TestAS · Medicine
Pressure and flow in vessels, electricity in nerves and the heart, waves, radiation and heat in the body.
testas.de lists this topic for the Medicine module. In the test it comes as testlets: a short text, table or formula, then 4 to 8 questions you answer from it.
Topic list: testas.de
Full exam-length testlets from the Kollego course, generated the same way as our mocks. Answer from the material, then open each answer to check.
Hydrostatic pressure in the body
A column of liquid exerts a pressure at its base that depends on its height, its density and the acceleration due to gravity. This hydrostatic pressure is the reason why blood pressure is not the same everywhere in the body of a standing person. For blood, a height difference of about 13 cm corresponds to a pressure difference of about 10 mmHg.
Hydrostatic pressure = density × g × height for blood: about 10 mmHg per 13 cm of height
When a person stands, the arteries and veins of the feet lie about 120 cm below the heart. The pressure in them is therefore about 90 mmHg higher than at heart level, in both arteries and veins. The arterial pressure in the feet of a standing person with a mean pressure of 95 mmHg at the heart is thus about 185 mmHg, while the arteries of the head, about 40 cm above the heart, have a mean pressure of only about 65 mmHg. Lying down, these differences almost disappear.
This has practical consequences. Blood pressure is measured with the cuff at heart level; if the arm hangs down or is raised, the reading changes by about 10 mmHg for every 13 cm. When a person stands up quickly, blood pools in the veins of the legs, the return of blood to the heart falls and blood pressure drops for a moment; normally the baroreceptor reflex corrects this within seconds. In the veins of the legs, valves divide the column of blood into short segments, and the contraction of the leg muscles pumps blood upwards. If the valves fail, the full hydrostatic pressure acts on the veins, which dilate and become varicose.
The same principle drives gravity infusions. The fluid in a bag hung above the patient flows into the vein because the hydrostatic pressure of the column of fluid exceeds the pressure in the vein, which is usually only a few mmHg. Because infusion fluid has about the same density as water, each 13.6 cm of height produces about 10 mmHg. If the bag is hung too low, below the level needed to overcome the venous pressure, blood can flow back into the tubing.
| Site | Height relative to the heart | Mean arterial pressure |
|---|---|---|
| Head | 40 cm above | About 65 mmHg |
| Heart | 0 cm | 95 mmHg |
| Knee | 70 cm below | About 150 mmHg |
| Foot | 120 cm below | About 185 mmHg |
1. A nurse measures a patient's blood pressure with the arm raised 26 cm above the heart. How does the reading compare with the true value at heart level?
D: About 20 mmHg too low
Above the heart, the pressure is lower: 26 cm is two steps of 13 cm, so about 2 × 10 = 20 mmHg less than at heart level.
2. Why do the head and feet of a standing person have such different arterial pressures, although the heart pumps the same blood to both?
A: The weight of the blood column adds pressure below the heart and reduces it above
The text explains the difference by hydrostatic pressure: the blood column adds about 10 mmHg per 13 cm below the heart and removes the same above it.
3. Why does blood pressure drop for a moment when a person stands up quickly?
B: Blood pools in the leg veins and less returns to the heart
The text says when a person stands up quickly, blood pools in the leg veins, the return to the heart falls and blood pressure drops until the baroreceptor reflex corrects it.
4. What is the main reason that leg veins become varicose when their valves fail?
A: The full column of blood now presses on the vein walls
The text says valves divide the column into short segments; if they fail, the full hydrostatic pressure acts on the veins, which dilate and become varicose.
5. An infusion bag hangs 68 cm above the vein. About what pressure does the fluid column produce?
B: About 50 mmHg
For fluid with the density of water, 13.6 cm gives about 10 mmHg. 68 ÷ 13.6 = 5, so about 5 × 10 = 50 mmHg.
6. A patient lies flat in bed. What happens to the pressure difference between head and feet?
A: It almost disappears
The text says lying down, these differences almost disappear, because head, heart and feet are at about the same height.
The eye as an optical system
The human eye forms an image of the world on the retina, a layer of light-sensitive cells at the back of the eye. Light is refracted, that is bent, when it passes from one transparent medium into another with a different refractive index. Most of the refraction happens at the front surface of the cornea, where light passes from air into the eye; the lens adds the rest and, importantly, can change its shape. Together they act like a single converging lens with a total refractive power of about 60 dioptres in a relaxed, normal eye.
The refractive power D of a lens is the reciprocal of its focal length f in metres: D = 1 ÷ f. A lens with a focal length of 0.5 m has a power of 2 dioptres. Converging lenses have positive power, diverging lenses negative power. When lenses are placed close together, their powers add, which makes dioptres convenient for describing glasses and contact lenses.
D = 1 ÷ f (f in metres) thin lens: 1 ÷ f = 1 ÷ object distance + 1 ÷ image distance
To see objects at different distances sharply, the eye changes the power of its lens, a process called accommodation. When the ciliary muscle contracts, the elastic lens becomes rounder and more powerful, so that near objects are focused. The nearest point that can be seen sharply is the near point; in young adults it is about 10 to 15 cm from the eye. With age, the lens loses elasticity, the near point moves away, and most people need reading glasses from about the age of 45; this condition is called presbyopia.
In myopia (short-sightedness), the eyeball is usually too long, or the refractive power too high, so that light from distant objects is focused in front of the retina. Distant objects appear blurred, while near objects are seen clearly. It is corrected with diverging lenses of negative power. In hyperopia (long-sightedness), the eyeball is too short, and light from near objects would be focused behind the retina; the eye can compensate by accommodating, but this is tiring, and near vision suffers first. It is corrected with converging lenses.
A simple rule applies to myopia: the far point, the greatest distance at which an uncorrected myopic eye sees sharply, determines the correction. The glasses must make distant objects appear to be at the far point, so a lens of focal length equal to minus the far-point distance is needed, if the distance between glasses and eye is neglected.
| Patient | Finding |
|---|---|
| A | Far point 50 cm; near vision normal |
| B | Aged 55; distant vision sharp, cannot read small print at 30 cm |
| C | Eyeball shorter than normal; near vision blurred, tired eyes when reading |
1. Which part of the eye contributes most of the refraction?
D: The cornea
The text: "Most of the refraction happens at the front surface of the cornea, where light passes from air into the eye"; the lens only adds the rest.
2. What lens does patient A need for distance vision?
D: A diverging lens of −2 dioptres
The text: for myopia the lens needs a focal length of minus the far-point distance. Far point 50 cm, so f = −0.5 m and D = 1 ÷ (−0.5) = −2 dioptres, a diverging lens.
3. What is the most likely cause of patient B's problem?
D: Presbyopia, a loss of lens elasticity with age
Table: B is 55, sees far sharply but cannot read at 30 cm. The text: with age the lens loses elasticity, the near point moves away, reading glasses from about 45: presbyopia.
4. What correction does patient C need?
A: Converging lenses
Table: C has a short eyeball and blurred near vision. The text: that is hyperopia, and it is corrected with converging lenses.
5. What happens in the eye when a person looks from a distant tree to a book?
C: The ciliary muscle contracts and the lens becomes rounder
Looking at the book means focusing near. The text: for near objects the ciliary muscle contracts and the elastic lens becomes rounder and more powerful (accommodation).
6. A contact lens of +3 dioptres is placed on an eye with a total power of 60 dioptres. What is the combined power?
A: 63 dioptres
The text: when lenses are placed close together, their powers add. 60 + 3 = 63 dioptres.
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