The Urinary System

Every day the kidneys filter nearly 200 liters of fluid from the bloodstream, allowing toxins, metabolic wastes, and excess ions to leave the body in urine while returning needed substances to the blood. The kidneys are usually unappreciated until they malfunction and body fluids become contaminated.Functions of the kidneys1 - The major excretory organs ,although the lungs and skin also participate in excretion. 2 -They also act as essential regulators of: a- the volume and chemical makeup of the blood, b-proper balance between water and salts and c-between acids and bases.

Other renal functions include:• Gluconeogenesis during prolonged fasting• Producing the hormones renin and erythropoietin -Renin (re nin; ren = kidney) acts as an enzyme to ′

help regulate blood pressure. -Erythropoietin (ĕ-rith″ro-poi ĕ-tin) stimulates red ′

blood cell production.• Metabolizing vitamin D to its active form .

• Kidney Anatomy-The bean-shaped kidneys lie in a retroperitoneal position (between the dorsal body wall and the parietal peritoneum) in the superior lumbar region Extending approximately from T12 to L3, the kidneys receive some protection from the lower part of the rib cage .

-The right kidney is crowded by the liver and lies slightly lower than the left.

-An adult’s kidney has a mass of about 150 g and its average dimensions are 12 cm long, 6 cm wide, and 3 cm thick—about the size of a large bar of soap.

-The lateral surface is convex. -The medial surface is concave and has a vertical cleft

called the renal hilum .

• The ureter, renal blood vessels, lymphatics, and nerves all join each kidney at the hilum.

• Atop each kidney is an adrenal (or suprarenal) gland, an endocrine gland that is functionally unrelated to the kidney.

Two layers of supportive tissue surround each kidney:

1. The fibrous capsule, a transparent capsule that prevents infections in surrounding regions from spreading to the kidneys

2. The perirenal fat capsule, a fatty mass that attaches the kidney to the posterior body wall and cushions it against blows

• HOMEOSTATIC IMBALANCE The fatty around the kidneys is important in holding the kidneys in their normal body position. If the amount of fatty tissue decreases (as with rapid weight loss), one or both kidneys may drop to a lower position, an event called renal ptosis (to sis; “a fall”). Renal ptosis may ′cause a ureter to become kinked, which creates problems because the urine, unable to drain, backs up into the kidney causing hydronephrosis (hi″dro-n ĕ-fro sis; “water in the kidney”). Hydronephrosis can ′severely damage the kidney, leading to necrosis (tissue death) and renal failure.

• Internal AnatomyA frontal section through a kidney reveals three distinct regions: cortex, medulla, and pelvis. The most superficial region, the renal cortex, is light in color and has a granular appearance.

Deep to the cortex is the darker, reddish-brown renal medulla, which exhibits cone-shaped tissue masses called medullary or renal pyramids.

The broad base of each pyramid faces toward the cortex, and its apex, or papilla (“nipple”), points internally.

• The pyramids appear striped because they are formed almost entirely of parallel bundles of microscopic urine-collecting tubules and capillaries.

• The renal columns, inward extensions of cortical tissue, separate the pyramids.

• Each pyramid and its surrounding cortical tissue constitutes one of approximately eight lobes of a kidney.

• The renal pelvis, a funnel-shaped tube, is continuous with the ureter leaving the hilum. Branching extensions of the pelvis form two or three major calyces (ka lih-s ĕz; singular: calyx), ′each of which subdivides to form several minor calyces, cup-shaped areas that enclose the papillae. The calyces collect urine, which drains continuously from the papillae, and empty it into the renal pelvis into the ureter, which moves it to the bladder to be stored. The walls of the calyces, pelvis, and ureter contain smooth muscle that contracts rhythmically to propel urine along its course by peristalsis.

HOMEOSTATIC IMBALANCE Infection of the renal pelvis and calyces produces the condition called pyelitis (pi″ĕ-li tis). Infections or inflammations that ′affect the entire kidney are pyelonephritis (pi″ĕ-lo-n ĕ-fri tis). Kidney infections in females are usually ′caused by fecal bacteria that spread from the anal region to the urinary tract. Less often they result from bloodborne bacteria (traveling from other infected sites) that lodge and multiply in the kidneys. In severe cases of pyelonephritis, the kidney swells, abscesses form, and the pelvis fills with pus.

