Chapter 44 (5)

In mammals, both the volume and osmolarity of urine are adjusted according to an animal's water nd salt balance and its rate of __ production. In situations of high salt intake and low water availability, a mammal can excerete ure and salt in small volumes of __ urine with minimal water loss. If salt is scarce and fluid intake is high, hte kidney instead get rid of the excess water with little salt loss by producing large volumes of __. At such times, the urin can be as dilute as 70 mOsm'L, commpared with an osmolarity of 300 mOsm/L for human blood.

Digesting proteins generates large quantities of __, but roosting bats lack access to the drinking water necessary to dilute it. Instead, their kidneys shift to producing small quantities of highly concentrated __, an adjustment that disposes of the urea load while conserving as much water as possible.

A combo of nervous and hormonal controls manages the osmoregulatory function of the mammalian kidney. One key hormone in this regulatory circuitry is __, also called __. __ is produced in the hypothalamus of the brain and stored in the posterior pituitary gland, located just below the hypothalamus. Osmoreceptor cells in the hypothalamus monitor the osmolarity of blood and regulate release of __ from the posterior pituitary.

In response to an increase in osmolarity above the set point of 300 mOsm/L, more __ is released into the bloodstream. When ADH reaches the kidney, its main targets are the distal tubules and collecting ducts. There, __ brings abbout changes that make the epithelium more permeable to water. The resulting increase in water reabsorptionn concentrates urine, reduces urine volume, and lowers blood osmolarity back toward the set point. As the osmolarity of the blood subsides, a negative feedback mechanism reduces the activity of osmoreceptor cells in the hypothalamus, and __ secretion is reduced.

A reduction in blood osmolarity below the set point has the opposite set of effects.
- __ influences water uptake in the kidney by regulating the water-selective channels formed by aquaporins. Binding of __ to receptor molecules leads to a temporary increase in the number of aquaporin molecules in the membranes of collecting duct cells. Additional channels recapture more water, reducing urine volume.

Mutatiosns that prevent __ production or tat inactivate the __ receptor gene block the increase in channel number and thus the __ response. The resulting disorder can cause severe dehydration and solute imbalance due to production of urine that is abnormally large in volume and very dilute. These symptoms give the condition its name: __.

Normally, blood osmolarity, ADH release, and water reabsorption in the kidney are all linked in a feedback loop that contributes to __.

A second regulatory mechanism thta helps maintain homeostasis is the __. THe __ involves a specialized tissue calle dthe __, located near the afferent arteriole that supplies blood to the glomerulus. When blood pressure or blood volume in the afferent arteriole drops, the JGA releases the enzyme renin, which initiates chemical reactions that cleave a plasma protein called angiotensinogen, yielding a peptide called __.

Functioning as a hormone, __ raises BP by constricting arterioles, which decreases blood flow to many capillaries, including those of hte kidney. __ also stimulates the adrenal glands to release a hormone __. This hormone acts ont eh nephron's distal tubules, making them reabsorb more sodium and water and increasing blood volume and pressure.

Because __ acts in several ways that increase blood pressure, drugs that block __ production are widely used to treat hypertension.

Many of these drugs are specific inhibitors of __, which catalyzes the second step in the production of an __. Renin released from the JGA acts on a circulating substrate, __, forming __. ACE in vascular endothelium then splits off two amino acids from __, forming active __. Blocking ACE activity with drugs prevents __ rpoduction and tehreby often loewrs BP into the normal range.

The __ system operates as part of a complex feedback circuit that results in homeostasis. A drop in BP and blood volume triggers renin release fromt eh JGA. In turn, the rise in BP nadd volume resulting fro the various actions of __ and aldosterone reduces the release of renin.

THe functions of __and the __ may seem to be redundant, but this is not the case. Both increase water reabsorption, but they counter different osmoregulatory problems. The release of __ is a response to an inccrease in blood osmolarity, as wehn the body is dehydrated from excessvie water loss or inadequate water intake. However, a situation that causes an excessive loss of both salt and body fluids- a major wound or severe diarrhea for ex- will reduce blood volume w/o increasing osmolarity. THis will not affect __ release, but the _ will respond to the drop in blood volume and pressure by increasing water and Na+ reabsorption.

Thus, __ and the __ are partners in homeostasis. ADH alone would lower blood Na+ concentration by stimulating water reabsorption in the kidney, but the __ helps maintain the osmolarity of body fluids at the set point by stimulating Na+ reabsorption.

Anothe rhormone, __, opposes the RAAS. THe walls of the atria of the heart release __ in response to an increase in blood volumje and pressure. __ inhibits the release of renin from the JGA, inhibits NaCl reabsorption by the collecting ducts, and reduces aldosterone release from the adrenal glands. These actions lower blood volume and pressure. THus, __, the __ and __ provide an elaborate sstem of checks and balances that regulate the kidneys ability to control the osmolarity, salt concentration, volume and pressure of blood. The precise regulatory role of ANP is area of active research.