Apago PDF Enhancer I Chapter 50 The Circulatory System Chapter Outline 50.1 The Components of Blood 50.2 Invertebrate Circulatory Systems 50.3 Vertebrate Circulatory Systems 50.4 The Four-Chambered Heart and the Blood Vessels 50.5 Characteristics of Blood Vessels 50.6 Regulation of Blood Flow and Blood Pressure Introduction In multicellular organisms, oxygen obtained by the respiratory system and nutrients processed by the digestive system must be transported to cells throughout the body. Conversely, carbon dioxide and other waste products produced within the cells must be returned to the respiratory, digestive, and urinary systems for elimination from the body. These tasks are the responsibility of the circulatory system. All multicellular organisms have a heart that pumps fluids through the body. Many invertebrates have an open system in which fluids move through the body cavity. Vertebrates also have a system like this that moves lymph through the body; however, the primary circulatory fluid is blood, which means through a closed system of blood vessels. platelets, although included in figure 50.1, are not complete cells; rather, they are fragments of cells that are produced in the bone marrow. (We describe the action of platelets in blood clotting later in this section.) Circulating blood has many functions: Transportation. 1. All of the substances essential for cellular metabolism are transported by blood. Red blood cells transport oxygen attached to hemoglobin; nutrient molecules are carried in the plasma, sometimes bound to carriers; and metabolic wastes are eliminated as blood passes through the liver and kidneys. Regulation. 2. The cardiovascular system transports regulatory hormones from the endocrine glands and also 50.1 The Components of Blood Learning Outcomes Describe the functions of circulating blood. 1. Distinguish between the types of formed elements. 2. Delineate the process of blood clotting. 3. Blood is a connective tissue composed of a fluid matrix, called plasma, and several different kinds of cells and other formed elements that circulate within that fluid (figure 50.1). Blood CHAPTER Apago PDF Enhancer Figure 50.1 Composition of blood. participates in temperature regulation. Contraction and dilation of blood vessels near the surface of the body, beneath the epidermis, helps to conserve or to dissipate heat as needed. Protection. 3. The circulatory system protects against injury and foreign microbes or toxins introduced into the body. Blood clotting helps to prevent blood loss when vessels are damaged. White blood cells, or leukocytes, help to disarm or disable invaders such as viruses and bacteria (see chapter 52). Blood plasma is a fluid matrix Blood plasma is the matrix in which blood cells and platelets are suspended. Interstitial (extracellular) fluids originate from the fluid present in plasma. Although plasma is 92% water, it also contains the follow- ing solutes: Nutrients, wastes, and hormones. 1. Dissolved within the plasma are all of the nutrients resulting from digestive breakdown that can be used by cells, including glucose, amino acids, and vitamins. Also dissolved in the plasma are wastes such as nitrogen compounds and CO 2 produced by metabolizing cells. Endocrine hormones released from glands are also carried through the blood to their target cells. Ions. 2. Blood plasma is a dilute salt solution. The predominant plasma ions are Na + , Cl – , and bicarbonate ions (HCO 3 – ). In addition, plasma contains trace amounts of other ions such as Ca 2+ , Mg 2+ , Cu 2+ , K + , and Zn 2+ . Proteins. 3. As mentioned earlier, the liver produces most of the plasma proteins, including albumin, which constitutes most of the plasma protein; the alpha (α) and beta (β) globulins, which serve as carriers of lipids and steroid hormones; and fibrinogen, which is required for blood clotting. Blood plasma with the fibrinogen removed is called serum. Formed elements include circulating cells and platelets The formed elements of blood cells and cell fragments include red blood cells, white blood cells, and platelets. Each element has a specific function in maintaining the body’s health and homeostasis. Erythrocytes Each microliter of blood contains about 5 million red blood cells, or erythrocytes. The fraction of the total blood volume that is occupied by erythrocytes is called the blood’s hematocrit; in humans, the hematocrit is typically around 45%. Each erythrocyte resembles a doughnut-shaped disk with a central depression that does not go all the way through. Ma- ture mammalian erythrocytes lack nuclei. The erythrocytes of vertebrates contain hemoglobin, a pigment that binds and transports oxygen. (Hemoglobin was described more fully in the previous chapter when we discussed respiration.) In verte- brates, hemoglobin is found only in erythrocytes. In inverte- brates, the oxygen-binding pigment (not always hemoglobin) is also present in plasma. Leukocytes Less than 1% of the cells in human blood are white blood cells, or leukocytes; there are only 1 or 2 leukocytes for every 1000 erythrocytes. Leukocytes are larger than erythrocytes and have nuclei. Furthermore, leukocytes are not confined to the blood as erythrocytes are, but can migrate out of capillaries through the intercellular spaces into the surrounding intersti- tial (tissue) fluid. Leukocytes come in several varieties, each of which plays a specific role in defending against invading microorganisms and other foreign substances, as described in chapter 52. Granular leukocytes include neutrophils, eosinophils, and ba- sophils, which are named according to the staining properties of granules in their cytoplasm. Nongranular leukocytes in- clude monocytes and lymphocytes. In humans, neutrophils are the most numerous of the leukocytes, followed in order by lym- phocytes, monocytes, eosinophils, and basophils. chapter 50 The Circulatory System 1019 www.ravenbiology.com
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