LEC 7

• The kidney plays a role in water homeostasis
• The renal mechanism for the excretion of dilute urine can be explained
• Counter-current multiplier


  Mechanism involving the loop of Henle and vasa recta that helps establish an osmotic gradient in the renal medulla
• Counter-current exchanger


  Mechanism involving the loop of Henle and vasa recta that helps establish an osmotic gradient in the renal medulla
• The mechanism of production of concentrated urine can be explained
• The causes for disorders of urine concentrating/diluting ability of the kidney can be outlined
• Osmotic diuresis


  A type of diuresis
• Water diuresis


  A type of diuresis
• Osmolarity


  The concentration of a solution expressed as the total number of solute particles per litre
• Diluted urine


  Urine with an osmolarity of 50 mOsM
• Concentrated urine


  Urine with an osmolarity of 1200 mOsM
• Hyperosmotic


  A condition in which the total amount of solutes in a solution is greater than that of another solution, with higher osmotic pressure
• Hyposmotic


  A condition in which the total amount of solutes in a solution is lower than that of another solution, with lower osmotic pressure
• Body fluid


  A substance, usually a liquid, that is produced by the body and consists of water and dissolved solutes
• Intracellular fluid (ICF)


  The body fluid inside cells, making up 2/3 of total body fluid
• Extracellular fluid (ECF)


  The body fluid outside cells, making up 1/3 of total body fluid
• Interstitial fluid


  The ECF that occupies the microscopic spaces between tissue cells, making up 80% of ECF
• Blood plasma


  The liquid portion of the blood, making up 20% of ECF
• Sources of daily water input

  • Ingested liquids and moist foods absorbed from the gastrointestinal (GI) tract (2200 mL/day)
  • Metabolic water produced in the body (300 mL)
• Daily water loss


  • Urine (1500 mL)
  • Feces (100 mL)
  • Insensible loss from lungs and nonsweating skin (900 mL)
  • Menstrual flow in females
• Vasopressin


  Also known as antidiuretic hormone (ADH), produced by the hypothalamus and stored in the posterior pituitary gland
• Aquaporins


  Water pores, a family of membrane channels that allow water to move across cell membranes
• Aquaporin-2 (AQP2)


  The water channel regulated by vasopressin in the kidney
• Action of vasopressin


  Vasopressin binds to V2 receptor on basolateral side of cell, activating G-protein/cAMP system, causing AQP2 vesicles to fuse with apical membrane, increasing water permeability of collecting duct
• Factors that stimulate vasopressin secretion

  • Plasma osmolarity
  • Blood volume
  • Blood pressure
• Thirst


  The subjective sensation that drives water intake, controlled by the hypothalamus
• Hypothalamic osmoreceptors


  Neurons that monitor osmolarity of surrounding fluid and send excitatory input for vasopressin secretion and thirst
• Increased ECF osmolarity


  Stimulates vasopressin secretion and thirst, increasing fluid reabsorption and intake to restore volume
• Decreased ECF osmolarity


  Suppresses vasopressin secretion and thirst, increasing fluid excretion to reduce water load
• Aldosterone


  Hormone synthesized in the adrenal cortex that regulates sodium reabsorption in the distal tubules and collecting ducts
• Renin-Angiotensin-Aldosterone System (RAAS)


  Multistep pathway for maintaining blood pressure, involving renin, angiotensin, and aldosterone
• Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are peptide hormones produced in the heart that enhance sodium and water excretion
• Countercurrent multiplier


  Process involving the loop of Henle that helps establish an osmotic gradient in the renal medulla
• Atria (ANP) and ventricle (BNP)


  Parts of the heart
• Increased blood volume


  1. Stretches the myocardial cells

  2. Releases ANP and BNP
• ANP


  • Enhances Na+ and water excretion
  • Promotes natriuresis (excretion of large amounts of sodium in the urine)
• Natriuretic peptides (NPs)


  • Increase GFR by dilating the afferent arterioles and constrict the efferent arterioles
  • Act indirectly to increase Na+ and water excretion by inhibiting the release of renin, aldosterone, and vasopressin
  • Act directly on the cardiovascular control center of the medulla to lower blood pressure
• Countercurrent multiplication


  The process by which a progressively increasing osmotic gradient is formed in the interstitial fluid of the renal medulla as a result of countercurrent flow
• Descending limb of the loop of Henle (LOH)

  • Highly permeable to water
  • Does not actively extrude Na+, (does not reabsorb Na+)
• Ascending limb of the LOH


  • Actively transports NaCl out of the tubular lumen into the surrounding interstitial fluid
  • Always impermeable to water, so salt leaves the tubular fluid without water osmotically following along