Osmolarity and Tonicity

Question 1

Diagram showing osmotic pressure

Answer 1

Question 2

Osmolarity of saline

Answer 2

Question 3

Isotonic saline is ___

Answer 3

Isotonic saline is ~300 mOsm/L

Isotonic saline is 0.9%

9 grams of salt in 1000 grams of water

How is 9 grams of NaCl in 1 L = 0.9%?

Question 4

Osmolarity definition

Answer 4

The total solute concentration of a solution

Question 5

What is an osmol?

Answer 5

• One osmol is equal to 1 mol of solute particles.
• Therefore, a 1 M solution of glucose has a concentration of 1 Osm (1 osmol per liter), whereas a 1 M solution of NaCl contains 2 osmol of solute per liter of solution.
• A liter of solu- tion containing 1 mol of glucose and 1 mol of NaCl has an osmolarity of 3 Osm.
• A solution with an osmolarity of 3 Osm may contain 1 mol of glucose and 1 mol of NaCl, or 3 mol of glucose, or 1.5 mol of NaCl, or any other combination of solutes as long as the total sol- ute concentration is equal to 3 Osm.

Question 6

Explaining osmolarity using movement of water across a membrane

Answer 6

• The diagram shows two 1 L compartments separated by a membrane permeable to both solute and water.
• Initially, the concentration of solute is 2 Osm in compartment 1 and 4 Osm in compartment 2.
• This difference in solute concentration means there is also a difference in water concentration across the membrane: 53.5 M in compartment 1 and 51.5 M in compartment 2.
• Therefore, a net diffusion of water from the higher concentration in compartment 1 to the lower concentration in compartment 2 will take place, and a net diffusion of solute in the opposite direction, from 2 to 1.
• When diffusion equilibrium is reached, the two compartments will have identical solute and water concentrations, 3 Osm and 52.5 M, respectively.
• One mol of water will have diffused from compartment 1 to compartment 2, and 1 mol of solute will have diffused from 2 to 1.
• Because 1 mol of solute has replaced 1 mol of water in compartment 1, and vice versa in compartment 2, no change in the volume occurs for either compartment.

Question 7

What happens if the membrane is replaced by one permeable to water but impermeable to solute? (addition to previous flashcard)

Answer 7

• The same concentrations of water and solute will be reached at equilibrium as before, but a change in the volumes of the compartments will also occur.
• Water will diffuse from 1 to 2, but there will be no solute diffusion in the opposite direction because the membrane is impermeable to sol- ute.
• Water will continue to diffuse into compartment 2, therefore, until the water concentrations on the two sides become equal.
• The solute concentration in compartment 2 decreases as it is diluted by the incoming water, and the solute in compartment 1 becomes more concentrated as water moves out. When the water reaches diffusion equilibrium, the osmolarities of the compartments will be equal; therefore, the solute concentrations must also be equal.
• To reach this state of equilibrium, enough water must pass from compartment 1 to 2 to increase the volume of compartment 2 by one-third and decrease the volume of compartment 1 by an equal amount.
• Note that it is the presence of a membrane impermeable to solute that leads to the volume changes associated with osmosis.

Question 8

Diagram showing he movement of water across a membrane that is permeable to water but not to solute leads to an equilibrium state involving a change in the volumes of the two compartments

Answer 8

Question 9

What is osmotic pressure?

Answer 9

When a solution containing solutes is separated from pure water by a semipermeable membrane (a membrane permeable to water but not to solutes), the pressure that must be applied to the solution to prevent the net flow of water into it is known as the osmotic pressure of the solution.

Question 10

The greater the osmolarity of a solution, the ___ the osmotic pressure

Answer 10

Greater

Question 11

Isotonic, hypertonic, and hypotonic solutions

Answer 11

Question 12

What is the difference between osmolarity and tonicity?

Answer 12

Osmolarity considers all solutes; tonicity considers non-penetrating solutes.

