2. Urinary system:
physiology
• The major work of the urinary system is
done by the nephrons.
• The other parts are primarily passageways
and storage areas.
• The eliminated materials are collectively
collected urine.
3. Functions of the urinary system
The primary function of the kidneys is to
maintain a stable internal environment
(homeostasis) for optimal cell and tissue
metabolism.
4. Functions of the urinary system
six important roles of the kidneys are
1. Regulation of plasma osmolarity.
2. Regulation of plasma volume
3. Regulation of plasma hydrogen ion
concentration (pH).
4. Removal of metabolic waste products and
foreign substances from the plasma
5. Secretion of Hormones
6. Regulation of plasma ionic composition
5. Formation of Urine
• Urine is formed in three steps:
– Filtration
– Reabsorption, and
– Secretion.
6. Urine formation
Glomerular Filtration
• Blood enters the afferent arteriole and
flows into the glomerulus.
• Blood in the glomerulus has both:
– Filterable
– Non-filterable
blood components
Filterable blood components move toward the
inside of the glomerulus while non-filterable blood
components bypass the filtration process by exiting
through the efferent arteriole.
7. Filtration cont….
filterable blood
components are
•water
• nitrogenous waste
•nutrients and
•salts (ions)
Nonfilterable blood
components include formed
elements such as
•blood cells and platelets
along with plasma
proteins.
8. Volume of blood filtered
• Kidneys are 1% of body weight and use
20-25% of the oxygen.
• ¼ of the cardiac output is delivered to the
kidneys each minute
• 1000 to 1200ml of blood go through each
glumeruli of each nephron every minute.
– Equals 650 ml of plasma (blood cells cannot
be filtered)
9. Volume of blood filtered cont…
• 1/5 of this goes through the renal tubules.
• The entire plasma volume is filtered 60X a
day or 180 liters of water per day.
– Normal urine output = 0.5ml/kg body weight/hr
– 30 to 40 ml per hour for an adult
Not all of the filtered material is excreted
10. • The volume and concentration of urine is
controlled by:
– Antidiuretic hormone
– Aldestrone
– The Renin – angiotensin mechanism
11. Urine formation:
Tubular Reabsorption
• Glomerular filtrate proximal convoluted
tubule
• Molecules and ions are reabsorbed back
into the blood.
– Peritubular capillary network
– This reabsorption process allows water (H2O)
to pass from the glomerular filtrate back into
the circulatory system
13. Urine formation:
Tubular Secretion
• Some substances are removed from blood
through the peritubular capillary network
into the distal convoluted tubule or
collecting duct.
• These substances are
– Hydrogen ions
– creatinine and
– drugs
14. Tubular Secretion cont…
• Urine is a collection of substances that
have not been re-absorbed during
glomerular filtration or tubular re-
absorbtion.
15. • Urine flows into the ureter, that connects
each kidney to the bladder.
– Is facilitated by peristaltic waves from
contraction of the smooth muscle in the ureter
wall.
Bladder:
– Adult bladder capacity is about 300 to 600 ml
– Functions as a muscular sac that allow storage
of urine until voiding is convenient.
16. Urethra
• The urethra is a muscular tube that drains
urine from the body.
• Female
– 3-4 cm long
• Male
– 20 cm long
17. Micturition
• Micturition:
– Is the process by which the urinary bladder
empties when it becomes filled.
• This involves two main steps:
1. The bladder fills progressively until the
tension in its walls rises above a threshold
level.
2. a nervous reflex called the micturition reflex
that empties the bladder.
18. Facilitation or Inhibition of
Micturition by the Brain
• The micturition reflex is a completely
autonomic spinal cord reflex, but it can be
inhibited or facilitated by centers in the brain.
– strong facilitative and inhibitory centers in the
brain stem, located mainly in the pons,
and
– Several centers located in the cerebral cortex that
are mainly inhibitory but can become excitatory
