section epub:type=”chapter” id=”c0065″ role=”doc-chapter”> The kidneys have a profound effect on recovery from surgery and anesthesia. An understanding of renal anatomy and physiology can prevent damage and provide a basis for immediate interventions. absorption excretion; acid-base balance; acute kidney injury; diuretics; filtration The importance of the renal system in the postanesthesia care unit (PACU) is focused on the ability of the kidneys to metabolize and excrete drugs and to maintain acid-base, electrolyte, and fluid volume balance. The cardiovascular and respiratory systems rely on the ability of the kidneys to maintain homeostasis through physiologic mechanisms. Adequate kidney function is imperative to ensure positive outcomes for the patient recovering from anesthesia in the PACU. When kidney function is impaired, anesthetic drugs cannot be metabolized, which leads to a prolonged emergence. Impaired kidney function can compromise cardiovascular function when fluids cannot be removed and electrolytes are not balanced. With the profound effects the kidneys have on the patient’s recovery, assessment of kidney function is an important consideration in the assessment of the perianesthesia patient. An understanding of renal anatomy and physiology is important to facilitate perianesthesia patient recovery. Definitions Acetonuria The appearance of acetone in the urine. Acetonuria is present when excessive fats are consumed or when an inadequate amount of carbohydrates is metabolized. Acute Kidney Injury The abrupt loss of kidney function resulting in the retention of serum urea and the dysregulation of extracellular volume and electrolytes. The loss reflects a continuum of injury that can lead to complete renal failure. Albuminuria The presence of protein in the urine, also called proteinuria. Albumin is the most common protein found in the urine. This condition usually indicates a malfunction in glomerular filtration. Anuria Lack of urine production, less than 50 mL/day. Azotemia The presence of nitrogenous products in the blood, usually because of decreased kidney function. Cystitis An inflammation of the bladder. Diuresis An increase in amount and frequency of urination and the physiologic processes that produces such an increase. Can be further qualified by etiology such as water, osmotic, cold-induced, or rebound diuresis. Dysuria Painful or difficult urination. Enuresis Involuntary discharge of urine. Glomerular Filtration Rate The volume of fluid filtered through the kidney. Glycosuria The presence of glucose in the urine. Hematuria The presence of blood in the urine. Nephritis Inflammation of the kidney. Nephrosis Degeneration of the kidney without the occurrence of inflammation. Oliguria A decreased urine formation of less than 500 mL/day or 0.5 mL/kg hour in an adult. Pyelonephritis An inflammation of the renal pelvis and calices. Stricture An abnormal narrowing in the urinary tract or a narrowing of the ureter or urethra. Uremia The presence of nitrogenous substances in the blood. Urinary Incontinence The inability to retain urine in the bladder. Urinary Retention Failure to expel urine from the bladder. The kidneys are two bean-shaped organs in the retroperitoneal spaces at the level of the twelfth thoracic to third lumbar vertebrae. The right kidney is slightly lower than the left. Each kidney weighs approximately 150 g. The notched portion of the kidney is called the hilum, which is where the ureter, renal vein, and renal artery enter the kidney (Fig. 13.1). The kidney is divided into an outer cortex and an inner medulla. A) Structure of abdomen and urinary tract shows labels (clockwise) as follows: Diaphragm, abdominal aorta, adrenal gland, 10th rib, 11th rib, 12th rib, left kidney, ureter, urinary bladder, prostate gland (male), urethra, inferior vena cava, right kidney, renal vein, renal artery, and liver. B) Structure of kidney shows labels (clockwise) as follows: Cortex, pyramids, medulla, minor calix, major calix, ureter, hilum (renal artery, renal vein), and pelvis. Blood is supplied to each kidney by a renal artery arising from each side of the abdominal aorta. The rate of blood flow through both kidneys of a man who weighs 70 kg is approximately 1200 mL/min, or approximately 25% of the cardiac output.1 As the renal artery enters the kidney at the hilum, it divides into the interlobar arteries. Branches from the interlobar arteries divide into afferent arteries that supply the capillaries of the nephrons. The capillaries form the efferent arterioles and divide to form the peritubular capillaries that help supply the nephron (Figs. 13.2 and 13.3). Human kidney section shows labels (clockwise) as follows: Arcuate arteries and veins, interlobular arteries and veins, segmental arteries, renal vein, renal artery, and interlobar arteries and veins. Nephron unit shows labels starting from top right as follows: Proximal tubule, cortical collecting tubule, loop of Henle, collecting duct, peritubular capillaries, distal tubule, arcuate vein, arcuate artery, afferent arteriole, juxta-glomerular apparatus, efferent arteriole, glomerulus, and Bowman’s capsule. The nephron is the functional unit of the kidney. Each kidney contains approximately 1 million closely packed nephrons.1 Each nephron consists of a glomerulus, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule, and collecting ducts. Blood enters the afferent arteriole and goes into the glomerulus, which is located in the cortex. The glomerulus is a compact network of capillaries encased in a double-layered capsule, the Bowman capsule. Filtered blood flows out of the glomerulus and into the efferent arteriole. The portion of the blood that is filtered, 25%, drains into the proximal convoluted tubule.1 The renal tubules begin in the Bowman’s capsule. The pressure gradient caused by renal artery blood flow forces fluid to leave the glomerulus and enter the Bowmans capsule. The filtrate flows into the proximal convoluted tubule in the cortex of the kidney and then into the loop of Henle. The proximal loop of Henle is thick-walled but becomes thin at the distal segment in the medulla of the kidney. The filtrate then flows into the distal convoluted tubule, located in the cortex of the kidney, and passes into the collecting ducts. The collecting ducts traverse the cortex to the medulla, where they merge into the renal pelvis by way of the renal calyces. In the collecting ducts, the filtrate is termed urine. The renal pelvis is a wide, funnel-shaped