evaporation does not remove all the sweat that has beenexcreted, the sweat collects in beads on the surface ofthe skin.CERUMINOUS GLANDS.—Ceruminousglands are modified sweat glands found only in theauditory canal. They secrete a yellow, waxy substancecalled cerumen that protects the eardrum.THE CIRCULATORY SYSTEMLEARNING OBJECTIVE: Identify the partsof the circulatory system, and recognize theirmajor components and functions.The circulatory system, also called the vascularsystem, consists of blood, heart, and blood vessels.The circulatory system is close circuited (i.e., there isno opening to external environment of the body). Thefunction of this system is to move blood between thecells and the organs of the integumentary, digestive,respiratory, and urinary system that communicate withthe external environment of the body. This function isfacilitated by the heart pumping blood through bloodvessels. The blood travels throughout the bodytransporting nutrients and wastes, and permitting theexchange of gases (carbon dioxide and oxygen).BLOODBlood is fluid tissue composed of formed elements(i.e., cells) suspended in plasma. It is pumped by theheart through arteries, capillaries, and veins to all partsof the body. Total blood volume of the average adult is5 to 6 liters.PlasmaPlasma is the liquid part of blood (fig. 1-31).Plasma constitutes 55 percent of whole blood (plasmaand cells). It is a clear, slightly alkaline, straw-coloredliquid consisting of about 92 percent water. Theremainder is made up mainly of proteins. One of theseproteins, fibrinogen, contributes to coagulation.Blood CellsThe blood cells suspended in the plasma constitute45 percent of whole blood. Its cells, which are formedmostly in red bone marrow, include red blood cells(RBCs) and white blood cells (WBCs). The blood alsocontains cellular fragments called blood platelets.When blood components are separated, the WBCs andplatelets form a thin layer, called the buffy coat,between the layers of plasma and RBCs. These layersare illustrated in figure 1-31.RED BLOOD CELLS.—Red blood cells, orerythrocytes, are small, biconcave, nonnucleateddisks, formed in the red bone marrow (fig. 1-32).Blood of the average man contains 5 million red cellsper cubic millimeter. Women have fewer red cells, 4.5million per cubic millimeter. Emotional stress,strenuous exercise, high altitudes, and some diseasesmay cause an increase in the number of RBCs.During the development of the red blood cell, asubstance called hemoglobin is combined with it.Hemoglobin is the key of the red cell's ability to carryoxygen and carbon dioxide. Thus, the main function oferythrocytes is the transportation of respiratory gases.The red cells deliver oxygen to the body tissues,holding some oxygen in reserve for an emergency.Carbon dioxide is picked up by the same cells anddischarged via the lungs.The color of the red blood cell is determined by thehemoglobin content. Bright red (arterial) blood is dueto the combination of oxygen and hemoglobin. Dark1-24BLOODSAMPLELIQUID(PLASMA)"BUFFYCOAT"RED BLOODCELLS55%45%HM3F0131Figure 1-31.—Blood sample illustrating blood components.HM3F0132PLATELETSWHITEBLOODCELLSREDBLOODCELLSPERIPHERAL BLOOD SMEARFigure 1-32.—A blood smear showing red blood cells, whiteblood cells, and platelets.
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