Text C. Vitamin Deficiencies.

Besides iron, bone marrow needs both vitamin B12 and folic acid to produce red blood cells. If either is lacking, megaloblastic anemia can de­velop. In this type of anemia, the bone marrow produces large, abnormal red blood cells (megaloblasts). White blood cells and platelets also are usually abnormal.

Although megaloblastic anemia is most often caused by a lack of vitamin B12 or folic acid in the diet or an inability to absorb these vitamins, it is sometimes caused by drugs used to treat cancer, such as methotrexate, hydroxyurea, fluorouracil, and cytarabine.

Vitamin B₁₂ Deficiency Anemia

Vitamin B_l2 deficiency anemia (pernicious ane­mia) is a megaloblastic anemia caused by a lack of vitamin B_l2.

Inadequate absorption of vitamin B12 (cobalamin) causes pernicious anemia. This vitamin, available in meat and vegetables, normally is readily absorbed in the ileum (the last part of the small intestine, leading to the large intestine). However, to be absorbed, the vitamin must com­bine with intrinsic factor, a protein produced in the stomach, which then carries the vitamin to the ileum, through its wall, and into the blood­stream. Without intrinsic factor, vitamin B12 remains in the intestine and is excreted in the stool. In pernicious anemia, the stomach doesn't pro­duce intrinsic factor, vitamin B12 isn't absorbed, and anemia develops even if large amounts of the vitamin are taken in with food. But because the liver stores a large amount of vitamin B12, anemia doesn't develop until 2 to 4 years after the body stops absorbing vitamin B12.

Although lack of intrinsic factor is the most common cause of vitamin B₁₂ deficiency, other possible causes include abnormal bacterial growth in the small intestine that prevents vita­min B12 absorption, certain diseases such as Grohn's disease, and surgery that removes the stomach or the part of the small intestine where vitamin B12 is absorbed. A strict vegetarian diet also may cause B_l2 deficiency.

Besides decreasing red blood cell production, vitamin B₁₂ deficiency affects the nervous system, leading to tingling in the hands and feet, loss of sensation in the legs, feet, and hands, and spastic movements. Other symptoms may include a pe­culiar type of color blindness involving yellow and blue, a sore or burning tongue, weight loss, darkened skin, confusion, depression, and de­creased intellectual function.

Diagnosis

Usually, vitamin B12 deficiency is diagnosed during routine blood tests for anemia. Megaloblasts (large red blood cells) are seen when a blood sample is examined under a microscope. Changes in white blood cells and platelets also can be detected, especially when a person has had anemia for a long time.

When this deficiency is suspected, the blood level of vitamin B₁₂ is measured. If a deficiency is confirmed, tests to determine the cause may be performed. Generally, the tests focus on intrinsic factor. First, a blood sample is usually drawn to check for antibodies to intrinsic factor, which are found in about 60 to 90 percent of the people who have pernicious anemia. The second more specific, test is a gastric analysis. A slender, flexible tube called a nasogastric tube is inserted through the nose, down the throat, and into the stomach. Then pentagastrin (a hormone that stimulates in­trinsic factor secretion) is injected into a vein. A sample of the stomach contents is withdrawn and tested for intrinsic factor.

If still uncertain about the mechanism that pro­duced the vitamin B₁₂ deficiency, the doctor may order a Schilling test. First, a person is given a tiny amount of radioactive vitamin B12 by mouth, and its absorption is measured. Then intrinsic factor is given along with vitamin B₁₂, and its absorption is measured again. If vitamin B12 is absorbed with but not without intrinsic factor, the diagnosis of pernicious anemia is confirmed. Other tests are rarely needed.

Treatment

The treatment of vitamin B12 deficiency or per­nicious anemia consists of replacing vitamin B12. Because most people who have this deficiency can't absorb vitamin B12 taken by mouth, they must take it by injection. At first, injections are given daily or weekly for several weeks until the blood levels of vitamin B_l2 return to normal; then injections are given once a month. People who have this deficiency must take vitamin B_l2 supple­ments for life.

 

Notes:

intrinsic flexible tiny

внутренний гибкий; гнущийся; мягкий, эластичный очень маленький, крошечный

Bleeding Disorders

 

Bleeding disorders, characterized by a ten­dency to bleed easily, may result from defects in the blood vessels or from abnormalities in the blood itself. The abnormalities may be in the blood clotting factors or the platelets. Normally, blood is confined to the blood vessels—arteries, capillaries, and veins. When bleeding (hemorrhaging) occurs, blood spills from these vessels, inside or outside of the body. The body prevents or controls bleeding in several ways.

Hemostasis is the body's way of stopping in­jured blood vessels from bleeding. It involves three major processes: (1) constriction of the blood vessels, (2) activty of the platelets (irregularly shaped cell-like particles in the blood that are involved in clotting), and (3) activity of the blood clotting factors (proteins dissolved in plasma, the liquid part of blood). Abnormalities in these processes can lead to either excessive bleeding or excessive clotting, both of which can be dangerous.