Kidney Function and Fluid Balance

Fluid and Electrolyte Homeostasis

• Renal, respiratory, and cardiovascular systems control fluid and electrolyte balance.
• Renal compensation is slower than respiratory and cardiovascular compensation.
• Renal/kidney function is controlled by endocrine/neuroendocrine systems.
• Respiratory and cardiovascular systems are controlled by neural systems.
• High osmolarity equals low water volume, leading to less urine output.
• Low osmolarity equals high water volume, leading to more urine output.
• Kidneys are the primary route for ion and water excretion.

Countercurrent Multiplier in the Loop of Henle

• The loop of Henle creates high osmolarity in the interstitial fluid of the medulla by actively transporting Na+, Cl-, and K+ out of the nephron in the thick ascending limb.
• This process is required for concentrating urine as the filtrate flows through the collecting duct.
• Pumping ions establishes a gradient enabling other substances to passively follow.
• The vasa recta capillaries act as a countercurrent exchanger, removing water that leaves the nephron, thus preventing dilution of the medulla interstitial fluid.
• Urea plays a significant role in increasing interstitial osmolarity.

Water Balance and Vasopressin

• Vasopressin (AVP/ADH) controls water permeability in the collecting duct, impacting urine concentration based on body water needs.
• Increased vasopressin leads to increased water reabsorption, resulting in concentrated urine.
• Decreased vasopressin leads to decreased water reabsorption, resulting in dilute urine.
• Vasopressin release is triggered by high plasma osmolarity, low blood volume, or low blood pressure, stimulating osmoreceptors in the hypothalamus and baroreceptors in the heart and blood vessels.

Renin-Angiotensin-Aldosterone System (RAAS)

• Aldosterone secretion is stimulated by angiotensin II.
• Low blood pressure triggers the release of renin from granular cells in the kidney.
• Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II by ACE.
• Angiotensin II stimulates aldosterone secretion from the adrenal cortex.
• Aldosterone increases sodium reabsorption and potassium secretion in principal cells of the distal tubule and collecting duct.
• RAAS responses help regulate blood pressure, maintaining sodium balance, and controlling ECF volume.
• Low blood pressure triggers renin production via three mechanisms: increasing sympathetic activity, direct stretch sensing in granular cells, and low glomerular filtration rate leading to decreased NaCl transport in the macula densa.

Natriuretic Peptides

• Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are released in response to high blood volume and stretch.
• They promote sodium excretion and water loss.
• ANP and BNP decrease renin and aldosterone release, and increase glomerular filtration rate.
• They act to lower blood volume and blood pressure.

Potassium Balance

• Potassium homeostasis keeps plasma K+ concentrations in a narrow range.
• Hyperkalemia (high potassium) can lead to cardiac arrhythmias.
• Hypokalemia (low potassium) can lead to muscle weakness and respiratory muscle failure.

Acid-Base Balance and Compensation

• CO2 from respiration is a major source of acid (H+).
• CO2 combines with water to produce H+ and bicarbonate (HCO3–).
• The body uses three mechanisms to regulate pH:
• Buffers (proteins, phosphate ions, and bicarbonate) to immediately neutralize pH changes.
• Ventilation adjusting CO2 levels influences the acid/base balance.
• Kidneys regulating bicarbonate reabsorption and H+ secretion.
• Abnormal pH can alter the nervous system's function, resulting in acidosis (CNS depression) or alkalosis (CNS or muscle hyperexcitability).

Renal Compensation

• Kidneys adjust pH by regulating H+ secretion and HCO3-- excretion within the nephrons.
• Type A intercalated cells in the kidney help correct acidosis.
• Type B intercalated cells help correct alkalosis.

This quiz covers the essential roles of the kidneys in maintaining fluid and electrolyte balance in the body. Topics include the effects of osmolarity on urine production, the functions of hormones like aldosterone and vasopressin, and the physiological mechanisms behind thirst and urine concentration. Test your knowledge on how various systems respond to fluid imbalances.