AP Biology : Understanding Osmotic and Oncotic Pressure

Study concepts, example questions & explanations for AP Biology

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Understanding Osmotic And Oncotic Pressure

Which of the following pressure changes would result in decreased fluid movement into the interstitium?

Possible Answers:

A decrease in interstitial hydrostatic pressure.

An increase in capillary hydrostatic pressure.

An increase in interstitial osmotic pressure.

An increase in capillary osmotic pressure.

Correct answer:

An increase in interstitial osmotic pressure.

Explanation:

As fluid moves through the capillary, the hydrostatic pressure decreases from the arteriole end to the venule end (fluid exits the capillary along the gradient). The osmotic pressure in the interstitium is relatively constant, and will be stronger than capillary hydrostatic pressure near the venule end. As a result, an increase in the interstitial osmotic pressure would cause less fluid to enter the interstitium, because there is less area in the bed where the capillary hydrostatic pressure is greater than the interstitial osmotic pressure.

Example Question #2 : Understanding Osmotic And Oncotic Pressure

As blood enters the arteriole end of a capillary some fluid generally exits into the interstium. When the blood flows through the venule end of the capillary some of this fluid is returned to the vessel. What best explains this transition?

Possible Answers:

Capillary osmotic pressure becomes stronger than interstitial hydrostatic pressure

Capillary hydrostatic pressure becomes stronger than the interstitial osmotic pressure

Interstitial osmotic pressure becomes stronger than capillary hydrostatic pressure

Interstitial hydrostatic pressure becomes stronger than the capillary osmotic pressure

Correct answer:

Interstitial osmotic pressure becomes stronger than capillary hydrostatic pressure

Explanation:

The capillary is the site of fluid exchange with the body's tissues. This fluid transfer is moderated by two factors: hydrostatic pressure and osmotic pressure. Hydrostatic pressure is the "pushing" force on water due to the presence of more fluid in one region than another. In general, larger fluid volumes generate higher hydrostatic pressure. Osmotic pressure is the "pulling" force on water due to the presence of solutes in solution. Albumin proteins are the main source of osmotic pressure in capillaries, pulling water into the blood.

At the arteriole end of the capillary, the hydrostatic pressure is stronger than the interstitial osmotic pressure and fluid is forced into the interstitium. Osmotic pressure remains relatively constant over the length of the capillary, but hydrostatic pressure drops sharply as it nears the venule end due to the initial loss of fluid volume. At that point, the interstitial osmotic pressure becomes stronger than the capillary's hydrostatic pressure. This forces fluid back into the capillary.

Example Question #3 : Understanding Osmotic And Oncotic Pressure

Why does fluid reenter the capillary from the interstitium at the venule end of the capillary bed?

Possible Answers:

Osmotic pressure is lower at the venule end of the capillary

Osmotic pressure is greater at the venule end of the capillary

Hydrostatic pressure is greater at the venule end of the capillary

Hydrostatic pressure is lower at the venule end of the capillary

Correct answer:

Hydrostatic pressure is lower at the venule end of the capillary

Explanation:

Hydrostatic pressure is the force of the fluid volume against a membrane, while osmotic pressure is related to the protein concentration on either side of a membrane pulling water toward the region of greater concentration.

When fluid enters the capillaries, it is initially pushed out because the hydrostatic pressure pressing outward is greater than the osmotic pressure pushing inward. Although osmotic pressure stays constant throughout the capillary length, hydrostatic pressure decreases towards the venule end of the capillary. This makes the osmotic pressure larger than the hydrostatic pressure, and pushes the fluid back into the capillary. 

Example Question #3 : Understanding Osmotic And Oncotic Pressure

Osmolarity plays an important role in how water travels within our body. Osmolarity describes the concentration of solutes within a solvent and is expressed as the amount of solutes divided by the volume of solvent:

 

The higher the amount of solute is within a volume of solvent and/or the lower the volume of solvent is, then the osmolarity will be greater. In regard to fluid movement, water will move from an area with a low osmolarity to an area with a high osmolarity. Human blood has an osmolarity of roughly:

 

At this concentration, the osmolarity inside the cell is equal to the osmolarity of the surrounding environment; therefore, it is considered to be in an isotonic solution. When the osmolarity around the cell is higher, then water will flow out of the cell and into the blood. This type of solution is called a hypertonic solution. Conversely, a hypotonic solution exits when the osmolarity of the fluid surrounding the cell is lower than that inside the cell. In this case, water will flow from the surrounding environment and into the cell.

James was stranded on an island. He was thirsty and decided to drink the water from the sea (which has a very high osmolarity). After drinking the water, his body became even more dehydrated and he began urinating more frequently. Which of the follow choices best explains why James urinated more frequently and became more dehydrated despite drinking the seawater?

Possible Answers:

The seawater could not be processed by the kidneys because James' malnourished body lacked proper amounts of glucose in the pancreas

The seawater pulled water from James' cells, which left him more dehydrated and caused him to urinate more

James' body was malnourished and unable to absorb the seawater, which left as urine

None of these

The seawater was not processed by the kidneys because James' body lacked electrolytes

Correct answer:

The seawater pulled water from James' cells, which left him more dehydrated and caused him to urinate more

Explanation:

Ocean water has a higher osmolarity (more units of solute per unit of solvent) than human blood. When James drank the ocean water, it was absorbed into his circulatory system and it pulled water from the cells. Water flows from an area of low osmolarity to an area of high osmolarity. When water was pulled from the cells, the fluid volume in James’ blood increased. As blood reached James’ kidneys, the extra fluid from the tissues was filtered into the urine and caused him to urinate more frequently. 

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: