Introduction to Respiratory system Structure and Function of the Respiratory System

We cannot teach people anything; we can only help them discover itwithin themselves.

Galileo Galilei

Introduc ti on to Respiratory system

Structure and Func ti on of the Respiratory System

Learning objectives

• Recognise the Physioanatomy of the upper and lower respiratory tract (Weibel model)

• Understand Air flow and factors affecting airway resistance.

• Understand the characteristic of innervation and blood supply of the lung.

• Discuss Lung defence mechanism.• Metabolic ,endocrine and other functions of the

respiratory system

Upper Airways-Nose, Sinuses, Larynx ,vocal cords•The major function of the upper airways is to "condition" inspired air so that by the time it reaches the trachea, it is at body temperature and fully humidified. •The nose -filter, entrap, and clear particles larger than 10 μm in size.•Nose provides the sense of smell.•Resistance to airflow in the nose during quiet breathing accounts for approximately 50% of the total resistance of the respiratory system.

• The sinuses lighten the skull, which makes upright posture easier, and they offer resonance to the voice.

• The major structures of the larynx include the epiglottis, arytenoids, and vocal cords . With some infections, these structures can become edematous (swollen) and contribute significantly to airflow resistance.

• The epiglottis and arytenoids "hood" or cover the vocal cords during swallowing.

Lower Airways-Trachea, Bronchi, Bronchioles, Respiratory Unit

IN THE CLINIC

• Asthma involves both large and small airways and is characterized by inflammation, predominantly mediated by lymphocytes and eosinophils, in the airways and reversible airway smooth muscle constriction (bronchospasm).

• Bronchiolitis is a disease of the small airways. It usually occurs in young infants and is caused by viruses, most commonly respiratory syncytial virus.

• Chronic bronchitis, a disease of smokers, is associated with a marked increase in mucus-secreting cells in the airways and an increase in mucus production.

Airflow in Airways

Two major factors determine the speed at which gas flows into the airways for a given pressure change:

• the pattern of gas flow- laminar/turbulent• resistance to airflow by the airways.

Reynolds number (Re)

• The Reynolds number (Re) is a dimensionless value that expresses the ratio of two dimensionally equivalent terms (kinematic/viscosity).

•

where d is the fluid density, v is the average velocity, r is the radius, and η is the viscosity.

• In straight tubes, turbulence occurs when the Reynolds number is greater than 2000.

AIRFLOW, PRESSURE, AND RESISTANCE RELATIONSHIPS

Airway Resistance• (Q = ∆P/R). Resistance is determined by

Poiseuille's law

• The medium sized bronchi are the sites of highest airway resistance.

• smallest airways are arranged in parallel [total resistance is less than individual resistances] and the total cross sectional area for smallest airway is greater than that for medium sized bronchi.

Factors affecting airway resistance1. Smooth muscle:

Automatic control• Parasympathetic • Sympathetic

local factors

2. Lung volume:

3.Mucus, Edema

Neuro-humoral Regulation of Airway Resistance

AIRWAY DILATION

• Sympathetic activation causes bronchioles to dilate, mainly by inhibiting ACh release rather than through direct effects on the musculature.

• SNS terminals release norepinephrine, which binds to a presynaptic B2-adrenergic receptor. This receptor is particularly sen-sitive to epinephrine release from the adrenal medulla during SNS activation. SNS-mediated bronchodilation is important for facilitating increased airfow to the blood–gas interface during exercise

AIRWAY CONSTRICTION

• Parasympathetic nerve fibers from the vagus nerve release acetylcholine (ACh) from their termi-nals when active. ACh binds to M3 muscarinic ACh receptors and causes bronchoconstriction, which reduces airflow.

• Agents such as histamine, acetylcholine, thromboxane A2, prostaglandin F2, and leukotrienes released by airway cells in response to various triggers, such as allergens and viral infections

Factors That Contribute to Airway Resistance

•Dec in lung volume•mucus, edema• contraction of bronchial smooth muscle

BLOOD SUPPLY TO THE LUNG

PULMONARY CIRCULATION

• The pulmonary circulation brings deoxygenated blood from the right ventricle to the gas-exchanging units for removal of CO2 and oxygenation before blood is returned to the left atrium for distribution to the rest of the body.

BRONCHIAL CIRCULATION

• The bronchial circulation arises from the aorta and contains 1% to 2% of cardiac output

• They nourish the walls of the bronchi, bronchioles, blood vessels, and nerves, and they perfuse the lymph nodes and most of the visceral pleura

Lung Interstitium• Fibroblasts -synthesize and secrete collagen and elastin• Collagen is the major structural component of the lung

that limits lung distensibility. • Elastin is the major contributor to elastic recoil of the

lung.• Cartilage is a tough, resilient connective tissue that

supports the conducting airways of the lung and encircles about 80% of the trachea.

• Kult schitzky cells, neuroendocrine cells, are found in clumps throughout the tracheobronchial tree and secrete biogenic amines, including dopamine and 5-hydroxytryptamine (serotonin).

Other Functions of the Respiratory System

Lung Defense mechanisms:

• Removal of Particles larger than 10 μm – by hairs of nostrils

• particles of 2 – 10 μm in diameter are removed by cilia – ciliary escalator;

• Particles less than 2 μm are removed by pulmonary alveolar macrophages (dustcells)].

Mucociliary Escalator The cilia are bathed in a periciliary fluid .On top of the periciliary layer and the beating cilia rests a mucus layer, a complex mixture of proteins and polysaccharides secreted from specialized cells, glands, or both in the conducting airway. This combination allows for the trapping of foreign particles (in the mucus) and their transport out of the airway (powered by ciliary beat)

Cystic fibrosis • Is a genetically inherited disease that adversely affects chloride

channels in exocrine glands.• Faulty regulation of Na+ absorption and inability to secrete Cl–

reduce the volume of liquid on airway surfaces; i.e., they are "dehydrated."

• Both the thickening of mucus and the depletion of the periciliary liquid lead to adhesion of mucus to the airway surface.

• Mucus adhesion leads to a failure to clear mucus from the airways both by ciliary and airflow-dependent (cough) mechanisms.

• This results in a reduced periciliary layer that inhibits function of the mucociliary escalator, and alters the local environment to reduce the effectiveness of antimicrobial secretions.

Tobacco smoke

• Impairs respiratory cilia function. • Ciliary arrest allows bacteria and other inhaled

particulates to accumulate in the lungs, causing local irritation and epithelial inflammation.

• Smokers are prone to coughing episodes and bronchitis as a result.

• Ciliary function is typically restored with tobacco cessation.

Airway epithelial cells- produce

• Secretory immunoglobulins (IgA),• Collectins (including Surfactant A and D),

Defensins • Peptides and proteases,• reactive oxygen species, and reactive nitrogen

species

Non Respiratory Functions

• Blood reservoir• lyses of clots in pulmonary vessels.• Small thrombi are filtered.• Plays a role in clotting mechanism – mast cells contain

heparin.• Excretion of volatile substances – ketone bodies• Helps in Phonation• Regulation of body temperature• Regulation of water balance• Regulation of acid base balance

Metabolic & Endocrine Functions of the Lungs