Question 13

When is a solution isotonic?

Answer 13

If the concentration of non-penetrating solutes is 300mOsm (regardless of the concentration of membrane-penetrating solutes present)

Question 14

When is a solution hypertonic?

Answer 14

If the concentration of non-penetrating solutes is greater than 300mOsm (regardless of the concentration of membrane-penetrating solutes present)

Question 15

When is a solution hypotonic?

Answer 15

If the concentration of non-penetrating solutes is less than 300mOsm (regardless of the concentration of membrane-penetrating solutes present)

Question 16

Isoosmotic solution

Answer 16

A solution containing 300mOsmol/L of solute, regardless of its composition of membrane-penetrating and nonpenetrating solutes

Question 17

Hyperosmotic solution

Answer 17

A solution containing greater than 300 mOsmol/L of solutes, regardless of its composition of membrane-penetrating and nonpenetrating solutes

Question 18

Hypoosmotic

Answer 18

A solution containing less than 300 mOsmol/L of solutes, regarding of its composition of membrane-penetration and nonpenetrating solutes

Question 19

Table summarizing tonicity and osmotic solutions

Answer 19

Question 20

Assumptions made when analyzing tonicity

Answer 20

1. Normal intracellular concentration is 300 mOsm

2. Typical solutes found inside and outside cells are non-penetrating (e.g. Na+, Cl-, K+, Proteins)

3. Water is freely permeable (many aquaporin channels)

Question 21

Why may transient changes in volume occur?

Answer 21

Water moves fast. Transient changes in volume may occur depending on solubility of solute.

Question 22

Osmotic pressure and the capillary

Answer 22

Question 23

Filtration in the capillaries- big picture and details

Answer 23

Question 24

Revised Story on Fluid Exchange in capillary bed (microvascular)

Answer 24

• Glycocalyx forms a semipermeable membrane along the walls of endothelial cells.
• The microenvironment beneath small pores is different from interstitial fluid.

Question 25

More detail on glycocalyx

Answer 25

• Glycocalyx forms a semipermeable membrane along the walls of endothelial cells.
• The microenvironment beneath small pores is different from interstitial fluid.
• Dynamic changes can result in reabsorption but in the steady state, filtration throughout capillary with lymph vessels carrying fluid back to veins.
• Absorption into capillaries maintained in some tissues (kidneys, intestine) due to epithelial cell secretion of proteins into the interstitial fluid.

Question 26

Graphs showing net fluid movement through capillaries

Question 27

What is tonicity?

Answer 27

• The concentration of nonpenetrating solutes in a solution, not the total osmolarity, determines its tonicity—isotonic, hypotonic, or hypertonic.
• By contrast, solutes that readily diffuse through lipid bilayers (penetrating solutes) do not contribute to the tonicity of a solution.
• This is so because the concentrations of penetrating solutes rapidly equilibrate across the membrane.

Question 28

What do the terms isoosmotic, hypoosmotic, and hyperosmotic denote?

Answer 28

• The osmolarity of a solution relative to that of normal extracellular fluid without regard to whether the solute is penetrating or nonpenetrating. The two sets of terms are therefore not synonymous.
• For example, a 1 L solution containing 150 mOsm each of nonpenetrating Na+ and Cl− and 100 mOsm of urea, which can rapidly cross plasma membranes, would have a total osmolarity of 400 mOsm and would be hyperosmotic relative to a typical cell.

Question 29

What is the osmolarity of extracellular fluid?

Answer 29

About 300 mOsm. Because water comes to diffusion equilibrium across cell membranes, the intracellular fluid has an osmolarity equal to that of the extracellular fluid.

Question 30

Examples of nonpenetrating fluids

Answer 30

Na+ and Cl− are the major effectively nonpenetrating solutes in the extracellular fluid; K+ and various organic solutes are the major effectively nonpenetrating solutes in the intracellular fluid