To maintain electrical neutrality, potassium or hydrogen are secreted into the tubule in exchange for sodium. Aldosterone stimulates sodium reabsorption in the distal convoluted tubule and collecting ducts, intestine and salivary glands, but is really more important in the long term control of blood potassium levels. If potassium is excreted in excess, then hypokalemia, manifested by muscle weakness and possibly heart fibrillation, may result. As the filtrate leaves the distal convoluted tubule it is reduced in volume but isotonic to hypotonic. Diuresis is the production and passage of large amounts of urine.
Osmolar diuresis is caused by a resorbable or nonresorbable solute in the tubular fluid.
For instance, in diabetes mellitus the tubular active transport mechanism for glucose may be exceeded such that glucose remains in the tubular lumen drawing water with it. This accounts for the increased urine volume seen in untreated diabetes mellitus. Diabetes insipidis occurs when ADH is lacking and facultative water is lost. Urination or Micturation Urine collects in the renal pelvis and then flows with the assistance of peristaltic contractions of the ureter to the bladder. When there is 150-300 ml of urine in the bladder, stretch receptors become active and a spinal reflex to void the bladder is initiated.