Quiz 108 · Regulation of Respiration

Answers to Quiz 107 · Transport of Gases in Blood

1. In arterial blood, roughly what share of the oxygen is carried dissolved in plasma?

   Answer: About 2%

   Dissolved O₂ is only 0.003 × 95 ≈ 0.3 mL/dL, against about 19.7 mL/dL bound to Hb. 7% and 23% are the CO₂ figures (dissolved and carbamino). Revise oxygen transport

2. One gram of fully saturated hemoglobin binds about how much oxygen?

   Answer: 1.34 mL

   1.34 mL/g. With Hb at 15 g/dL that makes an O₂ capacity of about 20 mL per 100 mL of blood; 20 mL is per decilitre of blood, not per gram. Revise oxygen transport

3. The normal P50 of adult hemoglobin at sea level is about:

   Answer: 27 mmHg

   P50, the PO₂ at 50% saturation, is 27 mmHg. At 40 mmHg (mixed venous) Hb is about 75% saturated; at 60 mmHg, where the plateau begins, about 90%. Revise oxygen–hemoglobin dissociation curve

4. Which of these shifts the O₂–Hb dissociation curve to the LEFT?

   Answer: Fetal hemoglobin

   The γ chains of HbF bind 2,3-DPG poorly, so HbF holds O₂ more tightly, and that lets the fetus draw O₂ from maternal blood. The other three all shift the curve right. Revise oxygen–hemoglobin dissociation curve

5. The Bohr effect is:

   Answer: The fall in hemoglobin's O₂ affinity caused by a rise in CO₂ and H⁺

   Bohr: CO₂ and H⁺ push O₂ off Hb in the tissues (right shift). Oxygenation lowering CO₂ content is the Haldane effect; the Cl⁻/HCO₃⁻ swap is the chloride shift; and 2,3-DPG lowers affinity, it does not raise it. Revise carbon dioxide transport

6. Most of the CO₂ in blood is carried as:

   Answer: Bicarbonate

   About 70% travels as bicarbonate, 23% as carbamino compounds and 7% dissolved. Carbonic acid is only a passing intermediate. Revise carbon dioxide transport

7. The chloride shift across the red-cell membrane is carried out by:

   Answer: Anion exchanger 1 (band 3 protein)

   AE1, the old band 3 protein, swaps HCO₃⁻ out of the cell for Cl⁻ in. Carbonic anhydrase makes the bicarbonate but does not move it. Revise carbon dioxide transport

8. In carbon monoxide poisoning, which of these stays normal?

   Answer: Arterial PO₂

   CO does not stop O₂ from dissolving in plasma, so PaO₂ is normal, which is why the chemoreceptors raise no alarm. O₂ content and HbO₂ fall, and the curve shifts left, so P50 falls. Revise oxygen–hemoglobin dissociation curve

9. The plateau of the O₂–Hb dissociation curve means that:

   Answer: A fall in PaO₂ from 95 to 60 mmHg lowers Hb saturation only slightly

   At 60 mmHg Hb is still about 90% saturated, so the O₂ carried falls only 5–10%: a safety margin. Since Hb is already about 97% full, extra O₂ adds little in a healthy person. Revise oxygen–hemoglobin dissociation curve

10. The Haldane effect:

    Answer: Helps blood give up CO₂ in the lungs as hemoglobin takes up O₂

    Haldane: oxygenated Hb holds less CO₂ and H⁺, so taking up O₂ in the lungs drives CO₂ out. CO₂ pushing O₂ off Hb in the tissues is the Bohr effect. Revise carbon dioxide transport

11. Compared with hemoglobin, the O₂ dissociation curve of myoglobin is:

    Answer: Hyperbolic, and shifted left

    Myoglobin binds a single O₂, so there is no cooperative binding and the curve is hyperbolic. Its high affinity puts it far to the left: it stores O₂ and releases it only at very low PO₂. Revise oxygen–hemoglobin dissociation curve

12. The hematocrit of venous blood is about 3% higher than that of arterial blood because:

    Answer: Venous red cells take up Cl⁻ and water, and swell

    The chloride shift leaves venous red cells with about 20% more Cl⁻. Cl⁻ is osmotically active, so water follows, the cells swell and the hematocrit rises. Revise carbon dioxide transport

13. A patient with Hb 7 g/dL has a normal PaO₂ and a normal SaO₂. Which of these is low?

    Answer: Arterial O₂ content

    Saturation is a percentage of whatever Hb there is, so it stays normal, while content in mL of O₂ per dL falls with the Hb. That is why content matters in anemia. (P50, if anything, rises in anemia as 2,3-DPG goes up.) Revise oxygen transport

14. Blood kept for some days in a blood bank delivers O₂ poorly just after transfusion, mainly because its red cells have:

    Answer: Less 2,3-DPG

    Stored red cells lose 2,3-DPG, which shifts the curve left: Hb holds on to its O₂ and gives less to the tissues. Storage in CPD loses less than storage in ACD. Revise oxygen–hemoglobin dissociation curve

15. A pulse oximeter estimates arterial O₂ saturation by measuring:

    Answer: The pulsatile part of red and infrared light absorbance

    Oxy- and deoxy-Hb absorb red and infrared light differently. Using only the pulsing part of the signal picks out arterial blood from tissue, capillary and venous blood. Revise oxygen transport

15 questions on chapter 108. Pick one answer for each, then save. Your answers stay in this browser, and the right ones are shown at the top of the next quiz.

