Respiratory System: Structure and Function

Respiration

• Respiration releases water vapor and other gases
• Gases are exchanged between the body and outside atmosphere in this life-sustaining process.

Structural Components

• The respiratory system is divided into an upper and lower system
• The upper respiratory system includes the nasal cavity, pharynx, and larynx
• The lower respiratory system includes the trachea, bronchi, and lungs

Supporting Structure

• The thoracic cage houses, protects, and facilitates respiratory function
• Muscles of breathing include the diaphragm and accessory muscles

Functions of the Respiratory System

• The primary function is to supply oxygen to the bloodstream and remove carbon dioxide
• It helps regulate blood pH
• It contains receptors for the sense of smell
• It filters, warms, and moistens inspired air
• It produces sounds
• It removes water and heat from the body via exhaled air
• Otorhinolaryngology is the branch of medicine for the diagnosis and treatment of ear, nose, and throat(ENT)

Organs of the Respiratory System

• The respiratory system has two zones: conducting and respiratory
• The conducting zone is a series of interconnected cavities and tubes outside and within the lungs
• The respiratory zone consists of tissues within the lungs, facilitating gas exchange between air and blood

Functions of the Upper Respiratory Tract

• Warms or cools inspired gases to body temperature (98.6°F [37°C])
• Filters particles from inspired gases
• Humidifies inspired gases to a relative humidity of 100%
• Provides for the sense of smell, or olfaction
• Produces sounds, or phonation
• Ventilates, or conducts, the gas down to the lower airways

The Nose

• The nose is a specialized organ at the entrance of the respiratory system
• The external nose is made of cartilage and skin and lined with mucous membrane
• The nasal cavity is split into right and left halve by the nasal septum
• The regions contained within are the vestibular, olfactory, and respiratory

The Pharynx

• The pharynx (throat) is a funnel-shaped tube
• It sits posterior to the nasal and oral cavities, and anterior to the cervical (neck) vertebrae
• It is made of skeletal muscle and lined with mucous membrane
• The pharynx functions as a passageway for air and food
• It is a resonating chamber for speech sounds
• It houses the tonsils in the division: the nasopharynx, oropharynx, and laryngopharynx
• The nasopharynx functions in respiration.
• The oropharynx and laryngopharynx function both in digestion and in respiration

The Larynx

• The larynx connects the pharynx and trachea and helps to conduct air
• It contains the vocal cords, which vibrate when air flows over to produce sound
• Larynx helps protect the trachea from aspirated food, when swallowing, the epiglottis closes

Larynx Composition

• It is comprised of the thyroid cartilage (Adam's apple), the epiglottis, the cricoid cartilage, arytenoid cartilages, false vocal cords, and true vocal cords.

The Trachea

• The trachea (windpipe) extends from the larynx to the primary bronchi
• It composed of smooth muscle and C-shaped rings of cartilage
• It is lined with pseudostratified ciliated columnar epithelium.
• At the level of vertebrae T4-5 the trachea bifurcates, into two bronchi , the right mainstem and the left mainstem.
• The site of bifurcation into the right and left lung is called the carina.

Bronchi and Bronchioles

• The right main (primary) bronchus goes to the right lung, and the left main (primary) bronchus, goes to the left lung
• The main bronchi divide to form the lobar (secondary) bronchi, one for each lobe of the lung
• The right lung has three lobes; the left lung has two.
• The inner lining of the trachea and bronchi is a layer of Ciliated Epithelium containing Goblet Cells.
• The glandular tissue and goblet cells secrete mucus, which is then wafted up the airways by the cilia which move in a synchronised pattern
• The mucus traps harmful substances and organisms, preventing them from entering the lungs

More on Bronchi and Bronchioles

• The lobar bronchi branch, forming smaller segmental (tertiary) bronchi, that divide several ti
• The bronchioles are narrower and contain less cartilage than the trachea and bronchi
• The walls are mainly smooth muscle and elastic fibres with ciliated epithelium.
• Bronchioles branch into terminal bronchioles, forming the bronchial tree