Blood and Nerve Supply The kidneys receive 25% of the total cardiac output

per minute. The vascular pathway through a kidney is as follows: renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomeruli → efferent arterioles → peritubular capillary beds → cortical radiate veins → arcuate veins → interlobar veins → renal vein. The nerve supply of the kidneys is derived from the renal plexus.

NephronsNephrons (nef ronz) are the structural and functional ′units of the kidneys. Each kidney contains over 1 million of these tiny units, which carry out the processes that form urine . Each nephron consists of a glomerulus ,which is a tuft of capillaries, and a renal tubule. The cup-shaped end of the renal tubule, the glomerular capsule (or Bowman’s capsule) is blind and completely surrounds the glomerulus.. The endothelium of the glomerular capillaries is fenestrated (penetrated by many pores), which makes them exceptionally porous. This allows large amounts of solute-rich, virtually protein-free fluid to pass from the blood into the glomerular capsule. This plasma-derived fluid or filtrate is the raw material that the renal tubules process to form urine.

• The remainder of the renal tubule is about 3 cm long and has three major parts. It leaves the glomerular capsule as the coiled proximal convoluted tubule (PCT), makes a hairpin loop called the loop of Henle ,and then winds and twists again as the distal convoluted tubule (DCT) before emptying into a collecting duct. The collecting ducts, each of which receives filtrate from many nephrons, run through the medullary pyramids and give them their striped appearance. As the collecting ducts approach the renal pelvis, they fuse together and deliver urine into the minor calyces via papillae of the pyramids.

• Cortical nephrons represent 85% of the nephrons in the kidneys. Except for small parts of their loops of Henle that dip into the outer medulla, they are located entirely in the cortex.

• remaining juxtamedullary nephrons (juks″tah-m ĕdul-ah-re) originate close to (juxta = near to) the ′

cortex-medulla junction, and they play an important role in the kidneys’ ability to produce concentrated urine. Their loops of Henle deeply invade the medulla, and their thin segments are much more extensive than those of cortical nephrons.

• Nephron Capillary BedsEvery nephron is closely associated with two capillary beds:

1-the glomerulus and the peritubular capillaries .The glomerulus is specialized for filtration. It differs from all other capillary beds in the body in that it is both fed and drained by arterioles—the afferent arteriole and the efferent arteriole, respectively.

Because (1) arterioles are high-resistance vessels and (2) the afferent arteriole has a larger diameter than the

efferent, the blood pressure in the glomerulus is extraordinarily high .

2- The peritubular capillaries arise from the efferent arterioles and empty into nearby venules. Most of the filtrate (99%) is reabsorbed by the peritubular capillary beds as they are low-pressure, porous capillaries that readily absorb solutes and water from the tubule cells .

Mechanisms of Urine Formation

• Urine formation and the adjustment of blood composition involve three major processes:

• Step 1: Glomerular Filtration -The glomeruli function as filtersdue to high glomerular

blood pressure (55 mm Hg), - Usually about 10 mm Hg, the net filtration pressure (NFP)

is determined by the difference between forces favoring filtration (glomerular hydrostatic pressure) and forces that oppose it (capsular osmotic pressure) .

- About 180 L/day is filtered from the glomeruli into the renal tubules.

- Strong sympathetic nervous system activation causes constriction of the afferent arterioles, which decreases filtrate formation and stimulates renin release .

- The renin-angiotensin mechanism raises systemic blood pressure via generation of angiotensin II, which promotes aldosterone secretion.

Step 2: Tubular Reabsorption During tubular reabsorption, needed substances are removed from

the filtrate by the tubule cells and returned to the peritubular capillary blood.

-Na+ by a Na+-K+ by active transport .- Water, many anions, and various other substances (for example, urea) are reabsorbed passively.- Certain substances (creatinine, drug metabolites, etc.) are not reabsorbed or are reabsorbed incompletely because of the lack of carriers.- The proximal tubule cells are most active in reabsorption. Most of the nutrients, 65% of the water and sodium ions, and the bulk of actively transported ions are reabsorbed in the proximal convoluted tubules.- Reabsorption of additional sodium ions and water occurs in the distal tubules and collecting ducts and is hormonally controlled. Aldosterone increases the reabsorption of sodium (and water that follows it); antidiuretic hormone enhances water reabsorption by the collecting ducts.

