Basic A & P II
Dr. L. Bacha
Chapter Outline (Martini & Nath 2010)
 list the three major functions of the urinary system:
 by examining Fig. 24-1, list the organs of the urinary system:
 describe the location of the kidneys relative to the vertebral column:
 they are retroperitoneal, which means they are situated between the parietal peritoneum
and muscles of the back
 describe the fibrous capsule:
 does adipose tissue surround the kidneys?
 a typical adult kidney is about the size of a large bar of soap!
 describe the hilum:
SECTIONAL ANATOMY OF THE KIDNEYS
 describe the following (and examine the kidney in Figs. 24-3 and 24-4)
renal sinus renal cortex renal medulla -
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renal papilla renal lobe: consists of one renal pyramid and the overlying area of renal cortex
 on Fig. 24-4, locate the following parts of the duct system that collects urine from the renal
papillae (a):
ureter
renal pelvis
minor calyx
major calyx
 about how many nephrons are in each kidney?!
BLOOD SUPPLY AND INNERVATION OF THE KIDNEYS
 what percent of the total cardiac output do the kidneys receive?
 examine Fig. 24-5 and identify the first 2 blood vessels listed below:
renal artery (one branches to each kidney from the abdominal aorta)
 various arteries which eventually branch into
 cortical radiate arteries (extend up into the cortex)
 afferent arterioles
 glomerulus = a ball-like network of glomerular capillaries within the glomerular capsule
 efferent arterioles
 peritubular capillaries (network of capillaries that surround the PCTs, DCTs and part of
collecting duct, which we will get to!) and vasa recta (capillary loops that surround the
loop of Henle and collecting ducts)
   various veins that eventually lead into  renal veins  inferior vena cava
THE NEPHRON
 nephrons are the microscopic, functional units of the kidneys
 about 1.25 million in each kidney!
 each nephron consists of what two parts?
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 Renal Corpuscle
- a renal corpuscle includes what two parts?
 Glomerular Capsule (Bowman’s capsule): The glomerular capsule consists of three parts
(see Fig. 24-8):
1. capsular epithelium = simple squamous epithelium; forms the outer wall of the
capsule
2. visceral epithelium - formed by unique, modified epithelial cells called podocytes (we’ll
get back to these!)
3. the capsular space - the space between the two epithelial layers into
which fluid filtered from the glomerular capillaries enters
 Glomerulus = a ball-like network of glomerular capillaries that are fenestrated capillaries
- the renal corpuscle is where blood is filtered during a process called glomerular filtration to
form fluid called glomerular filtrate
 Renal Tubule
◦ the renal tubule is the tubular portion of a nephron into which filtrate passes
◦ reabsorption and secretion occur here
◦ examine the parts of the renal tubule in Fig. 24-6
◦ parts of the renal tubule:
 proximal convoluted tubule (PCT)
What does “proximal” refer to
here? “distal”? “convoluted”?
The names of some of the parts
of the renal tubule make sense if
you know what the terms mean!
 loop of Henle
(1) thick descending limb
(2) thin segment
(3) thick ascending limb
 distal convoluted tubule [DCT]
the DCTs of several nephrons empty into a collecting duct
 Collecting duct - leads into a papillary duct that opens at the renal papilla
 List and summarize the three distinct processes of urine formation (page 840; Fig. 24-9!):
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I. GLOMERULAR FILTRATION
 during glomerular filtration, blood pressure forces water and small solutes across the
filtration membrane into the capsular space (larger solutes, such as plasma proteins, and
formed elements are excluded)
 glomerular filtration results in the formation of fluid called glomerular filtrate in the capsular
space
 The Filtration Membrane is what substances must pass through from the blood of the
glomerulus into the capsular space during glomerular filtration
The filtration membrane is formed by three components (see Fig. 24-8):
1. Fenestrations of the endothelial cells of the glomerular capillaries
 pores in the endothelial cells that prevent filtration of blood cells and platelets
2. Basement membrane of the glomerular capillaries
3. Filtration slits between the pedicels of the podocytes that form the visceral epithelium
of the glomerular capsule
 the visceral layer of the glomerular capsule consists of modified simple squamous cells
called podocytes; these podocytes have numerous footlike processes called pedicels
that wrap around the endothelial cells of the glomerular capillaries; the spaces
between the pedicels are called filtration slits
 Net Filtration Pressure
 glomerular filtration depends on three main pressures:
1. glomerular hydrostatic blood pressure (page 845)
 define glomerular hydrostatic pressure:
 how does it compare to capillary pressures elsewhere in the systemic circuit?