structure composed of the calyces draining the kidney. The pelvis drains into the ureter, which leads to the bladder. Urine is formed by processes of filtration, reabsorption, and secretion. Filtration occurs as the blood passes through the glomerulus. The force of filtration is a pressure gradient that pushes fluid through the glomerular membrane. Approximately 180 L of water along with other substances is filtered out of plasma by the glomeruli every 24 hours (Table 13.1). Blood cells and heavy particles including proteins are retained in the blood because they are too large to pass through the glomerular epithelium. The presence of red blood cells or protein in the urine usually indicates a pathologic process in the kidney. Reabsorption occurs in the proximal and distal tubules. Approximately 99% of the water filtered by the glomeruli is reabsorbed. Many substances in the water are reabsorbed with active or passive transport. Active transport requires energy for movement of the substance across the membrane. Passive transport can be regarded as simple diffusion that does not require energy. Important constituents of body fluids—substances such as glucose, amino acids, sodium, potassium, calcium, and magnesium—are almost entirely reabsorbed. Certain substances are reabsorbed in limited quantities, such as urea and phosphate, and consequently appear in the urine. In a healthy individual, creatinine is the only filtered substance not reabsorbed and entirely secreted, allowing creatinine to serve as an indicator of glomerular filtration ability. The last process in the formation of urine is secretion. Various substances, including hydrogen and potassium ions, are secreted directly into the tubular fluid through the epithelial cells that line the renal tubules. Secretion plays an important role in promoting the body’s acid-base balance. The formation of urine and the reabsorption of substances needed for body function are aided by three physiologic mechanisms: the countercurrent mechanism, autoregulation, and hormonal control. The kidneys use the countercurrent mechanism to concentrate urine. The vasa recta are special capillaries that, with the loop of Henle, form the countercurrent mechanism. Fluids flow in opposite directions between the ascending and descending loops and sections of the vasa recta. The osmolality or weight of particles in solution of the interstitial fluid increases as it moves deeper into the medulla. The increase in osmolality results in active transport of particles or solutes into the interstitial fluid. The countercurrent mechanism is useful when the body needs to excrete a large amount of waste products yet reabsorb the normal amount of solutes and when the water in the body needs to be conserved, while waste products are eliminated. Autoregulation helps keep the glomerular filtration at a near normal rate despite fluctuations in arterial pressure. Renal vascular resistance changes in proportion to the renal perfusion pressure. For most patients, kidney autoregulation occurs when the mean arterial pressure (MAP) is greater than 70 mm Hg.2 As arterial pressure increases, the sympathetic innervation to the afferent arterioles causes constriction, thus keeping the glomerular filtration rate constant. When the arterial pressure is low, dilatation of the afferent arterioles serves to maintain glomerular filtration. The risk of adverse outcomes doubles if the MAP falls below 65 for more than 20 minutes.2 Secretion of antidiuretic hormone (ADH) by the posterior pituitary gland is affected by plasma osmolality. When the blood becomes hypertonic, ADH is secreted and the kidneys retain water. If the blood is hypotonic, less ADH is formed, causing the kidneys to reabsorb less water and increasing urine formation. ADH acts on the distal tubules and collecting tubules by altering permeability to water. The juxtaglomerular complex is a group of cells, located just before the glomerulus and in close proximity to the distal tubule, which contain granules of inactive renin. Renin is released in response to reduced arterial blood pressure entering the afferent arteriole of the kidney or a low concentration of sodium in the distal tubule. The released renin acts as an enzyme to convert angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II by the angiotensin-converting enzyme. Angiotensin II stimulates the adrenal cortex to release aldosterone. Aldosterone signals the kidney tubules to increase the reabsorption of sodium and retention of water. Because the renin-angiotensin system causes this reabsorption of water and sodium, it plays a role in the control of arterial blood pressure and is important in the conservation of sodium and control of fluid volume in hypotensive states (Fig. 13.4). Mechanism for arterial pressure control begins with angiotensinogen secreted from liver, leads to the formation of angiotensin 1, angiotensin 2, and then aldosterone secretion in adrenal gland: cortex. Decrease in renal perfusion (juxtaglomerular apparatus) stimulates renin signal, followed by increased angiotensin 1. • Surface of pulmonary and renal endothelium A C E in lungs and kidney follows passive transport and stimulates angiotensin 2. • Angiotensin 2 undergoes passive transport and stimulates signal to increase Sympathetic activity. • Angiotensin 2 undergoes passive transport and stimulates tubular sodium ion, chloride ion reabsorption and potassium ion excretion. Water retention. • Angiotensin 2 undergoes passive transport, stimulates adrenal gland: cortex to secrete Aldosterone that further stimulates water retention. • Angiotensin 2 undergoes passive transport and stimulates arteriolar vasoconstriction. Increase in blood pressure. • Angiotensin 2 undergoes passive transport, stimulates pituitary gland: posterior lobe, and A D H secretion that further stimulates collecting duct to absorb water. • Text reads, Water and salt retention. Effective circulating volume increases. Perfusion of the juxtaglomerular apparatus increases that act as inhibitory signal for kidney.
13: The Renal System
Abstract
Keywords
Anatomy of the kidneys
Kidney physiology
Particle
Filtered (mEq/24 h/170 L)
Reabsorbed (mEq/24 h/169 L)
Excreted (mEq/24 h/45 L)
Sodium
24,500
24,350
150
Chloride
17,800
17,700
100
Bicarbonate
4900
4900
0
Potassium
700
600
100
Glucose
780
780
0
Urea
870
460
410
Creatinine
12
0
12
Regulation of kidney function
Countercurrent Mechanism
Autoregulation
Hormonal Control
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