Lungs

• The lungs are paired organs in the thoracic cavity enclosed by the pleural membrane
• The parietal pleura is the outer layer; the visceral pleura is the inner layer
• The right lung has three lobes separated by two fissures; the left lung has two lobes separated by one fissure plus the cardiac notch
• Each lobe consists of lobules:lymphatic vessels, arterioles, venules, terminal bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli
• Exchange of gases (O2, and CO₂) in the lungs occurs across the respiratory membrane

Structure of an Alveolus

• Alveoli are small sacs, surrounded by capillaries, for efficient gaseous exchange
• Oxygen diffuses from the alveoli to the blood in the capillaries
• Carbon Dioxide diffuses from the capillaries to the alveoli
• Though each alveolus is small, the total surface area of all alveoli in the lungs is large (about 70m2 in humans)
• Increases rate of gaseous exchange

Type I and II Alveolar Cells

• The walls of the alveoli are only one cell thick;
• The cells are specialised Squamous Cells
• Capillaries are very close to the alveoli walls, decreasing the rate of gaseous exchange
• The type I cell is a complex branched cell with multiple cytoplasmic plates, representing the gas exchange surface
• The type II cell acts as the "caretaker" of the alveolar, responding to damage of the vulnerable
• Type 1 cells divides and synthesises, stores and releases pulmonary surfactant

Pulmonary Ventilation

• Pulmonary ventilation (breathing) is the flow of air between the atmosphere and the lungs, due to air pressure differences
• Inhalation occurs when the pressure inside is less than the atmospheric air pressure
• Exhalation happens when the pressure inside the lungs is greater than the atmospheric air pressure
• Contraction and relaxation of skeletal create the air pressure changes

Muscles in Pulmonary Ventilation

• The diaphragm contracts when nerve impulses from the phrenic nerves are received
• The SCMs, scalenes, and pectoralis minors contribute to forced inhalation
• External intercostal muscles participate in relaxed inhalation
• Forced exhalation involves contraction of the internal intercostals, external oblique, internal oblique, transverse abdominis, and rectus abdominis

Pressure Changes during Breathing

• At rest, when the diaphragm is relaxed, alveolar pressure is equal to atmospheric pressure, and there is no air flow
• During inhalation, the diaphragm and external intercostals contract, expanding the chest cavity and dropping the alveolar pressure below the atmospheric pressure
• Air flows into the lungs
• The lung volume expands
• During deep inhalation, the sternocleidomastoid muscles expand the chest further
• During exhalation, the diaphragm and external intercostals relax
• The chest and lungs recoil, collapsing the chest cavity and increasing the alveolar pressure above atmospheric pressure
• Air flows out of the lungs
• Lung volume decreases
• During forced exhalations, the internal intercostals and abdominal muscles contract, further reducing the size of the chest cavity and creating a greater increase in alveolar pressure

Lung Volume and Capacity

• Air in the lungs is measured in terms of lung volumes and capacities
• Lung volumes (respiratory volumes) refer to the volume of gas at a given time during the respiratory cycle
• Volume measures the amount of air for one function (such as inhalation or exhalation)
• Lung capacities are derived from a summation of different lung volumes
• The average total lung capacity of an adult human male is about 6 litres of air

Measuring Lung Volumes

• Lung volumes measurement is an integral part of pulmonary function test
• The volumes vary, depending on the depth of respiration, ethnicity, gender, age, body composition and respiratory
• Spirometry measures lung volumes, such as Tidal volume, Inspiratory reserve volume, and Expiratory reserve volume
• A spirometer measures the volume of air inspired and expired by the lungs

Lung Volume Definitions

• Tidal volume: Volume that enters and leaves with each breath, from a normal quiet inspiration to a normal quiet expiration ~ 0.5 L
• Inspiratory reserve volume: Extra volume that can be inspired above tidal volume, from normal quiet inspiration to maximum inspiration ~ 2.5L
• Expiratory reserve volume: Extra volume that can be expired below tidal volume, from normal quiet expiration to maximum expiration ~ 1.5 L
• Residual volume/reserve volume: Volume remaining after maximum expiration ~ 1.5 L