• Step Three: Tubular SecretionIs a means of adding substances to the filtrate (from the blood or tubule cells). It is an active process that is important in eliminating drugs, certain wastes, and excess ions and in maintaining the acid-base balance of the blood. Tubular secretion is important for

1. Disposing of certain drugs .2. Eliminating of undesirable substances (urea and uric

acid).3. Ridding the body of excess K+. 4. Controlling blood pH. When blood pH drops toward

the acidic end of its homeostatic range, the renal tubule cells actively secrete more H+ into the filtrate and retain more HCO3

– (a base).

UrinePhysical Characteristics1-Color and Transparency-Freshly voided urine is clear and pale to deep yellow. Its yellow color is due to urochrome (u ro-krōm), a pigment that results from the body’s ′destruction of hemoglobin .

-The more concentrated the urine, the deeper the yellow color. - An abnormal color such as pink or brown, or a smoky tinge, may result

from eating certain foods or may be due to the presence in the urine of bile pigments or blood.

-Some commonly prescribed drugs and vitamin supplements alter the color of urine.

-Cloudy urine may indicate a urinary tract infection.2-Odor-Fresh urine is slightly aromatic, but if allowed to stand, it develops an ammonia odor as bacteria metabolize its urea solutes.

-Some drugs and vegetables alter the usual odor of urine, as do some diseases. For example, in uncontrolled diabetes mellitus the urine smells fruity because of its acetone content.

3- pH-Urine is usually slightly acidic (around pH 6), but changes in body metabolism or diet may cause the pH to vary from about 4.5 to 8.0.

- A predominantly acidic diet that contains large amounts of protein and whole wheat products produces acidic urine.

- A vegetarian (alkaline) diet, prolonged vomiting, and bacterial infection of the urinary tract all cause the urine to become alkaline.4- Specific GravityThe ratio of the mass of a substance to the mass of an equal volume of distilled water is its specific gravity. The specific gravity of distilled water is 1.0 and that of urine ranges from 1.001 to 1.035, depending on its solute concentration.

• Chemical Composition-Water accounts for about 95% of urine volume; the remaining 5% consists of solutes.

-The largest component of urine by weight, apart from water, is urea, which is derived from the normal breakdown of amino acids.

-Other nitrogenous wastes in urine include uric acid (an end product of nucleic acid metabolism) and creatinine (a metabolite of creatine phosphate, which is found in large amounts in skeletal muscle tissue).

-Normal solute constituents of urine, in order of decreasing concentration, are urea, Na+, K+, PO4

3–, SO42–, creatinine, and

uric acid. -Much smaller but highly variable amounts of Ca2+, Mg2+, and

HCO3– are also present in urine.

Abnormal Urinary constituents

UretersThe ureters are slender tubes that begins at the level of L2 as a continuation of the renal pelvis. From there, it descends behind the peritoneum and runs obliquely through the posterior bladder wall. This arrangement prevents backflow of urine during bladder filling because any increase in bladder pressure compresses and closes the distal ends of the ureters. The transitional epithelium of its lining mucosa is continuous with that of the kidney pelvis superiorly and the bladder. Incoming urine distends the ureter and stimulates its muscularis to contract, propelling urine into the bladder. (Urine does not reach the bladder through gravity alone

• HOMEOSTATIC IMBALANCE Calcium, magnesium, or uric acid salts in urine may

precipitate in the renal pelvis, forming renal calculi or kidney stones. Most calculi are under 5 mm in diameter and pass through the urinary tract without causing problems. However, larger calculi can obstruct a ureter and block urine drainage.

Predisposing conditions are frequent bacterial infections, urine retention, high blood levels of calcium, and alkaline urine.

Surgical removal of calculi has been almost entirely replaced by shock wave lithotripsy, a noninvasive procedure that uses ultrasonic shock waves to shatter the calculi.

Urinary BladderThe urinary bladder is a smooth, collapsible, muscular sac that stores urine temporarily. It is located retroperitoneally on the pelvic floor just posterior to the pubic symphysis. The prostate (part of the male reproductive system) surrounds the bladder neck inferiorly where it empties into the urethra. In females, the bladder is anterior to the vagina and uterus. The interior of the bladder has openings for both ureters and the urethra .The smooth, triangular region of the bladder base outlined by these three openings is the trigone (tri gōn; trigon = triangle), important ′clinically because infections tend to persist in this region.

