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Question 1

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Answer 1

Formation of Red Blood Cells
Because they are anucleate, RBCs are unable to synthesize proteins, grow, or divide. As they age, RBCs become rigid and begin to fall apart in 100 to 120 days. Their remains are eliminated by phagocytes in the spleen, liver, and other body tissues. Some RBC components are salvaged for reuse or recycled to their building blocks. Iron is bound to protein as ferritin, and the balance of the heme group is degraded to bilirubin, which is then secreted into the intestine by liver cells. There it becomes a brown pigment called stercobilin that leaves the body in feces. Globin is broken down to amino acids, which are released into the circulation.
Lost blood cells are replaced more or less continuously by the division of hemocytoblasts in the red bone marrow. The developing RBCs divide many times and then begin synthesizing huge amounts of hemoglobin. When enough hemoglobin has been accumulated, the nucleus and most organelles are ejected, and the cell collapses inward. The result is the young RBC, called a reticulocyte (rĕ-tik′u-lo-sīt) because it still contains some rough endoplasmic reticulum (ER). The reticulocytes enter the bloodstream to begin their task of transporting oxygen. Within 2 days of release, they have ejected the remaining ER and have become fully functioning erythrocytes. The entire developmental process from hemocytoblast to mature RBC takes 3 to 5 days.
The rate of erythrocyte production is controlled by a hormone called erythropoietin (ĕ-rith″ro-po-e′tin). Normally a small amount of erythropoietin circulates in the blood at all times, and red blood cells are formed at a fairly constant rate. Although the liver produces some, the kidneys play the major role in producing this hormone. When the blood level of oxygen begins to decline for any reason, the kidneys (a convenient location to monitor blood) step up their release of erythropoietin. Erythropoietin targets the bone marrow, prodding it into “high gear” to turn out more RBCs. (Follow this sequence of events in Figure 10.4.)