Regulation of Homeostasis by the Kidney Flashcards
Role of the kidney in volume regulation and acid base balance
Volume regulation:
Fluid balance: the amount of water gained by the body each day equals the amount lost
Electrolyte balance: the ion gain each day equals ion loss.
Control of acid base balance: Acid base balance: H+ gain is offset by H+ loss
Acid base balance
The lungs excrete a large amount of CO2 – this is a potential acid (CO2 reacts with water to form carbonic acid)
The kidneys also secrete and excrete non-volatile acids produced from normal metabolism, e.g. lactic acid, which the lungs can’t excrete
However, to maintain acid-base balance, the kidneys must also reabsorb virtually all filtered HCO3-
This is important as HCO3- acts as a physiological buffer
This control of acid-base balance prevents acidosis or alkalosis
Relationship between pH, HCO3- and CO2
Blood pH: 7.4, urine pH: 6.0 Blood HCO3-: 24mM, blood pCO2: 40mmHg Plasma osmolality: 285 mOsm/kg Urine osmolality: 600 mOsm/kg water Inverse relationship between pH and plasma concentration of CO2 pH = HCO3- / pCO2
Respiratory regulation of acid base balance
Lung regulates pCO2
Respiratory regulation of pH is achieved via the HCO3-/CO2 (carbonic acid) buffer system:
- as pCO2 levels increase, pH decreases
- as pCO2 levels decrease, pH increases
Changes in pH levels are detected by peripheral chemoreceptors
These then act on respiration centres in the brain to adjust respiration rates
Increased alveolar ventilation removes CO2
Decreased alveolar ventilation
Renal regulation of acid base balance
Kidneys are the most effective regulator of fluid pH
Most HCO3- in filtrate is reabsorbed, and H+ is secreted - Renal tubular acidosis may occur if the kidneys don’t do this effectively
If pH of ECF falls = more secretion of H+ into filtrate and reabsorption of HOC3- into ECF
If pH of ECF increases = secretion of H+ into filtrate and reabsorption of HCO3- declines
Intercalated cells help H+ secretion into tubular fluid and HCO3- reabsorption from tubular fluid, and HCO3- generation from amino acids
Acid base imbalances
Acidosis: below 7.35; solution = get rid of H+
Excrete it via lungs (as CO2) and kidneys (as H+); generate more buffer in kidneys (HCO3-)
Alkalosis: above 7.45; solution
Reduce excretion of CO2 via lungs; increase excretion of HCO3- via kidneys; increase generation of H+ by kidneys
Acidosis
Caused by inadequate ventilation – acute or chronic, or metabolic– always chronic
How can you gain H+: CO2 in blood (combines with H2O to form carbonic acid); Non-volatile acids from metabolism (e.g. lactic acid); Loss of HCO3- in diarrhoea or non-gastric GI fluids; Loss of HCO3- in urine.
How the body responds: Increased respiratory rate lowers PCO2; H+ secreted via kidneys; Buffer systems absorb H+ - generation of HCO3-
Treatment of acidosis
Treatment of Metabolic acidosis:
- Give IV isotonic HCO3-
- Give IV lactate solution (Ringer’s Lactate or Hartmann’s solution - converted to HCO3- buffer in liver)
Treatment of Respiratory acidosis:
- Restore ventilation
- Treat underlying dysfunction or disease
-Give IV lactate solution (converted to HCO3 buffer in liver)
Alkalosis
Respiratory: hyperventilation – acute or chronic
Metabolic: results from all conditions other than respiratory that increase pH – always chronic
How can you suffer a loss of H+: Use of H+ in metabolism of organic anions; Loss of H+ in vomit; Loss of H+ in urine; Hyperventilation (blows off CO2)
How the body responds
-Decreased respiratory rate – elevates PCO2
-Increased generation of H+ by kidneys
-Secretion of HCO3-
-HCO3- converted to NaHCO3
-Other buffer systems release H+
Treatment of alkalosis
Treatment of metabolic alkalosis -Give electrolytes to replace those lost -Give IV Cl- containing solution -Treat underlying disorder Treatment of respiratory alkalosis -Treat underlying cause -Breathe into a paper bag (increases CO2) -Give IV Cl- containing solution (increase HCO3- excretion)
