Physiology of the Lymphatic and Respiratory Systems

• Cell-Mediated Immunity: Driven by T-lymphocytes. Helper T cells (containing CD4 coreceptors) are activated when macrophages or other antigen-presenting cells secrete interleukins to coordinate the response. Once activated, cytotoxic T cells (containing CD8 coreceptors) work to recognize and destroy infected or abnormal body cells.
• Antibody-Mediated Immunity: Driven by B cells. When B cells recognize an antigen via their receptors, they differentiate into plasma cells that secrete antibodies.

Exam Tip: Primary vs. Secondary Response Remember that the primary immune response occurs upon the first exposure to a specific antigen, while the secondary immune response is faster and stronger because the system utilizes "memory" from the initial encounter to produce antibodies more efficiently.

Respiratory Physiology: Ventilation and Mechanics

The respiratory system is divided into the respiratory tract (anatomically split into the upper tract, including the nose and larynx, and the lower tract, including the trachea and bronchi) and the lungs. Functionally, the tract is divided into the conducting zone (passageways) and the respiratory zone, which contains the alveoli where gas exchange actually occurs.

Mechanisms of Breathing (Pulmonary Ventilation)

Breathing is driven by changes in the volume of the thoracic cage, which creates pressure gradients between the lungs and the atmosphere. These mechanics are governed by Boyle’s Law, which states that pressure ($P$) and volume ($V$) are inversely proportional:

$P \propto \frac{1}{V}$

• Inspiration (Inhalation): The diaphragm and external intercostals contract, increasing the thoracic volume. According to Boyle's Law, as volume increases, intrapulmonary (alveolar) pressure decreases relative to atmospheric pressure, causing air to flow into the lungs.
• Expiration (Exhalation): The diaphragm relaxes, decreasing thoracic volume. This increases intrapulmonary pressure, forcing air out of the lungs.

To keep the lungs inflated, intrapleural pressure (the pressure within the pleural cavity) must always remain lower than the intrapulmonary pressure.

Gas Exchange and Transport

Once air reaches the alveoli, gas exchange occurs through diffusion, a process governed by Henry’s Law. This law dictates that gases move down their partial pressure gradients—from an area of higher partial pressure to an area of lower partial pressure.

Alveolar vs. Systemic Exchange

1. Alveolar Gas Exchange: Occurs at the respiratory membrane. Oxygen diffuses from the alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.
2. Systemic Gas Exchange: Occurs at the tissue level. Oxygen moves from the blood into the systemic cells, and carbon dioxide moves from the cells into the blood.

Blood Transport Mechanisms

The blood must efficiently transport these gases to and from the tissues.

Oxygen Transport:

• Oxyhemoglobin: Over 98% of oxygen is bound to hemoglobin within red blood cells.
• Dissolved: Less than 2% is dissolved directly in the plasma.

Carbon Dioxide Transport:

• Bicarbonate ($\text{HCO}_3^-$): The majority (80–90%) is transported in the plasma as bicarbonate, a process facilitated by the enzyme carbonic anhydrase and the chloride shift.
• Carbaminohemoglobin: 5–10% is bound to hemoglobin.
• Dissolved: 5–10% is dissolved in the plasma.

Clinical Applications: Spirometry and Lung Disorders

To assess respiratory health, clinicians use spirometry to measure various pulmonary volumes and capacities. By analyzing the resulting flow-volume loops, doctors can differentiate between different types of lung diseases:

• Obstructive Lung Disorders: These are characterized by an airflow rate that is lower than normal (e.g., difficulty exhaling).
• Restrictive Lung Disorders: These present with a normal airflow rate but show significantly low vital capacity and low inspiratory/expiratory volumes.