PHGY 216 Final

• The kidneys are involved in many homeostatic functions including regulating electrolytes, acid-base control, blood volume control, and the regulation of blood pressure
• This module will focus on how kidneys maintain the stability of the extracellular fluid (ECF) volume, electrolyte composition, and osmolarity
• Intracellular fluid (ICF)


  The fluid within cells and comprises about two thirds of total body fluid
• Extracellular fluid (ECF)


  The fluid surrounding the cells which includes the plasma, the interstitial fluid, lymph, and transcellular fluid (such as cerebrospinal fluid) and comprises about one-third of total body fluid
• Plasma


  About one fifth of the ECF
• Interstitial fluid

  About four fifths of the ECF
• Lymph and transcellular fluid

  Considered negligible portions of the ECF
• Transcellular fluid


  The portion of total body water contained within epithelial lined spaces
• Barriers between body-fluid compartments

  • They limit the movement of water and solutes between the various compartments to differing degrees
• Barrier between plasma and interstitial fluid


  At the level of the capillaries, water and everything else in the plasma (except proteins) can freely exchange
• The composition of the plasma and the interstitial fluid are essentially identical, except for the plasma proteins
• Barrier between intracellular fluid and ECF

  The plasma membrane that surrounds each cell in the body
• The ICF contains proteins that do not exchange with the ECF
• There is an unequal distribution of ions across the barrier between ICF and ECF, with K+ greater in the ICF and Na+ greater in the ECF
• The barrier does not allow the passive movement of either ICF or ECF constituents across the plasma membrane, preventing them from equilibrating through the process of diffusion
• ECF Volume


  Closely regulated to maintain blood pressure, with maintenance of salt balance important for long-term regulation
• ECF Osmolarity


  Closely regulated to prevent the swelling or shrinkage of cells
• Baroreceptor reflex


  • Baroreceptors detect changes in arterial blood pressure and regulate blood pressure through effects on the autonomic nervous system, cardiac output, and total peripheral resistance
• Fluid shifts


  Temporary compensation for changes in plasma volume by fluid shifts between the plasma and interstitial compartments
• Long-term regulation of blood pressure is a primary function of the kidneys and the thirst mechanism, controlling fluid output and input, respectively</b>
• Salt input


  Poorly regulated, dependent on dietary salt intake to replace losses in feces and sweat
• Salt output


  Primarily regulated by the kidneys, which have the greatest role in salt elimination
• Hypotonicity of the ECF

  Usually associated with overhydration, or excess free H2O, and has three major causes: renal failure, rapid water ingestion, and over-secretion of vasopressin
• Hypothalamic osmoreceptors


  Monitor the osmolarity of the fluid surrounding them and stimulate vasopressin secretion and thirst in response to changes
• The kidneys are controlled by both neural and endocrine inputs and their primary function is to maintain the ECF volume, electrolyte composition, and osmolarity
• Major functions of the kidneys

  • Maintain water balance
  • Maintain body fluid osmolarity
  • Maintain proper plasma volume
  • Help maintain acid-base balance
  • Regulate ECF solutes
  • Excrete wastes of metabolism
  • Excrete foreign compounds ingested
  • Produce erythropoietin
  • Produce renin
  • Activate vitamin D
• Renal cortex


  The outside of the kidney
• Renal medulla


  The inner part of the kidney
• Renal pelvis

  The inner core of each kidney, through which the urine empties and is channeled to the ureter
• Nephron


  The functional unit of the kidney, of which there are more than one million in a healthy adult kidney
• Vascular component of the nephron

  • Supplies blood to the nephron, including the glomerulus which filters water and solutes from the plasma
• Tubular component of the nephron


  • Carries the filtrate throughout the nephron
• Types of nephrons


  • Cortical nephrons
  • Juxtamedullary nephrons
• Glomerular filtration (GF)


  About 20% of the blood that flows through the glomerular capillaries is filtered into Bowman's capsule
• Tubular reabsorption (TR)


  Important substances are returned to the peritubular capillaries as the filtrate flows through the tubules
• Tubular secretion (TS)


  Selective transfer of substances from the peritubular capillaries into the tubules
• Glomerular filtration rate (GFR)


  The rate at which blood is filtered through all the glomeruli, a measure of overall renal function
• Layers of the glomerular membrane

  • Glomerular capillary wall
  • Basement membrane
  • Inner layer of Bowman's capsule (podocytes)
• Glomerular filtration


  1. Fluid passes through three layers of the glomerular membrane

  2. Glomerular capillary wall

  3. Basement membrane

  4. Inner layer of Bowman's capsule
• Glomerular capillary wall


  • Single layer of endothelial cells with many large pores that make it 100 times more permeable to fluids and solutes than regular capillaries
  • Large plasma proteins cannot pass through, but smaller ones like albumin can