Human Lung Structure & Function Lecture Notes PDF

Dr. Gerd Xuereb’s Lecture Notes Structure & Function of the Human Lung  Lecture One Respiration Respiration can be divided into: Respiration Cellular Respirat...

Dr. Gerd Xuereb’s Lecture Notes Structure & Function of the Human Lung  Lecture One Respiration Respiration can be divided into: Respiration Cellular Respiration External Respiration The intracellular reaction of: Movement of gases between Oxygen + Glucose the environment and the body’s cells CO2 + water + energy (ATP) External respiration is made up of 4 processes: 1. Ventilation/Breathing - air exchange between atmosphere and lungs. The mechanics of breathing are the mechanisms by which breathing takes place. 2. Gas Exchange - oxygen and carbon dioxide between the lungs and the blood. 3. Transport - oxygen and carbon dioxide transported by the blood. 4. Gas Exchange - oxygen and carbon dioxide between blood and cells. The Respiratory Tract is divided into the upper & lower parts The Upper Respiratory Tract consists of the: 1. Mouth 2. Nasal Cavity - nose filters air from large particles 3. Pharynx - common passageway for food + liquids & air 4. Larynx - contains vocal cords. The vocal cords are connective tissue bands that vibrate and tighten to create sound as air passes through them Dr. Gerd Xuereb 2 of 8  Lecture One The Lower Respiratory Tract consists of the: 1. Trachea - semi-flexible tube made up of 15-20 C-shaped cartilage rings 2. Primary Bronchi (x2) - semi-rigid tubes supported by cartilage 3. Branches of bronchi (see below) 4. Lungs The Airways The airways become narrower, shorter and more numerous as they get deeper into the lung. The airways can be divided into 2 zones: Conducting zone Airways within the conducting zone lead inspired air to the gas-exchanging regions of the lung. Within this zone, the trachea divides into right and left main bronchi, which in turn divide into lobar, then segmental bronchi. This process continues down to the terminal bronchioles, which are the smallest airways without alveoli. The conducting airways lack alveoli and hence do not participate in gas exchange and thus form the anatomic dead space (volume is typically ≈150ml). Respiratory zone The region of the lung where gas exchanges occurs. 1. Terminal bronchioles branch into respiratory bronchioles which have occasional alveoli budding from their walls. 2. Respiratory bronchioles in turn lead to the alveolar ducts, which are completely lined with alveoli. The portion of the lung distal to a terminal bronchiole is called an acinus or a lung unit. The respiratory zone makes up most of the lung (volume is ≈2.5 – 3L during rest). Dr. Gerd Xuereb 3 of 8  Lecture One The Weibel model states that the conducting zone is made up of the first 16 airway divisions whereas the last 7 airway divisions make up the respiratory zone. The diameter of the airways becomes progressively smaller with every division however the number of airways increases geometrically, resulting in an increase in total cross-sectional area with each airway division. Air flows down to about the terminal bronchioles by bulk flow whilst diffusion is the main mechanism of air flow in the respiratory zone. Note: The velocity of gas falls rapidly in the region of the terminal bronchioles, inhaled medium-sized dust particles (1-5μm) often settles out there. Function of the Airways a.k.a. respiratory ‘pipes’ prepare the air for gas exchange in the lungs 1. Passage for air to flow through to reach the alveoli 2. Air Conditioning - involves 3 steps i. Air Warming Air is warmed to 37’C (body temperature). The air is warmed by the body’s heat. This is important since it avoids alveolar damage by cold air and also does not cause changes in core body temperatures. The 3 Functions of an Air-conditioner: 1. Changes the temperature 2. Filters the air 3. Dehumidifies air Dr. Gerd Xuereb 4 of 8  Lecture One ii. Air Moistening Air reaches 100% humidity in the airways thanks to water evaporation from the mucosal lining of the airway. This is important since the gas exchange epithelium must remain moist. iii. Air Filtration Air is filtered from foreign bodies so that viruses, bacteria and inorganic particles do not reach the alveoli. Both trachea and bronchi are lined with columnar epithelial cells containing cilia bathed in a watery saline layer, on top of which lies a sticky layer of mucus secreted by goblet cells. This mucus layer traps most inhaled medium-sized particles (1-5μm) and contains immunoglobulins that can disable trapped pathogens. Mucus is moved continuously towards the pharynx by the upward motion generated by beating cilia (a process called the mucociliary escalator). Once mucus reaches the pharynx, it can be expectorated (spitting) or swallowed (stomach acid and enzymes destroy any remaining microorganisms). The Blood-Gas Barrier This is where gas exchange occurs. It is formed from alveoli and capillaries and hence two systems make up the blood-gas barrier 1. Respiratory system (Alveoli) 2. Circulatory system The Design of Alveoli Alveoli are small air sacs covered with capillaries, which together form the blood-gas barrier. Therefore, their main function is the exchange of gases. Since they lack cilia, alveoli also contain macrophages which engulf particles (