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McKinley/O’Loughlin
Human Anatomy, 2nd Edition
CHAPTER 27
Answers to “What Did You Learn?”
1.
The organs of the urinary system are the kidneys, ureters, urinary bladder, and
urethra. Urinary system organs function to filter waste products from the
bloodstream, convert the filtrate into urine, and transport the urine out of the body.
2.
The kidneys control blood volume under the direction of specific hormones, and
they help regulate erythrocyte production by secreting the hormone erythropoietin
if blood oxygen levels decrease. The erythropoietin stimulates stem cells in the
bone marrow to increase erythrocyte production.
3.
The renal cortex is located immediately internal to the kidney’s lateral border, and
the renal medulla is located immediately internal to the renal cortex.
4.
The right kidney is positioned about 2 centimeters inferior to the left kidney
because of the large size of the right lobe of the liver.
5.
The renal corpuscles of juxtamedullary nephrons lie adjacent to the
corticomedullary junction, and their relatively long nephron loops extend deep
into the medulla. The renal corpuscles of cortical nephrons are near the peripheral
edge of the cortex, and their relatively short nephron loops barely penetrate the
medulla.
6.
Filtration is the process by which water and some dissolved solutes in the blood
plasma passively move out of blood vessels into the glomerular space.
Reabsorption occurs when substances originally appearing in the filtrate move by
diffusion or active transport ot of the nephron back into the blood.
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7.
Human Anatomy, 2nd Edition
The components of a nephron are the renal corpuscle, composed of a glomerulus
and a glomerular capsule, and the renal tubule, composed of the proximal
convoluted tubule, the nephron loop, and the distal convoluted tubule. The filtrate
is produced in the renal corpuscle. As it flows through renal tubule the filtrate is
modified by the diffusion and active transport of substances across the walls of
the renal tubule back into the blood in the process of tubular reabsorption. Once
modification of the filtrate begins, it is called tubular fluid. The tubular fluid is
also modified by tubular secretion, which is the active transport of solutes out of
the blood into the tubular fluid.
8.
The posterior pituitary secretes antidiuretic hormone (ADH) as a consequence of
dehydration. ADH secretion results in decreased water loss in urine by acting on
the collecting duct epithelium to make it more able to absorb water from the
tubular fluid. Thus, the urine becomes more concentrated.
9.
The juxtaglomerular apparatus consists of juxtaglomerular cells and the macula
densa. The juxtaglomerular cells are modified smooth muscle cells in the wall of
the afferent arteriole. The macula densa cells are modified epithelial cells that
line the distal convoluted tubule at the point of contact between the afferent
arteriole and the distal convoluted tubule. The juxtaglomerular apparatus
continuously monitors the fluid electrolyte concentration in the distal convoluted
tubule (by macula densa cells). Under conditions that reduce blood volume or
solute concentration, the macula densa cells stimulate the juxtaglomerular cells to
release renin. Ultimately aldosterone is produced, which increases blood
electrolyte concentrations and blood volume.
McKinley/O’Loughlin
10.
Human Anatomy, 2nd Edition
The middle tunic of the ureter is called the muscularis. It consists of an inner
longitudinal and an outer circular layer of smooth muscle fibers. Additionally, the
smooth muscle in the inferior one-third of the ureter consists of another outer
longitudinal layer of smooth muscle. These muscle layers produce peristaltic
waves that proceed along the length of the ureter from the kidney toward the
urinary bladder.
11.
The ureters are innervated by both sympathetic fibers from the T11-L2 segments
of the spinal cord, and parasympathetic fibers from CN X and the pelvic
splanchnic nerves.
12.
The trigone is a posteroinferior triangular area of the urinary bladder wall. It is
formed by imaginary lines connecting the two posterior ureteral openings and the
anterior urethral opening. Because the trigone remains immovable during shape
changes as the urinary bladder fills and evacuates, it funnels urine into the urethra
as the bladder wall contracts to evacuate the stored urine.
13.
The initiating stimulus in the micturition reflex is the filling of the bladder with
urine resulting in the distension of its wall and subsequent activation of stretch
receptors.
14.
The shortest and least distensible part of the male urethra is the membranous
urethra, which extends from the inferior surface of the prostate gland through the
urogenital diaphragm.
15.
Structural changes in the kidneys affect urine production. With age, a gradual
reduction in kidney size accompanies both a reduction in blood flow to the
kidneys and a decrease in the number of functional nephrons in the kidney. These
McKinley/O’Loughlin
Human Anatomy, 2nd Edition
changes lead to reduced functional capability. Reduced blood flow also
contributes to a decreased glomerular filtration rate; consequently, both
reabsorption and secretion are reduced. The loss of nephrons results in
diminished ability to filter and cleanse the blood. Additionally, the possibility of
incontinence (the inability to control the passage of urine) increases with age.
16.
The urinary bladder, urethra, rectum, and anal canal are formed by the cloaca.
Answers to “Content Review”
1.
The urinary system filters water and waste from the blood plasma; it stores some
water and waste products as urine until the body is ready to expel the urine; it
excretes the urine from the bladder to outside the body; it helps maintain blood
volume by preventing excess water loss; and it is responsible for regulating
erythrocyte production.
