Renal Physiology

Kidney Regulation

• Blood volume (plasma volume) and pressure.

  1. Water concentration and fluid volume.

  2. Inorganic ion composition.

• These two factors keep pressure under homeostatic conditions.

Kidneys Balance

• They help regulate the acid-base balance.

Kidneys Excretion

• Urea, uric acid, creatinine, and bilirubin (breakdown product of Hb).

• Removal of foreign chemicals (i.e. drugs, food addictives, pesticides.

Uric Acid

• Produced from the breakdown of nitrogenous bases.

Creatinine

• Byproduct of muscle metabolism.

Kidneys Synthesis

• Glucose (gluconeogenesis).

Kidneys Secretion

• Acts as a endocrine organ, secreting cytokines or hormones.

• Erythropoietin (EPO) → synthesized in the kidney.

• Renin.

• 1,25-dihydroxy Vitamin D (inactive form).

Fluid Compartments

• Total body water: 42L, around 60% body weight.

• Plasma is 3L, the interstitial fluid is 11L, and the intercellular fluid 28L.

• Functions of the kidneys is to maintain the plasma volume within a narrow range.

Fluid Volume Changes

• During various health disorders.

• By rapid movement water (osmosis).

Intracellular Fluid (ICF)

• The fluid inside the cell.

Extracellular Fluid (ECF)

• Fluid outside the cell.

• Plasma + interstitial fluid + cerebrospinal fluid.

Plasma

• Non-cellular part of blood, fluid inside blood vessels.

Kidney Regulates ECF

• The ECF is made up of:

  • Plasma.

  • Interstitial fluid.

  • Cerebrospinal fluid.

Dominant Ions in ECF

• Na⁺, HCO₃⁻, Cl⁻.

ICF Dominant in ICF

• K⁺.

Diffusion

• Rate of water diffusion that will affect fluid volume.

• A low and finite degree of H₂O can diffuse through the tissues.

• Aquaporins: water channels.

Osmoles

• 1 osmoles (osm) is equal to 1 mole of solute particles.

Osmolarity

• Number of solutes per unit volume of solution expressed in moles per liter.

• The addition of solute lowers the water concentration.

• Addition of more solute would increase the solute concentration and further reduce the water concentration.

Diffusion

• Is the movement of molecules from one location to another due to their random thermal motion.

• Molecules initially move from a region of higher concentration to lower concentration.

• Over time, solute molecules placed in a solvent evenly distribute themselves.

Diffusional Equilibrium

• Is the result of diffusion.

• Happens over time as solute molecules placed in a solvent evenly distribute themselves.

Osmosis

• Net diffusion of water across a selectively permeable membrane from a region of high water concentration → lower water concentration.

Osmotic Pressure

• The pressure necessary to prevent solvent movement (osmosis).

Reaching Diffusional Equilibrium

• The partition between the compartments is permeable to water and solute.

• After diffusional equilibrium has occurred, movement of water and solutes has equalized their concentrations on both sides.

Reaching Osmosis

• The partition between the compartments is permeable to water only.

• After diffusional equilibrium has occurred, movement of water has equalized solute concentration.

• The opposing pressure required to stop osmosis completely is equal to osmotic pressure.

Tonicity

• Is determined by the concentration of non-penetrating solutes of an extracellular solution relative to the intracellular environment of a cell.

• The solute concentrations may influence changes in cell volume.

Isotonic (Isoosmotic)

• Same osmolarity outside and inside the cell.

Hypertonic (Hyperosmotic)

• Higher osmolarity than inside of the cell.

Hypotonic (Hypoosmotic)

• Lower osmolarity than inside of the cell.

Osmolarity Gradient

• Water flows from osmolarity to higher osmolarity.

• Normal osmolarity inside a cell is about 300mOsm/L.

Non-Penetrating Solutes

• Solutes that cannot easily cross the cell membrane and establish osmotic gradients, thereby influencing cell volume and tonicity.

• They are crucial for maintaining fluid balance.

  • i.e. Na⁺ and K⁺.

Penetrating Solutes

• Solutes that can freely cross cell membranes, meaning they do not establish a sustained osmotic gradient.

• Do not directly influence cell volume or tonicity.

  • i.e. urea.

Changes in Cell Volume

• In a hypertonic solution the cell shrinks.

• In a isotonic solution the cell volume does not change.

• In a hypotonic solution the cell swells.

Absorption

• Movement of solute/water into the blood (plasma).

Filtration

• Movement of solute/water out of blood (plasma).

Plasma Proteins + Movement

• Plasma proteins in the blood create osmotic pressure.

• This draws water into the capillary from the interstitial space.

• Some plasma proteins may escape into the interstitial fluid.

Proteins

• Big molecules that are sometimes charged.

• Due to their size and charge, they cannot move in & out of capillaries easily.

  • Unable to cross capillary walls.

Capillary Hydrostatic Pressure (P_C)

• Pressure exerted on inside of capillary walls by blood.

• Favors fluid movement out of capillary.

Interstitial Fluid Hydrostatic Pressure (P_IF)

• Fluid pressure exerted on the outside of the capillary wall by interstitial fluid.

• Favors fluid movement into capillary; pressure is negligible & does not contribute.

Blood Colloid Osmotic Pressure (π_C)

• Osmotic pressure due to nonpermeating plasma proteins inside the capillaries.

• Favors fluid movement into the capillaries.

Interstitial Fluid Colloid Osmotic Pressure (π_IF)

• Small amount of plasma proteins may leak out of capillaries into interstitial space.

• Favors fluid movement out of capillaries; negligible.

Starling Forces

• Net filtration pressure = P_C + π_IF - P_IF - π_C.

• Govern the movement of fluid across capillary walls, determining whether fluid moves out of the capillary (filtration) or back into it (reabsorption).

Fluid Movement in Capillaries

• At the arterial end, starling forces favour filtration. at the venous end, they favour reabsorption.

• Filtration dominates at the arterial end; reabsorption at the venous end.

• Overall, slightly more fluid is filtered than reabsorbed, with the excess drained by the lymphatic system.

Homeostasis

• Total body balance of any substance; keeping levels constant and maintained.

Urinary System

• The kidneys are retroperitoneal (behind the peritoneum) in location.

• Other structures associated with the urinary system include the ureters, bladder, and urethra.

Ureters

• Drain the formed urine from the kidneys.

• Travel to the bottom of the abdominal cavity and empties into the bladder.

Bladder

• Storage organ or sac.

• Innervated by the ANS.

• Voiding of the bladder is controlled by SNS/PSNS.

• Bladder empties out of the body through the urethra.

Hilum

• Inner concave part of kidneys.

• From the area two tubes emerge; these tubes are called ureters.

Micturition

• The process of releasing the urine outside the body, or urination.

• Bladder emptying with the help of autonomic control.

Kidney Anatomy

• Covered with a capsule-like structure.

• 2 regions:

  • Outer portion: cortex.

  • Inner portion: medulla.

Nephron

• Functional units of the kidney; where the urine is made.

• 1 million nephrons in your kidneys.

• The parts of the nephrons form parts of the cortex and medulla.

• Contains: renal corpuscle and renal tubule.

Renal Corpuscle

• Bulb-like structure; attached to renal corpuscle is a long tube called the renal tubule.

Renal Tubule

• Attached to the renal corpuscle; a long tube.

• Found mostly in the medullary portion.

Collecting Ducts

• Urine starts forming in the nephrons which fuse together and form ducts.

• Empty their contents into the renal pelvis.

• The formed urine enters into the ureter to be taken away to the bladder.