Urinary System and Excretion
Organs
Urine Formation
Homeostatic Mechanisms
Path of Urine through the Urinary System
Kidneys
produce urine
Ureters
transport urine
Urinary bladder
stores urine
Urethra
passes urine
to outside
Applying Your Knowledge
1.
2.
3.
4.
Ureters
Bladder
Kidney
Urethra
A. Where is urine stored?
B. Which structure passes urine to the outside
of the body?
C. Which structure transports urine to the
bladder?
D. Where is urine produced?
Functions of the Urinary System
• Excretion: removal of metabolic wastes
– Urea: breakdown product of amino acids
– Creatinine: breakdown product of creatine
phosphate, a high energy molecule in muscles
– Uric acid: breakdown product of nucleotides
Ammonium from
amino acids used for
energy
Structure of the Kidney
• Renal Cortex: outer layer
• Renal Medulla: inner layer
• Renal Pelvis: central space
Nephrons are
microscopic tubules
that produce urine
Structure of the Nephron
Glomerular
capsule
Proximal
convoluted
tubule
Distal
convoluted
tubule
Peritubular
capillary network
Glomerulus
(capillaries)
Descending
limb
Collecting Duct
Ascending
limb
Loop of the
nephron
Urine Formation: Glomerular Filtration
Cells, cell fragments
and proteins remain in
the bloodstream
Water, nutrients, salts,
nitrogenous wastes
enter nephron
• Formation of a
filtrate containing
some components
from whole blood
• Water and small
molecules enter
glomerular capsule
from the glomerulus
• Proteins, cell
fragments and cells
are excluded from
the filtrate
Urine Formation: Tubular Reabsorption
Water, nutrients, salt
reenter the bloodstream
Nitrogenous waste,
excess salts, some water
remain in nephron
Peritubular
capillary network
• Molecules and ions
are reabsorbed into
the blood, either
actively or passively
• Reabsorption occurs
from the proximal
convoluted tubule
into the peritubular
capillary network
• Water, nutrients and
salts are reabsorbed
into the bloodstream
Urine Formation: Tubular Secretion
H+, creatinine, drugs
enter the nephron
Peritubular
capillary network
• Substances are
actively transported
from the blood
(peritubular capillary
network) into the
kidney tubule
• Involves transport of
hydrogen ions,
creatinine, penicillin
and other drugs
• Water is reabsorbed into
the bloodstream from all
parts of the nephron,
and the collecting duct
• Occurs by osmosis
through aquaporins,
membrane channel
proteins
• Controlled by
release is inhibited by alcohol
Urine Formation:
Water Reabsorption
Urine has a lower
water concentration
than blood
– Reabsorption of salt
– Gradient of NaCl in
medulla tissue
surrounding the loop
– ADH, antidiuretic
hormone, released from
pituitary when water
intake is low, stimulates
aquaporin activity in distal
convoluted tubule and
collecting duct
Applying Your Knowledge
1.
2.
3.
4.
5.
Distal Convoluted Tubule
Glomerular Capsule
Loop of the Nephron
Proximal Convoluted Tubule
Collecting Duct
A. Where is glucose returned to the bloodstream?
B. Which structures does ADH influence? (two answers)
C. Which structure forms a NaCl gradient to influence
water reabsorption?
D. Which structure produces a blood filtrate that
contains water, nutrients and waste?
Urine Formation: Salt Reabsorption
• Sodium: >99% reabsorbed
– reabsorption in PCT,
DCT, collecting duct
– active transport in
ascending limb of loop
of nephron
• Other reabsorbed or
excreted ions
– potassium ions
– bicarbonate ions
– magnesium ions
Hormonal Control of Sodium Reabsorption
If blood pressure decreases
Angiotensinogen
Renin
Angiotensin I
Angiotensin II
Stimulates sodium
reabsorption from DCT,
water follows sodium
Aldosterone
(from adrenal glands)
constricts
blood vessels
Hormonal Control of Sodium Excretion
If blood pressure increases
prevents renin
secretion
Angiotensinogen
Renin
X
Angiotensin
X I
Angiotensin
X II
Heart atrial cells
Atrial Natriuretic
Hormone (ANH)
Sodium is excreted in urine,
water follows
Aldosterone
X
(from adrenal glands)
Functions of the Urinary System
• Maintenance of Water-Salt Balance
important for blood pressure and blood
volume
– Water reabsorption
• Loop of the Nephron
• Collecting Duct
– Sodium reabsorption
or excretion
• Proximal convoluted tubule
• Distal convoluted tubule
Functions of the Urinary System
• Maintenance of Acid-Base Balance
– Blood pH is controlled by the
bicarbonate buffering system
• Buffer: substance that resists pH change
water
hydrogen ion
bicarbonate ion
carbonic acid
carbon dioxide
– When lungs release carbon dioxide,
H+ has been used to produce water,
so pH remains about neutral (7.4)
Functions of the Urinary System
• Maintenance of Acid-Base Balance
– Kidney
• excretes excess hydrogen ions (H+)
• reabsorbs bicarbonate ions (HCO3-)
Functions of the Urinary System
• Assistance with Hormone Release and
Secretion of Hormones
– Renin (enzyme) influences secretion of
aldosterone from adrenal glands
located at top of kidneys
– Aldosterone regulates water/salt
balance of blood by influencing ion
exchange at the distal convoluted
tubule
– Erythropoietin: stimulates red blood
cell production
Functions of the Urinary System
• Assisting other body systems
– Kidneys regulate the level of calcium
ions in blood
• Convert Vitamin D to a form that assists
with calcium absorption
• Regulate the excretion of calcium
– Kidneys regulate the sodium and
potassium ions in blood, important for
nerve conduction, heart and skeletal
muscle contraction
Applying Your Knowledge
1.
2.
3.
4.
5.
Aldosterone
Antidiuretic Hormone (ADH)
Renin
Angiotensin II
Erythropoietin
A. Which one affects activity of aquaporins?
B. Which one is made by the kidney in response to
decreased blood pressure?
C. Which one stimulates red blood cell production?
D. Which one stimulates reabsorption of sodium?