Please take notes on urine formation as homework (May 14th). You will be allowed to use these notes and the urinary structure notes to complete an in-class “quiz” May 16th. No notes/homework done means you cannot bring these in. I will print a companion diagram sheet for you and give it to you on May 16th.

Note: this video goes deeper than needed and into the final set of our notes – hormones and regulation!

Formation of Urine

Three stages of urine formation:

  1. Filtration – Removing maximum waste from blood into nephron and creating a filtrate.
  2. Reabsorption – Bringing useful molecules back into the blood
  3. Secretion (aka tubular excretion) – Bringing as much harmful molecules from the blood as possible.
Figure 1: Simplified nephron/capillary interaction

There is a countercurrent exchange occurring between the nephron and the peritubular capillary network.

  • This allows for osmotic gradients to be maintained throughout the formation process.
  • More Efficient
Figure 2: Countercurrent example

1. Filtration

At the glomerulus there is very high pressure, thus this type of filtration is called pressure filtration.

  • The substances removed create a plasma-like filtrate in the Bowman’s capsule

Things that are filtered into the Bowman’s capsule from the blood:

  • Water
  • NaCl
  • Glucose
  • H+
  • Urea/Uric acid

Things that are not filtered into the Bowman’s capsule from the blood:

  • Plasma proteins (too big)
  • Blood cells (too big)
  • Some water, salts, glucose, amino acids, and H+ stay
Figure 3: Comparison of solutes in the glomerulus and Bowman’s capsule

2. Reabsorption

Occurs at the proximal convoluted tubule and the Loop of Henle.

In the proximal convoluted tubule:
  • Selective reabsorption: Nephron actively transports glucose, amino acids, and Na+ ions back into the blood (useful molecules – takes ATP).
  • Negative ions (i.e. Cl-) follow the positive ion (Na+) passively

More ions/molecules moving back into the blood concentrates the blood making an osmotic gradient (Difference in concentration between two solutions )

  • This causes water to reenter the blood vis osmosis.
  • This causes the filtrate to become concentrated as it moves through the proximal convoluted tubule.
Figure 4: Example of an osmotic gradient
In the Loop of Henle:
  • In the descending loop: not permeable to ions, permeable to water.
  • Water leaves nephron, urine becomes more concentrated
  • In the ascending loop: permeable to ions, not permeable to water.
  • Na+ leaves the nephron, fluid around descending loop becomes concentrated
  • This allows for more water reabsorption (back into the blood) anytime the nephron passes back into that region (even the collecting duct!)
Figure 5: Creating a “salty” medulla of the kidney

3. Secretion

Occurs in the distal convoluted tubule (+ little in collecting duct).

  • Movement of waste still in blood into nephron
  • Active Transport: Urea, Uric acid, excess K+, vitamin C, drugs, H+.
  • Some water enters the urine again
  • The urine is now collected in the collecting ducts and carried to the bladder through the ureter for excretion.
Figure 6: Urinary system structures

Summary:

  1. Urine is formed through a modified capillary fluid exchange between the blood and the nephron.
  2. Filtration: Filtrate enters the nephron from the Glomerulus (very high pressure).
  3. Filtration: Water, NaCl, Glucose, H+, Urea/Uric acid, enter nephron.
  4. Reabsorption – Proximal Tubule: Selective reabsorption of glucose, amino acids, and Na+ (this is active transport – takes energy) Cl- passively follows.
  5. Reabsorption – Loop of Henle: Descending loop – water leaves nephron (osmosis), enters blood.
  6. Reabsorption – Loop of Henle: Ascending loop – Na+ leaves nephron enters blood (lower, thin section – Na leaves passively, higher, thick section – Na+ leaves with active transport.
  7. Secretion (Tubular excretion): Occurs in distal convoluted tubule. Active Transport of Urea, Uric acid, excess K+, vitamin C, drugs, H+ back into the nephron.
Figure 7: Summary of urine formation
Figure 8: Another summary of urine formation