 what is the average glomerular hydrostatic pressure?
2. capsular hydrostatic pressure
 capsular hydrostatic pressure is the pressure exerted against the filtration membrane
by the fluid that is already in the capsular space and renal tubule
 does this pressure oppose or enhance filtration?
 what is the average capsular hydrostatic pressure?
3. blood colloid osmotic pressure
 blood colloid osmotic pressure is the osmotic pressure in the blood of the glomerulus
resulting from the presence of what?
 does this pressure oppose or enhance filtration?
 what is the average blood colloid osmotic pressure?
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 net filtration pressure is the total pressure that promotes filtration
 stay tuned…we will write an equation that shows how a normal NFP is calculated:
 Glomerular Filtration Rate
 define glomerular filtration rate (GFR):
 what is the average GFR?
read about the value of a creatinine clearance test:
 Control of GFR
(p. 846)
 homeostasis of body fluids requires that the kidneys maintain a relatively constant GFR
- what happens if glomerular filtration does not occur (or if the GFR is too low)?
 filtration depends on what:
 there are three mechanisms that stabilize GFR:
(1) Autoregulation of GFR
- what does autoregulation maintain?
- maintenance of GFR is accomplished mainly by changing the diameter of the afferent
arterioles
e.g., decrease in systemic blood pressure  afferent arterioles dilate  blood flow
increases through the glomerulus  increases GFR to previous level
e.g., elevated systemic blood pressure  afferent arterioles contract  narrows the
arteriole’s lumen  blood flow decreases through the glomerulus  reduces GFR to
previous level
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(2) Hormonal Regulation of GFR
- when blood volume or hydrostatic blood pressure are low, the cells of the kidney secrete
a hormone called renin into the blood, which ultimately leads to the formation of
angiotensin II

angiotensin II causes:
1. vasoconstriction of systemic arterioles, which increases systemic blood pressure
2. the adrenal cortex to release aldosterone
∙ aldosterone increases the reabsorption of sodium ions and water by the kidneys to
increase blood volume and blood pressure back to normal
- atrial natriuretic peptide - is released when the walls of the atria are stretched; it
increases GFR, so that the kidneys excrete more sodium and water into urine; this
results in a decrease in blood volume and, therefore, hydrostatic blood pressure
(3) Autonomic Regulation of GFR
- sympathetic nerves cause vasoconstriction of afferent arterioles, decreasing the GFR and
slowing the production of filtrate
e.g., with exercise or hemorrhage  increase sympathetic stimulation of smooth muscle of
afferent arterioles  vasoconstriction of afferent arterioles  decrease blood flow
through glomerulus  decreases GFR  this lowering of renal blood flow, and
therefore of GFR, has two consequences: it decreases urine output to help conserve
blood volume and it allows greater blood flow to other parts of the body
II. TUBULAR REABSORPTION (this is my summary of all the detail in the book!!!)
 as tubular fluid flows along the renal tubule and collecting duct, substances in the fluid
pass from the lumen of the tubule  through the epithelial cells lining the tubule 
into the blood in surrounding peritubular capillaries and vasa recta
 different substances get reabsorbed through different parts of the renal tubule and by
different mechanisms (active transport; diffusion, etc.)
 e.g. of substances that get reabsorbed:
A. electrolytes
B. amino acids
C. glucose (I will go over this in class)
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Once the
glomerular
filtrate begins to
flow from the
capsular space
down through the
renal tubule, it is
referred to as
tubular fluid.
