Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 44.5 Hormonal Circuits Link Kidney Function, Water Balance, and Blood Pressure
The kidney plays a key role in regulating water balance and blood pressure. Hormonal systems including ADH, the RAAS, and ANP interact to adjust urine concentration, fluid volume, and salt levels in response to internal and external changes.
1. ADH and Water Reabsorption
- Antidiuretic hormone (ADH) is released by the posterior pituitary when blood osmolarity rises (e.g. after sweating or salty meals)
- ADH binds to receptors on collecting duct cells, triggering insertion of aquaporin channels, which increase water reabsorption
- Results in concentrated urine and lowered blood osmolarity
- A negative feedback loop halts ADH release when osmolarity returns to normal
- When overhydrated, ADH release drops → fewer aquaporins → dilute urine
- Alcohol inhibits ADH, causing dehydration and hangover symptoms
- Diabetes insipidus occurs when ADH production or aquaporin function is defective, leading to excessive dilute urination
2. The RAAS System
- Renin-angiotensin-aldosterone system (RAAS) responds to low blood pressure or volume (e.g. due to blood loss or diarrhea)
- Juxtaglomerular apparatus (JGA) detects drop and releases renin
- Renin converts angiotensinogen (from liver) to angiotensin I, which is converted to angiotensin II by ACE
- Angiotensin II causes vasoconstriction, raising blood pressure
- It also triggers the adrenal glands to secrete aldosterone, which promotes Na⁺ and water reabsorption in distal tubules and collecting ducts
- Net effect: increased blood volume and pressure
3. ANP: A Balancing Hormone
- Atrial natriuretic peptide (ANP) is released from heart atria when blood pressure/volume is high
- ANP opposes RAAS:
- Inhibits renin release from JGA
- Inhibits NaCl reabsorption
- Reduces aldosterone secretion
- ANP promotes salt and water excretion, lowering blood pressure
4. Integration and Thirst
- ADH and RAAS both enhance water retention, but RAAS also increases Na⁺ reabsorption, maintaining osmolarity
- Thirst, regulated by hypothalamic neurons, complements these hormonal responses
- Specific neurons trigger drinking behavior or suppress it depending on hydration state
In a Nutshell
The kidney's ability to regulate water and salt balance is controlled by interacting hormonal systems. ADH adjusts water retention based on osmolarity, RAAS responds to blood volume and pressure changes by promoting Na⁺ and water reabsorption, and ANP provides a check against overhydration. These circuits ensure homeostasis of fluid volume, pressure, and osmotic balance in vertebrates.