McKenna's Pharmacology for Nursing, 2e

C H A P T E R 5 0 Introduction to the kidneys and the urinary tract 795

T he renal system is composed of the kidneys and the structures of the urinary tract: the ureters, the urinary bladder and the urethra. This system has four major functions in the body: • Maintaining the volume and composition of body fluids within normal ranges, including the following functions: • clearing nitrogenous wastes from protein metabolism • maintaining acid–base balance and electrolyte levels • excreting various drugs and drug metabolites • Regulating vitamin D activation, which helps to maintain and regulate calcium levels • Regulating blood pressure through the renin– angiotensin–aldosterone system • Regulating red blood cell production through the production and secretion of erythropoietin THE KIDNEYS The kidneys are two small organs that make up about 0.5% of total body weight but receive about 25% of the cardiac output. Approximately 1600 L of blood flows through these two small organs each day for cleansing. Most of the fluid that is filtered out by the kidneys is returned to the body, and the waste products that remain are excreted in a relatively small amount of water as urine. Urinary: Renal function Structure The kidneys are located under the ribs, for protection from injury. They have three protective layers that make up the renal capsule: a fibre layer, a perirenal, or brown fat, layer and the renal parietal layer. The capsule contains pain fibres, which are stimulated if the capsule is stretched secondary to an inflammatory process. The kidneys have three identifiable regions: the outer cortex, the inner medulla and the renal pelvises. The renal pelvises drain the urine into the ureters. The ureters are muscular tubes that lead into the urinary bladder, where urine is stored until it is excreted (Figure 50.1). Nephron The functional unit of the kidneys is called the nephron . There are approximately 2.4 million nephrons in an adult. All of the nephrons filter fluid and make urine, but only the medullary nephrons can concen- trate or dilute urine. It is estimated that only about 25% of the total number of nephrons are necessary to maintain healthy renal function. That means that the renal system is well protected from failure with a large

Adrenal medulla Adrenal cortex

Renal capsule

Renal cortex

Renal sinus

Renal pelvis

Renal medulla

Ureter

Urinary bladder Ureter Fundus of bladder Epithelial layer Opening of ureter Muscle layer Neck of bladder Urethral sphincter Prostate gland Urethra FIGURE 50.1  The kidney and organs of the urinary tract.

backup system. However, it also means that by the time a person manifests signs and symptoms, suggest- ing failure of the kidneys, extensive kidney damage has already occurred. The nephron is basically a tube that begins at Bowman’s capsule and becomes the proximal and then distal convoluted tubule (Figure 50.2). Bowman’s capsule has a fenestrated or “window-like” epithelium that works like a sieve or a strainer to allow fluid to flow through but keep large components (e.g. proteins) from entering. The tube exits the capsule curling around in a section called the proximal convoluted tubule. From there, it narrows to form the descending and ascending loop of Henle. It widens as the distal convoluted tubule and then flows into the collecting ducts, which meet at the renal pelvises. Each section of the tubule functions in a slightly different manner to maintain fluid and electro­ lyte balance in the body.