External and Internal Respiration

• Air is a mixture of gases: nitrogen, oxygen, water vapor, carbon dioxide
• Each gas contributes to the total air pressure.
• The pressure of each specific gas is called partial pressure
• O2 and CO₂ move from areas of higher partial pressure to areas of lower partial pressure: External and Internal
• External Respiration (pulmonary gas exchange) is the exchange of gases between alveolar air and pulmonary blood capillaries
• Internal Respiration (systemic gas exchange) is gas exchange between systemic tissue capillaries and systemic tissue cells

Blood Gas Partial Pressure

• Deoxygenated blood contains a PO₂ = 40 mm Hg and a PCO₂ = 45 mm Hg
• Atmospheric air contains a PO₂ = 159 mm Hg and a PCO₂ = 0.3 mm Hg
• Inhaled alveolar air contains a PO₂ = 105 mm Hg and a PCO₂ = 40 mm Hg
• Oxygenated blood contains a PO₂ = 100 mm Hg and a PCO₂ = 40 mm Hg
• Systemic tissue cells need to contain a PO₂ = 40 mm Hg and a PCO₂ = 45 mm Hg

Blood Transport of Gases

• Blood transports gases between the lungs and body tissues
• 98.5% of blood O₂ is bound to hemoglobin in red blood cells
• Association of O2 and hemoglobin is affected by Po₂, pH, temperature, and Pco₂
• Carbon dioxide is transported in three ways: About 7% is dissolved in plasma, 23% combines with the globin of hemoglobin, and 70% is converted to bicarbonate ions (HCO3-)

Regulation of Breathing - Factors

• Carbon dioxide influences respiratory rate- The brainstem detects increased carbon dioxide and increases the respiratory rate to eliminate the excess
• Blood oxygen is a secondary influencer on respiratory rate
• An abnormally low blood pH increases the respiratory rate

Regulation of Breathing - Control

• The respiratory system

1. The medullary respiratory center in the medulla oblongata
2. The pontine respiratory group in the pons

Regulation of Breathing - Feedback

• Stimuli disrupts homeostasis by increasing arterial blood PCO2 (or decreasing pH or PO2)
• Receptors from the medulla and carotid bodies send nerve impulses
• Dorsal respiratory group in medulla oblongata receive impulses
• Nerves send output impulses to effectors to alter breathing
• Muscles of inhalation and exhalation contract more forcefully and more frequently (hyperventilation)
• Then arterial blood PCO2, pH, and PO2 return back to normal

Impact of Exercise on Respiratory System

• Exercise causes cardiovascular and respiratory systems make adjustments
• The abrupt increase is due to neural changes sending impulses to inspiratory area in medulla oblongata
• The gradual increase is due to chemical and physical changes in the bloodstream

Aging and the Respiratory System

• The airways and tissues of the respiratory tract, including the alveoli, become less elastic and more rigid
• The chest wall becomes more rigid as well
• Decrease lung capacity
• Elderly are susceptible to pneumonia, emphysema, bronchitis, and other pulmonary disorders.

Homeostasis - Gas Exchange

• Gas exchange in the lungs is one way that the respiratory system helps maintain homeostasis
• Several other strategies keep the body in equilibrium
• The mouth and nose are the first lines of defence against pathogen
• The trachea has a thin coating of mucus to catch and hold particulates until expelled

Homeostasis - Acid-Base

• Gas exchange in the lungs maintains acid-base balance
• If the pH of the blood becomes too acidic, the breathing rate increases
• The lungs secrete an antibody known as IgA and cytokines to destroy pathogen.
• Lymphoid tissue lines the respiratory system: lymphocytes recognise and deactivate microbes entering

Disorders that Affect the Respiratory System

• COPD prevalence in 2020, across both males and females, was 10.6%, which translates to 480 million cases
• Chronic obstructive pulmonary disease is the third leading cause of death worldwide, and caused 3.23 million deaths in 2019
• More than 90% of COPD deaths occur in low and middle-income countries