• The bladder has mucosa containing transitional epithelium .The muscular layer, called the detrusor muscle, consists of intermingled smooth muscle fibers arranged in inner and outer longitudinal layers and a middle circular layer. When empty, its walls are thick and thrown into folds (rugae). As urine accumulates, the bladder expands, the muscular wall stretches and thins, and rugae disappear. These changes allow the bladder to store more urine without a significant rise in internal pressure.

• A moderately full bladder is about 12 cm long and holds approximately 500 ml of urine, but it can hold nearly double that if necessary.

• When tense with urine, it can be palpated well above the pubic symphysis. The maximum capacity of the bladder is 800–1000 ml and when it is overdistended, it may burst.

• Although urine is formed continuously by the kidneys, it is usually stored in the bladder until its release is convenient.

• UrethraThe urethra is a thin-walled muscular tube that drains urine from the bladder and conveys it out of the body. At the bladder-urethra junction a thickening of the detrusor smooth muscle forms the internal urethral sphincter .This involuntary sphincter keeps the urethra closed when urine is not being passed and prevents leaking between voiding. This sphincter is unusual in that contraction opens it and relaxation closes it. The external urethral sphincter surrounds the urethra as it passes through the urogenital diaphragm. This sphincter is formed of skeletal muscle and is voluntarily controlled. The levator ani muscle of the pelvic floor also serves as a voluntary constrictor of the urethra .

• The length and functions of the urethra differ in the two sexes. In females the urethra is only 3–4 cm (1.5 inches) long and straight while in males the urethra is approximately 20 cm (8 inches) long , curved and has three regions:

- The prostatic urethra, about 2.5 cm (1 inch) long, runs within the prostate.

-The membranous urethra, which runs through the urogenital diaphragm, extends about 2 cm from the prostate to the beginning of the penis.

-The spongy urethra, about 15 cm long, passes through the penis and opens at its tip via the external urethral orifice.

- The male urethra has a double function: It carries semen as well as urine out of the body.

-The male urethra opens away from anus while the female’s external orifice is close to the anal opening.

• HOMEOSTATIC IMBALANCE in females, improper toilet habits (wiping back to front after defecation) can easily carry fecal bacteria into the urethra. Overall, 40% of all women get urinary tract infections. The urethral mucosa is continuous with that of the rest of the urinary tract, and an inflammation of the urethra (urethritis) can ascend the tract to cause bladder inflammation (cystitis) or even renal inflammations (pyelitis or pyelonephritis).

Symptoms of urinary tract infection include dysuria (painful urination), urinary urgency and frequency, fever, and sometimes cloudy or blood-tinged urine. When the kidneys are involved, back pain and a severe headache often occur.

MicturitionMicturition (mik″tu-rish un; mictur = urinate), also ′called urination or voiding, is the act of emptying the bladder. Stretching of the bladder wall by accumulating urine initiates the micturition reflex, in which parasympathetic fibers, in response to signals from the micturition center of the pons, cause the detrusor muscle to contract and the internal urethral sphincter to open.

Because the external sphincter is voluntarily controlled, micturition can usually be delayed temporarily.

• HOMEOSTATIC IMBALANCE Incontinence occurs when we are unable to voluntry control

the external urethral sphincter. It isnormal in children 2 years old or younger.

After the toddler years, incontinence is usually a result of emotional problems, physical pressure during pregnancy, or nervous system problems.

In urinary retention, the bladder is unable to expel its contained urine. Urinary retention is normal after general anesthesia (it seems that it takes a little time for the detrusor muscle to regain its activity). Urinary retention in men often reflects hypertrophy of the prostate, which narrows the urethra, making it difficult to void. When urinary retention is prolonged, a slender rubber drainage tube called a catheter (kath ĕ-ter) must be inserted through the urethra to drain the ′urine and prevent bladder trauma from excessive stretching.

• Because its bladder is very small and its kidneys are less able to concentrate urine for the first two months, a newborn baby voids 5 to 40 times daily, depending on fluid intake. By 2 months of age, the infant is voiding approximately 400 ml/day, and the amount steadily increases until adolescence, when adult urine output (about 1500 ml/day) is achieved. Incontinence, the inability to control micturition, is normal in infants because they have not yet learned to control the external urethral sphincter.

Control of the voluntary urethral sphincter goes hand in hand with nervous system development. By 15 months, most toddlers know when they have voided. By 18 months, they can usually hold urine for about two hours.

Daytime control usually is achieved first; it is unrealistic to expect complete nighttime control before age 4.

• HOMEOSTATIC IMBALANCE Three of the most common congenital abnormalities of the urinary system are horseshoe kidney, hypospadias, and polycystic kidney.

