MCAT Review

Expiration

Exhalation

Inspiration

Inhalation

How might central and peripheral chemoreceptors compare with regard to their role in the detection of respiratory gases resulting from a prolonged period of hypoventilation?

Central chemoreceptors and peripheral chemoreceptors may both respond to the resultant increase in pCO2, but only peripheral chemoreceptors could respond to the decrease in pO2

If the mouth and nose are closed at the peak of a complete inspiration, but before expiration, and the breath is held, what is the pressure of gases within the alveoli relative to the pressure of atmospheric air?

Alveolar pressure is equal to the pressure of atmospheric air

The partial pressures of carbon dioxide (pCO2) and oxygen (pO2) in the atmosphere at sea level are 0.3 mmHg and 160 mmHg respectively, but the partial pressures of these gases in blood leaving the lungs are 40 mmHg (pCO2) )and 95 mmHg (pO2). What factor most likely accounts for this difference?

CO2 is more soluble in blood than O2 The lungs do not mechanically discriminate between gasses. Both carbon dioxide and oxygen 'penetrate' throughout the entire lung. Hint #22 / 5 Henry's law states that the amount of gas that dissolves in a given liquid is directly proportional to the partial pressure of the gas in equilibrium with that liquid. Hint #33 / 5 If the only factor influencing dissolved gas concentrations is the partial pressure of the gas in the air in contact with the liquid, then we would assume that the ratio of oxygen to carbon dioxide would be equal both within and outside of the liquid. Hint #44 / 5 However, if a given gas were more soluble in a given liquid, a greater proportion of the gas could be dissolved within that liquid. Hint #55 / 5 CO_2CO ​2 ​​ C, O, start subscript, 2, end subscript is more soluble in the blood than O_2O ​2 ​​ O, start subscript, 2, end subscript.

Interstitial lung disease (ILD) refers to a set of conditions which affect the pulmonary interstitium-- the area of tissue and space which lies between the alveoli and alveolar capillaries. What factor in the setting of severe ILD, would NOT decrease the extent to which oxygen passes from the air sacs of the lungs into the blood?

Decreased interstitial thickness There are two ways of answering this question. First, what would most probably be the effects of ILD? Second, what would restrict the ability of gas in the lungs to reach the blood? The second method may be easier. Hint #22 / 6 Fick's law describes the rate of diffusion of particles across a membrane, and considers wall thickness, wall surface area, pressure gradient across the wall, and the diffusion constant of the molecule moving across the membrane. Hint #33 / 6 Increased lung elastic recoil and increased alveolar surface tension are more or less synonymous. Increasing either increases the pressure of the alveolar wall on the gas within the alveolar space. Hint #44 / 6 ILD produces a thick, scarred interstitium, which restricts the ability of the lungs to expand. Therefore, less air is inspired with each breath, and less gas reaches the respiratory zone. Hint #55 / 6 A decreased interstitial thickness, or decreased alveolar/capillary wall thickness, would allow for easier diffusion of gasses between these spaces. Hint #66 / 6 Decreased interstitial thickness would NOT decrease the extent to which oxygen passes from the air sacs of the lungs into the blood.

What produces the force which drives normal exhalation, and is the process active or passive?

Elastic Force, passive Contraction of the diaphragm produces a negative pressure in the space between the lungs and the abdominal wall. Hint #22 / 6 Contraction of the diaphragm drives normal inhalation. This is an active process. Hint #33 / 6 The intercostal muscles elevate the thoracic wall and ribcage, and assist with respiration under conditions of increased work of breathing, such as in disease, intense exercise, or forced respiration. Hint #44 / 6 Normal exhalation is a passive process. Hint #55 / 6 During exhalation, the diaphragm relaxes, and the recoil force of the thoracic wall and elastic force of the alveoli produce a positive pressure change on the air in the lungs. This pressure exceeds that of atmospheric air, and thus air flows outward. Hint #66 / 6 Elastic forces drive normal exhalation, and this is a passive process.

In a situation where the respiratory bronchioles become inflamed and narrowed, such as is seen in asthma, which aspect of respiration would be most mechanically impaired?

