Understanding the Respiratory System: From Ventilation to Disorders

Posted on Aug 26, 2024 in Biology

Pulmonary Ventilation

Respiration

Respiration is the process of gas exchange within the body. It encompasses several key processes:

  • Pulmonary Ventilation (Breathing): The flow of air into and out of the lungs.
  • External Respiration: The exchange of oxygen and carbon dioxide between the alveoli (tiny air sacs in the lungs) and the circulatory system.
  • Internal Respiration: The exchange of oxygen and carbon dioxide between the capillaries (tiny blood vessels) and the cells.
  • Cellular Respiration: The breakdown of glucose to produce ATP (energy). This process requires oxygen and produces carbon dioxide.

Inspiration (Inhalation)

Inspiration is the process of bringing air into the lungs. Here’s how it works:

  1. Boyle’s Law: The pressure of a gas is inversely proportional to the volume of the container (lungs), at a constant temperature. This means that when the volume of the lungs increases, the pressure inside decreases.
  2. Normal inspiration begins with the contraction of the diaphragm and the external intercostal muscles. This contraction expands the chest cavity (increases intrathoracic space).
  3. As the chest cavity expands, the pressure inside the alveoli (alveolar pressure) decreases to a level lower than the atmospheric pressure. This difference in pressure causes air to rush into the lungs.
  4. Muscles located above the rib cage can further assist in inspiration by enlarging the intrathoracic space.

Expiration (Exhalation)

Expiration is the movement of air out of the lungs. It is typically a passive process:

  1. During normal exhalation, the diaphragm and external intercostal muscles relax. This relaxation allows the chest wall to recoil to its original position, decreasing the intrathoracic space.
  2. As the chest cavity decreases in volume, the pressure inside the alveoli (alveolar pressure) increases to a level higher than the atmospheric pressure. This pressure difference forces air out of the lungs.
  3. Forced exhalation, such as during exercise or coughing, involves the contraction of muscles located below the rib cage, further decreasing the chest cavity volume.

Factors Affecting Pulmonary Ventilation

Several factors can influence the efficiency of pulmonary ventilation:

  • Surface Tension of Alveolar Fluid: The natural attraction of water molecules to each other creates surface tension within the alveoli. This tension can cause the alveoli to collapse. However, a substance called surfactant, produced by specialized cells in the alveoli, reduces surface tension and prevents alveolar collapse.
  • Compliance: Compliance refers to the ease with which the lungs and chest wall can expand. Factors like lung tissue elasticity and chest wall flexibility influence compliance.
  • Airway Resistance: Airway resistance is the hindrance to airflow within the respiratory passages, particularly in the bronchioles. Factors like bronchiole diameter and mucus production can affect airway resistance.

Lung Volumes and Capacities

Understanding lung volumes and capacities is crucial for assessing respiratory health:

  • Tidal Volume: The volume of air inhaled or exhaled during a normal breath (approximately 500 ml).
  • Inspiratory Reserve Volume: The maximum amount of air that can be forcefully inhaled after a normal inhalation (approximately 3100 ml).
  • Expiratory Reserve Volume: The maximum amount of air that can be forcefully exhaled after a normal exhalation (approximately 1200 ml).
  • Residual Volume: The volume of air remaining in the lungs after a maximal exhalation (approximately 1200 ml). Residual volume helps keep the alveoli inflated.
  • Vital Capacity: The maximum amount of air that can be exhaled after a maximal inhalation. It is calculated as: Tidal Volume + Inspiratory Reserve Volume + Expiratory Reserve Volume.
  • Total Lung Capacity: The total volume of air the lungs can hold. It is calculated as: Vital Capacity + Residual Volume (approximately 6000 ml).
  • Anatomic Dead Space: The volume of air in the conducting airways (nose, trachea, bronchi) where gas exchange does not occur (approximately 150 ml). This means that only about 350 ml of a normal breath (70% of tidal volume) reaches the alveoli for gas exchange.

Exchange and Transportation of Gases

Dalton’s Law

Dalton’s Law states that each gas in a mixture of gases exerts its own pressure, known as partial pressure, independent of the other gases present. The partial pressure of a gas is calculated as:

Partial Pressure = Percentage of Gas in Mixture x Total Pressure of Mixture

For example, the partial pressure of oxygen in the atmosphere (where oxygen makes up about 20.9% and atmospheric pressure is 760 mmHg) is:

Partial Pressure of Oxygen = 0.209 x 760 mmHg = 158.8 mmHg

Henry’s Law

Henry’s Law states that the amount of gas that dissolves in a liquid (like blood) is directly proportional to the partial pressure of that gas and its solubility coefficient (a measure of how well the gas dissolves in the liquid) at a constant temperature.

Oxygen Transport

  • Over 98% of oxygen in the blood is transported bound to hemoglobin, a protein found in red blood cells, forming oxyhemoglobin. Only a small amount (about 1.5%) is dissolved directly in the blood plasma.
  • The binding of oxygen to hemoglobin is influenced by the partial pressure of oxygen. As the partial pressure of oxygen increases, more oxygen binds to hemoglobin. Conversely, as the partial pressure of oxygen decreases, oxygen is released from hemoglobin, making it available for diffusion into the tissues.
  • Other factors affecting oxygen-hemoglobin binding include:
    • Acidity (pH): A lower pH (more acidic) reduces oxygen binding to hemoglobin (Bohr effect).
    • Partial Pressure of Carbon Dioxide: Increased carbon dioxide levels decrease blood pH, further reducing oxygen binding.
    • Temperature: Higher temperatures decrease oxygen binding to hemoglobin.
    • 2,3-Bisphosphoglycerate (BPG): Increased levels of BPG, a molecule produced in red blood cells, decrease oxygen binding to hemoglobin. BPG levels are elevated at higher altitudes.

