The Respiratory System and Gas Exchange

Fundamental Functions of the Respiratory System

• Gas Concentration Control: The primary job of the respiratory system is to control the concentrations of gases in the blood, specifically Oxygen ($O_2$) and Carbon Dioxide ($CO_2$).

• Acid-Base Balance: The system acts as a major component in controlling the hydrogen ion ($H^+$) concentration in the blood, which determines pH.

• The Respiratory Membrane: This is the interface where gas diffusion occurs between the air in the alveolus and the blood in the capillary.

Pulmonary Ventilation and Mechanics

• Definition: Pulmonary ventilation is the clinical term for breathing, consisting of two phases: inhalation (moving air in) and exhalation (moving air out).

• Pressure Relationships:

  • Atmospheric Pressure: The pressure of the air outside the body.

  • Intra-alveolar (Intrapulmonary) Pressure: The pressure inside the lungs (specifically the alveoli).

  • Intrapleural Pressure: The pressure within the potential space between the visceral pleura and the parietal pleura.

  • Physics of the "Bottle" Model: The thoracic cavity acts like a jar or bottle. The visceral and parietal pleura are "vacuumed" together, making intrapleural pressure negative. When the volume of the "bottle" (chest cavity) increases, the pressure drops below atmospheric levels, causing air to rush in. When the volume decreases, internal pressure exceeds atmospheric pressure, and air rushes out.

• Muscles of Respiration:

  • Diaphragm: The primary muscle that serves as the "floor" of the thoracic bottle.

  • External Intercostals: Muscles that swing the ribs out to increase volume.

  • Internal Intercostals: Muscles that swing the ribs back in.

  • Accessory Muscles: Various muscles that pull or push on the ribs during forced breathing.

Resistance and Compliance Factors

• Boyle’s Law: Illustrated by the metaphor of three-year-old boys in a room; as the volume of the container decreases, the molecules (boys) bounce off the walls more frequently, increasing pressure.

• Airway Resistance: Flow through the respiratory passageways can be enhanced or constricted. The diameter of the bronchioles is the primary site of resistance. Since bronchioles lack cartilage, their diameter is controlled by smooth muscle regulated by the autonomic nervous system.

• Alveolar Surface Tension: Without surfactant, the surface tension of water in the alveoli would cause them to snap closed (collapse).

• Lung Compliance: Refers to the "stretchiness" of the lungs. This is maintained by elastic fibers around the alveoli and is affected by various pathologies:

  • Chest Wall Deformities: Restrict the expansion of the "bottle."

  • Lung Cancer/Fibrosis/Scarring: These conditions decrease the stretchiness of the lung tissue.

  • Emphysema: Characterized by the destruction of alveoli and capillaries. While it initially may increase compliance (loss of elastic recoil), it ultimately destroys the gas exchange surface area, creating large, useless air spaces.

• Pneumothorax: Occurs when the seal between the visceral and parietal pleura is broken, causing the lung to collapse. Treatment typically requires a chest tube attached to a vacuum to remove air and reseal the membranes.

Respiratory Volumes and Capacities

• Standard Values (Must Know):

  • Tidal Volume ($V_T$): A normal, calm breath, approximately $500\,mL$.

  • Total Lung Capacity ($TLC$): The maximum amount of air the lungs can hold, typically $5,000\,mL$ to $6,000\,mL$ depending on body size.

• Definitions of Volumes:

  • Inspiratory Reserve Volume ($IRV$): The extra air taken in during forced inspiration.

  • Expiratory Reserve Volume ($ERV$): The extra air pushed out during forced expiration (e.g., playing a brass instrument like a tuba or trumpet).

  • Vital Capacity ($VC$): The total amount of exchangeable air ($V_T + IRV + ERV$).

  • Residual Volume ($RV$): The air remaining in the lungs after the most forceful expiration, approximately $1,200\,mL$. This prevents lung collapse.

• Dead Space:

  • Anatomical Dead Space: About $150\,mL$ of air that stays in the conducting zone (passageways) and never reaches the alveoli.

  • Physiological Dead Space: Occurs when there is a mismatch between air in the alveoli and blood in the capillaries (ventilation-perfusion mismatch).

Gas Laws and Exchange

• Dalton’s Law of Partial Pressures: The total pressure in a mixture of gases is the sum of the pressures of each individual gas.

  • Metaphor: If you have a room filled with three-year-old boys, three-year-old girls, and puppies, the total pressure on the walls is the combined force of each group bouncing off the walls. Each group provides a "partial pressure."

• Composition of Air (at Sea Level):

  • Oxygen ($O_2$): Approximately $21\%$.

  • Nitrogen ($N_2$): The majority of the air. It is ignored in standard physiology because it stays constant, except in diving contexts (where it can cause the "bends").

  • Other components: Carbon dioxide ($CO_2$), water vapor (humidity), and pollutants.

• Measurements: Gas pressures are measured in millimeters of mercury ($mm\,Hg$). Oxygen at $21\%$ in the atmosphere translates to a partial pressure ($P_{O_2}$) of approximately $160\,mm\,Hg$ ($0.21 \times 760\,mm\,Hg$), but it drops when it reaches the alveoli.

• Types of Respiration:

  • External Respiration: Gas exchange between the alveoli and the blood in the pulmonary capillaries.

  • Internal Respiration: Gas exchange between the blood in systemic capillaries and the tissue spaces.

  • Cellular Respiration: Exchange of gases into the cell and eventually to the mitochondria.

Partial Pressure Gradients

• External Respiration (Lungs):

  • In Alveolus: $P_{O_2} = 100\,mm\,Hg$; $P_{CO_2} = 40\,mm\,Hg$.

