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The Urinary System
The Urinary System
Urinary System – includes
the paired kidneys and
ureters and the single
urethra
Functions:
• Excretion – the elimination
of wastes and toxins from
the body via urine
(regulating water and ion
balance and blood volume)
• Hormone production –
kidneys produce renin to
regulate blood pressure and
EPO to trigger erythropoiesis
Figure 25.1a
The Kidneys
Kidneys – the functional organs of the urinary system, which
are paired and retroperitoneal
• Size: 12 cm length x 6 cm width
• Mass – 150 g
Figure 25.2
The Kidneys
Fibrous capsule – the
dense connective tissue
layer giving the kidneys
shape and support
Renal hilum – the medial,
concave indentation
where blood vessels and
ducts access the organs
Figure 25.3b
The Kidneys
Kidney Layers:
• Cortex - outer, lighter
region where majority of
filtration occurs
• Medulla - inner, darker
region of alternating renal
pyramids and renal columns
• Renal pyramids - a
collection of collecting
ducts that drain urine
from the cortex
• Renal column - tissue
between pyramids where
blood vessels pass to
access the cortex
Figure 25.3b
The Kidneys
Renal Papilla - bumpshaped structure at the
tip of renal pyramid with
pores that drain urine
from the pyramids into
the renal sinus
Figure 25.3b
The Kidneys
Renal sinus - series of hollow spaces that collect urine and
drain it into a ureter, includes the renal pelvis and calyces
Figure 25.3
Nephrons
Nephrons – functional units of the
kidneys, acting as individual filters,
numbering 1 million per kidney.
Parts of a Nephron:
• Glomerulus- porous capillary bed
that leaks plasma and solutes, now
called filtrate
• Renal Tubule – highly folded
passage that converts filtrate into
urine
Figure 25.4
Nephrons
Parts of the renal tubule:
• Glomerular (Bowman’s)
capsule – start of the tubule
that surrounds the glomerulus
to catch its leaked fluids and
solutes
• Proximal convoluted tubule
(PCT) – first coiled area in the
cortex
• Loop of Henle – long region
that dips into medulla, includes
descending and ascending
limbs
• Distal convoluted tubule
(DCT) – second coiled area in
the cortex
Figure 25.4
Nephrons
Collecting duct – tubule
that collects urine from
many nephrons for
transport through a
pyramid to the renal sinus
Types of nephrons:
Cortical – located higher
in the cortex, only a small
dip into the medulla
Juxtamedullary – sit
lower in the cortex, larger
dip into the medulla
Figure 25.5a
Nephrons
Other Nephron
Capillaries – perform
reabsorption of valuable
nutrients from the filtrate,
returning them to the
plasma
• Peritubular capillaries
– surrounds the regions of
the renal tubule in the
cortex
• Vasa recta – surrounds
the regions of the renal
tubule in the medulla
Figure 25.5a
Nephrons
Glomerulus – bed of fenestrated capillaries surrounded by a glomerular capsule;
together called a renal corpuscle
• supplied by an afferent arteriole and drained by an efferent arteriole
• Podocytes – epithelial cells covering the glomerular capillaries, forming the
filtration membrane
Figure 25.7a
Nephrons
Podocytes – epithelial cells covering the glomerular capillaries, forming the
filtration membrane
• Foot processes – finger-like extensions of the podocytes that loosely intertwine
• Filtration slit – small spaces between the foot processes through which plasma
leaks into the capsular space, becoming filtrate
Figure 25.7a
Nephrons
Filtration membrane – the wall through which plasma must cross in order to
become filtrate and enter the bowman’s capsule. Consists of the fenestrated
endothelium of the glomerular capillaries, the podocytes, and the basement
membrane that holds them together
Figure 25.7b, c
Juxtaglomerular Apparatus
JG apparatus – group of receptor cells that regulate the nephrons’ rate of
filtration
• Granular cells (JG cells) – stretch receptors in the wall of the afferent
arteriole. Increased stretch causes vasoconstriction, and decreased stretch
causes vasodilation
Figure 25.6
Juxtaglomerular Apparatus
JG apparatus
• Macula densa cells – chemoreceptors for sodium in the wall of the renal
tubule. High sodium levels triggers vasoconstriction of the afferent arteriole,
and low sodium levels triggers vasodilation.
Figure 25.6
Urine Formation
Urine is formed in 3 steps:
1. Glomerular filtration – plasma
and solutes (valuable and waste)
leak from the glomerulus into the
bowman’s capsule, forming
filtrate
2. Tubular reabsorption – valuable
solutes are transported from the
filtrate back to the plasma of the
peritubular capillaries
3. Tubular secretion – additional
wastes are added to the filtrate
from the plasma of the peritubular
capillaries. The filtrate is now
changed to urine
Figure 25.8
Glomerular Filtration
• Glomerular capillaries are highly
porous, leak bout 20% of their plasma
• Higher pressure than other capillary
beds (55 mmHg)
• Net filtration pressure – balance of
hydrostatic and osmotic pressures in
the glomerulus, forces plasma out to
form filtrate
• Glomerular filtration rate (GFR) –
volume of filtrate formed each minute,
normally 125 mL/min
Figure 25.9
Regulating Glomerular Filtration Rate
Intrinsic Regulation:
• Myogenic control –
any rise in pressure
stretches the afferent
arteriole, and JG cells
trigger vasoconstriction.