1. The central pattern generator that sets the basic breathing rhythm is located in the:

Bötzinger complex of the medullaNucleus tractus solitarius of the medullaPre-Bötzinger complex of the medullaKölliker–Fuse nucleus of the pons

2. The dorsal respiratory group (DRG) of the medulla:

Lies around the nucleus ambiguus and is mainly expiratoryLies around the nucleus tractus solitarius and is mainly inspiratoryLies in the rostral pons and switches inspiration offLies in the caudal pons and prolongs inspiration

3. The pneumotaxic center is made up of the:

Nucleus parabrachialis medialis and Kölliker–Fuse nucleusNucleus retroambiguus and nucleus para-ambiguusNucleus tractus solitarius and nucleus ceruleusPre-Bötzinger complex and lateral reticular nucleus

4. In an animal, the brainstem is cut in the upper pons, just below the pneumotaxic center, and both vagi are then cut. Breathing becomes:

Completely arrestedUnchanged from normalProlonged inspiratory gasps with brief expirations (apneusis)Rhythmic but irregular

5. Loss of vagal feedback from the lungs changes the phrenic nerve discharge in each breath by:

Shortening it and making it smallerMaking it larger without changing its lengthProlonging it without changing its sizeAbolishing its rhythm altogether

6. Blood flow through the carotid body is about:

2 L per 100 g per min, about 4 times that of the brain2 L per 100 g per min, about 40 times that of the brain20 mL per 100 g per min, about half that of the brain200 mL per 100 g per min, about 4 times that of the brain

7. Ventilation rises only a little as arterial PO₂ falls from 100 to 60 mmHg. One reason is that:

The extra breathing lowers PCO₂, which damps the driveHypoxia makes the CO₂ response curve less steepOxyhemoglobin is a weaker acid than reduced hemoglobinThe peripheral chemoreceptors do not fire above 60 mmHg

8. A patient with carbon monoxide poisoning has a much reduced arterial O₂ content but a normal PaO₂. His breathing is barely stimulated because:

Carbon monoxide raises the pH of the cerebrospinal fluidThe peripheral chemoreceptors respond to PaO₂, not to O₂ contentThe central chemoreceptors are the main sensors of hypoxiaCarbon monoxide blocks the glossopharyngeal nerve endings

9. The central chemoreceptors respond quickly to a rise in arterial PCO₂ but hardly at all to a metabolic fall in blood pH because:

The central chemoreceptors respond to CO₂ itself, not to H⁺H⁺ crosses into the CSF easily, while CO₂ enters the brain very slowlyCSF holds more protein buffer than plasma doesCO₂ crosses into the CSF easily, while H⁺ enters the brain very slowly

10. A man with long-standing COPD has a raised PaCO₂ and a low PaO₂. He is given a high concentration of O₂ alone and becomes drowsy. The most likely reason is that:

Removing his hypoxic drive has made him hypoventilate and retain more CO₂Oxygen has directly stimulated his central chemoreceptorsHis CSF bicarbonate has fallen suddenly, lowering CSF pHOxygen has set off the Hering–Breuer reflex and shortened each breath

11. The Hering–Breuer inflation reflex:

Is set off by rapidly adapting receptors and causes a gaspIs set off by slowly adapting stretch receptors and lengthens expirationIs carried by J receptors reached from the pulmonary capillariesControls tidal volume in awake adults during quiet breathing

12. Juxtapulmonary capillary (J) receptors are:

Unmyelinated endings beside the alveoli, most sensitive to edema and congestionMyelinated endings in airway smooth muscle, most sensitive to lung stretchMyelinated endings in the large airways, most sensitive to smoke and dustUnmyelinated endings in the bronchial walls, most sensitive to chemicals

13. After a heart–lung transplant, which of these is lost?

The Hering–Breuer reflexCough on stimulating the tracheaThe resting pattern of breathingNormal yawns and sighs

14. During swallowing, breathing is briefly inhibited and the glottis closes. This protective response is carried by afferents from the:

Carotid and aortic bodiesViscera and diaphragmJuxtapulmonary capillary receptorsIntercostal muscle spindles

15. An obese man snores loudly, wakes many times a night, and has morning headaches and daytime sleepiness. The basic fault is most likely:

Excess firing of J receptors from congestion of the lungsLoss of pharyngeal muscle tone in REM sleep, blocking the airway in inspirationReduced output from the medullary centers to the phrenic motor neuronsAn overactive Hering–Breuer reflex that ends each inspiration early

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