Urine Formation in the Nephron
Four processes crucial to urine formation:
1. Glomerular Filtration: Moves water, solutes (excluding proteins and blood cells),
and plasma from the blood into the nephron, producing filtrate.
2. Tubular Reabsorption: Removes useful substances/solutes from the filtrate and
returns them to the blood through active and passive transport mechanisms.
3. Tubular Secretion: Transfers additional waste substances, such as antibiotics,
from the blood into the filtrate, along with potassium (K+) and hydrogen (H+) ions
to maintain osmotic pressure and pH balance.
4. Water Reabsorption: The collecting duct passively reabsorbs water from the
filtrate through osmosis, returning it to the blood for use by the body.
Glomerular Filtration: Filters the Blood
The formation of urine begins with glomerular filtration, where water and dissolved
substances pass from the blood into Bowman's capsule. Two factors contribute to this
process:
● The capillaries of the glomerulus are highly permeable, allowing water and
dissolved substances to pass through, while larger substances like proteins and
blood cells remain impermeable.
● The high blood pressure within the glomerulus provides the force necessary for
filtration.
Tubular Reabsorption
Approximately 65% of the filtrate passing through the proximal tubule and loop of Henle
is reabsorbed and returned to the body. This process involves active and passive
transport mechanisms. Key points include:
● Proximal tubule cells contain numerous mitochondria, which generate ATP for
active transport of substances across concentration gradients.
● Hydrogen ions (H+) are actively secreted into the tubule.
● Loop of Henle's function is to reabsorb water and sodium (Na+) ions from the
filtrate into the blood.
● Descending limb: Permeable to water, slightly permeable to ions; water diffuses
from the filtrate to the capillaries due to the salty environment in the renal
medulla.
● Ascending limb: Permeability changes; the thin portion is impermeable to water
but allows solutes to pass, causing Na+ to diffuse into the capillaries and
interstitial fluid. The thick portion actively transports Na+ ions from the filtrate to
the capillaries.
Four processes crucial to urine formation:
1. Glomerular Filtration: Moves water, solutes (excluding proteins and blood cells),
and plasma from the blood into the nephron, producing filtrate.
2. Tubular Reabsorption: Removes useful substances/solutes from the filtrate and
returns them to the blood through active and passive transport mechanisms.
3. Tubular Secretion: Transfers additional waste substances, such as antibiotics,
from the blood into the filtrate, along with potassium (K+) and hydrogen (H+) ions
to maintain osmotic pressure and pH balance.
4. Water Reabsorption: The collecting duct passively reabsorbs water from the
filtrate through osmosis, returning it to the blood for use by the body.
Glomerular Filtration: Filters the Blood
The formation of urine begins with glomerular filtration, where water and dissolved
substances pass from the blood into Bowman's capsule. Two factors contribute to this
process:
● The capillaries of the glomerulus are highly permeable, allowing water and
dissolved substances to pass through, while larger substances like proteins and
blood cells remain impermeable.
● The high blood pressure within the glomerulus provides the force necessary for
filtration.
Tubular Reabsorption
Approximately 65% of the filtrate passing through the proximal tubule and loop of Henle
is reabsorbed and returned to the body. This process involves active and passive
transport mechanisms. Key points include:
● Proximal tubule cells contain numerous mitochondria, which generate ATP for
active transport of substances across concentration gradients.
● Hydrogen ions (H+) are actively secreted into the tubule.
● Loop of Henle's function is to reabsorb water and sodium (Na+) ions from the
filtrate into the blood.
● Descending limb: Permeable to water, slightly permeable to ions; water diffuses
from the filtrate to the capillaries due to the salty environment in the renal
medulla.
● Ascending limb: Permeability changes; the thin portion is impermeable to water
but allows solutes to pass, causing Na+ to diffuse into the capillaries and
interstitial fluid. The thick portion actively transports Na+ ions from the filtrate to
the capillaries.
