Renal Physiology and Acid-Base Balance Notes

Fluid and Electrolyte Homeostasis

• Major fluid Compartments

  • Intracellular fluid

  • Extracellular fluid

    • Interstitial fluid

    • Blood plasma

  • ECF volume is sensed by baroreceptors in the vascular system

• Obligatory water loss: Must excrete ~ 600 mOsm worth of urea, SO4, PO4, ions each day (0.5L)

Na+ and Water, K+, Ca2+, and H+ and HCO3–

• Na^+ and water: ECF volume and osmolarity

• K^+: Cardiac and muscle function

• Ca^{2+}: Exocytosis, muscle contractions, and other functions

• H^+ and HCO_3^-: pH balance

• Body must maintain mass balance

• Excretion routes: kidney and lungs

Fluid and Electrolyte Homeostasis and Blood Volume & Pressure

• Blood volume

• Blood pressure

• Volume receptors in atria and carotid and aortic baroreceptors trigger homeostatic reflexes

• Cardiovascular system: Cardiac output, vasoconstriction

• Kidneys: Conserve H_2O to minimize further volume loss

• Behavior: Thirst causes water intake (+ ECF and ICF volume, + Blood pressure)

• Cardiovascular system: Cardiac output, vasodilation

• Kidneys: Excrete salts and H_2O in urine

Renin-Angiotensin Mechanisms

• Renin released when:

  • blood pressure drops

  • sympathetic innervation

  • osmolarity of the tubular fluid is too low (through macula densa)

• Angiotensin II restores blood pressure

  • Arterioles vasoconstrict

  • Aldosterone secretion

  • CNS (thirst and ADH release)

• Negative Sodium Balance

Positive Sodium Balance

• Homeostatic responses to salt ingestion

  • Ingest salt (NaCl)

  • No change in volume, increase osmolarity

  • Vasopressin secreted

    • Renal water reabsorption

    • Kidneys conserve water

  • Thirst increases water intake (+ ECF Volume, + Blood Pressure)

  • Kidneys excrete salt and water (slow response)

  • Cardiovascular reflexes lower blood pressure (rapid response)

  • Osmolarity returns to normal

  • Volume and blood pressure return to normal

Natriuretic Peptides

• Promote salt and water excretion

  • Increased blood volume causes increased atrial stretch.

  • Myocardial cells stretch and release natriuretic peptides

    • Hypothalamus: Less vasopressin

    • Kidney

      • Tubule: - Na^+ reabsorption

      • Afferent arteriole dilates: Increased GFR

      • Decreased renin

    • Adrenal cortex: Less aldosterone

    • Medulla oblongata: Decreased sympathetic output

    • Increased NaCl and H_2O excretion

    • Decreased blood volume

    • Decreased blood pressure

Cardiovascular, Renin-Angiotensin, and Hypothalamic Mechanisms

• Dehydration leads to:

  • Decreased Blood volume/Blood pressure

  • Cardiovascular Mechanisms:

    • Carotid and aortic baroreceptors

    • CVCC

    • Sympathetic output increases and Parasympathetic output decreases

    • Heart + Rate and + Force

    • Arterioles Vasoconstriction

    • Peripheral resistance increases

    • Cardiac output increases

    • Blood pressure increases

  • Renin-Angiotensin System:

    • Granular cells release Renin; Flow at macula densa stimulates Renin release

    • Angiotensinogen converted to Angiotensin I by Renin

    • ACE converts Angiotensin I to Angiotensin II

    • Adrenal cortex releases Aldosterone

    • Distal nephron: + Na^+ reabsorption

    • Volume is conserved.

  • Hypothalamic Mechanisms:

    • Atrial volume receptors; Carotid and aortic baroreceptors are activated

    • Hypothalamic osmoreceptors are activated.

    • Hypothalamus releases + Vasopressin from posterior pituitary

    • Distal nephron: + Osmolarity and +H_2O reabsorption

    • Thirst increases, leading to + H_2O intake

  • What is the main difference between responses to severe dehydration and to hemorrhage?

Acid-Base Balance

pH Control and Imbalances

• Lungs provide a faster way to help control blood pH.

• Extracellular buffers: Hb, plasma protein, phosphate

• Kidneys ultimately remove H^+ and HCO_3^- ions that build up.

• Gastrointestinal losses can also affect arterial pH, e.g., Vomiting- loss of H^+ leading to alkalosis and Diarrhea: Loss of HCO_3^- leading to acidosis

• HCO3^- ightharpoonup CO2 + H^+ : phosphor acid, NH4^+ ightharpoonup HCO3^- + H^+

• Organs help enhance the homeostatic function of the buffers.

Diagnosing Acid-Base Imbalances

• To diagnose an acid-base imbalance, ask 3 questions:

  • Does the pH indicate acidosis or alkalosis?

  • Is the cause of the pH imbalance respiratory or metabolic?

  • Is there compensation for the acid- base imbalance?

Clinical Scenarios and Acid-Base Disorders

• 60-year-old diabetic with a long history of not taking her insulin. She is admitted to the hospital with the following arterial pH parameters: pH 7.26, PaCO2 42, HCO_3^- 17

  • diagnosis: metabolic acidosis

• A 1st year graduate student anxious about their midterm performance begins to feel lightheaded and tingling in their hands so they go to the clinic. A workup revealed: pH 7.48, PaCO2 30, HCO_3^- 23

• An undergraduate student celebrated too much on St. Patrick's day. After a weekend of atonement, his lab values are: pH 7.48, PaCO2 51, HCO_3^- 29