14 Questions
What is the main determinant of the number of osmotically active particles in a fluid?
The number of moles per liter of water of the solute multiplied by the number of osmotically active particles released into solution
What is the main contributor to the osmolality of the ECF?
Sodium ions, chloride ions, and bicarbonate ions
Why do the ICF and ECF have the same osmolarity despite their differences in composition?
Because the capillary endothelium and cell membranes are freely permeable to water
What is the approximate total osmolality of the ECF and ICF?
290-300 mOsmol/kg H2O
Which of the following body fluids can have a significantly different osmolality from 290 mOsmol/kg H2O?
Peritubular interstitial fluid of the renal medulla
What is the range of osmolality for urine?
70-1200 mOsmol/kg H2O
What is the primary function of aquaporins in the plasma membrane?
To facilitate the flow of water molecules across the membrane
What determines the magnitude of the osmotic pressure of a solution?
The concentration of solute particles
What is the primary function of oncotic pressure?
To pull water towards large molecules
What is the unit of osmolarity?
mol/L H2O
What is the primary difference between osmolarity and osmolality?
Osmolarity is measured in mol/L H2O, while osmolality is measured in mol/Kg H2O
What happens when there is a high concentration of solute in a solution?
Water molecules flow into the solution
What is the direction of water flow due to osmotic pressure?
From a solution with a lower solute concentration to a solution with a higher solute concentration
What is the role of the plasma membrane in osmosis?
To regulate the flow of water molecules across the membrane
Study Notes
Osmosis and Osmotic Pressure
- Osmosis is the diffusion of water molecules across a semipermeable membrane into a region with a high concentration of solutes, regulated by cells through the addition or removal of aquaporins.
- Aquaporins are channel proteins in the plasma membrane that facilitate osmosis.
Osmotic Pressure
- Osmotic pressure is an attractive force between solute particles and water molecules that drives water movement from a solution with low solute concentration to one with high solute concentration.
- The magnitude of osmotic pressure depends on the number of solute particles in a solution.
- Oncotic pressure, or colloid osmotic pressure, is a type of osmotic pressure exerted by large molecules like proteins in blood vessels' plasma or interstitial fluid.
Osmolarity and Osmolality
- Osmolarity is the concentration of osmotically active particles in one liter of a solution (mol/L H2O).
- Osmolality is the concentration of osmotically active particles in one kilogram of a solution (mol/Kg H2O).
- Osmotically active particles are molecules or particles that attract water to them.
- The number of osmotically active particles in a fluid is determined by the number of moles per liter (or kg) of water and the number of particles released into solution when the solute is dissolved.
Body Fluids and Osmolality
- The osmolality of plasma and extracellular fluid (ECF) is mainly due to sodium ions, chloride ions, bicarbonate ions, and other ions, urea, glucose, and proteins.
- The osmolality of intracellular fluid (ICF) is mainly due to potassium ions, magnesium ions, organic phosphates, proteins, and other nitrogen-containing solutes.
- Despite differences in composition, the body fluids have essentially identical total osmolalities of about 290-300 mOsmol/kg H2O.
- The osmolality of other body fluids can vary significantly, such as peritubular interstitial fluid of the renal medulla and certain transcellular fluids like urine.
Understanding osmosis, osmotic pressure, and osmolarity in the human body, including the role of aquaporins and semi-permeable membranes.
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