McKenna's Pharmacology for Nursing, 2e

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P A R T 9  Drugs acting on the renal system

a person develops renal failure and the production of erythropoietin drops, the production of red blood cells falls and the person becomes anaemic.

including Escherichia coli , which can cause frequent bladder infections or cystitis. In the male, the urethra is much longer and passes through the prostate gland , a small gland that produces an alkaline fluid that is important in maintaining the sperm and lubricating the tract. Enlargement and infection in the prostate gland are often problems in older men. ■■ The ureters, urinary bladder and urethra make up the rest of the urinary tract. ■■ The shorter female urethra leads from the urinary bladder to the outer body into an area rich in gram- negative bacteria. Cystitis, or infection of the urinary bladder, is a common problem for women. ■■ The longer male urethra passes through the prostate gland, which may enlarge or become infected, a problem often associated with advancing age. CHAPTER SUMMARY ■■ The functional unit of the kidneys is called the nephron; it is composed of Bowman’s capsule, the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct. ■■ The blood flow to the nephron is unique, allowing autoregulation of blood flow through the glomerulus. ■■ Sodium levels are regulated throughout the tubule by active and passive movement and are fine tuned by the presence of aldosterone in the distal tubule. ■■ The countercurrent mechanism in the medullary nephrons allows for the concentration or dilution of urine under the influence of ADH secreted by the hypothalamus. ■■ Potassium concentration is regulated throughout the tubule, with aldosterone being the strongest influence for potassium loss. ■■ The kidneys play a key role in the regulation of calcium by activating vitamin D to allow GI calcium reabsorption and by reabsorbing or excreting calcium from the tubule under the influence of parathyroid hormone. ■■ The kidneys influence blood pressure control, releasing renin to activate the renin–angiotensin system, which leads to increased blood pressure and volume and a resultant increased blood flow to the kidney. The balance of this reflex system can lead to water retention or excretion and has an impact on drug therapy that promotes water or sodium loss. ■■ The ureters, urinary bladder and urethra make up the rest of the urinary tract. The longer male urethra passes through the prostate gland, which may enlarge or become infected, a problem often associated with advancing age. KEY POINTS

KEY POINTS

■■ The kidneys are two small, bean-shaped organs that receive about 25% of the cardiac output. ■■ The nephron is the functional unit of the kidneys and is involved in three processes: glomerular filtration, tubular secretion and tubular reabsorption. ■■ The kidney plays a key role in regulating body fluid volume and maintaining blood pressure, red blood cell production, acid–base balance and electrolyte stability. ■■ The renin–angiotensin–aldosterone system is activated when blood flow to the nephron is decreased and renin is released. The end result is increased vasoconstriction and increased blood pressure and sodium and water retention, which increase blood volume and pressure. ■■ Red blood cell production is controlled by erythropoietin released from the juxtaglomerular apparatus when oxygen delivery to the nephron is decreased. Erythropoietin stimulates the bone marrow to produce red blood cells to increase oxygen delivery to the nephrons. THE URINARY TRACT As noted previously, the urinary tract is composed of the ureters, urinary bladder and urethra (see Figure 50.1). Ureters One ureter exits each kidney, draining the filtrate from the collecting ducts. The ureters have a smooth endothelial lining and circular muscular layers. Urine entering the ureter stimulates a peristaltic wave that pushes the urine down towards the urinary bladder. Urinary bladder The urinary bladder is a muscular pouch that stretches and holds the urine until it is excreted from the body. Urine is usually a slightly acidic fluid; this acidity helps to maintain the normal transport systems and to destroy bacteria that may enter the bladder. Control of bladder emptying is learned control over the urethral sphincter; once it is established, a functioning nervous system is necessary to maintain control. Urethra In the female, the urethra is a very short tube that leads from the bladder to an area populated by normal flora,