24.1 Introduction to the Urinary System PDF

Page 948 24.1 Introduction to the Urinary System LEARNING OBJECTIVES 1. Identify the structures that compose the urinary system, and describe the general function of each. 2. List the functions of the kidneys. The urinary (ūr′i-nār-ē) system is collectively composed of the kidneys, ureters, urinary...

Page 948 24.1 Introduction to the Urinary System LEARNING OBJECTIVES 1. Identify the structures that compose the urinary system, and describe the general function of each. 2. List the functions of the kidneys. The urinary (ūr′i-nār-ē) system is collectively composed of the kidneys, ureters, urinary bladder, and urethra ( figure 24.1). One of the primary functions of the kidneys is to filter blood and convert the filtrate into urine. This liquid waste is then transported by the ureters from the kidneys to the urinary bladder. The urinary bladder is an expandable, muscular sac that stores as much as 1 liter (L) of urine until it is eliminated from the body through the urethra. Page 949 Figure 24.1 Urinary System. The urinary system is composed of two kidneys, two ureters, a single urinary bladder, and a single urethra, shown here in (a) anterior views (illustration and photo) and (b) posterior view. (Components of urinary system are labeled in bold font.) (a) ©McGraw-Hill Education/Christine Eckel APR Module 13: Urinary: Animations: Urinary System Overview Several essential physiologic processes occur in the kidney when the filtrate is being converted to urine: Elimination of metabolic wastes. The kidneys remove waste that is present in the filtrate (e.g., urea, uric acids) to prevent these substances from reaching toxic levels within the blood. Regulation of ion levels. The kidneys help control the blood’s ion balance, such as sodium ions (Na+), potassium ions (K+), calcium ions (Ca2+), and phosphate ions (PO43–) by eliminating varying amounts of these substances in the urine, depending upon dietary intake. Regulation of acid-base balance. The kidneys aid in maintaining acid-base balance by altering blood levels of both hydrogen ions (H+) and bicarbonate ions (HCO3–) by eliminating varying amounts of these ions. Regulation of blood pressure. The kidneys help regulate blood pressure by excreting fluid in the urine. Regulating fluid lost in the urine helps to regulate blood volume. The kidneys also release the enzyme renin, which is required for production of angiotensin II, a hormone that increases blood pressure (see section 20.6b and table R.7). Arguably, blood pressure regulation is one of the kidney’s most important functions and is described in detail in section 25.4. Elimination of biologically active molecules. Small molecules that are biologically active (e.g., hormones, drugs) are filtered, but are not reclaimed, and then become part of urine. The kidneys perform other functions, however, in addition to their role in filtering blood and processing filtrate to form urine. As you read through these functions, you may remember that many of these have been described in previous chapters. The other functions of the kidney include the following: Formation of calcitriol. The kidneys synthesize the final enzyme in calcitriol hormone formation. Calcitriol increases the absorption of calcium from the small intestine to increase blood calcium concentration (see section 7.6). Production and release of erythropoietin. As the kidneys are filtering the blood, they are also indirectly measuring the oxygen level of blood. In response to low blood oxygen levels, cells within the kidney secrete erythropoietin (EPO) hormone. EPO stimulates red bone marrow to increase its rate of erythrocyte formation (see section 18.3b and table R.6). The greater numbers of erythrocytes transport additional oxygen from the lungs to systemic cells (see section 23.7a). Potential to engage in gluconeogenesis. During prolonged fasting or starvation, the kidneys may engage in gluconeogenesis to produce glucose from noncarbohydrate sources; this process helps to maintain normal blood glucose levels during periods of extreme nutrient deprivation (see section 2.7c). A general observation can be made about the various functions of the kidneys. They take care of our blood. The kidneys remove unwanted materials from the blood, maintain blood plasma concentration of ions (e.g., Na+, K+, Ca2+), help regulate blood pH (i.e., H+ and concentration), alter blood volume, regulate the number of erythrocytes, and help maintain blood glucose levels during severe limitation on nutrient intake. Healthy blood is intimately dependent upon healthy kidneys. WHAT DO YOU THINK? 1 Which of the following would accompany a loss of kidney function: (a) accumulation of wastes, (b) anemia, (c) changes in blood pressure, or (d) acid-base imbalance? WHAT DID YOU LEARN? 1 Which structure of the urinary system forms urine, and which structure stores urine? 2 What are the two means by which the kidney helps to regulate blood pressure?