McKennon_Ch01.indd15.1 Anatomy and Physiology of the Renal System The renal system (see Figure 15.1), also called the urinary system, is responsible for filtering and clearing metabolic by-products and waste products from the blood while maintaining the body’s proper fluid, pH, and electrolyte balance. If these waste prod- ucts are not eliminated from the body, they build up and become toxic.
The kidneys are the primary organs for this waste filtration process, commonly referred to as elimination. Blood flows through the kidneys, which filter these waste products and produce urine, a modified filtrate of plasma. Urine formation is essen- tial for normal body function because it enables the blood to reabsorb necessary nutri- ents, water, and electrolytes. Large molecules, such as plasma proteins, cannot cross the glomerular basement membranes (the barrier that separates the vasculature from the urinary space) to be filtered from the blood, whereas small molecules—such as water, ions, and glucose—do pass through the membranes to be reabsorbed later into the blood.
FIGURE 15.1 The Renal System
The renal sys- tem includes the kidneys, important organs that main- tain the balance of water, electro lytes, and acids and bases in extra- cellular fluid.
right kidney
left kidney
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In the process of urine formation, the kidneys regulate the following metabolic functions:
• volume of blood plasma, which contributes significantly to the regulation of blood pressure
• concentration of waste products in the blood
• concentration of electrolytes in plasma, such as sodium (Na+), potassium (K+), bicarbonate (HCO3
–), calcium (Ca2+), and phosphate (PO4 3–)
• acid-base balance (pH) of plasma
• regulation of necessary hormones such as renin, erythropoietin, and vitamin D
After the kidneys perform their filtering function, the ureters transport the urine to the urinary bladder. There, the urine is held until voided, when the urine exits the body through the urethra.
Kidneys A kidney is one of two bean-shaped organs which are located in the upper abdomi- nal region. Although the kidneys are in the abdominal region, they are not inside the peritoneal cavity, where the stomach, pancreas, and intestines are located. On the tops of the kidneys sit the adrenal glands, which are part of the endocrine sys- tem, not the renal system. Hormones released by the adrenal glands regulate physi- ologic functions of several body systems. (To learn more about the adrenal glands, see Chapter 13.)
The blood is filtered by the kidneys. The renal artery, which branches off of the abdominal aorta, brings blood into the kidneys. The filtered blood returns to the bloodstream via the renal vein. The renal cortex is the outer layer of a kidney and is responsible for filtration. The renal medulla, in the body of each kidney, also performs filtration. Both the renal cortex and the renal medulla are made up of microscopic nephrons. A nephron is the functional filtering unit of the kidney (see Figure 15.2).
FIGURE 15.2 Anatomy of the Kidney
The kidney con- tains about 2 mil- lion microscopic nephrons, which produce urine and maintain constancy in the body’s internal environment.
arteries and veins
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A normal human kidney contains approximately 2 million microscopic nephrons (see Figure 15.3). The nephrons work to produce urine and thereby maintain constancy in the body’s internal environment. The renal tubules of the nephrons produce urine through three processes: filtration, reabsorption, and secre- tion. The role that individual nephrons play in these processes is outlined below.
Blood containing fluid and waste products enters the nephron through the afferent arteriole into the Bowman’s capsule. Here, the capillaries are tightly folded, forming the glomerulus. The tight folding in the glomerulus and the small amount of space inside the capsule create the high pressure that forces fluid and other substances out of the blood. Glomerular filtration is the initial step in urine production and the maintenance of fluid balance. Large molecules, such as proteins, are not filtered
glomerulus
Filtration
glucose
hydrogen
ammonia
water
potassium
hydrogen
sodium
FIGURE 15.3 Anatomy of a Nephron
Each part of the microscopic-sized nephron performs one or more of the following func- tions: filtration; reabsorption; and secretion of select electrolytes, fluids, and other sub- stances.
Pharmacy Technician Series © Paradigm Education Solutions Pharmacology for Technicians, Seventh Edition: Chapter 15, Section 15.1
out in the glomerulus, but most fluids and other smaller substances are. Blood leaves the Bowman’s capsule via the efferent arteriole. Filtrate, the fluids and by-products filtered from the blood in the glomerulus, continues through the nephron.
The filtrate passes through the distal and proximal convoluted tubules and the loop of Henle, a long, U-shaped structure from which water and salts are reabsorbed into the blood (see Figure 15.3). As filtrate passes through these structures, mol- ecules are selectively exchanged between the tubules, loop of Henle, and surround- ing capillaries through several mechanisms. Some substances are reabsorbed into the blood through simple diffusion. Others are exchanged between the blood and filtrate through secretion, an active transport process. Still others move across the mem- branes due to force of pressure, which is another way to describe filtration. Those substances that are filtered out of the blood or secreted into the filtrate (but do not reenter the blood) are then eliminated from the body as urine.
Proper urine production and maintenance of fluid balance rely on the tubular reabsorption and secretion processes. Reabsorption is the process by which sub- stances are pulled back into the blood after waste products have been removed during the formation of urine. In fact, reabsorption of water and sodium into the blood is essential for maintaining good hydration. When the kidneys are not func- tioning correctly, the proper balance of reabsorption and secretion is not maintained; as a result, toxins build up in the blood and begin to poison the body.
Ureters and Urinary Bladder A ureter is one of two paired muscular ducts that extend from the renal pelvis (see Fig- ure 15.2) to the urinary bladder. The main function of the ureters is to move urine from the kidneys to the urinary bladder. This movement is facilitated by smooth muscle contraction in the walls of the ureters.
The urinary bladder is located in the pelvic region. This organ collects and holds urine until the fluid exits the body during urination. The bladder is made of stretchy epithelial and smooth muscle cells (see Figure 15.4), which allow it to expand and hold up to 1 L of fluid. However, the functional capacity of the bladder (the volume held before voluntary voiding) is much smaller: approximately 300–400 mL in adults.
FIGURE 15.4 Anatomy of the Bladder
Although the blad- der can hold up to 1 L of urine, stretch receptors typically trigger the urge to uri- nate when only 30%–40% of that volume has accu- mulated.
ureter
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Once urine flows down the ureters and into the urinary bladder, the internal urethral sphincter, an involuntary muscle, relaxes as the bladder fills to allow urine into the urethra. The external urethral sphincter, a voluntary muscle, holds the urine in the bladder before it exits the body. A detrusor muscle is smooth muscle in the bladder that contracts to help push urine out. When the bladder is full and dis- tended, stretch receptors sense the pressure and cause the detrusor muscles in the bladder to contract and the internal and external urethral sphincter to relax. Urine is pushed out, and the bladder empties. This urination process is called micturition.
Pharmacy Technician Series © Paradigm Education Solutions Pharmacology for Technicians, Seventh Edition: Chapter 15, Section 15.1