Notes- Urinary System
Parts of the urinary system:
-A pair of kidneys, a pair of ureters, a urinary
bladder, and a urethra
Functions of the Kidneys:
1. remove salts and other nitrogenous wastes
2. form urine
3. maintain normal concentrations of water and
electrolytes within body fluids
4. control the rate of red blood cell formation
5. maintain blood pressure
6. regulate the volume, composition, and pH of
body fluids
7. secrete the hormone erythropoietin
8. play a role in the activation of vitamin D
Kidneys:
-Reddish brown, bean-shaped organ
-12cm long by 6 cm wide by 3 cm thick
-Enclosed in a fibrous capsule
-Located retroperitoneally (behind the parietal
peritoneum) on either side of the vertebral column
on the posterior wall of the abdominal cavity
-Left kidney is slightly higher than the right
Kidney Structure:
-Lateral surface is convex; medial side is concave
-Medial side depression leads to a hollow chamber
called the renal sinus
-Hilum= the entrance to the sinus region where blood
vessels, nerves, lymphatic vessels, and ureters
enter/exit
-Renal pelvis- a funnel-shaped sac inside the renal
sinus at the superior end of the ureter
-Two distinct regions inside each kidney:
a. renal medulla- inner area with renal pyramids
b. renal cortex- outer area with renal columns
made up of nephrons (working units of the
kidneys)
Nephron Structure:
-Approximately 1 million nephrons per kidney
-Each nephron has two main parts: a renal corpuscle
and a renal tubule
-Renal corpuscle- made of a tangled cluster of
capillaries called a glomerulus; these capillaries filter
fluid (first step in urine formation); and a glomerular
capsule (thin-walled sac-like structure) surrounds
each glomerulus
-Renal tubule- begins at the distal end of the
glomerular capsule; made up of a proximal
convoluted (highly coiled) tubule, a descending limb
of the nephron loop, an ascending limb of the
nephron loop, and a distal convoluted tubule
-All distal convoluted tubules merge into collecting
ducts
Urine Formation:
- A series of three processes:
A. Filtration: Occurs in the Bowman’s
capsule/Glomerular capsule
1. Glomerular filtrate = mostly water and the
same components as blood plasma that are
small enough to go through the membrane
walls (Exception = protein molecules- too
large!)
2. Filtration Rate:
a. Filtration rate is directly proportional
to the net filtration pressure
b. A change in the diameter of the
arterioles (vasodilation or
vasoconstriction) can change how
much filtrate there is
c. The number of plasma proteins in the
blood can change how much filtrate
there is
d. An obstruction (like kidney stones or
an enlarged prostate) can cause fluid to
back up which can slow down
filtration
B.
Reabsorption:
-Definition: the process by which substances are
transported out of the tubular fluid, through the
epithelium of the renal tubule, and into the
interstitial fluid
1. Most occurs in the proximal convoluted
tubule
2. The process returns substances to the
internal environment
3. Contain microvilli (increase the surface area
and reabsorption)
4. Substances- water, glucose, urea, vitamin C,
and many ions
5. Mechanisms- active transport (glucose,
amino acids, Na+, and approx. 90% of uric
acid), osmosis (water), and passive transport
(various negatively charged ions and 50% of
urea)
C. Secretion:
-Definition: When certain substances move from
the plasma of the peritubular capillary into the
fluid of the renal tubule
1. Occurs in the tubule
2. Opposite process of reabsorption (the unneeded molecules that could not fit through
the tubule into the capillaries during
filtration are secreted into the tubular fluid).
Urine Formation Summary:
1. Glomerular filtration of materials from blood
plasma
2. Reabsorption of substances, including glucose,
water, urea, proteins, creatine, amino acids, lactic
acid, citric acid, uric acid, phosphate, sulfate,
calcium, potassium, and sodium ions
3. Secretion of substances including penicillin,
histamine, phenobarbitol, hydrogen ions,
ammonia, and potassium ions
Urine Composition:
- Urine contains approx. 95% water, wastes (urea and
uric acid), trace amounts of amino acids, and a
variety of electrolytes
- Approximately 0.6-2.5 liters of urine are produced
each day
- Amount of urine formed depends on many factors:
fluid intake, environmental temperature, relative
humidity, a person’s emotional state, respiratory
rate, and body temperature
- Urine output should be approximately 50-60 mL per
hour; Less than 30 mL may indicate kidney failure
Urea and Uric Acid Excretion:
-Urea is a by-product of amino acid catabolism; its
plasma concentration reflects the amount of protein
in the diet; approx. 50% is excreted in the urine
- Uric acid is a product of the metabolism of certain
organic bases in nucleic acids; only a small amount
is excreted in the urine
Ureters:
- Two tubular organs approx. 25 cm each that reach
from kidneys to the bladder
Urinary Bladder:
- A hollow, distensible (stretchy), muscular organ
which is the temporary storage site for urine. The
trigone area is a triangular area which contains the
two openings from the ureters and one opening for
the urethra
- The detrusor muscle suurounds the neck of the
bladder and forms the internal urethral sphincter;
contraction of this muscle prevents the bladder from
emptying until pressure within the bladder increases
to a certain level
- The bladder can hold up to 600mL of urine before
stimulating pain receptors; but the urge to urinate
begins when the bladder has approximately 150mL;
At 300mL of urine, the sensation of fullness
intensifies and the bladder begins contractions to
force the internal urethral sphincter open, the
external urethral sphincter to relax, and allows the
bladder to empty
Micturition:
- Another word for urination (the process that expels
urine from the urinary bladder)
- The external urethral sphincter is under voluntary
control (this allows us to “hold it” when we have to
go)
Urine Elimination:
nephrons collecting ducts calyx of renal pelvis
ureter  urinary bladder
Urethra- carries urine from the bladder to the outside
of the body