Urinary System Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 18, 2011

Urine Concentration

The kidneys produce either a concentrated or dilute urine depending on the operation of a countercurrent exchange mechanism in the nephron loop, and the amount of circulating antidiuretic hormone (ADH) secreted from the posterior pituitary.

  1. A concentration gradient exits in the renal medulla due to active transport of Cl- out of the tubular fluid in the ascending limb of the nephron loop, and the movement of Na+ ions out of the tubule. The ascending limb is impermeable to water and as Na+ and Cl- move out, the fluid in the ascending limb becomes more dilute.
  2. Na+ and Cl- diffuse into the descending limb. The descending limb is permeable to water, and as water diffuses out into the interstitium as a result of the osmotic gradient, the tubular fluid in the descending limb becomes more concentrated.
  3. Ions are actively transported into the intersititium from the collecting duct; urea passively diffuses out of the collecting duct into the interstitium.
  4. Thin-wall vessels, the vasa recta, parallel the course of the nephron loops. Na+, Cl-, and water diffuse into the descending vasa recta and Na+ and Cl- diffuse out of the ascending vasa recta. These vessels function as countercurrent exchangers.
  5. The amount of water reabsorbed form the distal convoluted tubules and collecting ducts is dependent on levels of ADH present. When low levels of ADH are secreted from the posterior, these tubules are impermeable to water, and a dilute urine is excreted. When ADH levels are high, these tubules are highly permeable to water, which is forced by the osmotic gradient out into the interstitium and a more concentrated urine is excreted.

Acid-Base Balance

In acidosis, increased amounts of H+ are secreted into the kidney tubule and bicarbonate ions in the tubule are reabsorbed. In alkalosis, decreased amounts of H+ are secreted and less bicarbonate is reabsorbed. Two buffer systems in the tubular fluid carry excess H+ into the urine:

    Phosphate buffer system: HPO32 + H+ → H2PO4
    Ammonia buffer system: NH3 + H+ → NH4+


Micturition is the physiological process of urination. Distention of the urinary bladder sends signals via sensory neurons to the spinal cord and up to the brain. Parasympathetic impulses stimulate the detrusor muscle to contract and the internal urethral sphincter to relax. If the decision is to urinate, the external urethral sphincter is relaxed and micturition results.

Practice problems for these concepts can be found at:

Urinary System Practice Problems

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