Chapter 15 The Urinary System
The kidneys, which maintain the purity and constancy of our internal fluids, are
perfect examples of homeostatic organs. Much like sanitation workers who keep a
city's water supply drinkable and dispose of its waste, the kidneys are usually
unappreciated until there is a malfunction and "internal garbage" piles up. Every
day, the kidneys filter gallons of fluid from the bloodstream. They then process this
filtrate, allowing wastes and excess ions to leave the body in urine while returning
needed substances to the blood in just the right proportions. Although the lungs
and the skin also play roles in excretion, the kidneys bear the major responsibility
for eliminating nitrogenous (nitrogen-containing) wastes, toxins, and drugs from
the body.
Kidneys
 Describe the functions and location of the kidneys in the body.
o To maintain the purity and constancy of our internal fluids – to eliminate nitrogenous
wastes, toxins, and drugs from the body – to regulate the blood’s volume and chemical
makeup – to producing the enzyme renin that helps regulate blood pressure – to produce
the hormone erythropoietin that stimulates red blood cell production in bone marrow – to
convert vitamin D to its active form
o The kidneys lie against the dorsal body wall in a retroperitoneal position (beneath the
parietal peritoneum in the superior lumbar region
 They extend from the T12 to the L3 vertebra – receive some protection from the lower
part of the rib cage
 The right kidney is positioned slightly lower than the left due to crowding by the liver
 Identify the following regions of a kidney (longitudinal section): hilus, cortex, medulla, medullary
pyramids, calyces, pelvis, and renal columns.
o See figures 15.1 on page 481 and figure 15.2 on page 482
o Hilus – medial indentation where the ureters, the renal blood vessels, and nerves enter or
exit the kidney
o Renal cortex – the outer region, which is light in color as seen when the kidney is cut
lengthwise
o Renal medulla – deep to the cortex, a darker reddish-brown area
o Medullary pyramids –contains triangular regions with a striped appearance
o Apex – the broader base of each pyramid faces toward the cortex, its tip
o Renal columns – extensions of cortex-like tissue that separate the pyramids
o Renal pelvis – medial to the hilus, a flat, basin-like cavity – continuous with the ureter
leaving the hilus
o Calyces – extensions of the pelvis that form cup-shaped areas that enclose the tips of the
pyramids – collect urine, which continuously drains from the tips of the pyramids into the
renal pelvis then into the ureter, which transports it to the bladder for temporary storage
 Recognize that the nephron is the structural and functional unit of the kidney and describe its
anatomy.
o Nephrons are responsible for forming the urine product
o Each consist of two main structures – glomerulus, which is a knot of capillaries and a renal
tubule
 The closed end of the renal tubule is enlarged and cup-shaped and completely
surrounds the glomerulus – this portion of the renal tubule is called the
glomerular, or Bowman’s capsule
The inner (visceral) layer of the capsule is made up of highly modified cells
called podocytes that have long branching processes called pedicels that
intertwine and cling to the glomerulus
 Filtration slits, openings, exist between the extensions so the
podocytes form a porous, or holey, membrane around the glomerulus
 The renal tubule extends from the glomerular capsule and enters the collecting
duct
 Three regions – proximal convoluted tubule (PCT), loop of Henle, distal
convoluted tubule (DCT)
 The lumen surfaces of the tubule cells in the proximal convoluted tubules are
covered with dense microvilli to increase the surface area – also present in
other regions to a lesser degree
o Most nephrons are called cortical nephrons because they are located within the cortex
while a few are called juxtamedullary nephrons because they are close to the cortexmedulla junction
 The collecting ducts, which receive urine from many nephrons, run downward
through the medullary pyramids, giving them the striped appearance and deliver the
final urine product into the calyces and renal pelvis
o Each nephron is associated with two capillary beds – the glomerulus and the peritubular
capillary bed
 The glomerulus is both fed and drained by arterioles – the afferent arteriole
comes from the interlobular artery is the feeder vessel and the efferent arteriole
receives blood that has passed through the glomerulus
 The afferent arteriole has a larger diameter and blood pressure in the
glomerulus is extraordinarily high forcing fluid and solutes out of the blood
into the glomerular capsule – most of the filtrate is reclaimed by the renal
tubule cells and returned to the blood in the peritubular capillary beds
 The peritubular capillaries come from the efferent arteriole that drains the
glomerulus and are low pressure, porous vessels that are for absorption instead of
filtration – by clinging to the renal tubule, they receive solutes and water from the
tubule cells as they are reabsorbed from the filtrate – they drain into interlobular
veins leaving the cortex
 Describe the process of urine formation, identifying the areas of the nephron that are responsible
for filtration, reabsorption, and secretion.
