Page 132 - 84 human physiolofy part-1

Chapter 7

peoples.

Genetics

Sickle-cell disease is inherited in the autosomal recessive pattern, depicted above. The allele
responsible for sickle cell anemia is autosomal recessive. A person who receives the defective gene
from both father and mother develops the disease; a person who receives one defective and one healthy
allele remains healthy, but can pass on the disease and is known as a carrier. If two parents who are
carriers have a child, there is a 1-in-4 chance of their child developing the illness and a 1-in-2 chance of
their child just being a carrier.

Polycythemia

Polycythemia is a condition in which there is a net increase in the total circulating erythrocyte (red
blood cell) mass of the body. There are several types of polycythemia.

Primary Polycythemia

In primary polycythemia, there may be 8 to 9 million and occasionally 11 million erythrocytes per
cubic millimeter of blood (a normal range for adults is 4-5 million), and the hematocrit may be as high
as 70 to 80%. In addition, the total blood volume can increase to as much as twice as normal. The
entire vascular system can become markedly engorged with blood, and circulation times for blood
throughout the body can increase up to twice the normal value. The increased numbers of erythrocytes
can increase the viscosity of the blood to as much as five times normal. Capillaries can become plugged
by the very viscous blood, and the flow of blood through the vessels tends to be extremely sluggish.

As a consequence of the above, people with untreated Polycythemia are at a risk of various
thrombotic events (deep venous thrombosis, pulmonary embolism), heart attack and stroke, and have a
substantial risk of Budd-Chiari syndrome (hepatic vein thrombosis). The condition is considered
chronic; no cure exists. Symptomatic treatment (see below) can normalize the blood count and most
patients can live a normal life for years.

Secondary polycythemia

Secondary polycythemia is caused by either appropriate or inappropriate increases in the
production of erythropoietin that result in an increased production of erythrocytes. In secondary
polycythemia, there may be 6 to 8 million and occasionally 9 million erythrocytes per cubic millimeter
of blood. A type of secondary polycythemia in which the production of erythropoietin increases
appropriately is called physiologic polycythemia. Physiologic polycythemia occurs in individuals
living at high altitudes (4275 to 5200 meters), where oxygen availability is less than at sea level. Many
athletes train at higher altitudes to take advantage of this effect — a legal form of blood doping. Actual
polychthemia sufferers have been known to use their condition as an athletic advantage for greater
stamina.

Other causes of secondary polycythemia include smoking, renal or liver tumors, or heart or lung
diseases that result in hypoxia. Endocrine abnormalities, prominently including pheochromocytoma
and adrenal adenoma with Cushing's Syndrome, are also secondary causes. Athletes and bodybuilders

132 | Human Physiology