Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 211:

  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?

  A. The arterial pressure equals the osmotic pressure.

  B. The weight of the gut equals the weight of the mass (m).

  C. The net force on the beam is zero.

  D. The net torque on the beam is zero.

  Correct Answer: D

  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. torque = Frsin where F is the force appliedr is the distance between the force and the fulcrum is the 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.

• Question 212:

  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

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

  A. Mass of solid dissolved, volume of solvent, temperature

  B. Molecular formula of solute, mass of solvent

  C. Number of particles in solution, volume of solution, temperature

  D. Number of moles of solid dissolved, density of solution, temperature

  Correct Answer: C

  The osmotic pressure of a solution is given in the passage by the following equation: = MRT where M is the molarity of particles in solutionR is the universal gas constantT 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.

• Question 213:

  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:

  A. higher because the fluid is less dense.

  B. lower because the fluid is less dense.

  C. the same because the pressure being measured is the same.

  D. the same because the velocity of blood flow in the capillary bed is the same.

  Correct Answer: A

  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.

• Question 214:

  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

  An arteriole is almost completely blocked by a blood clot as shown below. At which point will the velocity of blood flow be the greatest? (Assume ideal laminar flow throughout.)

  

  A. A

  B. B

  C. C

  D. D

  Correct Answer: B

  The stem says to assume laminar flow, indicating that we should consider the blood as a simple homogeneous fluid. Using the continuity equation it is clear that the greatest velocity will occur in the region of smallest cross sectional area.

  

  continuity equation where v is the velocityA is the cross sectional area Note that following the divergence of the arteriole into two branches, the total cross sectional area has not changed greatly so the velocity in each branch will not be different from the velocity at C. Choices A, C, and D are incorrect because the cross sectional area is greater for each than at B, so the velocity at these locations is lower.

• Question 215:

  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 a mass (m) of 0.2 kg is placed 25 cm to the right of the fulcrum. A section of gut, initially weighing 0.1 kg is placed 50 cm to the left of the fulcrum. During the experiment, the mass m is seen to descend. In order to maintain the balance level the following action should be taken:

  A. the mass m should be moved away from the fulcrum.

  B. the arterial pressure should be decreased.

  C. the venous pressure should be increased.

  D. an inert fluid with higher density than that of blood should be used in the pipette.

  Correct Answer: C

  According to the passage, when fluid flows out of the gut (because the hydrostatic pressure in the capillary bed is lower than the osmotic pressure), the gut will become lighter and will ascend. Thus, the mass m will descend. The question stem indicates that the section of gut has become lighter. Fluid must be returned to the gut section to make it heavier and maintain the balance level. Increasing the venous pressure will increase the hydrostatic pressure in the capillary bed, leading to flow of fluid from the capillary into the gut section. Note that no quantitative analysis (calculation) is necessary to answer this question. Choice A is incorrect because moving the mass m away from the fulcrum will increase the distance between the force applied and the fulcrum, thus increasing the torque produced by the mass m causing it to descend more. The equation for torque is shown below: torque = Frsin where F is the magnitude of the force appliedr is the distance between the force and the fulcrum is the angle between the force and the vector r Choice B is incorrect because decreasing the arterial pressure will decrease the hydrostatic pressure in the capillary bed causing more fluid to be lost from the gut. The gut will become lighter and will ascend, causing the mass m to descend more. Choice D is a distractor choice: the pipette (from Method 1) does not come into play in this set-up, which deals with Method 2.

• Question 216:

  Which of the following species exists as a resonance hybrid?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  A resonance hybrid is a molecule that cannot be represented by just one Lewis structure; the molecule is said to be a composite of the possible structures. Choice A is wrong because all the atoms have a complete octet--hydrogen having its two electrons--and only one Lewis structure is possible:

  

  Choice B is wrong because it too has only one structure:

  

  Choice C is correct because it does exist as a resonance hybrid:

  

  Keep in mind that the nitrite ion does not exist as one or the other of the above resonance structures but is a composite of them, having a structure somewhere in-between the two. Choice D is wrong because it has only one Lewis structure as well:

  

• Question 217:

  An automobile reaches a velocity of 30 m/s after accelerating at 4 ms-2 for 5 seconds. What was its initial velocity?

  A. -10 m/s

  B. 0 m/s

  C. 10 m/s

  D. 20 m/s

  Correct Answer: C

  

  

• Question 218:

  When the temperature of an iron bar is raised, its length expands. If the temperature of a second iron bar, which is initially twice as long as the first bar, is raised by twice as much, what is the ratio of the change in length of the first bar to the change in length of the second bar?

  A. 1:1

  B. 1:2

  C. 1:4

  D. 2:1

  Correct Answer: C

  To answer the question, apply the formula for the thermal expansion of a solid. This formula is given by L = LT, where L is the change in length, L is the original length, T is the change in temperature, and is some constant which depends on the type of material. This formula makes sense intuitively: as the length increases, there is more material available to expand, and as the temperature increases, the more vigorously the material expands.

  

  Compared with the first iron bar, the second iron bar is twice as long, so L2 = 2L1 , and is raised twice as much in temperature, so . Note that is the same for both bars because they are made of the same material. Therefore, the ratio of the first bar's length change to the second bar's length change is

  

  so choice C is correct.

• Question 219:

  Light traveling from air into a new medium is refracted away from the normal. This medium might be:

  A. glass.

  B. water.

  C. steel.

  D. a vacuum.

  Correct Answer: D

  The question is about refraction, which is the process that bends light as it travels from one medium into another. Snell's law n1sin 1 = n2 sin 2 describes this process, where n1 and n2 are the indices of refraction of the two media, and 1 and 2 are the angles that the light rays make with the normal to the interface between the two media. This law predicts that light traveling into a medium with a higher index of refraction will be refracted towards the normal. Light traveling into a medium with a lower index of refraction will be refracted away from the normal. In the question, the light is refracted away from the normal, so the new medium must have an index of refraction lower than air. The vacuum has the lowest index of refraction possible, n = 1, so choice D is correct.

  Steel is opaque, so light will not travel through it at all. The steel surface will absorb or reflect light, not refract it, so choice C is wrong. Water and glass both have indices of refraction that are higher than air, so both choices A and B are wrong. Note that even if you did not remember whether higher indices cause refraction towards or away from the normal, you could have eliminated choices A and B because they both would bend the light in the same direction, and both cannot be right.

• Question 220:

  Which expression correctly expresses the Ksp of a solution of XmYn?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  The Ksp, or solubility product constant, is found by multiplying together the concentrations of the dissociated ions, each raised to a power equal to the number of ions in one formula unit of the salt. When a mole of the salt, XmYn, dissolves, m

  

  moles of Xn+ ions and n moles of Ym- ions enter solution. So, the coefficient of X is m, and the coefficient of Y is n. The expression is as follows:

  Choice B is therefore the correct answer. Choice A is wrong because it has the coefficients reversed. Choice C and choice D can be eliminated because the Ksp is not a fraction.