1-When ascending from the pelvis the kidneys are very close together, and in 1 out of 600 people they fuse across the midline, forming a single, U-shaped horseshoe kidney.

2- Hypospadias (hi″po-spa de-as), found in male infants ′only, is the most common congenital abnormality of the urethra. It occurs when the urethral orifice is located on the ventral surface of the penis. This problem is corrected surgically when the child is around 12 months old.

3- Polycystic kidney disease (PKD) is a group of disorders characterized by the presence of many fluid-filled cysts in the kidneys, which interfere with renal function, ultimately leading to renal failure.

• Developmental Aspects of the Urinary System - Three sets of kidneys (pronephric, mesonephric, and

metanephric) develop from the intermediate mesoderm. - The kidneys of newborns are less able to concentrate urine; their bladder is small and voiding is frequent. Neuromuscular maturation generally allows toilet training for micturition to begin by 18 months of age.-The most common urinary system problems in children and young to middle-aged adults are bacterial infections.- Renal failure has serious consequences: the kidneys are unable to concentrate urine, nitrogenous wastes accumulate in the blood, and acid-base and electrolyte imbalances occur.- With age, nephrons are lost, the filtration rate decreases, and tubule cells become less efficient at concentrating urine.- Bladder capacity and tone decrease with age, leading to frequent micturition and (often) incontinence. Urinary retention is a common problem of elderly men.

Fluid, Electrolyte, and Acid-Base BalanceBody Fluids Body Water Content Water accounts for 45–75% of body weight, depending on age, sex, and amount of body fat.Fluid Compartments

• About two-thirds (25 L) of body water is found within cells ,intracellular fluid (ICF) compartment; the extracellular fluid (ECF) compartment (15 L) is about one-third. The ECF includes plasma (3 L)and interstitial fluid(12 L).Composition of Body Fluids Solutes dissolved in body fluids include electrolytes and nonelectrolytes. Electrolyte concentration is expressed in mEq/L. Plasma contains more proteins than does interstitial fluid; otherwise, extracellular fluids are similar. The most abundant ECF electrolytes are sodium, chloride, and bicarbonate ions. Intracellular fluids contain large amounts of protein anions and potassium, phosphate, and magnesium ions.

Fluid Movement Among Compartments Fluid exchanges between compartments are regulated

by osmotic and hydrostatic pressures: (a) Filtrate is forced out of the capillaries by

hydrostatic pressure and pulled back in by colloid osmotic pressure.

(b) Water moves freely between the ECF and the ICF by osmosis, but solute movements are restricted by size, charge, and dependence on transport proteins.

(c) Water flows always follow changes in ECF osmolality. Plasma links the internal and external environments.

Water Balance -Sources of body water are ingested foods and fluids and

metabolic water. - Water leaves the body via the lungs, skin, gastrointestinal

tract, and kidneys.Regulation of Water Intake Increased plasma osmolality triggers the thirst mechanism, mediated by hypothalamic osmoreceptors. Thirst, inhibited by distension of the gastrointestinal tract by ingested water and then by osmotic signals, may be damped before body needs for water are met.Regulation of Water Output Obligatory water loss is unavoidable and includes insensible water losses from the lungs, the skin, in feces, and about 500 ml of urine output daily.-The volume of urinary output depends on water intake and loss via other routes and reflects the influence of antidiuretic hormone( most filtered water is reabsorbed) and aldosterone (Na and water reabsorption)on the renal tubules.

Disorders of Water Balance • Dehydration occurs when water loss exceeds water intake over

time. It is evidenced by thirst, dry skin, and decreased urine output. A serious consequence is hypovolemic shock.

• Hypotonic hydration occurs when body fluids are excessively diluted and cells become swollen by water entry. The most serious consequence is cerebral edema.

• Edema is an abnormal accumulation of fluid in the interstitial space, which may impair blood circulation.

Regulation of Sodium Balance Sodium ion balance is linked to ECF volume and blood pressure

regulation and involves both neural and hormonal controls.-Aldosterone promotes Na+ reabsorption and H2O conservation, unless other mechanisms favor water excretion.

-Declining blood pressure and falling filtrate osmolality stimulate the kidney cells to release renin. Renin, via angiotensin II, enhances systemic blood pressure and aldosterone release.

