Transcript Chapter 16: Urinary System and Excretion

Chapter 16: Urinary System
and Excretion
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Urinary System
Urinary Organs
The urinary system consists of the kidneys,
ureters, urinary bladder, and urethra.
The bean-shaped kidneys are at the back of
the abdominal wall beneath the
peritoneum, protected by the lower rib
cage.
The renal artery and renal vein along with
ureters exit the kidney at the hilum.
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The kidneys produce urine which is
conducted by two muscular tubes
called ureters to the urinary bladder
where it is stored before being released
through the urethra.
Two urethral sphincters control the
release of urine.
In females, the urethra is 4 cm long; in
males, the urethra is 20 cm long and
conveys both urine and sperm during
ejaculation.
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The urinary system
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Urination
As the bladder fills with urine, sensory
impulses travel to the spinal cord
where motor nerve impulses return and
cause the bladder to contract and
sphincters to relax.
With maturation, the brain controls this
reflex and delays urination, the release
of urine, until a suitable time.
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Urination
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Functions of the Urinary System
Excretion refers to the elimination of
metabolic wastes that were cell
metabolites; this is the function of the
urinary system.
Kidneys play a role in homeostasis of the
blood by excreting metabolic wastes,
and by maintaining the normal watersalt and acid-base balances of blood.
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Excretion of Metabolic Wastes
Kidneys excrete nitrogenous wastes,
including urea, uric acid, and creatinine.
Urea is a by-product of amino acid
metabolism.
The metabolic breakdown of creatine
phosphate in muscles releases
creatinine.
Uric acid is produced from breakdown of
nucleotides.
Collection of uric acid in joints causes
gout.
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Maintenance of Water-Salt
Balance
Kidneys maintain the water-salt balance
of the body which, in turn, regulates
blood pressure.
Salts, such as NaCl, in the blood cause
osmosis into the blood; the more salts,
the greater the blood volume and also
blood pressure.
Kidneys also maintain correct levels of
potassium, bicarbonate, and calcium
ions in blood.
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Maintenance of Acid-Base
Balance
The kidneys regulate the acid-base
balance of the blood.
Kidneys help keep the blood pH within
normal limits by excreting hydrogen
ions (H+) and reabsorbing bicarbonate
ions (HCO3-) as needed.
Urine usually has a pH of 6 or lower
because our diet often contains acidic
foods.
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Secretion of Hormones
Kidneys secrete or activate several
hormones:
1) They secrete the hormone
erythropoietin to stimulate red blood
cell production,
2) They activate vitamin D to the hormone
calcitriol needed for calcium
reabsorption during digestion, and
3) They release renin, a substance that
leads to the secretion of aldosterone.
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Kidney Structure
The kidneys filter wastes from the blood,
and thus the renal arteries branch
extensively into smaller arteries and
then arterioles inside each kidney.
Many venules unite to form small veins,
which merge to become the renal vein.
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Blood supply in a kidney
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There are three regions to a kidney: an
outer renal cortex, an inner renal
medulla, and a central space called the
renal pelvis.
Microscopically, each contains over one
million nephrons.
The nephrons produce urine which flows
into a collecting duct; several
collecting ducts merge and drain urine
into the renal pelvis.
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Gross anatomy of a kidney
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Anatomy of a Nephron
Each nephron has its own blood supply.
An afferent arteriole approaches the
glomerular capsule and divides to
become the glomerulus, a knot of
capillaries.
The efferent arteriole leaves the capsule
and branches into the peritubular
capillary network.
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Nephron anatomy
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Parts of a Nephron
The closed end of the nephron is a cuplike
glomerular capsule.
Spaces between podocytes of the
glomerular capsule allow small
molecules to enter the from the
glomerulus via glomerular filtration.
The cuboidal epithelial cells of the
proximal convoluted tubule have many
mitochondria and microvilli to carry out
active transport (following passive
transport) from the tubule to blood.
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Proximal convoluted tubule
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The descending loop of the nephron
allows water to leave and the
ascending portion extrudes salt.
The cuboidal epithelial cells of the distal
convoluted tubule have numerous
mitochondria but lack microvilli.
They carry out active transport from the
blood to the tubule or tubular secretion.
Collecting ducts gather in the renal
medulla and form the renal pyramids.
