Composition of inhaled air vs exhaled air: Oxygen = Inhaled ~21%, Exhaled ~16% (O2 absorbed by body). Carbon Dioxide = Inhaled ~0.04%, Exhaled ~4% (CO2 produced by cellular respiration). Nitrogen = Inhaled ~78%, Exhaled ~78% (unchanged - not used by body). Water vapour = Exhaled air is saturated with water vapour (hence breath fogs cold glass). So we exhale significantly more CO2 than we inhale, as CO2 is the waste product of cellular respiration (aerobic metabolism).

Aerobic Respiration: C6H12O6 + 6O2 > 6CO2 + 6H2O + 38 ATP - occurs in mitochondria; produces large amounts of energy. Anaerobic Respiration (in humans): C6H12O6 > 2 Lactic acid + 2 ATP - occurs during intense exercise when O2 supply is insufficient; produces small amounts of energy; lactic acid builds up causing muscle fatigue and cramps. In yeast/microorganisms: Glucose > Ethanol + CO2 + energy (fermentation). Oxygen debt = extra O2 needed after anaerobic exercise to break down accumulated lactic acid.

Sleep Apnoea is a disorder where breathing repeatedly stops (apnoea episodes) during sleep. Types: Obstructive Sleep Apnoea (OSA) - most common, throat muscles relax and block the airway; associated with obesity, snoring. Central Sleep Apnoea - brain fails to send proper signals to breathing muscles. Complex/Mixed - combination of both. Symptoms: loud snoring, daytime sleepiness, morning headaches, poor concentration, witnessed breathing pauses. Diagnosis: Polysomnography (sleep study). Treatment: CPAP (Continuous Positive Airway Pressure) machine, weight loss, positional therapy, surgery.

The respiratory system's primary function is gas exchange (respiration) - taking in oxygen (O2) from the environment and expelling carbon dioxide (CO2) produced by cellular metabolism. Other functions include: regulation of blood pH (by controlling CO2 levels), voice production (phonation), olfaction (sense of smell), and protection against inhaled pathogens and particles. The process involves: Breathing (ventilation), Gas exchange (in alveoli), Transport (by blood), and Cellular respiration.

The respiratory centre that controls the rhythm and rate of breathing is located in the brainstem: Medulla oblongata contains the Dorsal Respiratory Group (DRG) and Ventral Respiratory Group (VRG) - sets the basic rhythm of breathing. Pons contains the Pneumotaxic centre (limits inspiration) and Apneustic centre (prolongs inspiration). The respiratory centre responds to chemoreceptors that monitor blood CO2, O2, and pH levels. Rising CO2 (falling pH) is the strongest stimulus to increase breathing rate - detected by central chemoreceptors in the medulla.

Emphysema is a type of COPD where the walls between alveoli are destroyed, forming large, non-functional air spaces. This leads to: Reduced surface area for gas exchange (hypoxia), Air trapping (hyperinflation - barrel chest), Loss of lung elasticity (difficulty exhaling), and Pink Puffer appearance (breathless but maintain normal blood O2 by increased breathing rate). Primary cause: cigarette smoking destroys the alveolar walls via proteases (anti-trypsin deficiency is a genetic cause). Diagnosis: Spirometry shows reduced FEV1/FVC ratio.

Vibrissae (nasal hairs) are the coarse hairs at the entrance of the nostrils. They form the first line of defence against inhaled particles by filtering large particles (dust, pollen, insects, debris) and acting as a physical barrier at the nasal entrance. After the vibrissae, smaller particles are trapped by the mucous membrane of the nasal cavity. Even smaller particles (<10 microm) may reach the bronchi, and the finest particles (<2.5 microm - PM2.5) can reach the alveoli - these are most dangerous to health. Breathing through the nose (not mouth) maximises filtration efficiency.

The Lungs are the primary organs of the respiratory system. They are a pair of large, spongy, cone-shaped organs located in the thoracic cavity (chest), protected by the rib cage. The right lung has 3 lobes (superior, middle, inferior) and the left lung has 2 lobes (superior, inferior) - the left lung is smaller to accommodate the heart. The lungs contain millions of tiny air sacs called alveoli, which are the actual sites of gas exchange between air and blood.

Asthma is a chronic inflammatory disease of the airways (bronchi and bronchioles) characterised by airway inflammation (swelling of airway walls), bronchoconstriction (narrowing of airways due to smooth muscle contraction), excessive mucus production (further blocking airflow), and reversible airway obstruction (key feature - distinguishes from COPD). Symptoms: wheeze, cough, chest tightness, shortness of breath. Triggers: allergens (pollen, dust mites), exercise, cold air, infections, smoke. Treatment: Bronchodilators (relievers) - salbutamol (beta-2 agonist), Corticosteroids (preventers).

The intercostal muscles are located between the ribs and play an important role in breathing. External intercostal muscles: Contract during inhalation, ribs move upward and outward, increases thoracic volume, air flows in. They are accessory muscles of inspiration. Internal intercostal muscles: Contract during forced exhalation, ribs move downward and inward, decreases thoracic volume, air flows out. During quiet breathing, exhalation is passive (muscles relax). During exercise or respiratory distress, accessory muscles like sternocleidomastoid and scalenes also assist.