The Urinary System

Document Sample
The Urinary System Powered By Docstoc
					                 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 kidneys
1 - 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
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 Anatomy
  A 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
• The pyramids appear striped because they are
  formed almost entirely of parallel bundles of
  microscopic urine-collecting tubules and
• 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.
Nephrons (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
• 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
  ĕ′dul-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
• Nephron Capillary Beds
   Every nephron is closely associated with two capillary
    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 Secretion
  Is 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
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).
 Physical Characteristics
 1-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.
 -Fresh urine is slightly aromatic, but if allowed to stand, it
 develops an ammonia odor as bacteria metabolize its urea
 -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 Gravity
 The 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+, PO43–,
   SO42–, creatinine, and uric acid.
  -Much smaller but highly variable amounts of Ca2+,
   Mg2+, and HCO3– are also present in urine.
Abnormal Urinary constituents
The 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
  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 Bladder
The 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.
• Urethra
  The 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
  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.
Micturition (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
  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
   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
   - 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 Balance
   Body 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
(c) Water flows always follow changes in ECF
   Plasma links the internal and external
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
 -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
  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
- 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
  -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
- 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

Shared By:
jiang lifang jiang lifang