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Urine Formation
Glomerular Filtration
During glomerular filtration, small
molecules including water, wastes, and
nutrients are forced from the blood
inside the glomerulus to the inside of
the glomerular capsule.
Blood cells, platelets, and large proteins
do not move across.
About 180 liters of water are filtered daily.
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Reabsorption from Nephrons
Substance Amount
Filtered
Water, L
180
Amount
Excreted
1.8
Reabsorption (%)
99.0
Sodium, g
630
3.2
99.5
Glucose, g
180
0.0
100.0
Urea, g
54
30.0
44.0
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Tubular Reabsorption
During tubular reabsorption, certain
nutrients, water and some urea moves
from the proximal convoluted tubule into
the blood of the peritubular capillary
network.
Tubular reabsorption is a selective
process because only molecules
recognized by carrier molecules are
actively reabsorbed.
The rate of this process is limited by the
number of carriers.
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Tubular Secretion
During tubular secretion, specific
substances such as hydrogen ions,
creatinine, and drugs such as penicillin
move from the blood into the distal
convoluted tubule.
In the end, urine contains substances
that have undergone glomerular
filtration but have not been reabsorbed,
and substances that have undergone
tubular secretion.
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Steps in urine formation
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Maintaining Water-Salt Balance
The kidneys maintain the water-salt
balance of the blood within normal
limits.
By doing so, they also maintain blood
volume and blood pressure.
Most of the water and salt (NaCl) present
in the filtrate is reabsorbed across the
wall of the proximal convoluted tubule.
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Reabsorption of Water
Salt passively diffuses out of the lower
portion of the ascending limb of the
loop; the upper thick portion actively
extrudes salt into the tissue of the outer
renal medulla.
Water is reabsorbed by osmosis from all
parts of the tubule.
The ascending limb of loop of the nephron
establishes an osmotic gradient that
draws water from the descending limb of
the nephron and the collecting duct.
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The permeability of the collecting duct is
under the control of antidiuretic hormone
(ADH).
Diuresis is an increase in urine flow and
antidiuresis is a decrease.
When ADH is present, more water is
reabsorbed, blood volume and blood
pressure rise, and there is a decreased
amount of urine.
If there is insufficient water intake, the
posterior pituitary releases ADH, causing
more water to be reabsorbed with a
decreased urine output.
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Reabsorption of water
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Reabsorption of Salt
Kidneys regulate salt balance by controlling
excretion and reabsorption of ions.
Two hormones, aldosterone and atrial
natriuretic hormone (ANH), control the
kidneys’ reabsorption of sodium (Na).
When the juxtaglomerular apparatus
detects low blood volume, it secretes
renin that eventually results in the adrenal
cortex releasing aldosterone that restores
blood volume and pressure through
reabsorption of sodium ions.
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Juxtaglomerular apparatus
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Reabsorption of salt increases blood
volume and pressure because more
water is also reabsorbed.
ANH is secreted by the atria of the heart
when cardiac cells are stretched by
increased blood volume.
ANH inhibits secretion of renin; the
resulting excretion of sodium also
causes excretion of water and blood
volume drops.
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Diuretics
Diuretics are chemicals that lower blood
pressure by increasing urine output.
Alcohol inhibits secretion of ADH;
dehydration after drinking may
contribute to the effects of a hangover.
Caffeine increases the glomerular
filtration rate and decreases tubular
reabsorption of sodium.
Diuretic drugs inhibit active transport of
Na+ so a decrease in water reabsorption
follows.
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Maintaining Acid-Base Balance
Kidneys rid the body of acidic and basic
substances.
If the blood is acidic, hydrogen ions (H+)
are excreted and bicarbonate ions
(HCO3-) are reabsorbed.
If the blood is basic, H+ are not excreted
and HCO3- are not reabsorbed.
Breathing also ties up H+ when carbon
dioxide is exhaled.
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Acid-base balance
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Chapter Summary
The urinary system has organs
specialized to produce, store, and rid
the body of urine.
Kidneys excrete nitrogenous wastes and
maintain the water-salt and the acidbase balance of the blood within
normal limits.
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Kidneys have a macroscopic anatomy
and a microscopic anatomy.
Urine is produced by many microscopic
tubules called nephrons.
Urine formation is a multistep process.
Kidneys are under hormonal control as
they regulate the water-salt balance of
blood.
Kidneys excrete hydrogen ions and
reabsorb bicarbonate ions to regulate
the pH of blood.
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