Anatomy & Physiology

Anatomy & Physiology Study Guide materials from solution, and plasma and serum differ in several significant ways. Thus, the results of a blood test indicate whether the sample was plasma or serum. The remaining one percent of plasma proteins is composed of specialized proteins whose levels vary widely. Peptide hormones—including insulin, prolactin (PRL), glycoproteins thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH)—are normally present in circulating blood. Their plasma concentrations fluctuate from day to day or even hour to hour. 18.4 Red Blood Cells The most abundant blood cells are the erythrocytes or red blood cells (RBCs), which account for 99.9 percent of the formed elements. These cells contain the red pigment hemoglobin, which binds gases, transports oxygen and carbon dioxide, and gives whole blood its deep red color. A standard blood test reports the number of RBCs per microliter (µL) of whole blood as the red blood cell count. In adult males, one microliter, or one cubic millimeter (mm3), of whole blood contains 4.5– 6.3 million RBCs; in adult females, 1 microliter contains 4.2–5.5 million. A solitary drop of whole blood contains approximately 260 million RBCs, and the blood of an average adult has 25 trillion RBCs. RBCs thus account for one-third of all cells in the human body. The hematocrit is the percentage of whole blood volume contributed by formed elements, 99.9 percent of which are red blood cells. The standard hematocrit in adult males’ average 46 (range: 40– 54); the average for adult females is 42 (range: 37–47). The hematocrit is determined by centrifuging a blood sample so that all the formed elements come out of suspension. Amid the most specialized cells of the body, the shape of a RBC is a biconcave disc with a thick outer margin and thin central region. An average RBC has a diameter of 7.8 µm and a maximum thickness of 2.85 µm, although the center narrows to about 0.8 µm. During their differentiation, the RBCs of humans and other mammals lose most of their organelles, including nuclei; the cells retain only the cytoskeleton. Without nuclei and ribosomes, circulating RBCs cannot divide, replicate, or synthesize enzymes or structural proteins. As a result, the RBCs cannot perform repairs; hence, their life span is only about 120 days. With few organelles, their energy demands are low, and they obtain their energy through the anaerobic metabolism of glucose absorbed from the surrounding plasma. Hemoglobin In effect, a developing red blood cell loses any organelle not directly associated with its primary function: the transport of respiratory gases. Greater than 95 percent of an RBC’s intracellular proteins are molecules of hemoglobin. The hemoglobin content of whole blood is reported in grams of Hb per deciliter (100 mL) of whole blood (g/dL). Normal ranges are 14–18 g/dL in males and 12–16 g/dL in females. Hemoglobin is responsible for the ability of the cell to transport oxygen and carbon dioxide. Hb molecules have complex quaternary structures. Each Hb molecule has two alpha (α) chains and two beta (β) chains of polypeptides. Globular protein subunits make up each chain and resemble the Achieve Page 224 of 368 ©2018