A lowered pressure
triggers vasodilation
• Tubuloglomerular
control – a decrease in
GFR causes a decrease
in filtrate NaCl levels.
Macula densa cells then
trigger vasodilation.
Vasoconstriction is
performed when GFR is
high and NaCl levels
are increased.
Figure 25.10
Regulating Glomerular Filtration Rate
Extrinsic Regulation:
• Renin - Angiotensin
mechanism –
decreased blood
pressure causes JG
cells to secrete Renin,
causing Angiotensin II
production. Causes
vasoconstriction and
water retention to
elevate pressure
• Neural control decreased blood
pressure causes
sympathetic nervous
system activity to
elevate pressure
Figure 25.10
Tubular Reabsorption
Reabsorption pathways:
• Paracellular – movement of
materials between the cells of the
renal tubule to access the
peritubular capillaries
• Transcellular - movement of
materials through the epithelial
cells of the renal tubule to access
the peritubular capillaries
• A healthy kidney should reabsorb
all nutrients, most water and ions,
and some urea. The remaining
urea an other nitrogenous wastes
are not reabsorbed.
Figure 25.11
Tubular Reabsorption
• Na+ ions are pumped out of the filtrate via active transport by carriers that
simultaneously pump amino acids, glucose and other nutrients
• Water, negative ions, some urea, and lipid soluble solutes follow this salt into
the peritubular capillaries
• Transport maximum – the limited activity of a set number of carriers,
additional solutes beyond this amount wind up in the urine
Figure 25.12
Tubular Reabsorption
• PCT – allows majority of reabsorption including all glucose and amino acids,
65% of sodium, 60% of chloride and more
• Loop of Henle – descending limb allows water reabsorption, the ascending
limb allows sodium and other ion reabsorption
• DCT – allows limited water an ion reabsorption, more if specific hormones
are present
Figure 25.12
Tubular Secretion
• Movement of material from the
peritubular capillaries into the
filtrate, mostly occurring at the
PCT
• Secretes some of the urea that
was reabsorbed earlier, excess K+
ions, creatinine, and certain drugs
• Also secretes excess H+ ions to
regulate blood pH
Figure 25.8
Regulating Urine
Concentration
Osmolarity – the number of
solutes dissolved in one kg of
water.
• Note how the gradient of
osmolarity increases from cortex to
medulla
Counter current mechanism –
interaction between two adjacent
tubules whose contents flow in
opposite direction
• The two limbs of the loop of henle
passing through the osmotic
gradient helps with reabsorption
and allows the regulation of urine
concentration
Figure 25.13
Regulating Urine Concentration
• The descending limb is permeable to water but not solutes. The filtrate
therefore loses water and becomes more concentrated as it descends
• The ascending limb is permeable to solutes but nor water. The concentrate
filtrate now loses salt and becomes more dilute as it enters the collecting duct.
Figure 25.14
Regulating Urine Concentration
• In the absence of Antidiuretic Hormone (ADH), the dilute filtrate can be
drained as dilute urine
• In the presence of ADH, the collecting duct becomes permeable to water
which leaks out of the filtrate to form concentrated urine. Some urea follows
this water, but this maintains the osmotic gradient
Diuretics – chemicals
that increase urinary
output by inhibiting
ADH secretion or
sodium reabsorption
Figure 25.15
Urine
Urine Characteristics:
• Color – ranges from clear to deep yellow. Yellowness is due to the
presence of urochrome, a pigment from the breakdown of bilirubin
• pH – urine is normally slightly acidic, pH near 6, but diet can shift this
toward more acidic or alkaline
Urine Composition :
• 95% water
• 5% solutes
• Urea – from the breakdown of amino acids
• Uric acid – from the breakdown of nucleic acids
• Creatinine – from the breakdown of creatine phosphate
• Ions – sodium, potassium, bicarbonate, calcium, H+
Figure 25.16
Ureters
Ureter – tube to transport
urine from the kidneys to
the urinary bladder
• Lined with a transitional
epithelium to allow stretch
• Smooth muscle layer
moves urine via peristalsis
Figure 25.17
Urinary Bladder
Urinary bladder – muscular
organ that stores urine
• Lined with a transitional
epithelium to allow stretch
• Trigone – triangular region
outlined by the openings to
the 2 ureters and the 1
urethra
• Detrusor muscle – muscle
in the bladder wall that
contracts during emptying
Figure 25.18b
Urethra
Urethra – tube that drains urine from
the bladder and transports it out of
the body. Much longer in males
because it courses through the penis
Internal urethral sphincter –
smooth muscle structure surrounding
the urethra at the base of the bladder
External urethral sphincter –
skeletal muscle structure
surrounding the urethra in the
urigenital diaphragm
Figure 25.18a
Micturition Reflex
Micturition (voiding) – the act of
emptying the bladder
Micturition reflex:
1. When about 200 mL of urine is
collected, stretch of the bladder
initiates the reflex causing
contractions of the detrusor muscle
and relaxation of the internal
urethral sphincter
2. Relaxation of the external urethral
sphincter can allow micturition, OR
contraction of the external urethral
sphincter can temporarily inhibit the
reflex
3. After an additional 200 mL of urine
is collected, the reflex restarts.
Figure 25.20a