What Is Blood? What Does Blood Do?

Blood is a combination of plasma (watery liquid) and cells that float in it. It is a specialized bodily fluid that supplies essentials substances and nutrients, such as sugar, oxygen, and hormones to our cells, and carries waste away from those cells, this waste is eventually flushed out of the body in urine, feces, sweat, and lungs (carbon dioxide). Blood also contains clotting agents.

Plasma constitutes 55% of blood fluid in humans and other vertebrates (animals with a backbone, spinal column).

Apart from water, plasma also contains:

• Blood cells
• Carbon dioxide
• Glucose (sugar)
• Hormones
• Proteins

Types of blood cells:

• Red blood cells - also known as RBCs or erythrocytes. They are shaped like slightly indented, flattened disks. These are the most abundant cells, and contain hemoglobin (Hb or Hgb).
  
  Hemoglobin is a protein which contains iron; it transports oxygen from the lungs to body tissues and cells. 97% of a human's red blood cell's dry content is protein.
  
  Each RBC has a life span of about 4 months; at the end of their lives they are degraded by the spleen and the Kupffer cells in the liver. The body continuously replaces the ones that die.
• White blood cells (leukocytes) - these are the cells of our immune system; they defend the body against infections and foreign materials. Lymphocytes and ganulocytes (types of white blood cells) can move in and out of the bloodstream to reach affected areas of tissue.
  
  White blood cells will also fight abnormal cells, such as cancer cells.
  
  There are normally between 4x10¹⁰ white blood cells in one liter of blood (making up about 1% of total blood) in a healthy individual.
• Platelets (thrombocytes) - are involved in the clotting (coagulation) of blood. When we bleed the platelets clump together to help form a clot.
  
  If exposed to air the platelets break down and release fibrinogen into the bloodstream, this sets off a series of reactions which results in the clotting of blood in, for example on a skin wound. A scab is formed.

When hemoglobin is oxygenated human blood is bright red.

The heart pumps blood around the body through blood vessels. Oxygen-laden arterial blood is carried from the lungs to the rest of the body, and carbon dioxide laden blood (venous blood) is returned to the lungs where the carbon dioxide is exhaled. Carbon dioxide is a waste product produced by cells during metabolism.

According to Medilexicon's medical dictionary:

Blood is The "circulating tissue" of the body; the fluid and its suspended formed elements that are circulated through the heart, arteries, capillaries, and veins; blood is the means by which 1) oxygen and nutritive materials are transported to the tissues, and 2) carbon dioxide and various metabolic products are removed for excretion. Blood consists of a pale yellow or gray-yellow fluid, plasma, in which are suspended red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.

What is hematology?

Hematology is the diagnosis, treatment and prevention of blood and bone marrow diseases, as well as immunologic, blood clotting (hemostatic) and vascular systems. A doctor who is specialized in hematology is called a hematologist.

Functions of blood

• It supplies oxygen to cells and tissues.
• It supplies essential nutrients to cells, such as amino acids, fatty acids, and glucose.
• It removes carbon dioxide, urea and lactic acid (waste products)
• Its white blood cells have antibodies which defend us from infection and foreign bodies.
• It has specialized cells, such as platelets, which help the blood to clot (coagulate) when we are bleeding.
• It transports hormones - chemicals released by a cell in one part of the body that sends out messages that affect cells elsewhere in the body.
• It regulates our acidity (pH) levels.
• It regulates our body temperature. When the weather is very warm or during strenuous exercise there will be increased blood flow to the surface, resulting in warmer skin and faster heat loss. When environmental temperatures drop, blood flow focuses more on the important organs deep inside the body.
• It also has hydraulic functions - when a human is sexually aroused, engorgement (filling the area with blood) will result in a male erection and swelling of the female's clitoris.

Blood cells are produced in the bone marrow

White cells, red cells and platelets are made in the bone marrow - a jellylike substance that fills the cavities of bones. Bone marrow consists of fat, blood, and special cells (stem cells) that turn into the various kinds of blood cells. The main areas of bone marrow involved in the formation of blood cells are in the vertebrae, ribs, sternum, skull and hips.

There are two types of marrow, red marrow and yellow marrow. Most of our red and white blood cells, as well as platelets are made in the red marrow.

Blood cells in babies and very young children are made in the bone marrow of most of the bones in the body. As we get older, some of the bone marrow converts to yellow marrow, and just the bones that make up the spine (vertebrae), ribs, pelvis, skull and sternum contain red marrow.

If a human experiences severe blood loss, the body is able to convert yellow marrow back to red marrow as it tries to boost blood cell production.

Blood groups

Humans can have one of four main blood groups, either RhD positive or negative:

• Group A - RhD negative or positive
  A antigens are found on the surface of blood cells. Anti-B antibodies are found in the plasma.
• Group B - RhD negative or positive
  B antigens are found on the surface of blood cells. Anti-A antibodies are found in the plasma.
• Group AB - RhD negative or positive
  A and B antigens are found on the surface of blood cells. There are no antibodies are found in the plasma.
• Group O - RhD negative or positive
  There are no antigens are found on the surface of blood cells. Both Anti-B and Anti-A antibodies are found in the plasma.

Another protein, an antigen, may be found on some red blood cells - called Rh factor. Blood cells that have Rh factor are RhD positive, those that don't are RhD negative. According to the National Health Service (NHS), UK, 85% of the British population is Rhd positive.

When someone receives a blood transfusion, the medical team has to make sure it is a compatible blood group.

• Group O can be given to people of virtually any blood type.
• Patients with Group A can only accept groups A or O.
• Patients with Group AB + (RhD positive) can generally receive blood from any group.

If a pregnant woman is RhD negative, but her child has inherited RhD positive from the father, the baby will need treatment.