Respiratory System: Structures and Functions

Respiratory System Overview

• The respiratory system is covered in weeks 3-5.
• Week 4 includes lecture 1 (respiratory physiology intro), lecture 2 (respiratory physiology measurement and control), tutorial 1 and lecture 3 (respiratory physiology diseases).
• Week 5 includes tutorial 2 and respiratory practical.
• Objectives include describing the basic structures of the respiratory system, outlining function, explaining processes of normal breathing, and familiarizing with respiratory system vocabulary.
• Dr Penny Lympany can be contacted at [email protected].
• Her office is located in 28AY04.

Respiratory System Structures

• The upper respiratory tract includes the nose, nasopharynx, pharynx, and larynx.
• The lower respiratory tract includes the trachea, lungs, bronchi, and alveoli.

Functions of the Respiratory Tract

• Primary functions: exchange of oxygen and carbon dioxide between blood and the atmosphere, olfaction, and voice.
• Secondary functions: warming and humidifying incoming air, moistening cell lining, keeping lining clean, keeping airways open during pressure changes, and keeping alveoli open against surface tension.
• Humans exhale up to 400 mL of water per day at rest.

Anatomy of the Upper Respiratory Tract

• Nasal cavity features include blood supply and hairs.
• Sinuses are air cavities in the cranial bones (lat: pocket).
• The larynx enables speech, i.e. pitch and volume are adjusted in the larynx.
• The larynx prevents material from reaching the lower respiratory tract.
• Ingested matter stimulates the larynx to produce strong cough reflex via vagal receptors.

Anatomy of the Lower Respiratory Tract

• Each lung is surrounded by two membranes: the parietal pleura (outer) and the visceral pleura (inner).
• The space between the membranes is filled with fluid, reducing friction via surface tension.
• Elephants have tough connective tissue in their pleural cavity that allows them to snorkel and withstand pressure differences.
• The trachea bifurcates into two main bronchi at the level of the sternal angle and is supported by horseshoe-shaped cartilaginous rings.
• The left main bronchus is longer than the right.
• Bronchi subdivide into lobar and segmental until terminal bronchioles.

The Pulmonary Airway Tree

• The pulmonary airway tree consists of segmental bronchi, large subsegmental bronchi, small bronchi, terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveolar sacs.

Bronchi and Bronchioles

• Bronchi contain cartilage,.
• Bronchioles lack cartilage.
• The terminal bronchiole supplies the acinus.

Alveoli

• Each lung has 300-350 million alveoli with a diameter of 0.1-0.2 microns.
• Alveoli are lined with a single layer of flattened epithelial cells.
• Alveoli contain Type I and Type II pneumocytes.

Surfactant

• Pulmonary surfactant is a liquid in the lungs that helps keep the air sacs open for breathing, secreted by Type II pneumocytes.
• It is composed of lipids, proteins, and carbohydrates, and molecules are hydrophilic.
• Surfactant lowers surface tension between water molecules in the lungs, allowing the lungs to expand and take in air.
• Surfactant coats the air sacs in the lungs, preventing them from collapsing.
• Surfactant protects the lungs from inhaled particles and microorganisms.

Surfactant - Surface Tension

• Surface tension is the force acting on the surface of a liquid.
• Laplace's Law states that the smaller the bubble, the greater the inward pressure trying to collapse the bubble.
• On inhalation, the surface area of alveoli increases.
• On exhalation, the surface area of alveoli decreases, which results in an increase in inward pressure trying to collapse the lungs.
• Surfactant acts like detergent in water and reduces pressure which is trying to collapse the bubble.

Cytology - Nasal Cavity Lining

• The nasal cavity lining includes hairs, olfactory mucosa (small area, roof of cavity, pseudostratified epithelium), and respiratory mucosa (cilia, goblet cells, serous & mucous glands, venous plexus).
• Goblet cells produce mucus for clearance.
• Conducting airways in the lower respiratory tract have pseudostratified (basal cells) epithelium.
• Cytology features simple cuboidal epithelium too

Cytology - Conducting airways and Cilia

• Cilia are covered with a thin layer of mucus.
• Cilia move in coordinated waves via the mucociliary escalator.

Surfactant Production and Components

• Type II epithelial cells produce surfactant, which contains phospholipids, lipoproteins, and essential ions.
• Functions: aid expansion of alveoli and transport solutes out of the alveolar space.
• Surfactant decreases surface tension of the fluid, reducing the chance of alveolar collapse.

Summary

• Objectives include describing the basic structures of the respiratory system, outlining function, identifying upper respiratory tract components and external respiration roles, and describing the role of lower respiratory tract components in extracellular respiration.

Glossary

• Alveolar Pressure: pressure of the air within the alveoli.
• Alveolar Ventilation Rate: speed of flow of fresh air that reaches the alveoli in volume/minute.
• Anatomic Dead Space (VD): spaces of the nose, pharynx, larynx, trachea, bronchi and bronchioles totalling about 150ml in a quiet breath (tidal volume), which does not reach the alveoli to participate in gas exchange.
• Capacity: The sum of 2 or more volumes
• Carbamino Compounds: compound created through the addition of CO2 with a free amino group in an amino acid or a protein e.g. haemoglobin converts to carbaminohemoglobin.
• Compliance: The ability of the lung to stretch.
• External Respiration: the exchange of respiratory gases between the lungs and the blood, also called pulmonary respiration.
• Elastic Recoil: lung's intrinsic tendency to deflate following inflation, responsible for normal resting expiration in combination with abdominal organs and from surface tension.
• Expiration: The movement of air into the lungs.
• Hypoxia: Lack of adequate oxygen at the tissue level.
• Hypercapnia: condition of abnormally elevated carbon dioxide (CO2) levels in the blood, also known as hypercarbia.
• Inspiration: The movement of air into the lungs.
• Intra Pleural Pressure: pressure within the pleural fluid.
• Internal Respiration: exchange of respiratory gases between the blood and body cells, also called tissue respiration or systemic gas exchange.
• Minute Volume: the total volume of air entering the lungs in a minute, also known as Minute ventilation [VE]).
• Passive: Without the need for an energy source/expenditure of energy.
• Partial Pressure: pressure of a single gas.
• Pulmonary Ventilation: inflow (inhalation) and outflow (exhalation) of air between the atmosphere and the lungs, also known as breathing.
• Respiratory Membrane: membrane separating air within the alveoli from the blood within pulmonary capillaries.
• Tidal Volume: volume of air that moves in a single normal inspiration or expiration (during quiet breathing).
• Ventilation: see pulmonary ventilation.