Osmolarity and Tonicity in Kidney Function

Osmolarity and Tonicity

Understand osmolarity and tonicity.
Know the anatomy of the kidneys and the different sections of the nephron.
Know the steps of urine formation and in which parts of the nephron it takes place.
Understand how glomerular filtration rate is regulated.
Understand the functions of the kidneys in excretion, pH regulation, and blood pressure regulation.
Predict how disturbances in the Renin-Angiotensin-Aldosterone system affect blood volume and blood pressure.
Understand how hormones, such as anti-diuretic hormone and aldosterone, can affect osmolarity and fluid regulation.

Diffusion: Process through which atoms or molecules intermingle due to their random thermal motion, also known as Brownian movement.
Net Diffusion: Directional movement due to a greater concentration of particles in one location compared to another.
- Continues until the concentration gradient is eliminated.

Area of the Plane of Contact: A greater area will result in a faster rate of diffusion.
Concentration Gradient: A larger concentration gradient will increase the rate of diffusion.
Size of the Molecule: Smaller molecules will move faster.
Distance: Diffusion is fast for short distances and slow as distances increase.

Water moves from a solution that has less solute to a solution that has more solute.
Osmolarity: Defined as the number of particles in a solution, expressed as osmol/L.
- Does not describe the composition of particles.
- Example: A 1 mOsm solution can be composed of glucose, Na extsuperscript{+}, Cl extsuperscript{-}, or a combination of all.

Non-electrolytes: Do not dissociate in solutions.
- Example: Sucrose, Urea.
- Molarity = Osmolarity:
  - 150 mM sucrose = 150 mOsm sucrose.
Electrolytes: Dissociate in solutions.
- Example: NaCl → Na extsuperscript{+} + Cl extsuperscript{-}. Both generate osmotic pressure.
- Example: CaCl extsubscript{2} → Ca extsuperscript{2+} + 2Cl extsuperscript{-}.
  - Molarity x Dissociation # = Osmolarity:
  - 150 mM NaCl = 300 mOsm NaCl.
- Osmolarity of most fluids in the human body is approximately 300 mOsm.

Isosmotic: Two solutions have an equal number of solute particles per unit volume and generate the same osmotic pressure (∏).
- Example:
- Solution A → 2 mM sucrose = 2 mOsm.
- Solution B → 1 mM NaCl = 2 mOsm.
- Solution A is isosmotic to Solution B.
Hyperosmotic: One solution generates a higher osmotic pressure.
- Example:
- Solution A → 2 mM sucrose = 2 mOsm.
- Solution B → 0.5 mM NaCl = 1 mOsm.
- Solution A is hyperosmotic to Solution B.
Hyposmotic: One solution generates a lower osmotic pressure.
- Example:
- Solution B is hyposmotic to Solution A.

Osmosis: This is defined as the net movement of water molecules from an area of greater concentration to an area of lesser concentration.

Tonicity refers specifically to the effect of a solution on a cell, focusing on how the solution compares to the internal osmolarity of the cell.
Kidneys play a crucial role in maintaining fluid balance and regulating osmolarity.

Regulation of extracellular fluid volume and blood pressure.
Regulation of ion levels in the blood (Na extsuperscript{+}, K extsuperscript{+}, Ca extsuperscript{2+}).
Homeostatic regulation of pH.
Excretion of wastes in urine (ammonia, urea, uric acid, bilirubin, hormones).
Production of hormones such as erythropoietin, calcitriol, and renin.

Renal Cortex: The outer part of the kidney.
Renal Medulla: The inner part of the kidney.
Nephrons: The functional unit of the kidneys, closely associated with the circulatory system.

Renal Corpuscle: Consists of the glomerulus and Bowman's capsule.
Renal Tubule: Comprises the proximal convoluted tubule, nephron loop (loop of Henle), and distal convoluted tubule.
Additional structures include the ascending and descending limbs as well as the collecting duct.

Filtration: The movement of fluid from the blood into the lumen of the nephron.
Reabsorption: The process of moving molecules or fluid from the filtrate back into the blood.
Secretion: Selected molecules are moved from the blood into the filtrate.

Definition: The amount of filtrate formed in both kidneys every minute.
GFR = 125 ml/min.
Net Filtration Pressure (NFP): Calculated with the following pressures:
- Pc (capillary hydrostatic pressure) = 55 mm Hg
- Pfluid (Bowman's capsule pressure) = 15 mm Hg
- = NFP = Pc - Pfluid = 55 mm Hg - 15 mm Hg = 40 mm Hg.
Blood composition upon filtration:
- Filtered: Water, salts, HCO3 extsuperscript{-}/H extsuperscript{+}, urea, glucose, amino acids.
- Not filtered: Cells and plasma proteins.
- Initial osmolarity of filtrate = 300 mOsm.

In a day, approximately 180 L of filtrate are produced, but only about 1.5 L are excreted as urine.
The composition of the filtrate differs from that of blood due to reabsorption and secretion processes.
The final volume and osmolarity of urine depend on the need to conserve or excrete water and solutes.

Reabsorption: Moving materials from the lumen of the renal tubule back to the blood.
Secretion: Moving materials from the blood to the lumen of the renal tubule to be excreted.
Sodium (Na extsuperscript{+}) moves down its concentration gradient via various channels.
- The concentration gradient is established by the Na extsuperscript{+}-K extsuperscript{+} ATP pump.
- Other solutes such as glucose and amino acids use sodium's driving force for movement.
- Water follows due to the osmotic pressure generated by solute movement.
- Urea moves along its concentration gradient.
- Bicarbonate (HCO3 extsuperscript{-}) is reabsorbed to maintain pH balance, while H extsuperscript{+} is secreted to regulate pH.

Following the proximal tubule, the filtrate moves through the Loop of Henle down the renal medulla.
Osmotic gradients exist in the renal medulla:
- The descending limb of the Loop of Henle is only permeable to H2O, allowing significant reabsorption of water, concentrating the filtrate.
- At the bottom of the loop of Henle, the filtrate's osmolarity increases, making it more hyperosmotic relative to plasma.

This portion is not permeable to water.
Solutes such as Na extsuperscript{+}, K extsuperscript{+}, and Cl extsuperscript{-} are transported and reabsorbed from the filtrate, resulting in a diluted filtrate as it enters the distal tubule.

The processes of reabsorption of water and solutes in the distal tubule and collecting duct are reliant on body hydration and osmolarity and are under hormonal control.

Osmoreceptors in the hypothalamus detect changes in blood osmolarity (e.g., after heavy sweating).
The hypothalamus triggers the release of anti-diuretic hormone (ADH), which acts in the distal tubule and collecting duct to promote water reabsorption.
Response to Increased Blood Osmolarity:
- Specific neurons generate thirst, prompting water intake to reduce blood osmolarity.
- Normal blood osmolarity is maintained between 275-295 mOsm/L.

ADH Mechanism:
1. ADH binds to receptors in the distal tubule and collecting duct.
2. Stimulates the insertion of aquaporins into the cell membranes, facilitating water reabsorption.

With ADH:
- Concentrated urine forms, with osmolarities approaching 1200 mOsm.
Without ADH:
- Dilute urine is formed with lower osmolarities (e.g., 100 mOsm).