326 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. electrolytes, iron, lipids, and fat-soluble vitamins. Fibrinogen plays a vital role in blood clotting. Other organic compounds found in plasma include electrolytes, nutrients, regulatory substances, and waste products. The electrolytes include sodium, potassium, chloride, magnesium, and calcium, all of which help maintain fluid and electrolyte balance. Nutrients include lipids (cholesterol, fatty acids, glycerol), carbohydrates (mainly glucose), and amino acids. Regulatory substances include hormones, enzymes, and vitamins. Waste products such as urea, uric acid, creatinine, bilirubin, and ammonia are carried to various sites for disposal. The composition of plasma is similar to that of interstitial fluid because water, electrolytes, and small solutes are constantly exchanged between the walls of the capillaries and the interstitial fluid in the tissues. Homeostatic mechanisms monitor and maintain relatively constant plasma composition. Maintaining homeostasis requires constant effort by many of the body’s organs. Think about all the changes that take place in the body during a 30-minute run, for example. As the exercise session continues, blood glucose levels decline, blood pH becomes more acidic due to lactic acid accumulation, and blood temperature increases. The body’s feedback systems sense these changes and respond accordingly. To maintain homeostasis under these conditions, the liver begins gluconeogenesis to increase blood glucose levels. The kidneys and the respiratory system are activated to balance blood pH, the hypothalamus activates the sweat glands, and blood vessels conduct blood close to the surface of the skin. As the blood nears the surface of the body, it releases heat to the person’s surroundings to help cool the body. In addition, blood flow is shunted away from the organs to the active muscle to supply the cells with more oxygen and Blood is slightly salty, with a sodium chloride concentration of 0.9%. It has a pH between 7.35 and 7.45 and an average temperature of 38°C (100.4°F). The color of blood varies based on the oxygen level in the bloodstream. Oxygen-rich blood in the arteries is a brighter red than the oxygen-poor blood in the veins. The old saying that “blood is thicker than water” is literally true. At its average temperature of 38°C (100.4°F), blood is about five times more viscous (thicker and more resistant to flow) than water. This is because blood contains components such as the formed elements, plasma proteins, and electrolytes. Blood viscosity contributes to blood flow resistance. In general, the more viscous the blood, the harder the heart has to work to maintain blood flow. Factors that increase blood viscosity include dehydration, decreased blood temperature, prolonged exposure to a high altitude, and an elevated hematocrit level. Plasma Plasma is a pale yellow fluid composed of 90% water and 8% plasma proteins. The remaining 2% is a mixture of electrolytes, nutrients, ions, respiratory gases, hormones, and waste products. Blood plasma contains three primary types of proteins: albumin, globulin, and fibrinogen (Figure 11.4). The majority of plasma proteins are synthesized by the liver. The most abundant plasma protein, albumin, is a major factor in determining the blood’s osmotic pressure, which helps to maintain the water content of the blood. It also acts as a carrier for fatty acids and some steroidal hormones and serves as a pH buffer in the blood. Some globulins serve as antibodies (immunoglobulins) that protect the body from viruses and bacteria. Others, called transport globulins (alpha and beta globulins), carry hormones, Plasma Proteins Protein Average Percent of Plasma Composition Origin Function albumin 55%–60% liver regulates osmotic pressure of blood and blood volume transports lipids, hormones, and other solutes globulin alpha 35%–38% liver aids in blood clot formation transports lipids and fat-soluble vitamins beta liver transports lipids and fat-soluble vitamins gamma plasma cells helps fight infection fibrinogen 4%–6% liver aids in blood clot formation Figure 11.4 Goodheart-Willcox Publisher