The formation of urine by each kidney enables the body to regulate blood (water) volume, blood composition and blood pH. In this activity you will examine the effect of fluid intake on urine production.
Play the animation to review more about fluid intake and forming urine.
After you have completed this section, click on the Pre-Lab Quiz button at the top menu bar to assess your understanding.
Ideally, daily water intake and water loss should be equal. However, it is not unusual for our water intake to be either higher or lower than the amount of water lost. In order to maintain body fluid (water) volume, our kidneys adjust the amount of water lost in urine.
The functional unit of the kidney is the nephron. Each nephron filters blood and modifies that filtrate, via the processes of reabsorption and secretion, to form urine. Normally the kidneys produce 180 liters of filtrate but only 1 — 2 liters of urine per day. Since the filtrate is mainly water, the nephrons are reabsorbing about 178-179 liters of water each day. 90% of water reabsorption by the kidney nephrons is obligatory water reabsorption while only 10% is facultative water reabsorption.
Antidiuretic hormone (ADH) regulates body water content by controlling facultative water reabsorption. ADH increases the water permeability of the last part of the distal tubule and of the collecting ducts. Water reabsorption from the filtrate increases, resulting in a smaller volume of concentrated urine and less water loss in urine. The absence of ADH results in no facultative reabsorption of water and the production of a larger volume of dilute urine and greater water loss in urine. ADH secretion by the posterior pituitary is controlled by negative feedback. ADH is secreted in response to an increase in plasma and interstital osmolarity (decrease in water content). When plasma osmolarity increases by as little as 1%, osmoreceptors in the hypothalamus respond by sending nerve impulses to the posterior pituitary resulting in the release of ADH. ADH causes facultative reabsorption of water resulting in a decrease in plasma osmolarity and a decrease in ADH secretion.
Plasma and urine osmolarity is measured with an osmometer. An osmometer uses freezing point depression to measure osmolarity. Substances that have a high osmolarity freeze at a lower temperature than water.
