and subsequently classified them as four groups: A, B, AB and O. Type O is known as the universal donor because type O blood can be given to patients with any blood group whereas type AB is known as the universal recipient: patients with type AB blood can receive blood from any blood group. The table shows which group is compatible with which.

Type Can Give Blood To Can Receive Blood From
O Any blood group O
AB AB Any blood group
A A and AB A and O
B B and AB B and O

The genes from your parents determine your blood group.

What Is The Rhesus Factor?

The Rhesus factor (abbreviated as Rh or sometimes RhD) is an antigen (protein) found in the red blood cells of most people and animals. Blood that is described as rhesus positive contains this antigen, whereas rhesus negative blood does not. 85 percent of people are Rh+, the remainder are Rh-. The only time rhesus factor becomes an issue, generally, is in pregnancy. If the mother is Rh- and the baby is Rh+, the baby may develop rhesus disease (also called Rh disease or hemolytic disease). Only mothers who are Rh- are at risk - Rh+ women can carry a child with an opposite factor without problems. Although the mother will not be affected, a baby with Rh disease can suffer from anemia, jaundice and even heart or brain damage. Pregnant women should be tested for their Rh factor early in pregnancy. If found to be Rh-, she may require immunoglobulin injections in the third trimester to reduce the risk of Rh disease in her baby (works in 99 percent of cases).

The Cultural Significance of Blood

The use of the word 'blood' in the English language shows that, historically, the circulation has had a moral and social importance beyond its biological necessity, signifying family, connection and emotion. 'Blood is thicker than water' means that family duties and connections are stronger than any others. 'Blood money' refers to the compensation paid to the relatives of a murder victim in an attempt to stop them seeking revenge. If you have 'bad blood' you are likely to feel unwell and friends or relatives with 'bad blood' between them do not like each other. The term 'blue blood' comes from the Spanish sangre azul, which was used to describe aristocrats of 'purer' ancestry than those with 'mixed' blood. It is still used today to refer to the nobility and aristocracy. Since blood has such a strong link to family and emotions it is no surprise that 'cold-blooded' means lacking feeling and 'making one's blood boil' means causing anger.

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