Note 59 - Water Balance - Reading

Maintaining an Internal Balance

Water Balance

  • The body adjusts for increased water intake by increasing urine output.

  • Adjustments for exercise or decreased water intake involve decreasing urine output, managed by the nervous and endocrine systems.

Regulating ADH (Antidiuretic Hormone)

  • ADH regulates osmotic pressure of body fluids by increasing water reabsorption in the kidneys.

  • ADH is produced by nerve cells in the hypothalamus, sent to the pituitary gland for storage and release into the blood.

  • Osmoreceptors in the hypothalamus detect changes in osmotic pressure, signaling the release of ADH when blood solutes are concentrated (e.g., due to dehydration).

  • Increased ADH levels lead to more concentrated urine, conserving body water, while decreased ADH levels result in diluted urine.

Homeostasis and Thirst Response

  • Decreased water intake or increased loss (e.g., sweating) raises blood osmotic pressure, leading to osmoreceptor activation and the sensation of thirst.

  • Drinking water lowers blood osmotic pressure, leading to reduced ADH release and less water reabsorption from the nephrons.

ADH and the Nephron

  • About 85% of water is reabsorbed in the proximal tubule. Without ADH, the remaining 15% would be lost.

  • ADH enables permeability in the upper distal tubule and collecting duct, allowing water to exit nephron into interstitial spaces and enter blood, concentrating the urine and regulating body osmotic pressure.

Kidneys and Blood Pressure Regulation

  • Aldosterone increases Na+ reabsorption in the nephrons, impacting fluid volume and osmotic gradient.

  • As Na+ reabsorption increases, more water is drawn out of nephrons, affecting blood pressure levels.

Effects of Alcohol on Fluid Levels

  • Alcohol consumption decreases ADH release, leading to increased urine output and potential dehydration.

Homeostasis and Blood Pressure

  • Low blood pressure detected by juxtaglomerular apparatus can trigger renin release.

  • Renin converts angiotensinogen into angiotensin, causing blood vessel constriction and stimulating aldosterone release from adrenal glands, increasing Na+ and fluid reabsorption in kidneys.

pH Balance Regulation

  • Kidneys maintain blood pH between 7.3 and 7.5. Excess H+ ions from metabolic processes lower the pH.

  • Bicarbonate ions buffer excess H+ ions, maintaining acid-base balance through the bicarbonate-carbon dioxide buffer system.

Buffer System Restoration

  • The kidneys reverse buffer reactions by transporting carbon dioxide into nephron cells, generating bicarbonate for blood buffering and excreting excess H+ ions with filtrate.

Summary of Key Concepts

  • ADH and aldosterone regulate osmotic pressure, fluid volume, and blood pressure through negative feedback mechanisms.

  • Kidneys contribute to pH regulation and buffer restoration by excreting H+ ions and maintaining HCO3- levels.