Biology of Blood

Blood is a combination of liquid, cells, and cell-like particles that course through the arteries, capillaries, and veins, delivering oxygen and essential nutrients to tissues and carrying away carbon dioxide and other waste products.

Liquid Components

More than half of the blood consists of a liquid (plasma), which is mostly water containing dissolved salts and proteins. The major protein in plasma is albumin. Others are antibodies (immunoglobulins) and clotting proteins. Plasma also contains hormones, electrolytes, fats, sugars, minerals, and vitamins.

Plasma does much more than transport blood cells. It provides a reservoir of water for the body, prevents blood vessels from collapsing and clogging, and helps maintain blood pressure and circulation throughout the body. Even more important, the antibodies in plasma actively defend the body against foreign substances such as viruses, bacteria, fungi, and cancer cells, while the clotting proteins control bleeding. Besides transporting hormones and regulating their effects, plasma cools and warms the body as needed.

Cellular Components

The cellular components of blood are red blood cells, white blood cells, and platelets, all of which are suspended in the plasma.

Red blood cells (erythrocytes), the most numerous of the three cellular components, normally make up almost half of the blood's volume. These cells are filled with hemoglobin, which enables them to carry oxygen from the lungs and deliver it to all body tissues. Oxygen is consumed to provide energy to cells, leaving carbon dioxide as a waste product, which the red blood cells carry away from the tissues and back to the lungs.

White blood cells (leukocytes) are fewer in number, with a ratio of about 1 white blood cell to every 660 red blood cells. There are five main types of white blood cells that work together to provide the body's major mechanisms for fighting infections, including the production of antibodies.

Neutrophils, also called granulocytes because they contain enzyme-filled granules, are the most prevalent white blood cell type. They help protect the body against bacterial and fungal infections and ingest foreign debris. They consist of two types: band (immature) and segmented (mature) neutrophils.

Lymphocytes consist of two main types: T lymphocytes, which help protect against viral infections and can detect and destroy some cancer cells, and B lymphocytes, which develop into cells that produce antibodies (plasma cells).

Monocytes ingest dead or damaged cells and provide immunologic defenses against many infective organisms.

Eosinophils kill parasites, destroy cancer cells, and are involved in allergic responses. Basophils also participate in allergic responses.

Platelets (thrombocytes) are cell-like particles smaller than red or white blood cells. As part of the blood's protective mechanism for stopping bleeding, they gather at a bleeding site, where they are activated. Once activated, they become sticky and clump together to form a plug that helps seal the blood vessel and stop the bleeding. At the same time, they release substances that help promote clotting.

Red blood cells tend to flow smoothly through the bloodstream, but white blood cells do not. Many of them adhere to blood vessel walls or even penetrate the walls to enter other tissues. When white blood cells reach the site of an infection or other problem, they release substances that attract more white blood cells. The white cells function like an army, dispersed throughout the body but ready at a moment's notice to gather and fight off an invading organism.

Blood Cell Formation

Red blood cells, white blood cells, and platelets are produced in the bone marrow. In addition, lymphocytes are also produced in the lymph nodes and spleen, and T lymphocytes are produced and mature in the thymus, a small gland near the heart. The thymus gland is active only in children and young adults.

Within the bone marrow, all blood cells originate from a single type of cell called a stem cell. When a stem cell divides, it first becomes an immature red blood cell, white blood cell, or platelet-producing cell (megakaryocyte). Then the immature cell divides, matures further, and ultimately becomes a red blood cell, white blood cell, or platelet. The speed of blood cell production is controlled by the body's need.

When the oxygen content of body tissues or the number of red blood cells decreases, the kidneys produce and release erythropoietin, a hormone that stimulates the bone marrow to produce more red blood cells. The bone marrow produces and releases more white blood cells in response to infections and more platelets in response to bleeding.

Laboratory Blood Tests

Doctors depend on many different laboratory tests of blood samples to diagnose and monitor diseases. Some tests measure the components and function of the blood itself; others examine substances in the blood to determine how other organs are functioning.

