Page 106 - Alaska A & P Primer

18.2 Production of the Formed Elements
All formed elements arise from stem cells of the red bone marrow. Recall that stem cells undergo mitosis plus cytokinesis (cellular division) to give rise to new daughter cells: One of these remains a stem cell and the other differentiates into one of any number of diverse cell types. Stem cells may be viewed as occupying a hierarchal system, with some loss of the ability to diversify at each step. The totipotent stem cell is the zygote, or fertilized egg. The totipotent (toti- = “all”) stem cell gives rise to all cells of the human body. The next level is the pluripotent stem cell, which gives rise to multiple type of cells of the body and some of the supporting fetal membranes.
Prior to birth, hemopoiesis occurs in a number of tissues, beginning with the yolk sac of the developing embryo, and continuing in the fetal liver, spleen, lymphatic tissue, and eventually the red bone marrow. Following birth, most hemopoiesis occurs in the red mar- row, a connective tissue within the spaces of spongy (cancellous) bone tissue. In children, hemopoiesis can occur in the medullary cavity of long bones; in adults, the process is largely restricted to the cranial and pelvic bones, the vertebrae, the sternum, and the proxi- mal epiphyses of the femur and humerus. Throughout adulthood, the liver and spleen maintain their ability to generate the formed elements. This process is referred to as extra- medullary hemopoiesis (meaning hemopoiesis outside the medullary cavity of adult bones). When a disease such as bone cancer destroys the bone marrow, causing hemopoie- sis to fail, extramedullary hemopoiesis may be initiated.
18.3 Erythrocytes
The erythrocyte, commonly known as a red blood cell (or RBC), is by far the most common formed element: A single drop of blood contains millions of erythrocytes and just thou- sands of leukocytes. The primary functions of erythrocytes are to pick up inhaled oxygen from the lungs and transport it to the body’s tissues, and to pick up some (about 24 per- cent) carbon dioxide waste at the tissues and transport it to the lungs for exhalation.
Erythrocytes remain within the vascular network. Although leuko- cytes typically leave the blood ves- sels to perform their defensive functions, movement of erythro- cytes from the blood vessels is ab- normal. As an erythrocyte ma- tures in the red bone marrow, it extrudes its nucleus and most of its other organelles. During the first day or two that it is in the cir- culation, an immature erythro- cyte, known as a reticulocyte, will still typically contain remnants of organelles.
Erythrocytes are biconcave disks;
that is, they are plump at their pe-
riphery and very thin in the cen-
ter. Since they lack most organ-
elles, there is more interior space
for the presence of the hemoglobin
molecules that, as you will see shortly, transport gases. The biconcave shape also provides a greater surface area across which gas exchange can occur, relative to its volume; a sphere of a similar diameter would have a lower surface area-to-volume ratio. In the capillaries, the oxygen carried by the erythrocytes can diffuse into the plasma and then through the capillary walls to reach the cells, whereas some of the carbon dioxide produced by the cells as a waste product diffuses into the capillaries to be picked up by the erythrocytes.
Hemoglobin is a large molecule made up of proteins and iron. It consists of four folded chains of a protein called globin, designated alpha 1 and 2, and beta 1 and 2. Each of these globin molecules is bound to a red pigment molecule called heme, which contains an ion of iron (Fe2+). In the lungs, hemoglobin picks up oxygen, which binds to the iron ions, form- ing oxyhemoglobin. The bright red, oxygenated hemoglobin travels to the body tissues, where it releases some of the oxygen molecules, becoming darker red deoxyhemoglobin, sometimes referred to as reduced hemoglobin.
Populations dwelling at high elevations, with inherently lower levels of oxygen in the at- mosphere, naturally maintain a hematocrit higher than people living at sea level. Conse- quently, people traveling to high elevations may experience symptoms of hypoxemia, such as fatigue, headache, and shortness of breath, for a few days after their arrival. In response to the hypoxemia, the kidneys secrete EPO to step up the production of erythrocytes until homeostasis is achieved once again. To avoid the symptoms of hypoxemia, or altitude sick- ness, mountain climbers typically rest for several days to a week or more at a series of camps situated at increasing elevations to allow EPO levels and, consequently, erythrocyte counts to rise.
18.2 OBJECTIVES
1. Describe bone marrow and stem cells and hemopoiesis
18.3 OBJECTIVES
1. Describe elements of the erythrocytes cells
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State of Alaska EMS Education Primer - 2016
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