What is Blood?

Approximately 80% water and 20% solid, blood is a fluid tissue. Blood delivers oxygen and nutrients to the rest of the cells in the body, removes carbon dioxide and waste, fights infection and foreign matter, and protects the circulatory system from failure. It is composed of plasma, three different kinds of cells, and other components, including:


– A yellowish liquid containing water, salts, sugars, and proteins.

Red blood cells

– Bind and transport oxygen, carbon dioxide, and other molecules through the blood. A healthy adult has approximately 35 trillion red blood cells, each with a lifespan of ~120 days.

White blood cells

– Fight infection and are found in both the blood and lymphatic systems. Many different shapes and sizes of white blood cells are at work in the body, and their life spans vary by type from days to years.


– Are blood cells that clump together to stop bleeding from a damaged blood vessel. Platelets also may collect and form clots in diseased blood vessels, damaged heart valves, and implanted prosthetic devices. Their lifespan is ~10 days long.

Other components of blood

– Carbohydrates, proteins, fats, hormones, and gases.

What Does Blood Do?

  • Carries oxygen from the lungs to cells in the rest of the body
  • Carries nutrients from the digestive tract to cells in the rest of the body
  • Carries carbon dioxide and waste products out of the body via the kidneys
  • Helps regulate body temperature
  • Carries hormones to the body’s cells
  • Carries white blood cells to infections or foreign matter
  • Carries platelets to damaged areas to prevent bleeding and promote healing

Blood Types and Grouping

Red blood cell groups are determined genetically and divided into four categories based on whether or not they carry antigens (A, B, both, or neither). The typing of blood refers to the Rh factor, which is positive ( ) or negative (-). The four different groups of red blood cells are:

AB – Universal acceptor, can accept any blood type
O – Universal donor, can donate blood to anyone but only accept type O blood

Blood Transfusions

Red blood cells are made in the bone marrow, and blood lost through small cuts or wounds can usually be easily replaced. However, when a large blood volume is lost, it must be replaced using a blood transfusion (blood donated by others). In blood transfusions, the donor and recipient blood group and type must be compatible. Blood transfusions are often anticipated in open heart surgery, and some patients bank their own blood in case it is needed during their operation. A whole-blood transfusion is rarely given; usually red blood cells, platelets, plasma, or a combination of two components is given, and this decision is based on the medical condition. Transfusions may take from 30 minutes to 3 hours depending on the components given. Examples of conditions include:

Red Blood Cells – bleeding, anemia, chemotherapy, radiation, or any condition where blood oxygen is low.

Platelets – leukemia, blood clotting problems, damaged blood vessels.

Plasma – bleeding, blood clotting problems.

Blood Tests and Cardiovascular Conditions

Blood tests are often used to diagnose cardiovascular problems.

Arterial blood gas studies measure how well blood is being oxygenated in the lungs.

Blood clotting tests

Measure the blood’s ability to clot. Clotting stops the blood from flowing out of the body when a vein or artery is broken.

Blood cultures

Can be used to determine the presence of microorganisms (like the bacteria that causes endocarditis). After blood is drawn, it is placed on a culture dish and placed in a warm incubation oven. The culture is then analyzed to determine what type(s) of microorganism is present and what medicines might best be used to treat it.

Cardiac enzyme tests

Measure cardiac enzyme levels in the blood. If heart muscle (myocardium) has been damaged by a heart attack, it will release these enzymes, especially one called creatine kinase.

Cholesterol (lipoprotein) profiles

Measure how much fat or lipid is in the blood.

Troponin tests

Measure the amount of troponin (a protein) in the blood. Troponin affects how heart muscle contracts. High levels of troponin in the blood (troponin T or troponin I) indicate heart muscle damage, and the higher the troponin level, the higher the degree of damage that has been caused.


Texas Heart Institute 
American Red Cross
Mayo Clinic