Absorption Summary - Proximal tubule: Water and ions are reabsorbed - Descending loop of Henle: Only water is reabsorbed - Ascending loop of Henle: Ions, but no water is reabsorbed - Distal tubule and collecting duct: Fine tuning from hormones to increase or decrease water And/or solute reabsorption Sodium Potassium Chloride - Active reabsorption of ions in the thick ascending limb creates a dilute filtrate in the lumen Water Balance - The kidneys conserve volume but cannot replace lost volume - Diuresis is the removal of excess water in urine - Diuretics are drugs that promote the excretion of urine Diuretics - All diuretics lead to an increase in urine production - Diuretics have their effect in a variety of ways - Certain diuretics are used to treat high blood pressure. They are usually cheaper and nearly As effective Vasopressin and Water - Vasopressin makes the collecting duct permeable to water Aldosterone - Aldosterone - Endocrine pathway responsible for the regulation of sodium - Released from adrenal gland - Takes place in the distal tubule and first part of collecting duct - Higher aldosterone levels lead to increased reabsorption of sodium Sodium Balance - The renin-angiotensin-aldosterone system - Renin is released when the juxtaglomerular apparatus senses low blood pressure - Renin converts inactive angiotensinogen into angiotensin I - Angiotensin converting enzyme converts angiotensin I into angiotensin Ii - Angiotensin II leads to the release of aldosterone - Natriuresis is the loss of sodium and water in the urine - Natriuretic peptides promote salt and water excretion Angiotensin II has many effects - Angiotensin II leads to the release of aldosterone - Angiotensin II stimulates thirst - Angiotensin II is one of the most potent vasoconstrictors known in humans - Increases sympathetic output to the heart and blood vessels. Increases cardiac output Natriuretic Peptides antagonize Renin Angiotensin Aldosterone System - Atrial natriuretic peptide – peptide hormone produced in the atria of the heart - Enhance sodium and water excretion - Released in response to increased pressure in the atria - Increase GFR - Inhibit renin, aldosterone, and vasopressin Potassium Balance - Regulatory mechanisms keep plasma potassium in narrow range - Aldosterone plays a critical role - Hypokalemia (low potassium) - Muscle weakness and failure of respiratory muscles and the heart - Hyperkalemia (high potassium) - Can lead to cardiac arrhythmias - Causes include kidney disease, diarrhea, and diuretics Behavioral Mechanisms - Drinking replaces fluid loss - Low sodium stimulates salt appetite - Avoidance behaviors help prevent dehydration - Desert animals avoid the heat