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.
AcetonuriaThe 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 InjuryThe 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.
AlbuminuriaThe 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.
AnuriaLack of urine production, less than 50 mL/day.
AzotemiaThe presence of nitrogenous products in the blood, usually because of decreased kidney function.
CystitisAn inflammation of the bladder.
DiuresisAn 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.
DysuriaPainful or difficult urination.
EnuresisInvoluntary discharge of urine.
Glomerular Filtration RateThe volume of fluid filtered through the kidney.
GlycosuriaThe presence of glucose in the urine.
HematuriaThe presence of blood in the urine.
NephritisInflammation of the kidney.
NephrosisDegeneration of the kidney without the occurrence of inflammation.
OliguriaA decreased urine formation of less than 500 mL/day or 0.5 mL/kg hour in an adult.
PyelonephritisAn inflammation of the renal pelvis and calices.
StrictureAn abnormal narrowing in the urinary tract or a narrowing of the ureter or urethra.
UremiaThe presence of nitrogenous substances in the blood.
Urinary IncontinenceThe inability to retain urine in the bladder.
Urinary RetentionFailure 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.
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).
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.
Particles Filtered by the Glomerulus, Reabsorbed by the Tubules, and Excreted in the Urine
Filtered (mEq/24 h/170 L)
Reabsorbed (mEq/24 h/169 L)
Excreted (mEq/24 h/45 L)
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 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).