  • In Deoxygenated Blood entering Lungs: $P_{O_2} = 40\,mm\,Hg$; $P_{CO_2} = 45\,mm\,Hg$.

  • Diffusion: $O_2$ moves from alveolus to blood (steeper gradient). $CO_2$ moves from blood to lung.

• Internal Respiration (Tissues):

  • In Oxygenated Blood entering Tissues: $P_{O_2} \approx 100\,mm\,Hg$; $P_{CO_2} \approx 40\,mm\,Hg$.

  • In Tissue Spaces: $P_{O_2} \approx 40\,mm\,Hg$; $P_{CO_2} \approx 45\,mm\,Hg$.

  • Diffusion: $O_2$ moves into the tissues; $CO_2$ moves into the blood.

Gas Transport in Blood

• Erythrocytes (Red Blood Cells): These are essentially "bags of hemoglobin." They lack a nucleus and organelles to maximize space for gas transport.

• Hemoglobin Structure: Consists of four protein subunits. Each subunit has a heme group with an iron ion ($Fe^{2+}$) at its center. One hemoglobin molecule can carry four oxygen molecules.

• Limo Service Metaphor: Hemoglobin is like a car with four seats. Oxygen molecules sit in the seats. Carbon dioxide rides on the "roof rack" (different binding site) or in the "water" (plasma).

• Oxygen-Hemoglobin Dissociation Curve:

  • This S-shaped graph shows the relationship between $P_{O_2}$ and hemoglobin saturation.

  • In the lungs ($P_{O_2} = 100\,mm\,Hg$), saturation is nearly $100\%$.

  • In the tissues ($P_{O_2} \approx 40\,mm\,Hg$), hemoglobin "bails" or unloads much of its oxygen.

  • Carbon Monoxide ($CO$) Poisoning: $CO$ is a "terrorist" that sits in the seat and won't get out, keeping its butt in the car and preventing $O_2$ from binding. Because it does not dissociate, it eventually leaves no seats for oxygen, leading to death.

• Shifting the Curve (Affinity Modifications):

  • Right Shift (Decreased Affinity): Oxygen unloads more easily. Caused by increased temperature, increased hydrogen ions ($H^+$/acidity), or increased $2,3\text{-DPG}$. Tissues that are metabolically active (hot/acidic) get more oxygen.

  • Left Shift (Increased Affinity): Oxygen clings more tightly. Caused by decreased temperature, decreased $H^+$, or decreased $2,3\text{-DPG}$.

Carbon Dioxide Chemistry and Transport

• Transport Methods: Carbon dioxide is transported dissolved in plasma, bound to hemoglobin (carbaminohemoglobin), or as bicarbonate ions ($HCO_3^-$).

• The Carbonic Acid Reaction:

  • Equation: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

  • Carbonic Anhydrase: The enzyme that catalyzes the joining of $CO_2$ and water. The instructor compares it to a "glove" with slots for $CO_2$ and water to sit in.

  • Physiological Importance: Deeply linked to pH. High $CO_2$ leads to more $H^+$, making the blood acidic. Breathing allows the body to "blow off" $CO_2$, thereby removing $H^+$ and raising pH.

Neural Control of Ventilation

• Primary Brain Centers:

  • Medulla Oblongata: Contains the Dorsal Respiratory Group (DRG), which houses inspiratory neurons. These set the resting rate of $12$ to $20$ breaths per minute.

  • Pons: Contains the Pneumotaxic Center, which inhibits inspiration to prevent over-inflation of the lungs (volume control).

• Receptor Systems:

  • Peripheral Chemoreceptors: Located in the carotid sinus and the aortic arch. They monitor $O_2$, $CO_2$, and $H^+$.

  • Central Chemoreceptors: Located in the brainstem. They primarily monitor $CO_2$ and $H^+$/pH.

  • Brain's Priority: The brain is more sensitive to $CO_2$ levels than $O_2$ levels. You breathe primarily to get rid of $CO_2$.

• Higher Brain Influence:

  • Cerebral Cortex: Allows for voluntary control (e.g., holding your breath during a tantrum). However, if a child passes out, the brainstem takes back over automatically.

  • Limbic System: Fear, pain, and anxiety drive the respiratory rate up.

  • Hypothalamus: Controls body temperature; if the thermostat is turned up, the respiratory rate increases.

Clinical Pathologies and Scenarios

• Acute Mountain Sickness (AMS): Occurs at altitudes over $12,000\,feet$. Lower air pressure makes it hard to get enough oxygen (hypoxia). Can lead to cerebral edema (brain swelling) or pulmonary edema.

• Hyperventilation: Breathing too fast/deep, blowing off too much $CO_2$, causing pH to rise. Accompanied by anxiety and finger cramping. Treatment: Breathe into a paper bag to re-inhale $CO_2$.

  • Drowning Risk: Hyperventilating before swimming underwater is dangerous. It tricks the brain into thinking it has time ($CO_2$ is low), but the body runs out of $O_2$ and the swimmer passes out and drowns.

• Hypoventilation and Apnea:

  • Preemies: Their brainstems may not be mature enough, leading to apnea (stoppage of breathing). Monitored by apnea alarms.

  • Depressants: Alcohol, narcotics, benzodiazepines, and barbiturates can put the brainstem to sleep, causing a person to stop breathing.

  • Sleep Apnea: Often caused by upper airway abnormalities or weight. Treated by CPAP (Continuous Positive Airway Pressure), which keeps the airway splinted open with forced air.

Upcoming Tasks

• Monday: Quiz over the Respiratory System.

• Wednesday: Start the Digestive System and conduct the Midterm Lab Exam.