Blood Composition and Function

Blood Cells
Blood Cells. Credit: NATIONAL CANCER INSTITUTE/Science Photo Library/Getty Images

Blood Function

Our blood is a fluid that is also a type of connective tissue. It is composed of blood cells and an aqueous fluid known as plasma. Two major functions of the blood include transporting substances to and from our cells and providing immunity and protection against infectious agents such as bacteria and viruses. Blood is a component of the cardiovascular system. It is circulated through the body via the heart and blood vessels.

Blood Components

Blood consists of several elements. The major components of blood include plasma, red blood cells, white blood cells, and platelets.

  • Plasma: This major constituent of blood comprises about 55 percent of blood volume. It consists of water with several different substances dissolved within. Plasma contains salts, proteins, and blood cells. Plasma also transports nutrients, sugars, fats, hormones, gases, and waste material contained within blood.
  • Red Blood Cells (erythrocytes): These cells determine blood type and are the most abundant cell type in the blood. Red blood cells have what is known as a biconcave shape. Both sides of the cell's surface curve inward like the interior of a sphere. This flexible disc shape helps increase the surface area-to-volume ratio of these extremely small cells. Red blood cells do not have a nucleus, but they do contain millions of hemoglobin molecules. These iron containing proteins bind oxygen molecules obtained in the lungs and transport them to various parts of the body. After depositing oxygen to tissue and organ cells, red blood cells pick up carbon dioxide (CO2) for transportation to the lungs where the CO2 is expelled from the body.
  • White Blood Cells (leukocytes): These cells play an important role in the immune system and lymphatic system by defending the body against infection. These cells locate, destroy, and remove pathogens and foreign matter from the body. There are several different types of white blood cells, each with different functions. Examples include lymphocytes, monocytes, neutrophils, basophils, and eosinophils.
  • Platelets (thrombocytes): These cell components are formed from pieces of cells found in bone marrow called megakaryocytes. Fragments of the megakaryocytes circulate through the blood stream and play a major role in clotting. When platelets encounter an injured blood vessel, they clump together to block the opening in the vessel.

Blood Cell Production

Blood cells are produced by bone marrow within bone. Bone marrow stem cells develop into red blood cells, white blood cells, and platelets. Certain white blood cells mature in the lymph nodes, spleen, and thymus gland. Matured blood cells have varying life spans. Red blood cells circulate for about 4 months, platelets for about 9 days, and white blood cells range from about a few hours to several days. Blood cell production is often regulated by body structures such as the lymph nodes, spleen, liver, and kidneys. When oxygen in tissues is low, the body responds by stimulating bone marrow to produce more red blood cells. When the body is infected, more white blood cells are produced.

Blood Pressure

Blood pressure is the force at which blood exerts pressure against artery walls as it circulates throughout the body. Blood pressure readings measure systolic and diastolic pressures as the heart goes through the cardiac cycle.

In the systole phase of the cardiac cycle, the heart ventricles contract (beat) and pump blood to the arteries. In the diastole phase, the ventricles are relaxed and the heart fills with blood. Blood pressure readings are measured in millimeters of mercury (mmHg) with the systolic number reported before the diastolic number.

Blood pressure is not constant and can fluctuate depending on various conditions. Nervousness, excitement, and increased activity are a few things that can influence blood pressure. Blood pressure levels also increase as we get older. Abnormally high blood pressure, known as hypertension, can have serious consequences as it can lead to hardening of the arteries, kidney damage, and heart failure. Persons with elevated blood pressure often experience no symptoms. Elevated blood pressure that persists for the majority of the time can lead to increased risk for health issues.

Blood Type

Blood type describes how blood is classified. It is determined by the existence or lack thereof of certain identifiers (called antigens) located on red blood cells. Antigens help the body's immune system to identify its own red blood cell group. This identification is crucial so that the body will not build up antibodies against its own red blood cells. The four blood type groupings are A, B, AB, and O. Type A has A antigens on red blood cell surfaces, type B has B antigens, type AB has both A and B antigens, and type O has no A or B antigens. Blood types must be compatible when considering blood transfusions. Those with type A must receive blood from either type A or type O donors. Those with type B from either type B or type O. Those with type O can receive blood from only type O donors and type AB may receive blood from any of the four blood type groups.


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  • What Is High Blood Pressure? National Heart, Lung, and Blood Institute. Updated 08/02/12 (