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Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 15 )

Updated On: 12-Jan-2026
Page 15 of 164
PDF with 811 Questions

Historically, two different methods have been used to estimate the fluid pressure in capillary beds.
Method 1
A glass pipette is inserted into the capillary. The level of blood rising in the pipette is measured and used to calculate the pressure. Alternatively, an inert fluid of density can be placed in the pipette and its height h can be measured. The pressure in the capillary is given by gh, where g is the acceleration due to gravity.

Figure 1
Method 2
The pressure can be measured indirectly in the following way. A section of gut tissue is removed from a specimen and placed on a beam balance. Blood is circulated through the tissue by a pump. The arterial pressure is then decreased. This leads to a decrease in the capillary hydrostatic pressure in the gut capillaries. The constant osmotic pressure of plasma proteins in the capillary causes absorption of fluid from the gut section which will decrease its weight. To prevent a change in the weight of the gut section, the venous pressure is increased. This tends to increase the capillary pressure, reducing the flow of fluid from the gut tissue into the capillaries. The capillary pressure is thus held constant (and the balance kept level) as the arterial pressure is decreased and the venous pressure increased. The arterial and venous pressures meet at the capillary pressure being measured.

( = MRT, where is the osmotic pressure, M the molarity of the solutes, R the universal gas constant, and T the temperature in Kelvin.)

Figure 2
A researcher using Method 1 to determine the capillary pressure fills the pipette with an inert fluid less dense than blood. Compared to blood, the height of this fluid in the pipette will be:

  1. higher because the fluid is less dense.
  2. lower because the fluid is less dense.
  3. the same because the pressure being measured is the same.
  4. the same because the velocity of blood flow in the capillary bed is the same.

Answer(s): A

Explanation:

The passage states that using Method 1, the capillary pressure is given by P = gh. We are measuring the same capillary, so the measured pressure is constant. If we use a less dense fluid, the column will rise higher for a given pressure. Choice B is incorrect for the reasons described above. Using a fluid more dense than blood would give a column with a lower height. Choice C is incorrect, although the pressure is the same, the height of the column changes with respect to the density of the fluid as described above. Choice D is a distractor choice that may have been tempting if you remembered that Bernoulli's equation relates the velocity of fluid flow to the pressure. Again, if the velocity is the same, and the pressure is the same, then the height of the fluid will depend on its density.



Historically, two different methods have been used to estimate the fluid pressure in capillary beds.
Method 1
A glass pipette is inserted into the capillary. The level of blood rising in the pipette is measured and used to calculate the pressure. Alternatively, an inert fluid of density can be placed in the pipette and its height h can be measured. The pressure in the capillary is given by gh, where g is the acceleration due to gravity.


Figure 1
Method 2
The pressure can be measured indirectly in the following way. A section of gut tissue is removed from a specimen and placed on a beam balance. Blood is circulated through the tissue by a pump. The arterial pressure is then decreased. This leads to a decrease in the capillary hydrostatic pressure in the gut capillaries. The constant osmotic pressure of plasma proteins in the capillary causes absorption of fluid from the gut section which will decrease its weight. To prevent a change in the weight of the gut section, the venous pressure is increased. This tends to increase the capillary pressure, reducing the flow of fluid from the gut tissue into the capillaries. The capillary pressure is thus held constant (and the balance kept level) as the arterial pressure is decreased and the venous pressure increased. The arterial and venous pressures meet at the capillary pressure being measured.
( = MRT, where is the osmotic pressure, M the molarity of the solutes, R the universal gas constant, and T the temperature in Kelvin.)

Which of the following is sufficient information to determine the osmotic pressure of a solution? (Assume that the universal gas constant R is known.)

  1. Mass of solid dissolved, volume of solvent, temperature
  2. Molecular formula of solute, mass of solvent
  3. Number of particles in solution, volume of solution, temperature
  4. Number of moles of solid dissolved, density of solution, temperature

Answer(s): C

Explanation:

The osmotic pressure of a solution is given in the passage by the following equation: = MRT where M is the molarity of particles in solution, R is the universal gas constant, T is the temperature. Knowing the number of particles and the volume of the solution, it is possible to calculate the molarity of particles in solution. (The number of moles of particles in solution is found by dividing the number of particles by Avogardo's number.) If the temperature and the gas constant are known, the osmotic pressure can be calculated.
Choice A is incorrect because knowing the mass of the solid is not enough information to determine the number of particles in solution. We must also know its molecular weight (to determine the number of moles of solid) and its molecular formula (to determine how many particles it forms when it dissolves).
Choice B is incorrect because knowing only the molecular formula of the solute is not enough information to determine the number of particles in solution. We must also know the mass of solid dissolved.
Choice D is incorrect because knowing the moles of solid dissolved is not enough information to determine the concentration of particles in solution. We must know the volume in order to calculate the concentration. We must also know the molecular formula of the dissolved solid. For example, 1 mole of NaCl will give 2 moles of particles in solution while 1 mole of MgCl2 will give 3 moles of particles in solution.



Historically, two different methods have been used to estimate the fluid pressure in capillary beds.
Method 1
A glass pipette is inserted into the capillary. The level of blood rising in the pipette is measured and used to calculate the pressure. Alternatively, an inert fluid of density can be placed in the pipette and its height h can be measured. The pressure in the capillary is given by gh, where g is the acceleration due to gravity.

