29 Patho

Chapter 29: Structure and Function of the Respiratory System

Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. This chapter covers the anatomy, physiology, and mechanics of the respiratory system, including the movement of air, blood flow, and the cellular and molecular mechanisms involved in respiration.

Components of the Respiratory System

Conducting Airways

• Definition: The conducting airways are passages that move air between the atmosphere and the lungs but do not participate in gas exchange.

• Main Structures: Nasopharynx, Oropharynx, Larynx, Trachea, Bronchial tree.

• Function: Warm, humidify, and filter incoming air.

Pulmonary Circulation

• Definition: The movement of blood through the lungs for gas exchange.

• Pulmonary Artery: Delivers deoxygenated blood from the right ventricle to the lungs for oxygenation.

Bronchial Circulation

• Definition: Supplies oxygenated blood to the lung tissue itself (except the alveoli).

• Source: Arises from the thoracic aorta.

• Function: Provides nutrients to the conducting airways and warms/humidifies incoming air.

Structure of the Lungs and Larynx

Lung Anatomy

• Shape: Spongy, cone-shaped organs located in the chest cavity.

• Separation: Divided by the mediastinum.

• Lobes: Right lung has three lobes; left lung has two lobes.

• Apex: Upper part, near the thoracic cavity top.

• Base: Lower part, resting on the diaphragm.

Larynx Structure and Function

• Location: Connects the oropharynx to the trachea, positioned between the upper airways and the lungs.

• Functions:

  • Produces speech (phonation).

  • Protects the lungs from foreign substances.

Alveolar Structure

Types of Alveolar Cells

• Type I Alveolar Cells: Flat squamous epithelial cells where gas exchange occurs.

• Type II Alveolar Cells: Produce surfactant, a lipoprotein that decreases surface tension in the alveoli, allowing easier lung inflation.

Properties of Gases and Respiratory Pressures

Gas Laws in Respiration

• Airflow: Air moves from areas of high pressure to low pressure.

• Inhalation: Occurs when lung pressure drops below atmospheric pressure.

• Exhalation: Occurs when lung pressure rises above atmospheric pressure.

• Atmospheric Pressure: Total pressure from all gases in the air.

• Partial Pressure: Pressure from a single gas in a mixture (e.g., oxygen or nitrogen).

Respiratory Pressures

• Pulmonary (Alveolar) Pressure: Pressure inside the airways and alveoli.

• Pleural Pressure: Pressure in the pleural cavity.

• Thoracic Pressure: Pressure in the thoracic cavity.

Air Conditioning

• Air must be warmed and humidified to protect lung tissue and optimize gas exchange.

Lung Compliance and Airway Resistance

Lung Compliance

• Definition: The ease with which the lungs can be inflated.

• Formula:

$C = \Delta V / \Delta P$

• Where $\Delta V$ is the change in lung volume and $\Delta P$ is the change in respiratory pressure.

Airway Resistance

• Definition: Opposition to airflow through the airways.

• Directly related to the pressure difference between the lungs and the atmosphere.

• Inversely related to airway diameter; higher resistance (e.g., in asthma, COPD, inflammation) leads to lower airflow.

Lung Volumes and Capacities

Lung Volumes

• Tidal Volume (TV): Air moved in and out during normal breathing.

• Inspiratory Reserve Volume (IRV): Maximum air inhaled above normal TV.

• Expiratory Reserve Volume (ERV): Maximum air exhaled beyond normal TV.

• Residual Volume (RV): Air remaining after maximal exhalation.

Lung Capacities

• Vital Capacity (VC): $VC = IRV + TV + ERV$

• Inspiratory Capacity (IC): $IC = TV + IRV$

• Functional Residual Capacity (FRC): $FRC = RV + ERV$

• Total Lung Capacity (TLC): Sum of all lung volumes.

Pulmonary Function Studies

• Maximum Voluntary Ventilation: Maximum air moved in/out during maximal effort (12–15 seconds).

