• Some functions of the respiratory system:

• Exchange of O₂ and CO₂ between lungs and blood.

• Humidification and warming of inspired air.

• Filtration and cleaning of inspired air.

• Enhancement of venous return via the respiratory pump.

• Alteration of acid-base balance by affecting amount of CO₂ exhaled.

• Physical Aspects of Ventilation (Breathing) and Mechanics of Breathing

• Air moves down a pressure gradient from a region of high pressure to a region of low pressure due to changes in lung volumes.

• Relevant pressures:

• Atmospheric - Pressure exerted by gases in the atmosphere.

• Intrapulmonary (intra-alveolar) - Pressure within the alveoli.

• Intrapleural - Pressure within the pleural cavity.

• Transpulmonary (Transmural) - Pressure difference across the lung wall.  It is equal to intrapulmonary minus intrapleural pressure.  Keeps lungs against the chest wall during respiratory movements.

• If chest wall gets punctured and changes the pressure difference across the chest wall, then a pneumothorax can result.  This can cause the lung to collapse away from the chest wall and atelectasis occurs.  See clinical box on page 518.

• Pulmonary ventilation requires inspiration (inhalation) = breathing in and expiration (exhalation) = breathing out.

• For air to flow into the lungs, intrapulmonary pressure must be lower than atmospheric pressure.  This follows from Boyle's Law in which pressure decreases as the volume (of the chest cavity) increases.

• Therefore, lung expansion is not caused by movement of air into the lungs; it's due to the decrease in pressure that allows air to flow into the lungs.

• The lungs exhibit: 1) compliance (a measure of the ease at which the lung expands under pressure) and 2) elasticity (the tendency of a structure to return to its initial size after being stretched).

• To accomplish lung expansion, the diaphragm and external intercostals contract at the onset of inspiration, resulting in the enlargement of the thoracic cavity.

• Diaphragm descends downward.

• External intercostals elevate the ribs and subsequently the sternum upward and outward.

• Deeper inspirations require more forceful contractions of the diaphragm and external intercostals and the involvement of some neck and thoracic muscles.

• Inspiration is always an active process; it requires contraction of inspiratory muscles and energy utilization.

• During expiration

• Diaphragm and external intercostals relax.

• Lungs recoil due to their elasticity.

• Intrapulmonary pressure increases and air leaves the lungs down its partial pressure gradient.

• Expiration is considered a passive process during quiet breathing since it is accomplished by elastic recoil of the lungs on relaxation of the inspiratory muscles, with no muscular exertion or energy expenditure required.

• Forced expiration (now an active event) requires contraction of the expiratory muscles (the internal intercostals and, importantly, the abdominal muscles).

• Gases move down partial pressure gradients.

• Partial pressure - The pressure exerted by a gas within a mixture of gases.

• Atmospheric pressure at sea level is = 760 mmHg and decreases as one moves to higher altitudes.

• Dalton’s Law - The total pressure of a gas mixture is equal to the sum of the pressures that each gas independently exerts.

• O₂ constitutes 21% and N₂ 78% of the atmospheric gases.

• O₂ and CO₂ enter and leave the blood down partial pressure gradients.  See figure 16.23 on page 529.

• Rate of gas transfer between blood and alveoli is dependent on such factors as the:

• Partial pressure gradient of gases

• Solubility of gas (For example, CO₂ is 20X more soluble than O₂ in body tissues and can diffuse 20X more rapidly.)

• Temperature of solution (more gas can be dissolved in cold water than in warm water)

• Surface area of alveoli - Normally constant, but can change under abnormal physiological conditions such as emphysema.

• Thickness of barrier separating air and blood across alveolar membrane - Normally constant, but can change under abnormal physiological conditions such as pulmonary edema and pneumonia.

• In addition to proper ventilation of the lungs, blood flow (perfusion) in the lungs must be adequate and matched to air flow (ventilation) in order for adequate gas exchange to occur.  This is sometimes referred to as ventilation-perfusion matching.

• Most O₂ (98-99%) in blood is bound to hemoglobin (Hb) in RBC's. The rest is dissolved in the plasma.

• Adult hemoglobin contains 4 polypeptide chains (2 alpha and 2 beta), each with a heme group that contains a central Fe²⁺ atom.  See Fig. 16.33 on page 537.

• Fetal Hb has 2 alpha and 2 gamma chains.

• Each heme binds 1 O₂ molecule. Therefore, each Hb molecule binds 4 O₂ molecules.

• Oxyhemoglobin refers to when oxygen is bound and deoxyhemoglobin refers to when oxygen is unbound.

