Human Physiology Chapter 16: Respiratory Physiology

Flashcards covering human respiratory physiology, including functional anatomy, mechanics of ventilation, lung volumes, gas exchange, chemoreceptor regulation, hemoglobin dynamics, and blood acid-base balance.

What are the four primary steps of external and internal respiration?

1. Ventilation (gas exchange between the atmosphere and air in the lungs)
2. Diffusion of gases across pulmonary capillaries
3. Transport of gases in the blood
4. Diffusion of gases across systemic capillaries to tissues (and cellular respiration/oxygen utilization to make $ATP$)

What anatomical structures make up the conducting zone and the respiratory zone of the respiratory system?

The conducting zone consists of the trachea, primary bronchus, bronchial tree, and terminal bronchioles ($60,000$ branches). The respiratory zone consists of respiratory bronchioles ($500,000$ branches), alveolar sacs ($8\text{ million}$), and alveoli.

What are the primary functions of the conducting zone?

It transports air to the lungs; warms, humidifies, filters, and cleans air using mucus and the mucociliary escalator; and produces voice in the larynx as air passes over vocal folds.

What are the structural features and total surface area of the alveoli?

Alveoli are very small, thin-walled inflatable sacs that are one-cell layer thick. There are approximately $300\text{ million}$ per lung in human adults, providing a large surface area of $760\,ft^2$ to facilitate high rates of gas diffusion.

What are the distinct functions of Type I and Type II alveolar cells?

Type I alveolar cells cover $95\text{--}97\%$ of the total surface area and are the site where gas exchange occurs. Type II alveolar cells secrete pulmonary surfactant and reabsorb sodium and water to prevent fluid buildup.

Identify the pleural membranes and spaces shown in this transverse section of the thoracic cavity.

The parietal pleura lines the thoracic wall, the visceral pleura covers the lungs, and between them lies the intrapleural space (pleural cavity). The central mediastinum contains the heart, trachea, esophagus, and thymus.

Define compliance, elasticity, and surface tension in relation to lung physical properties.

• Compliance: The ability of the lungs to expand when stretched.
• Elasticity: The ability of the lungs to recoil after being stretched.
• Surface tension: The force that resists distension and ballooning of the alveoli.

According to Boyle's Law, how does changing lung volume drive ventilation?

Pressure is inversely proportional to volume ($P \propto \frac{1}{V}$). An increase in lung volume decreases intrapulmonary pressure below atmospheric pressure, causing air to flow in; a decrease in lung volume increases intrapulmonary pressure above atmospheric pressure, causing air to flow out.

What is transpulmonary pressure and what is its physiological role?

Transpulmonary pressure is the pressure difference between intrapulmonary pressure and intrapleural pressure ($IP\,Pr - Intrapleural\,Pr$). It keeps the lungs held against the thoracic wall and allows them to expand during inspiration.

How do intrapulmonary and intrapleural pressures change during rest, inspiration, and expiration?

At rest: intrapulmonary pressure is $0\,cmH_2O$ (same as atmospheric) and intrapleural pressure is $-5\,cmH_2O$. During inspiration: intrapulmonary pressure drops to $-1\,cmH_2O$ and intrapleural pressure drops to $-8\,cmH_2O$. During expiration: intrapulmonary pressure rises to $+1\,cmH_2O$ and intrapleural pressure returns to $-5\,cmH_2O$.

Which muscles contract during quiet inspiration versus forced expiration?

• Quiet inspiration (active): Diaphragm and external intercostals.
• Forced expiration: Internal intercostals and abdominal muscles (rectus abdominis, external abdominal oblique, internal abdominal oblique, transversus abdominis).

On a spirogram, what are the four nonoverlapping lung volumes?

1. Tidal volume ($TV$): Gas inspired or expired in an unforced cycle (~$500\,mL$).
2. Inspiratory reserve volume ($IRV$): Maximum gas inspired during forced breathing in addition to tidal volume.
3. Expiratory reserve volume ($ERV$): Maximum gas expired during forced breathing in addition to tidal volume.
4. Residual volume ($RV$): Gas remaining in lungs after a maximum expiration.

Define vital capacity, total lung capacity, inspiratory capacity, and functional residual capacity.

• Vital capacity ($VC$): Maximum gas expired after maximum inspiration ($TV + IRV + ERV$).
• Total lung capacity ($TLC$): Total gas in lungs after maximum inspiration ($VC + RV$).
• Inspiratory capacity ($IC$): Maximum gas inspired after normal tidal expiration ($TV + IRV$).
• Functional residual capacity ($FRC$): Gas remaining in lungs after normal tidal expiration ($ERV + RV$).

What is the One-second forced expiratory volume ($FEV_1$) test, and what diagnostic threshold indicates obstructive pulmonary disease?

The $FEV_1$ test measures the percentage of vital capacity exhaled in the first second of forced expiration. An $FEV_1 < 80\%$ indicates obstructive pulmonary disease (such as asthma, emphysema, or COPD).

What are the partial pressures of $O_2$ and $CO_2$ in atmospheric dry air versus alveolar air?

• Atmospheric air ($760\,mmHg$): $PO_2 = 159\,mmHg$ (~$21\%$) and $PCO_2 = 0.29\,mmHg$ / $0.3\,mmHg$ (~$0.04\%$).
• Alveolar air: $PO_2 = 105\,mmHg$, $PCO_2 = 40\,mmHg$, $PH_2O = 47\,mmHg$, and $PN_2 = 568\,mmHg$.