o Filtration – a nonselective, passive process done by the glomerulus – the filtrate that is
formed is blood plasma without blood proteins or blood cells because they are too big to
pass through the filtration membrane
o Reabsorption – tubular reabsorption begins as soon as the filtrate enters the proximal
convoluted tubule where tubule cells are transporters, taking up needed substances from
the filtrate and then passing them out their posterior aspect into the extracellular space,
from which they are absorbed into peritubular capillary blood
 Some reabsorption is passive (water passes by osmosis) while most substances
depend on active transport, which uses membrane carriers and is a selective process
(glucose and amino acids are reabsorbed while nitrogenous waste products such as
urea are not)
o Secretion – tubular secretion is reabsorption in reverse – some substances such as
hydrogen and potassium ions and creatinine, move from the blood of the peritubular
capillaries through the tubule cells or from the tubule cells themselves into the filtrate to
be eliminated in urine
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 Describe the function of the kidneys in excretion of nitrogen-containing wastes.
o Nitrogenous waste products are poorly reabsorbed by the kidneys including urea, which is
formed by the liver as an end product of protein breakdown when amino acids are used to
produce energy, uric acid is released when nucleic acids are metabolized and creatinine is
metabolized in the muscle tissue
 Define polyuria, anuria, oliguria, and diuresis.
o Polyuria – excretion of large volumes of urine with loss of tremendous amounts of salt and
water to urine – usually caused by Addison’s disease (hypoaldosteronism)
o Anuria – abnormally low urinary output – less than 100 ml/day – may be the result of
transfusion reactions, acute inflammation, or crush injuries to the kidneys
o Oliguria – abnormally low urinary output – between 100 – 400 ml/day – may indicate that
glomerular blood pressure is too low to cause filtration
o Diuresis – flow of urine from the kidney
 Describe the composition of normal urine.
o Generally clear and pale to deep yellow due to the pigment urochrome that results
from the body’s destruction of hemoglobin – the more solutes in the urine, the deeper
yellow in color it is – any other colors may be the result of eating certain foods or the
presence of bile or blood in the urine
o Urine is sterile and its odor is slightly aromatic – when let stand, it takes on an
ammonia odor caused by the action of bacteria on the urine solutes or some drugs,
vegetables, and various diseases
o Urine pH is around 6 but changes in body metabolism and certain foods may alter the pH
o Urine is water plus solutes, and thus weighs more than distilled water – the term used to
compare how much heavier urine is than distilled water is specific gravity
o Solutes normally found in urine include sodium and potassium ions, urea, uric acid,
creatinine, ammonia, bicarbonate ions, and various other ions depending on blood
composition
 List abnormal urinary components.
o Glucose, blood proteins (primarily albumin), red blood cells, hemoglobin, white blood cells
(pus) and bile are not normally found in urine and their presence may be the result of
problems – see table 15.1 on page 488
Ureters, Urinary Bladder, and Urethra
 Describe the general structure and function of the ureters, bladder, and urethra.
