McKenna's Pharmacology for Nursing, 2e

C H A P T E R 5 0 Introduction to the kidneys and the urinary tract 797

Maintenance of volume and composition of body fluids The kidneys regulate the composition of body fluids by balancing the levels of the key electrolytes, secreting or absorbing these electrolytes to maintain the desired levels. The volume of body fluids is controlled by diluting or concentrating the urine. Sodium regulation Sodium is one of the body’s major cations (positively charged ions). It filters through the glomerulus and enters the renal tubule; then it is actively reabsorbed in the proximal convoluted tubule to the peritubular cap- illaries. As sodium is actively moved out of the filtrate, it takes chloride ions and water with it. This occurs by passive diffusion as the body maintains the osmotic and electrical balances on both sides of the tubule. Sodium ions are also reabsorbed via a transport system that functions under the influence of the catalyst carbonic anhydrase . This enzyme speeds the combining of carbon dioxide and water to form carbonic acid. The carbonic acid immediately dissociates to form sodium bicarbonate, using a sodium ion from the renal tubule and a free hydrogen ion (an acid). The hydrogen ion remains in the filtrate, causing the urine to be slightly acidic. The bicarbonate is stored in the renal tubule as the body’s alkaline reserve for use when the body becomes too acidic and a buffer is needed. The distal convoluted tubule acts to further adjust the sodium levels in the filtrate under the influence of aldosterone (a hormone produced by the adrenal gland) and natriuretic hormone (probably produced by the hypothalamus). Aldosterone is released into the circu- lation in response to high potassium levels, sympathetic stimulation or angiotensin III. Aldosterone stimu- lates a sodium–potassium exchange pump in the cells of the distal tubule, causing reabsorption of sodium in exchange for potassium (see Chapter 7 for a review of the sodium-potassium pump). As a result of aldosterone stimulation, sodium is reabsorbed into the system and potassium is lost in the filtrate. Natriuretic hormone causes a decrease in sodium reabsorption from the distal tubules with a resultant diluted urine or increased volume. Natriuretic hormone is released in response to fluid overload or haemodilution. Countercurrent mechanism Sodium is further regulated in the medullary nephrons in what is known as the countercurrent mechanism in the loop of Henle. In the descending loop of Henle, the cells are freely permeable to water and sodium. Sodium is actively reabsorbed into the surrounding peritubu- lar tissue, and water flows out of the tubule into this sodium-rich tissue to maintain osmotic balance. The filtrate at the end of the descending loop of Henle is con- centrated in comparison to the rest of the filtrate.

components of the blood through the glomerulus into the nephron tubule), tubular secretion (active movement of substances from the blood into the renal tubule) and tubular reabsorption (movement of substances from the renal tubule back into the vascular system). Glomerular filtration The glomerulus acts as an ultrafine filter for all of the blood that flows into it. The semipermeable membrane keeps blood cells, proteins and lipids inside the vessel, whereas the hydrostatic pressure from the blood pushes water and smaller components of the plasma into the tubule. The resulting fluid is called the filtrate. Scarring, or swelling of, or damage to the semipermeable membrane leads to the escape of larger plasma compo- nents, such as blood cells or protein, into the filtrate. The large size of these components prevents them from being reabsorbed by the tubule, and they are lost in the urine. Thus a clinical sign of renal damage is the presence of blood cells or protein in the urine. Approximately 125 mL of fluid is filtered out each minute, or 180 L/day. About 99% of the filtered fluid is returned to the bloodstream as the filtrate continues its movement through the renal tubule. Approximately 1% of the filtrate—less than 2 L of fluid—is excreted each day in the form of urine. Tubular secretion The epithelial cells that line the renal tubule can secrete substances from the blood into the tubular fluid. This is an energy-using process that allows active transport systems to remove electrolytes, some drugs and drug metabolites, and uric acid from the surrounding cap- illaries and secrete them into the filtrate. For instance, the epithelial cells can use tubular secretion to help maintain acid–base levels by secreting hydrogen ions as needed. Tubular reabsorption The cells lining the renal tubule reabsorb water and various essential substances from the filtrate back into the vascular system. About 99% of the water filtered at the glomerulus is reabsorbed. Other filtrate compo- nents that are reabsorbed regularly include vitamins, glucose, electrolytes, sodium bicarbonate and sodium chloride. The reabsorption process uses a series of trans- port systems that exchange needed ions for unwanted ones (see Chapter 7 for a review of cellular transport systems). Drugs that affect renal function frequently overwhelm one of these transport systems or interfere with its normal activity, leading to an imbalance in acid–base or electrolyte levels. The precision of the reab- sorption process allows the body to maintain the correct extracellular fluid volume and composition.