-Cardiovascular system baroreceptors sense changing arterial blood pressure, prompting changes in sympathetic vasomotor activity. Rising arterial pressure leads to vasodilation and enhanced Na+ and water loss in urine. Falling arterial pressure promotes vasoconstriction and conserves Na+ and water.- Atrial natriuretic peptide, released by certain atrial cells in response to rising blood pressure (or blood volume), causes systemic vasodilation and inhibits renin, aldosterone, and ADH release. Hence, it enhances Na+ and water excretion, reducing blood volume and blood pressure.- Estrogens and glucocorticoids increase renal retention of sodium. Progesterone promotes enhanced sodium and water excretion in urine.

Acid-Base Balance - Acids are proton (H+) donors; bases are proton

acceptors. Acids that dissociate completely in solution are strong acids; those that dissociate incompletely are weak acids. Strong bases are more effective proton acceptors than are weak bases.- The homeostatic pH range of arterial blood is 7.35 to 7.45. A higher pH represents alkalosis; a lower pH reflects acidosis.- Some acids enter the body in foods, but most are generated by breakdown of phosphorus-containing proteins, incomplete oxidation of fats or glucose, and the loading and transport of carbon dioxide in the blood.- Acid-base balance is achieved by chemical buffers, respiratory regulation, and in the long term by renal regulation of bicarbonate ion (hence, hydrogen ion) concentration of body fluids.

• Chemical Buffer Systems Chemical buffers are single or paired sets (a weak acid and its salt) of molecules that act rapidly to resist excessive shifts in pH by releasing or binding H+.-Chemical buffers of the body include the bicarbonate, phosphate, and protein buffer systems. Respiratory Regulation of H+ .

• Respiratory regulation of acid-base balance of the blood utilizes the bicarbonate buffer system and the fact that CO2 and H2O are in reversible equilibrium with H2CO3. -Acidosis activates the respiratory center to increase respiratory rate and depth, which eliminates more CO2 and causes blood pH to rise.

-Alkalosis depresses the respiratory center, resulting in CO2 retention and a fall in blood pH.

Renal Mechanisms of Acid-Base Balance • The kidneys provide the major long-term mechanism for controlling acid-base

balance by maintaining stable HCO3– levels in the ECF. Metabolic acids (organic

acids other than carbonic acid) can be eliminated from the body only by the kidneys.-Secreted hydrogen ions come from the dissociation of carbonic acid generated within the tubule cells.-Tubule cells are impermeable to bicarbonate in the filtrate, but they can conserve filtered bicarbonate ions indirectly by absorbing HCO3

– generated within them (by dissociation of carbonic acid to HCO3

– and H+). For each HCO3–

(and Na+) reabsorbed, one H+ is secreted into the filtrate, where it combines with HCO3

–.-To generate and add new HCO3

– to plasma to counteract acidosis, either of two mechanisms may be used: Secreted H+, buffered by bases other than HCO3

–, is excreted from the body in urine (the major urine buffer is the phosphate buffer system).

-NH4+ (derived from glutamine catabolism) is excreted in urine.

-To counteract alkalosis, bicarbonate ion is secreted into the filtrate and H+ is reabsorbed.

Abnormalities of Acid-Base Balance - Classification of acid-base imbalances as metabolic or respiratory

indicates the cause of the acidosis or alkalosis. - Respiratory acidosis results from carbon dioxide retention;

respiratory alkalosis occurs when carbon dioxide is eliminated faster than it is produced.- Metabolic acidosis occurs when fixed acids (lactic acid, ketone bodies, and others) accumulate in the blood or when bicarbonate is lost from the body.

- Metabolic alkalosis occurs when bicarbonate levels are excessive. - Extremes of pH for life are 7.0 and 7.8.

Compensations occur when the respiratory system or kidneys act to reverse acid-base imbalances resulting from abnormal or inadequate functioning of the alternate system.

-Respiratory compensations involve changes in respiratory rate and depth.

-Renal compensations modify blood levels of HCO3–.

Developmental Aspects of Fluid, Electrolyte, and Acid-Base Balance

• Infants have a higher risk of dehydration and acidosis because of their low lung residual volume, high rate of fluid intake and output, high metabolic rate, relatively large body surface area, and functionally immature kidneys at birth.

• The elderly are at risk for dehydration because of their low percentage of body water and insensitivity to thirst cues.

• Diseases that promote fluid and acid-base imbalances (cardiovascular disease, diabetes mellitus, and others) are most common in the aged.