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Urinary System – Physiology
•The normal healthy adult produces 1-2 liters of urine a day.
Filtration:
The movement of fluid across the filtration
membrane as a result of pressure difference.
The fluid that is pulled from the blood becomes
The filtrate. 21% of the total blood is the Renal
Faction from the total cardiac output.
Due to glomerular blood pressure water and
small molecules move from the glomerulus to
the inside of the renal corpuscle.
Nephrons in the kidney filter 180 liters of water per
Day along with a considerable amount of small
molecules
(such as glucose) and ions (sodium). If the composition
Of urine were the same as that of the glomerular*
filtrate, The body would continually lose water, salts, and
Nutrients. Therefore, we can conclude that the
Composition of the filtrate must be altered as this fluid
Passes through the remainder of the tubule.
*A
glomerulus is a capillary tuft that performs
the first step in filtering blood to form urine.
Reabsorption:
When sodium ions (Na+) are actively reabsorbed,
Chloride ions (Cl-) follow passively. The reabsorption
Of salt (NaCl) increases the osmolarity of the blood
Compared to the filtrate, the therefore water moves
Passively from the tubule into the blood.
Where is this happening?
Renal tubules
Glucose and amino acids also return to the blood at
the PCT (proximal convoluted tubule).
Glucose is an example of a molecule that
ordinarily is completely reabsorbed because there
is a plentiful supply of carrier molecule for it.
There is a max – the rest ends up in the urine.
In diabetes mellitus – excess glucose occurs in the
blood and then the urine. This increases the
osmolarity compared to the blood and less water
gets reabsorbed – this results in frequent urination
and an increased thirst.
Secretion:
Hydrogen ions, potassium ions, creatinine, and some
Drugs are moved by active transport from the blood
Into the distal convoluted tubule.
Ammonia is synthesized in the epithelium of the
Nephron and diffuses into the lumen.
Water reabsorption:
ADH (Antidiuretic Hormone) is released by the
Posterior lobe of the pituitary gland. When ADH
Is present, more water is reabsorbed in the
Collecting ducts – resulting in an increase in
Blood volume and pressure and decrease
Urine production.
Micturation Reflex

Triggered by stretching of the urinary
bladder.