1. Kidneys and Homeostasis
How Else do Kidneys Help with Balance?

2. Maintaining H2O Concentration
Blood osmolarity is maintained by Hormonal control of the kidney by NEGATIVE feedback loops
One hormone important for regulating water balance is ANTIDIURETIC hormone or (ADH)
ADH is produced in the hypothalamus of the brain and stored and released from the pituitary gland, which lies just below the hypothalamus
Hypothalamus
Pituitary Gland

3. ADH Feedback Loop
Osmoreceptor cells in the hypothalamus monitor the osmolarity of the blood
When the blood osmolarity rises above a set point of 300 mosm/L, more ADH is released into the bloodstream and reaches the kidney.
ADH induces the epithelium of the distal tubules and collecting ducts to become more permeable to water.
This amplifies water reabsorption, reducing urine volume, and helps prevent further increase in blood osmolarity above the set point.

4. Looping Around
The subsiding osmolarity of the blood reduces the activity of the osmoreceptor cells in the hypothalamus, and less ADH is secreted.
ONLY water from food or drink can bring osmolarity back down to 300 mosm/L
ADH alone only prevents further movements from the set point

5. Large Amount of Water (Osmolarity Goes Below Set Point)
Very little ADH is released by the Pituitary Gland.
The permeability of the distal tubules and collecting ducts decreases thus water is not reabsorbed and DILUTE urine is excreted.
Normally, blood osmolarity, ADH release and water reabsorption in the kidney are all linked in a feedback loop that contributes to homeostasis.

6. Blame It On the Alcohol
Alcohol can disturb water balance by inhibiting the release of ADH
This causes excessive urinary water loss
This causes DEHYDRATION (causing some symptoms of a hangover)

7. Juxtaglomerular Apparatus (JGA)
This tissue is located near the afferent arteriole that supplies blood to the glomerulus
This tissue has cells that monitor blood pressure or volume

8. Renin-Angiotensin-Aldosterone System (RAAS)
JGA cells detect a drop in blood pressure or blood volume
The enzyme RENIN initiates chemical reactions that convert a plasma protein angiotensinogen to a peptide called angiotensin II
Acting as a hormone angiotensin II increases blood pressure by:
a) constricting arterioles
b) decreasing blood flow to many capillaries,
c) stimulates proximal tubules to reabsorb more NaCl and water
d) stimulates adrenal glands, (located on top of kidneys) to release a hormone called ALDEERSTERONE. This acts on the distal tubules, which reabsorb Na+ and water.

9. Comparison of ADH and RAAS System
Both systems increase water reabsorption they counter different osmoregulatory problems
Release of ADH is a response to an increase in osmolarity in the blood due to dehydration.
RAAS will detect a fall in blood pressure and volume which may be brought on by a loss of salt and body fluids due to severe diarrhea or injury. This will reduce blood volume without changing osmolarity.
Normally ADH and RAAS are partners in homeostasis
a) ADH alone would lower blood Na+ concentration by water reabsorption.
b) But RAAS helps maintain balance by stimulating Na+ reabsorption