2.
Several layers of connective tissue coverings surround the kidneys. An external,
fibrous capsule covers the outer surface of the kidney. It maintains kidney shape,
protects the kidney from trauma, and helps prevent the spread of infectious
pathogens. The perinephric fat (adipose capsule) is external to the fibrous capsule
and completely surrounds the kidney to offer cushioning and insulation. The
renal fascia is external to the perinephric fat. It anchors the kidney to the
posterior abdominal wall and the peritoneum. The paranephric fat is the outermost
layer that surrounds the kidney. It is composed of adipose connective tissue and
lies between the renal fascia and the peritoneum.
McKinley/O’Loughlin
3.
Human Anatomy, 2nd Edition
Blood is carried to a kidney within a renal artery. Up to five segmental (lobar)
arteries branch from the renal artery within the renal sinus. Thereafter, the
branching of arteries results in the interlobar arteries, the arcuate arteries, the
interlobular arteries, the afferent arterioles, the glomerulus, the efferent arteriole,
both the peritubular capillaries and the vasa recta, which drain into the
interlobular veins, the arcuate veins, the interlobar veins, and the renal vein.
Note that the efferent arteriole is still carrying blood high in oxygen. Gas and
nutrient exchange for the tissues in the kidney has not yet occurred. The blood in
all arteries, the arterioles, the glomerulus, the peritubular capillaries and the vasa
recta is high in oxygen. The blood in all veins is low in oxygen. The peritubular
and vasa recta capillary networks are responsible for the actual gas and nutrient
exchange, and picking up reabsorbed materials.
4.
The glomerulus is a tuft or thick tangle of fenestrated capillaries within an
epithelial capsule, which together form the renal corpuscle. The visceral layer of
the capsule lies directly on the surface of the glomerular capillaries. It is
composed of specialized cells called podocytes that have long processes called
pedicels (feet). Pedicels are separated by filtration slits. The filtration slits of the
podocytes and the fenestrated glomerulus endothelium compose the filtration
membrane. It is capable of selectively filtering materials from the blood.
5.
The proximal convoluted tubule is lined by a simple cuboidal epithelium with tall
microvilli on its apical (luminal) surface to markedly increase its reabsorption
capacity. Most of the reabsorption occurs in the proximal convoluted tubules,
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Human Anatomy, 2nd Edition
thus this apical surface modification is not required elsewhere in the nephron,
such as in the distal convoluted tubule.
6.
The structures of the juxtaglomerular apparatus work together to help regulate
blood pressure. The macula densa cells continuously monitor electrolyte
concentration in tubular fluid. If either blood volume or solute concentration is
reduced, the macula densa cells detect this change and stimulate the
juxtaglomerular cells to release renin. Recall from chapter 20, the renin activates
the renin-angiotensin pathway resulting in the production of aldosterone (see also
chapter 20) that causes electrolyte concentrations to increase in the blood and a
blood volume increase to restore normal rates of filtration.
7.
The ureters project through the bladder wall obliquely, consequently the ureteral
walls are compressed as the bladder distends. This decreases the likelihood of
urine refluxing into the ureters from the bladder.
8.
The ureters are innervated by the autonomic nervous system. Parasympathetic
fibers innervation has an unknown effect. Sympathetic fibers come from the T11L2 segments of the spinal cord. Pain from the ureter (such as from a kidney
stone) will be referred to the T11-L2 dermatomes. These dermatomes are along a
"loin to groin" region, so "loin to groin" pain typically means there is ureter
and/or kidney discomfort. The bladder is supplied by both sympathetic and
parasympathetic nerve fibers of the autonomic nervous system. The sympathetic
fibers are from the T11-L2 segments of the spinal cord. These fibers contract the
internal urethral sphincter and inhibit contraction of the detrusor muscle. Thus,
sympathetic fibers inhibit micturition (urination). The parasympathetic fibers
McKinley/O’Loughlin
Human Anatomy, 2nd Edition
come from the pelvic splanchnic nerves (S2-S4). They will relax the internal
urethral sphincter (so urine can pass through) and stimulate contraction of the
detrusor muscle. Thus, the parasympathetic fibers stimulate micturition.
9.
Water and waste products are pushed out from the glomerulus into the capsular
space of the glomerular capsule. This is called filtrate. Filtrate enters the
proximal convoluted tubule and is now called tubular fluid. Tubular fluid flows
through the proximal convoluted tubule, nephron loop, distal convoluted tubule,
and collecting duct where reabsorption and secretion occur. The tubular fluid
leaving the renal papilla is called urine. Urine passes from the renal papilla into
and then through the minor calyx, major calyx, renal pelvis, ureter, urinary
bladder, and finally through the urethra when the urine is voided from the body.
10.
A urinary tract infection (UTI) occurs when bacteria (usually E. coli) or fungi
enter the urinary tract and multiply there. Females are more prone to
developing UTIs because they have a short urethra and it is close to the anus.
This close proximity of urethra and anus facilitates bacterial entry into the
female urethra from the digestive tract. Sexual intercourse also increases the
risk of UTIs. In addition, UTIs are associated with the use of a urinary
catheter (a tube inserted through the urethra and into the urinary bladder to
help void urine).