o Ureters – slender tubes that run behind the peritoneum form the hilus of a kidney to the
posterior aspect of the bladder where it enters at a slight angle – essentially passageways
that carry urine from the kidneys to the bladder – smooth muscle layers in the walls
contract to propel urine into the bladder by peristalsis
o Bladder – a smooth collapsible, muscular sac that stores urine temporarily and located
retroperitoneally in the pelvis just posterior to the pubic symphysis
 Contains three openings – two ureter openings and a single opening of the urethra,
which drains the bladder
 The bladder wall contains three layers of smooth muscle, collectively called the
detrusor muscle, and its mucosa is a special type of epithelium, the transitional
epithelium – as the muscular wall stretches, the transitional epithelial layer thins,
allowing the bladder to store more urine without increasing its internal pressure
o Urethra – a thin-walled tube that carries urine by peristalsis from the bladder to the
outside of the body
At the bladder-urethra junction, a thickening of the smooth muscle forms the
internal urethral sphincter, an involuntary sphincter that keeps the urethra closed
when urine is not being passed
 A second sphincter, the external urethral sphincter, is fashioned by skeletal
muscle as the urethra passes through the pelvic floor and is voluntarily controlled
Compare the course and length of the male urethra to that of the female.
o In females, the urethra is 3 – 4 cm (1 ½ inches) long and its external orifice, or opening,
lies anteriorly to the vaginal opening – its function is to conduct urine to the body exterior
o In males, the urethra is 20 cm (8 inches) long and has three named regions – prostatic,
membranous, and spongy, or penile, urethrae – it opens at the tip of the penis after
traveling down its length – the urethra of the male has a double function, to carry urine out
of the body and to provide passage for sperm
Define micturition.
o Also known as voiding – the act of emptying the bladder – two sphincters, or valves,
control the flow of urine from the bladder
o The bladder can collect urine until about 200 ml have accumulated then the stretching of
the bladder wall activated stretch receptors and impulses transmitted to the sacral region
of the spinal cord and back to the bladder via the pelvic splanchnic nerves cause the
bladder to go into reflex contractions – as the contractions get stronger, stored urine is
forced past the internal sphincter into the upper part of the urethra and the urge to void is
felt
o The lower sphincter is skeletal muscle and thus voluntarily controlled and one can postpone
bladder emptying until it is convenient but eventually, micturition occurs whether one wills it
or not
Describe the difference in control of the external and internal urethral sphincters.
o The internal urethral sphincter is smooth muscle and thus involuntarily controlled while the
external urethral sphincter is skeletal muscle and thus voluntarily controlled
Name three common urinary tract problems.
o Kidney stones – renal calculi – caused when urine becomes extremely concentrated and
solutes such as uric acid salts form crystals that precipitate in the renal pelvis – frequent
bacterial infections of the urinary tract, urinary retention, and alkaline urine all favor calculi
formation
o Incontinence – occurs when we are unable to voluntarily control the external sphincter –
normal in children younger than two years because they have not yet gained control over
their voluntary sphincter – in older people, may be the result of emotional problems,
pressure (as in pregnancy), or nervous system problems such as stroke or spinal cord injury
o Urinary retention – a condition where the bladder is unable to expel the urine – often
caused by general anesthesia given during surgery or enlargement, or hypertrophy, of the
prostate gland in elderly men (as it enlarges, it narrows the urethra) – when retention is
prolonged, a slender rubber drainage tube called a catheter must be inserted through the
urethra to drain the urine and prevent bladder trauma from excessive stretching
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Fluid, Electrolyte, and Acid-Base Balance
 Name and localize the three main fluid compartments of the body.
o Water occupies three main locations within the body
 About two-thirds of body fluid is the intracellular fluid (ICF) that is contained
within the living cells
 The remainder is called extracellular fluid (ECF) and includes all body fluids
located outside the cells – includes blood plasma, interstitial (or tissue) fluid,
cerebrospinal and serous fluids, the humors of the eye, lymph, and others
 Explain the role of antidiuretic hormone (ADH) in the regulation of water balance by the kidney.