What does a blood gas electrode measure when analyzing arterial $PO_2$?

It measures only oxygen dissolved in the blood plasma; it does not measure oxygen bound to hemoglobin inside red blood cells.

How does pulmonary blood flow and vascular resistance compare to the systemic circulation?

Blood flow rate through the lungs equals systemic cardiac output ($5.5\,L/min$), but the pulmonary pressure gradient is only $10\,mmHg$ (compared to $100\,mmHg$ systemically). Pulmonary vascular resistance must be very low, which reduces the possibility of pulmonary edema.

How do pulmonary arterioles uniquely respond to low alveolar oxygen levels, and what is the cellular mechanism?

Pulmonary arterioles constrict in response to low alveolar $PO_2$ (hypoxic vasoconstriction) to match ventilation to perfusion. Mechanistically, low oxygen depolarizes smooth muscle cells by inhibiting outward $K^+$ flow, opening voltage-gated $Ca^{2+}$ channels and causing contraction.

What are the locations and functions of the brain stem respiratory centers?

• Medulla oblongata: Rhythmicity center containing inspiratory ($I$) neurons that stimulate the phrenic nerve and expiratory ($E$) neurons that inhibit it.
• Pons: Apneustic center promotes inspiration, while the pneumotaxic center inhibits inspiration.

How do central chemoreceptors in the medulla detect changes in arterial blood $PCO_2$?

$H^+$ cannot cross the blood-brain barrier, but $CO_2$ readily diffuses into the cerebrospinal fluid ($CSF$). In the $CSF$, $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$, and the resulting decrease in $CSF$/interstitial pH directly stimulates the central chemoreceptor neurons.

Under what condition does arterial $PO_2$ directly stimulate peripheral chemoreceptors to increase ventilation?

Only when arterial $PO_2$ falls below $70\,mmHg$ (hypoxemia) does low oxygen directly stimulate the carotid bodies to increase ventilation.

Describe the quaternary structure of hemoglobin and its maximum oxygen capacity.

Hemoglobin consists of $4$ polypeptide globin chains ($2$ alpha and $2$ beta chains) and $4$ iron-containing heme groups ($Fe^{2+}$). Each hemoglobin molecule can carry $4$ molecules of $O_2$. With $280\text{ million}$ hemoglobin molecules per $RBC$, each red blood cell can carry over $1\text{ billion}$ oxygen molecules.

What distinguishes methemoglobin and carboxyhemoglobin from normal oxyhemoglobin?

• Oxyhemoglobin: Iron is in reduced form ($Fe^{2+}$) and reversibly binds $O_2$.
• Methemoglobin: Iron is oxidized to ferric form ($Fe^{3+}$) and cannot bind $O_2$.
• Carboxyhemoglobin: Hemoglobin is bound to carbon monoxide ($CO$), which binds with much higher affinity than $O_2$.

How does blood pH affect the oxyhemoglobin dissociation curve (Bohr effect)?

A decrease in pH (acidosis, e.g., pH $7.20$) shifts the curve to the right, decreasing hemoglobin's affinity for $O_2$ and promoting oxygen unloading at tissues. An increase in pH (alkalosis, e.g., pH $7.60$) shifts the curve to the left, increasing oxygen binding affinity.

What are the four primary factors that promote oxygen unloading (shift curve right) in active tissues?

1. Decreased $PO_2$
2. Decreased pH / increased $H^+$
3. Increased $PCO_2$
4. Increased temperature and increased $2,3\text{-DPG}$ (diphosphoglyceric acid)

In what three forms is carbon dioxide transported in the blood, and what are their relative percentages?

1. Dissolved gas in plasma: $7\text{--}10\%$
2. Carbaminohemoglobin (bound to globin protein of $Hb$): $20\text{--}23\%$
3. Dissolved in plasma as bicarbonate ($HCO_3^-$): $70\%$

Describe the mechanism of the chloride shift occurring in systemic capillaries.

$CO_2$ released from tissues enters $RBCs$, where carbonic anhydrase converts it to $H_2CO_3$, dissociating into $H^+$ and $HCO_3^-$. $H^+$ binds to hemoglobin (facilitating $O_2$ release via the Bohr effect). $HCO_3^-$ exits the $RBC$ into the plasma via an antiporter while $Cl^-$ moves into the cell.

Describe the reverse chloride shift occurring in pulmonary capillaries.

$O_2$ binding to hemoglobin displaces $H^+$. Bicarbonate ($HCO_3^-$) re-enters the $RBC$ while chloride ($Cl^-$) exits into plasma. Carbonic anhydrase converts $HCO_3^- + H^+ \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O$, and $CO_2$ diffuses into alveoli to be exhaled.

What is normal blood pH, and what defines respiratory acidosis versus respiratory alkalosis?

Normal body fluid pH is $7.40$ (range $7.35\text{--}7.45$).

• Respiratory acidosis ($pH < 7.35$): Caused by hypoventilation leading to elevated arterial $PCO_2$ and increased carbonic acid.
• Respiratory alkalosis ($pH > 7.45$): Caused by hyperventilation leading to excessively low arterial $PCO_2$.