Figure 1
Method 2
The pressure can be measured indirectly in the following way. A section of gut tissue is removed from a specimen and placed on a beam balance. Blood is circulated through the tissue by a pump. The arterial pressure is then decreased. This leads to a decrease in the capillary hydrostatic pressure in the gut capillaries. The constant osmotic pressure of plasma proteins in the capillary causes absorption of fluid from the gut section which will decrease its weight. To prevent a change in the weight of the gut section, the venous pressure is increased. This tends to increase the capillary pressure, reducing the flow of fluid from the gut tissue into the capillaries. The capillary pressure is thus held constant (and the balance kept level) as the arterial pressure is decreased and the venous pressure increased. The arterial and venous pressures meet at the capillary pressure being measured.
( = MRT, where is the osmotic pressure, M the molarity of the solutes, R the universal gas constant, and T the temperature in Kelvin.)


Figure 2
Method 2 relies on keeping the beam balance level. Which of the following must be true if the balance is level?

  1. The arterial pressure equals the osmotic pressure.
  2. The weight of the gut equals the weight of the mass (m).
  3. The net force on the beam is zero.
  4. The net torque on the beam is zero.

Answer(s): D

Explanation:

The net torque determines the angular acceleration of the beam. If the net torque is zero then the beam will remain level. Recall the following equation for calculating the torque.
= Fr sin , where F is the force applied, r is the distance between the force and the fulcrum, is the torque
angle between the force and the vector r.
Choice A is incorrect because the passage does not describe any specific relationship between the arterial pressure and the osmotic pressure. It does state that when the capillary pressure equals the osmotic pressure, there is no net fluid flow into or out of the capillary bed and the gut section will not change weight, keeping the balance level. Note that the capillary pressure is related to both the osmotic pressure and the hydrostatic pressure. Choice B is incorrect because the net torque may not be zero even if the weight of the gut equals the weight of the mass (m). If the distance from the fulcrum r is not equal, then the net torque will not be zero and the beam will not remain level. Choice C is incorrect because the net torque may not be zero even if the net force on the beam is equal to zero. Note that when the beam is level, two forces are applied downwards (by the gut section and by the mass m) and the beam does not rotate. It is true that when the beam balance is not translated (moved without rotation), the net force is zero. Under these conditions, the fulcrum will apply an upwards force to counteract the two downwards forces described above. However, the beam balance can be translated, indicating a net force, and the beam will still remain level if the net torque is zero ­ consider pushing the beam balance around the room while the weights keep it level.



Historically, two different methods have been used to estimate the fluid pressure in capillary beds.
Method 1
A glass pipette is inserted into the capillary. The level of blood rising in the pipette is measured and used to calculate the pressure. Alternatively, an inert fluid of density can be placed in the pipette and its height h can be measured. The pressure in the capillary is given by gh, where g is the acceleration due to gravity.


Figure 1
Method 2
The pressure can be measured indirectly in the following way. A section of gut tissue is removed from a specimen and placed on a beam balance. Blood is circulated through the tissue by a pump. The arterial pressure is then decreased. This leads to a decrease in the capillary hydrostatic pressure in the gut capillaries. The constant osmotic pressure of plasma proteins in the capillary causes absorption of fluid from the gut section which will decrease its weight. To prevent a change in the weight of the gut section, the venous pressure is increased. This tends to increase the capillary pressure, reducing the flow of fluid from the gut tissue into the capillaries. The capillary pressure is thus held constant (and the balance kept level) as the arterial pressure is decreased and the venous pressure increased. The arterial and venous pressures meet at the capillary pressure being measured.
( = MRT, where is the osmotic pressure, M the molarity of the solutes, R the universal gas constant, and T the temperature in Kelvin.)

Figure 2
Assume that the beam balance of Method 2 is initially level. If the arterial pressure is decreased to a lower level while everything else is held constant, which graph best represents the change in the mass of the gut following the decrease in arterial pressure?

  1. A
  2. B
  3. C
  4. D

Answer(s): C

Explanation:

The passage states that if the arterial pressure is decreased fluid will leave the gut section and its mass will decrease. The mass will decrease until a certain amount of fluid is lost and a new equilibrium between hydrostatic pressure and osmotic pressure in the capillaries is reached. Choice C best shows the initially rapid decrease in mass and the gradual leveling out as a new equilibrium is reached. Choice A is incorrect because the mass of the gut will not decrease to zero. Also, the rate of mass loss will not be linear. There will be rapid mass loss followed by a gradual leveling out. Choice B is incorrect because the mass of the gut will decrease as fluid is lost from the gut section to the capillaries. Choice D is incorrect because the mass loss will occur rapidly at first, followed by a gradual leveling out as a new equilibrium is reached. Choice D indicates a gradual increase in the rate of mass loss which is the opposite of what actually happens.



The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteriA. Researchers thus turned their attention to chemical alternatives.
The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements.
The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.
2NaN3 2Na + 3N2
Reaction 1
The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).
10Na + 2KNO3 K2O + 5Na2O + N2
Reaction 2
K2O + Na2O + SiO2 glass
Reaction 3

In order for the decomposition reaction to spread throughout the sodium azide after ignition, the H and G for Reaction 1 must be, respectively:

  1. positive, negative
  2. positive, positive
  3. negative, negative
  4. negative, positive

Answer(s): C

Explanation:

The decomposition reactions require an activation energy. Initially, this is supplied by the electrical impulse. In order for the reaction to continue following this ignition, however, the reaction must release sufficient energy to overcome the activation barrier of subsequent decompositions. In other words, in order for the reaction to spread throughout the sample, it must produce enough heat to provide the activation energy for adjacent reactions. The reaction must be exothermic and spontaneous.



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