Carbon Dioxide Transport

Carbon dioxide is transported in the blood in three main forms:

  1. Dissolved in Plasma (9%): A small amount of carbon dioxide dissolves directly in the blood plasma.
  2. Carbamino Compounds (13%): Carbon dioxide can bind to amino acids and proteins in the blood, primarily to hemoglobin, forming carbaminohemoglobin.
  3. Bicarbonate Ions (78%): The majority of carbon dioxide is transported as bicarbonate ions (HCO3-). This process involves the following reaction within red blood cells:

CO2 + H2O <—-> H2CO3 <—-> H+ + HCO3-

Carbon Water Carbonic Hydrogen Bicarbonate Dioxide Acid Ion Ion

Respiratory Control

Breathing is regulated by a complex interplay of neural and chemical factors:

  • Respiratory Center: Clusters of neurons in the medulla oblongata and pons of the brainstem control the basic rhythm of breathing. These neurons send signals to the diaphragm and intercostal muscles, initiating inhalation and exhalation.
  • Chemical Regulation: Chemoreceptors located in the aortic arch and carotid arteries are highly sensitive to changes in blood pH, carbon dioxide levels, and oxygen levels. They send signals to the respiratory center to adjust breathing rate and depth to maintain homeostasis.
  • Cortical Stimulation: Conscious control from the cerebral cortex allows for voluntary changes in breathing, such as holding your breath or taking a deep breath.
  • Proprioceptor Stimulation: Proprioceptors, sensors located in muscles and joints, detect body movement and position. During physical activity, proprioceptor stimulation signals the respiratory center to increase breathing rate and depth to meet the increased oxygen demand.
  • Inflation Reflex: Stretch receptors in the lungs prevent overinflation. When the lungs are stretched beyond a certain point, the inflation reflex inhibits further inhalation, allowing for exhalation.
  • Other Influences: Factors like temperature changes, pain, and emotional states can also influence breathing patterns.

Aging and the Respiratory System

As we age, the respiratory system undergoes several changes:

  • Lung tissues lose elasticity and become more rigid, leading to a decrease in lung capacity and efficiency.
  • The overall ability of the respiratory system to fight off infections declines.
  • These age-related changes can significantly impact the body’s ability to deliver oxygen to tissues and remove carbon dioxide, especially during exertion.

Respiratory Disorders

The respiratory system is susceptible to various disorders, including:

Asthma

  • A chronic inflammatory disorder characterized by spasms and narrowing of the airways, primarily the bronchioles.
  • Causes difficulty breathing, wheezing, coughing, and chest tightness.
  • Triggered by various irritants, such as allergens, cold air, exercise, or stress.

Chronic Bronchitis

  • Characterized by a productive cough (cough with mucus) for at least three months a year for two consecutive years.
  • Causes excessive mucus production, shortness of breath, and wheezing.
  • Often associated with smoking.

Emphysema

  • Involves the destruction of alveolar walls, leading to enlarged air spaces and reduced surface area for gas exchange.
  • Results in shortness of breath, wheezing, and a characteristic “barrel chest” appearance.
  • Primarily caused by long-term exposure to irritants, such as cigarette smoke.

Lung Cancer

  • Uncontrolled growth of abnormal cells in the lungs.
  • Often caused by chronic irritation, with smoking being the most common risk factor.
  • Symptoms include persistent cough, bloody sputum, shortness of breath, chest pain, hoarseness, difficulty swallowing, weight loss, fatigue, and loss of appetite.
  • Has a high rate of metastasis (spreading to other parts of the body).

Pneumonia

  • An acute inflammation of the alveoli, often caused by bacterial or viral infections.
  • Fluid accumulation in the alveoli impairs gas exchange.
  • Symptoms include fever, chills, cough (dry or productive), fatigue, chest pain, and difficulty breathing.
  • Can be a primary infection or secondary to other respiratory illnesses.

Tuberculosis (TB)

  • A chronic infectious disease caused by the bacterium Mycobacterium tuberculosis.
  • Primarily affects the lungs but can spread to other organs.
  • Symptoms include fatigue, weight loss, lethargy, loss of appetite, night sweats, persistent cough, shortness of breath, chest pain, and coughing up blood.
  • Treatable with antibiotics, but drug-resistant strains are a growing concern.
  • Associated with weakened immune systems, such as in individuals with HIV/AIDS.

Common Cold (Coryza)

  • A viral infection of the upper respiratory tract, with over 200 different viruses identified as causative agents.
  • Symptoms include runny nose, nasal congestion, sneezing, sore throat, cough, and mild fatigue.
  • Highly contagious and can lead to secondary bacterial infections, such as sinusitis or ear infections.

Cystic Fibrosis

  • A genetic disorder that affects the production of mucus, sweat, and digestive fluids.
  • Causes the production of abnormally thick and sticky mucus that obstructs airways and other ducts in the body.
  • Leads to recurrent respiratory infections, lung damage, and digestive problems.
  • Life-shortening condition, with respiratory failure being a common cause of death.