The most commonly performed blood test is the complete blood cell count (CBC), which is a basic evaluation of the cellular components of blood. Automated machines perform this test in less than a minute on a small drop of blood. In addition to determining the number of blood cells and platelets, the percentage of each type of white blood cell, and the hemoglobin content, the complete blood cell count usually assesses the size and shape of red blood cells. Abnormal red blood cells may be fragmented or shaped like teardrops, crescents, or needles.

Knowing the specific abnormal shape or size can help a doctor diagnose a disease.

For example, sickle-shaped cells are characteristic of sickle cell disease, small red blood cells may signal an early stage of iron deficiency, and large oval red blood cells suggest folic acid or Vitamin B12 deficiency (pernicious anemia).

Other tests provide additional information about blood cells. The reticulocyte count is the number of newly-formed (young) red blood cells (reticulocytes) in a specified volume of blood. Reticulocytes normally make up about 1 per cent of the total number of red blood cells. When the body needs more red blood cells, as in anemia, the bone marrow normally responds by producing more reticulocytes. Thus, the reticulocyte count is a measure of bone marrow function. Tests to determine red blood cell fragility and membrane characteristics help doctors further evaluate the cause of the anemia.

White blood cells can be counted as a group (white blood cell count).

When more detailed information is needed, a doctor requests a count of the specific types of white blood cells (differential white blood cell count). Platelets also can be counted separately.

One of the most common tests performed on plasma is an analysis of electrolytes. Electrolytes include sodium, chloride, potassium, and bicarbonate, as well as less commonly measured substances such as calcium, magnesium, and phosphates. Other tests measure the amount of protein (usually albumin), sugar (glucose), and toxic waste products that the kidneys normally filter out (creatinine and blood urea nitrogen).

Most other blood tests help monitor the function of other organs. Because the blood carries so many substances essential to the body's functioning, blood tests can be used to find out what's happening in the body. In addition, testing blood is relatively easy.

For example, thyroid function can be evaluated more easily by measuring the level of thyroid hormones in the blood than by directly sampling the thyroid. Likewise, measuring liver enzymes and proteins in the blood is easier than sampling the liver.

Bone Marrow Examination

Sometimes a sample of bone marrow must be examined to determine why blood cells are abnormal. A doctor can take two different types of bone marrow samples: a bone marrow aspirate and a bone marrow core biopsy. Both types are usually taken from the hipbone (iliac crest), although aspirates are sometimes taken from the breastbone (sternum). In young children, they are taken from a backbone (vertebra) or leg bone (tibia).

Taking a Bone Marrow Sample

Bone marrow samples are usually taken from the hipbone (iliac crest). The person may lie on one side; facing away from the doctor, with the knee of the top leg bent. After numbing the skin and tissue over the bone with a local anesthetic, the doctor inserts a needle into the bone and withdraws the marrow.

Both types of samples usually are taken at the same time. After the skin and tissue over the bone are numbed with a local anesthetic, the sharp needle of a syringe is inserted into the bone. For a bone marrow aspirate, the doctor pulls back on the plunger of the syringe and draws out a small amount of the soft bone marrow, which can be spread on a slide and examined under a microscope.

Special tests, such as cultures for bacteria, fungi, or viruses and chromosomal analysis, can be performed on the sample.

Although the aspirate often provides enough information to make a diagnosis, the process of drawing the marrow into the syringe breaks up the fragile bone marrow. As a result, determining the original arrangement of the cells is difficult.

When the exact anatomic relationships of cells must be determined and the structure of the tissues evaluated, the doctor also performs a corebiopsy. a small core of intact bone marrow is removed with an internal coring device on the needle. This core is preserved and sliced into thin sections that are examined under a microscope.

A bone marrow sampling generally involves only a slight jolt of pain, followed by minimal discomfort. The procedure takes only a few minutes.

Submitted By
Prof. Paul h. Tanser Md, FRCP(c)
The author is Professor at Macmaster University, Canada.

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yes blood is most important in human life...........

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