o Reabsorption of water and electrolytes by the kidneys is regulated primarily by hormones –
highly sensitive cells in the hypothalamus called osmoreceptors react to the change in
blood composition by becoming very irritable and this results in nerve impulses being sent
to the posterior pituitary, which releases antidiuretic hormone (ADH) – prevents
excessive water loss in the urine by acting on the kidney’s collecting ducts, where it causes
the duct cells to reabsorb more water – ADH is released until solute concentration of the
blood drops too low and then the osmoreceptors are turned off and excess water is allowed
to leave the body in the urine
 Explain the role of aldosterone in sodium and potassium balance of the blood.
o Aldosterone is the major factor regulating sodium ion (the electrolyte most responsible for
osmotic water flows) content of the ECF and in the process helps regulate the concentration
of other ions (Cl-, K+, Mg2+) – produced by the adrenal cortex and its release is triggered by
renin-angiotensin mechanism mediated by the juxtaglomerular (JG) apparatus of the renal
tubules – see figure 15.9 on page 494 for more details
 When concentrations of aldosterone is high most of the remaining sodium is actively
reabsorbed in the distal convoluted tubules and the collecting ducts – for each
sodium ion reabsorbed, a potassium ion is secreted into the filtrate causing
the sodium content of the blood to increase while the potassium concentration
decreases
 Because sodium is reclaimed, water follows it passively back into the blood causing
an increase in water reabsorption by the tubule cells – water follows salt
 Compare and contrast the relative speed of buffers, the respiratory system, and the kidneys in
maintaining the acid-base balance of the blood.
o Blood buffers – chemical buffers are systems of one or two molecules that act to prevent
dramatic changes in hydrogen ion concentration when a strong acid or strong base is added
by binding to hydrogen ions whenever the pH drops and by releasing hydrogen ions when
the pH rises – act within a fraction of a second and thus are the first line of defense in
resisting pH changes
o Respiratory system controls – the respiratory system eliminates carbon dioxide from the
blood while it loads oxygen into the blood but as carbon dioxide enters the blood, it is
converted to bicarbonate ions for transport in the plasma with carbonic acid being
produced in the meantime = Co2 + H2O  H2CO3  H+ + HCO3- - as carbon dioxide
accumulates in the blood or more hydrogen ions are released to the blood by metabolic
processes, the chemoreceptors in the respiratory control centers of the brain are activated
causing the breathing rate and depth to increase to rid the body of excessive carbon dioxide
– act within a minute or so
o Renal mechanisms – only the kidneys can rid the body of other acids generated during
metabolism and only the kidneys have the power to regulate blood levels of alkaline
substances – act slowly and require hours or days to bring about changes in blood pH
but are the most potent of the mechanisms for regulating blood pH
 Excreting bicarbonate ions
 Conserving (reabsorbing) or generating more bicarbonate ions
Developmental Aspects of the Urinary System
 Describe two common congenital problems of the urinary system.
o Polycystic kidney – a degenerative condition that appears to run in families – one or both
kidneys are enlarged and have many blister-like sacs (cysts) containing urine that interfere
with renal function by obstructing urine drainage – renal failure is the eventual outcome
requiring kidney transplants
Hypospadias – found only in male babies – occurs when the urethral orifice is located on
the ventral surface of the penis – corrective surgery is generally done when the child is one
year old
 Describe the effect of aging on urinary system functioning.
o There is a progressive decline in kidney function by age 70 – the rate of filtrate formation is
only about half that of the middle-aged adult possibly due to impaired renal circulation
due to atherosclerosis, which affect the entire circulatory system of the aging person
 Also a decrease in the number of functional nephrons and the tubule cells become
less efficient in their ability to concentrate urine
o Bladder shrinkage and loss of bladder tone causes many elderly individuals to
experience urgency (a feeling that it is necessary to void) and frequency (frequent voiding
of small amounts of urine)
 Nocturia – the need to get up during the night to urinate is also common
 Incontinence is the final outcome of the aging process – loss of control
 Urinary retention – most often a result of hypertrophy of the prostate gland in
males
o