Blood Components: Plasma, Cells, and Platelets

Blood is a connective tissue made of a liquid, plasma, and three kinds of cell components: red blood cells, white blood cells and platelets. In brief: plasma makes up roughly 55 percent of blood volume and carries dissolved substances, red blood cells carry oxygen with the protein hemoglobin, white blood cells defend against infection and platelets help the blood to clot. An adult has about 5 liters of blood.

Blood links all the organs of the body and has several jobs: transport, defense and regulation. This guide describes each component, explains hemoglobin and blood clotting, compares the components in a table, works through a blood-count and hematocrit calculation, and points out common mistakes.

What is blood made of?

When a sample of blood is spun in a centrifuge, it separates into layers. The top layer is straw-colored plasma, about 55 percent of the volume. A thin middle layer, the “buffy coat”, contains white blood cells and platelets, less than 1 percent. The bottom layer, about 45 percent in an average adult, contains red blood cells. The percentage of blood volume taken up by red cells is called the hematocrit.

The main functions of blood are transport (oxygen, carbon dioxide, nutrients, hormones and wastes), protection (defense against pathogens and clotting after injury) and regulation (body temperature, pH and water balance).

What does plasma do?

Plasma is about 90 percent water. It also contains dissolved substances: proteins (albumin, antibodies and clotting proteins such as fibrinogen), glucose, amino acids, lipids, mineral ions, hormones, urea and dissolved carbon dioxide, mostly as hydrogencarbonate ions.

Plasma carries these materials around the body, and it distributes heat from active organs. Albumin helps to keep water in the blood vessels by osmosis. Serum is plasma from which the clotting proteins have been removed.

What do red blood cells do?

Red blood cells, or erythrocytes, are the most numerous blood cells, with about 4.5 to 6 million per microliter of blood in adults. Their job is to carry oxygen from the lungs to the tissues.

• Biconcave disc shape: increases the surface area for gas exchange and shortens the diffusion distance. It is also flexible enough to squeeze through narrow capillaries.
• No nucleus (in mammals): leaves more room for hemoglobin.
• Hemoglobin: a red protein that contains iron. Each molecule can bind up to four oxygen molecules. In the lungs, where oxygen is abundant, hemoglobin becomes oxyhemoglobin (bright red). In tissues, it releases oxygen.

Red blood cells live about 120 days and are made in the red bone marrow. Old cells are broken down in the liver and spleen. Anemia is a condition in which blood has too little hemoglobin or too few red cells, and it often causes tiredness. A common cause is lack of iron.

What do white blood cells do?

White blood cells, or leukocytes, are the cells of the immune system. They are fewer than red cells, about 4 000 to 11 000 per microliter, and have a nucleus. There are two main groups.

• Phagocytes (such as neutrophils and macrophages) engulf and digest pathogens in a process called phagocytosis.
• Lymphocytes (B cells and T cells) recognize specific pathogens. B cells produce antibodies, and T cells destroy infected cells or coordinate the response.

The number of white cells often rises during infection, which is why doctors check it. The immune system is covered in detail in separate articles.

What do platelets do?

Platelets (thrombocytes) are small cell fragments, about 2 to 3 micrometers across, which come from large cells in the bone marrow called megakaryocytes. They have no nucleus. A normal count is roughly 150 000 to 400 000 per microliter. They survive for about 8 to 10 days. Their job is to start the clotting process at the site of an injury.

How does blood clotting work?

Clotting, or coagulation, seals wounds, stops blood loss and blocks the entry of pathogens. The main steps are as follows.

1. When a vessel is damaged, platelets stick to the damaged wall and to each other and form a plug.
2. Damaged tissue and platelets release chemicals that start a chain of reactions involving clotting factors in the plasma.
3. The enzyme thrombin converts the soluble protein fibrinogen into insoluble fibrin threads.
4. The fibrin forms a mesh that traps red blood cells and platelets and forms a clot.
5. The clot dries into a scab, and the skin underneath heals.

Clotting needs vitamin K and calcium ions. In hemophilia, a genetic condition, one clotting factor is missing, and bleeding continues for a long time. Clots inside vessels can be dangerous when they block blood flow, as in a stroke or heart attack.

Blood components compared

Worked example: reading a blood test

A centrifuged blood sample of 10 mL shows 4.4 mL of packed red cells, 0.05 mL of white cells and platelets, and the rest is plasma. A separate count finds 300 000 platelets per microliter, and a blood volume of 5 liters.

1. Plasma volume: 10 − 4.4 − 0.05 = 5.55 mL, which is 55.5 percent of the sample.
2. Hematocrit: 4.4 ÷ 10 × 100 = 44 percent, which is within a normal range for adults (roughly 36 to 50 percent, depending on sex).
3. Total platelets: 5 liters is 5 000 000 µL, so 300 000 × 5 000 000 = 1.5 × 10¹² platelets in the whole body.

The percentages add to 44 + 0.5 + 55.5 = 100. The results are all normal, so there is no sign of anemia or infection. A hematocrit of 30 percent would suggest anemia, while very high values may suggest dehydration.

How was blood understood?

Red blood cells were first seen by Jan Swammerdam and Antonie van Leeuwenhoek in the seventeenth century. In 1901 Karl Landsteiner discovered the ABO blood groups, which made safe transfusions possible, and he was awarded the Nobel Prize in 1930. Hemoglobin’s structure was determined by Max Perutz in the 1950s and 1960s. The discovery of the role of platelets in clotting is credited to Giulio Bizzozero in 1882.

Which mistakes are common?

Mistake 1: Saying that blood is a fluid only

Blood includes cells and cell fragments suspended in plasma. It is classed as a connective tissue.

Mistake 2: Thinking that deoxygenated blood is blue

Blood is always red. Oxygen-rich blood is bright red, and oxygen-poor blood is darker red. Veins look blue through the skin because of the way light passes through tissue.

Mistake 3: Saying platelets are cells

They are fragments of cells and have no nucleus.

Mistake 4: Confusing red and white cell functions

Red cells transport oxygen, and white cells defend against infection.