• Forced Vital Capacity (FVC): Full inspiration followed by maximal expiration.

• Forced Expiratory Volume (FEV): Volume exhaled in a specific time period.

• Forced Inspiratory Flow (FIF): Maximal inspiratory flow during rapid inspiration.

Dead Space and Shunts

Types of Dead Space

• Anatomic Dead Space: Air in conducting airways not involved in gas exchange.

• Alveolar Dead Space: Air in alveoli not participating in gas exchange.

• Physiologic Dead Space: Sum of anatomic and alveolar dead space.

Types of Shunts

• Anatomic Shunt: Blood bypasses the lungs entirely (e.g., septal defects).

• Physiologic Shunt: Blood flows through unventilated parts of the lung (e.g., pneumonia, ARDS).

Factors Affecting Gas Exchange

• Surface Area for Diffusion: Decreases with age, reduced lung volume, or diseases (e.g., emphysema).

• Thickness of Alveolar-Capillary Membrane: Increases with fibrosis or edema, slowing diffusion.

• Partial Pressure of Gases: Depends on ventilation, perfusion, and diffusion.

• Gas Properties: Higher solubility and lower molecular weight increase diffusion rate (e.g., CO diffuses faster than O2 due to higher solubility).

Oxygen and Carbon Dioxide Transport

Oxygen Transport

• Normal Arterial O2: Above 80 mm Hg.

• 98–99%: Carried bound to hemoglobin (as oxyhemoglobin).

• 1–2%: Dissolved in plasma.

• Hemoglobin Affinity: Determines oxygen binding and release.

Carbon Dioxide Transport

• Normal Arterial CO2: 35–45 mm Hg.

• Transport Forms:

  • Dissolved in plasma.

  • Bound to hemoglobin (carbaminohemoglobin).

  • As bicarbonate (HCO3-) in plasma.

• Acid-Base Balance: Regulated by the ratio of dissolved CO2 to bicarbonate.

Control of Breathing

Neural Control

• Respiratory Center: Pacemaker in the brainstem controls breathing rhythm.

• Medullary Neurons: Generate the basic rhythm.

• Pneumotaxic Center (upper pons): Helps switch inspiration off.

• Apneustic Center (lower pons): Promotes inspiration.

• Phrenic Nerve: Stimulates the diaphragm (main breathing muscle).

Chemical and Voluntary Regulation

• Chemoreceptors: Detect changes in CO2, O2, and pH.

• Lung Receptors: Monitor lung function and breathing patterns.

• Voluntary Control: Temporary override by the motor cortex (e.g., speaking, singing).

Protective Reflexes and Abnormal Breathing Patterns

Cough Reflex

• Protects the lungs by expelling irritants and secretions.

Cheyne-Stokes Respiration

• Definition: Abnormal breathing pattern with alternating periods of rapid breathing (tachypnea) and apnea.

• Causes: Heart failure, stroke, brain injury, tumors, encephalitis.

Mechanisms of Dyspnea (Shortness of Breath)

• Increased CNS Sensitivity: Over-responsiveness to normal breathing signals.

• Decreased Breathing Capacity: Reduced respiratory reserve.

• Muscle and Joint Receptors: Discomfort and effort in breathing due to nerve endings in respiratory muscles.

• Associated Conditions: Primary lung disease, heart disease, neuromuscular disorders.

Summary Table: Lung Volumes and Capacities

Parameter	Definition	Typical Value (Adult)
Tidal Volume (TV)	Air in/out during normal breath	~500 mL
Inspiratory Reserve Volume (IRV)	Max air inhaled above TV	~3000 mL
Expiratory Reserve Volume (ERV)	Max air exhaled beyond TV	~1100 mL
Residual Volume (RV)	Air remaining after maximal exhalation	~1200 mL
Vital Capacity (VC)	IRV + TV + ERV	~4600 mL
Total Lung Capacity (TLC)	Sum of all volumes	~5800 mL

Additional info: Some values and explanations have been expanded for clarity and completeness based on standard academic sources.