• Methemoglobin - Hb in which the iron atom is in the Fe³⁺ form.  It cannot bind O₂ in that form.

• DeoxyHb + O₂ OxyHb      Loading and Unloading Reactions

• Which way the reaction proceeds depends on the partial pressure of oxygen and the affinity (bond strength) between Hb and oxygen.

• High P_O2 favors the loading reaction in lungs.

• Low P_O2 favors the unloading reaction in the tissues.

• About 97% of Hb leaving the lungs is oxyHb.

• About 22% of O₂  is unloaded to the tissues at rest while more is unloaded during exercise.

• Oxyhemoglobin Dissociation (Saturation) Curve - See Fig. 16.34 on page 538.

• Curve is sigmoidal (S-shaped).  Small changes in P_O2 produce large differences in % saturation on the steep part of the curve.

• Hb exhibits subunit cooperativity in which the binding of 1 O₂ facilitates the binding of subsequent O₂'s.

• Several factors shift the curve to the right to decrease affinity.  These include:

• Increased H⁺ (decreased pH) = Bohr effect

• Increased P_CO2

• Increased temperature (during increased metabolism and fever)

• Increased 2,3-DPG

• The product of a side reaction in RBC's.

• Inhibited by oxyHb.

• Increased during anemia or at high altitudes when P_O2 is low.

• Carbon monoxide (CO) has a much greater affinity than oxygen for binding to Hb = carboxyhemoglobin.  This shifts the curve to the left.

• Myoglobin

• Found in striated muscle (skeletal and cardiac).

• Has 1 heme; binds 1 O₂ molecule.

• Doesn't unload O₂ until the P_O2 gets very low.  See Fig. 16.38 on page 542.

• CO₂ is carried by the blood in 3 forms: (see Figs. 16.39 and 16.40)

• Dissolved in plasma = ~ 10% of total

• Bound to Hb (carbaminohemoglobin) = ~ 20% of total

• As bicarbonate (HCO_3-) = ~ 70% of total

• Within the RBC:

• Reverse chloride shift occurs in pulmonary capillaries.

• Normal arterial blood pH (a measure of H⁺ concentration) averages ~ 7.40 with a range of 7.35-7.45.

• Acidosis - Decreased pH (< 7.35) relative to the normal pH.

• Alkalosis - Increased pH (> 7.45) relative to the normal pH.

• Elimination of acids is accomplished by:

• Lungs - Via exhalation of CO₂.

• Kidneys - Via excretion of H⁺.

• Ventilation is normally adjusted to keep pace with metabolism so that arterial P_CO2 remains in the normal range.

• Hypoventilation, resulting from abnormal retention of CO₂, can cause respiratory acidosis.

• Hyperventilation, resulting from excessive loss of CO_2, can cause respiratory alkalosis.

• Why do individuals who are hyperventilating breathe into a paper bag?  _______________________________________________

• Respiratory centers in the brain stem establish a rhythmic (automatic) breathing pattern.

• Since respiratory muscles are skeletal muscles, they require nervous stimulation to contract.

• Primary respiratory rhythmicity center is in the medulla oblongata which controls the diaphragm and intercostal muscles.

• Pons contains the pneumotaxic and apneustic centers which may influence the medullary respiratory center.

• Cerebrum can also influence the medulla.

         An additional level of control is via the Hering-Breuer reflex which prevents overinflation of the lungs when pulmonary stretch receptors sense that the lungs are being overstretched as inspiration occurs.  This results in the inhibition of inspiration.

• The autonomic control of breathing is also affected by chemoreceptors sensitive to the P_CO2, pH, and P_O2 of the blood and/or cerebrospinal fluid (CSF).

• Central chemoreceptors - Located in medulla.

• Peripheral chemoreceptors - Include the aortic bodies (in the aortic arch) and carotid bodies (in the common carotid artery).

• P_CO2 and consequent changes in pH are of greater importance than P_O2 in regulating breathing.

• Decreases in blood P_O2 (hypoxemia) directly stimulate breathing only when the blood P_O2 is < 50 mmHg.

Decreased ventilation
Increased arterial PCO2 (hypercapnia)		Decreased Blood pH	+	Peripheral Chemoreceptors
				+
CO2 diffuses from blood into CSF				Respiratory centers in medulla
				+
CO2 + H2O H2CO3 H+ + HCO3-				Spinal cord motor neurons
				+
Central chemoreceptors				Respiratory Muscles
+				+
Respiratory centers in medulla				Increased ventilation
+
Spinal cord motor neurons				Decreased arterial PCO2
+
Respiratory muscles
+
Increased ventilation
Decreased arterial PCO2

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