Forced expiration Contraction of the diaphragm produces a negative pressure within the intrathoracic cavity. The negative pressure gradient pulls air into the lungs, the lungs expand, and thus the radius of respiratory bronchioles increases during inspiration. Hint #22 / 6 During expiration, the elastic force of the lungs and chest wall induce a positive pressure on the air in the lungs, which forces air out. Hint #33 / 6 The airways within the thoracic cavity (such as the respiratory bronchioles) are also acted upon by this positive pressure, and this force causes their radius to decrease. Hint #44 / 6 If these airways are inflamed and smaller than usual, they would be much more prone to collapsing entirely. In fact, this is precisely what occurs during expiration in asthma; inflamed airways within the thoracic cavity collapse, interfering with expiration and trapping air within the lungs. Hint #55 / 6 A forced expiration generates additional force on the air within the lungs; this pressure is not only a greater magnitude than normal exhalation, but also occurs much more rapidly. Therefore, an inflamed airway would collapse much sooner in this setting, and more air would be trapped. Hint #66 / 6 In a situation where the respiratory bronchioles become inflamed and narrowed, such as is seen in asthma, forced respiration would be most greatly impaired.

Many respiratory diseases affect pulmonary function by altering the ability of alveoli to participate in gas exchange. What physical change would most greatly reduce the degree to which a particular alveolus is ventilated?

Increased alveolar elastic recoil

Septic shock is a serious condition resulting from the body's response to systemic bacterial infections, which may impair oxygen uptake and delivery. What physiological change may result from septic shock which would decrease the ability of hemoglobin in the alveolar capillaries to become fully saturated with oxygen?

Increased capillary blood flow Two 'broad' categories must be considered which may affect hemoglobin saturation: 1. Is there enough oxygen present to saturate the hemoglobin, and 2. physiological factors which affect Hb's oxygen affinity. Hint #22 / 7 An increase in temperature, pCO_2pCO ​2 ​​ p, C, O, start subscript, 2, end subscript, and 2,3-BPG concentration, or a decrease in pH will decrease O_2O ​2 ​​ O, start subscript, 2, end subscript affinity. Hint #33 / 7 If the pO_2pO ​2 ​​ p, O, start subscript, 2, end subscript in the afferent capillary is low (the capillary coming TO the alveoli), concentration gradients will favor greater diffusion of oxygen into the vessel, thereby increasing the amount of oxygen available to bind. Hint #44 / 7 Factors affecting gas diffusion into the capillaries include wall thickness, wall surface area, partial pressure difference, and the ventilation-perfusion ratio. Hint #55 / 7 The longer blood 'hangs around' in the alveolar capillaries, the longer the hemoglobin has to recruit oxygen. If blood is flowing too quickly for ventilation to match it, the hemoglobin saturation will decrease. Hint #66 / 7 Septic shock may produce an increase in heart rate, and a profound decrease in blood pressure due to vasodilation. Hint #77 / 7 Increased capillary blood flow may decrease the ability of hemoglobin in the alveolar capillaries to become fully saturated with oxygen.

Bronchodilators are a class of drug often used in the treatment of asthma and COPD, which act on β-adrenergic receptors of the airways to induce smooth muscle relaxation. The anatomic distribution of these receptors is closely correlated to the function of each structural component of the lungs. What structural component(s) of the airway would be most affected by the use of a bronchodilator, and in what functional zone(s) are they found?

Lobar Bronchi, which are found in the conducting zone The conducting zone is simply a series of tubes through which gases travel, while the respiratory zone directly participates in gas exchange. Hint #22 / 5 The respiratory zone is made up of only the alveoli and the respiratory bronchioles/ alveolar ducts. Hint #33 / 5 The walls of the airways contain smooth muscle, which is the site of action of bronchodilators. Hint #44 / 5 The alveoli are made of only a single layer of cells, and their walls do not contain smooth muscle. Bronchodilators do not directly affect the alveoli. Hint #55 / 5 The lobar bronchi would be most affected by the use of a bronchodilator, and they are found in the conducting zone.

What is the pressure of gas within the alveoli at the peak of inspiration, just before expiration, relative to that of atmospheric air?

The same as atmospheric air The flow of air (and all gases) is determined by pressure gradients. Air flows from areas of higher pressure to lower pressure. Hint #22 / 6 During inspiration, the diaphragm contracts which generates a negative pressure within the alveoli, which draws air into the lungs. Hint #33 / 6 During expiration, the elastic force of the thoracic wall and the alveoli themselves exert a positive pressure on the air within the alveoli. This pressure is greater than that of atmospheric air, and the air flows out of the lungs. Hint #44 / 6 At the peak of inspiration or expiration, the airflow momentarily stops. However, the airway is still open to the atmosphere. Hint #55 / 6 The halt of airflow is because the alveolar pressure is 'zero' relative to the atmosphere. Hint #66 / 6 The pressure within the alveoli at the peak of inspiration is the same as atmospheric air.