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D. water - water reabsorption is so important it gets its own heading…
Regulation of Water Reabsorption (and therefore, Urine Volume and
Concentration)
1. ADH (antidiuretic hormone)
- ADH is a hormone produced by neurosecretory cells in the hypothalamus of the brain;
it is stored and released by the posterior pituitary gland (the neurohypophysis)
- target of ADH = epithelial cells of the distal convoluted tubules (DCT) and collecting duct
- effect of ADH = causes the cell membrane of the epithelial cells of the DCT and collecting
duct to be more permeable to water; this allows an increase in reabsorption of water.
(A diuretic is a substance that causes an increase in urine production.)
summary of water reabsorption and influence of ADH:
 80% of water in the filtrate is reabsorbed from the PCT to thin segment of the loop of Henle.
This occurs regardless of the volume and concentration of urine that is finally produced by the
kidney, and is referred to as obligatory water reabsorption.
 How much of the remaining 19% of the water that can be reabsorbed is variable and depends
on ADH. If ADH is present, water is reabsorbed from the DCT and collecting duct, so that more
water in the filtrate is reabsorbed and less water remains in the filtrate as part of urine. (The
following steps correspond to the diagram on the next page.)
1. NaCl in the filtrate entering the PCT is actively transported out of the PCT. Water follows by
osmosis.
2. The descending portion is freely permeable to water. Water moves out by osmosis, because
the interstitial tissue of the medulla of the kidney is hypertonic to the fluid in the tubule.
3. NaCl is actively transported from the ascending portion of the loop of Henle into the
interstitial tissue, but the ascending tubule is NOT permeable to water.
4. As tubular fluid passes through the DCT and the collecting duct, water reabsorption and,
therefore, urine volume and concentration now depend on antidiuretic hormone (ADH):
a. A drop in blood hydrostatic pressure or an increase in osmotic pressure of the blood
stimulates an increase in ADH secretion into the blood
 cells of the collecting duct and DCT are more permeable to water
 more water moves out of the DCT and collecting duct by osmosis into the interstitial
fluid and into surrounding blood vessels
 this increase in water reabsorption causes the excretion of a smaller volume of more
 concentrated urine  the body conserves water
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b. A rise in blood hydrostatic pressure or a decrease in osmotic pressure of the blood
causes a decrease in ADH secretion
 cells of the collecting duct and DCT are less permeable to water and water remains
in these tubules as it passes through them
 decrease in water reabsorption and excretion of larger volume of more dilute urine
 the body gets rid of excess water
NOTE: The ascending
portion of the loop of
Henle actively
transports NaCl from
the tubule into the
interstitial fluid, but is
impermeable to
water. Therefore,
large quantities of
NaCl accumulate in
the interstitial fluid in
increasing
concentration from
the cortex through
the medulla, creating
a hypertonic
environment.
What if you are dehydrated on a hot summer day…
would more or less ADH be secreted?
would more or less water be reabsorbed?
would more or less urine be produced?
What if you drink too much water…
would more or less ADH be secreted?
would more or less water be reabsorbed?
would more or less urine be produced?
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2. Renin-Angiotensin-Aldosterone
A decrease in blood hydrostatic pressure stimulates cells of the kidney to secrete an
enzyme called renin into the blood which leads to the formation of angiotensin II
angiotensin II:
a. is a potent vasoconstrictor that increases blood hydrostatic pressure
b. stimulates the cells of the adrenal cortex to increase aldosterone secretion
 aldosterone increases Na+ reabsorption by the DCT and collecting duct
 more water is reabsorbed due to osmosis
 blood hydrostatic pressure increases
c. increases the sensation of thirst, salt desire, and ADH secretion
3.
Diuretics
Diuretics are agents that increase the rate of urine formation. Examples of diuretics are:
a. alcohol - inhibits secretion of ADH
b. sodium ion reabsorption inhibitors (loop diuretics; e.g., furosemide)
- decrease the reabsorption of Na+ and CL- from the thick ascending portion of the
loop of Henle and, therefore, decrease the amount of water that moves out of the
filtrate (decrease reabsorption of water)
c. caffeine
- promotes renal vasodilation, which increases GFR
- decreases reabsorption of NaCl → less water is reabsorbed
d. osmotic diuretics (e.g., mannitol)
- increase the osmotic pressure of the filtrate and, therefore decrease the amount of
water that moves out of the filtrate (decrease reabsorption of water)
4.
Temperature
An increase in body temperature causes:
a. cutaneous blood vessels to dilate
b. vessels to the abdominal organs constrict  decrease in blood flow to the kidney
 decrease in GFR  decrease in volume of urine produced
A decrease in body temperature causes:
a. cutaneous blood vessels to constrict
b. vessels to the abdominal organs dilate  increase in blood flow
to the kidney → increase in GFR → increase in volume of urine produced
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III. TUBULAR SECRETION
 tubular secretion is the process of adding substances from the blood into the tubular
fluid within the renal tubule
 some substances that get secreted are ammonia, K+, H+
 tubular secretion is important for maintaining blood pH by excreting H ions or retaining
them
The Composition of Normal Urine
(p.857)
 Characteristics of Normal of Urine
 from Table 24-5, list these characteristics and indicate the normal range for each:
 Urinalysis
Components of Urine
 water (95%)
 electrolytes
 organic waste products excreted in the urine (see the right column on page 840)
- urea - an organic waste from the breakdown of what?
- creatinine - what is creatinine generated in?
- uric acid
- what is uric acid formed during?
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 URETERS (p.860)
∙ 2 ureters, one from each kidney
∙ what do the ureters extend between?
∙ about how many inches long is each ureter?
∙ they enter the urinary bladder medially from the posterior side
∙ urine moves by peristalsis, pressure and gravity to the urinary bladder
 URINARY BLADDER
∙ describe the urinary bladder and indicate what it functions as:
∙ what is the trigone of the urinary bladder?
∙ describe the location of the internal urethral sphincter:
- it is formed by what type of muscle tissue?
- what do the smooth muscle fibers of this sphincter provide?
Histology of the Urinary Bladder
∙ the muscularis of the urinary bladder consists of layers of smooth muscle; collectively, these
layers form what muscle?
- what is the effect of contraction of this muscle?
 URETHRA
∙ small tube leading from the neck of the urinary bladder to the exterior of body.
∙ what does the urethra transport?
∙female - 1.5 inches long; male - about 8 in. long.
∙ note the two urethral sphincters shown in Fig. 24-18 (c)
∙ the external urethral sphincter is formed by what type of muscle tissue?
- so, is it voluntary or involuntary?
∙ the opening of the urethra to the outside is the external urethral orifice
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The Micturition Reflex and Urination
 urine reaches the urinary bladder by what?
 what is this process coordinated by?
 micturition is commonly known as urination
 when the urinary bladder is stretched (with about 200 ml of urine), stretch receptors send
impulses to the sacral region of the spinal cord
 the sacral region of the spinal cord sends parasympathetic impulses that cause:
 relaxation of the internal urethral sphincter
 contraction of the smooth muscle of the bladder wall
 interneurons in the spinal cord relay impulses to the cerebral cortex, where the sensation
of fluid pressure in the urinary bladder is perceived  the cerebral cortex sends somatic
motor impulses that cause:
 relaxation of the external urethral sphincter
 urination occurs !
 urination is a reflex, but after about 2 years of age, it can be initiated or stopped voluntarily by
impulses from the cerebral cortex
 why do infants lack voluntary control over urination?
 define incontinence:
THE END!
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