Respiratory System

Respiratory system

Respiration
Taking oxygen and giving CO2
Types:

• External in the alveoli of the lungs
• Internal in the mitochondria

Organs

• Nose and paranasal sinuses
• Pharynx
• Larynx
• Trachea
• Bronchi and their branches
• Lungs and their coverings
• Muscles of respiration

Nasal cavity

• 1st organ of respiration
• Cavity divided into two by nasal septum

Boundaries:

• Anterior- nostrils
• Posterior- posterior choana
• Medial- septum
• Lateral- bony conchae
• Roof- cribriform plate of ethmoid bone
• Floor- palate

The septum has two parts:

• Bony part- made by nasal bone , ethmoid, maxilla, vomer  and palatine.
• Anterior cartilagenous part  made of septal cartilage and with anteriormost fibrofatty plate called columella

The anterior part of the nose is the commenest site of bleeding. This part of nose ismella called as kisselbach’s or little’s area. 

The bleeding from the nose is called as epistaxis.

• The nose is lined by cilliated columnar epithelium.
• The lateral wall has the three bones, superior, middle and inferior conchae.
• The superior and middle conchae are the projections from the maxilla, whereas the inferior conchae is an independent bone.
• The space below these conchae are called meatus. So we have three meatuses.
• Superior meatus- opening of posterior ethmoidal sinus
• Middle meatus- opening of maxillary, anterior ethmoidal and frontal sinus
• Inferior meatus- opening of nasolacrimal duct.
• Sphenoidal sinus opens in the roof of the nasal cavity near the posterior choana through sphenoethmoidal recess.

Functions:

• First part of respiratory passage.
• It filters the air, it traps the particles >10 microns.
• It humidifies the inhaled air.
• It warms the air.
• Smell perception.

Paranasal sinus 

• They are the air containing cavities in the cranial bones.
• They open in the lateral wall of the nose.
• They are also lined by the respiratory epithelium.

They are:
Frontal sinus- a pair in the frontal bone
Maxillary sinus- a pair in the maxilla, they are the largest ones.
Ethmoid sinus- multiple in the ethmoid bone, and are multiple.

Sphenoid sinus- they are single in the ethmoid bone.

Maxillary sinus 

• This is the first sinus to be devloped in the human being.
• This is a pyramidal-shaped sinus occupying the cavity of the maxilla. 
• Its medial wall forms part of the lateral face of the nasal cavity and bears on it the inferior concha.
• Above this concha is the opening, or ostium, of the maxillary sinus into the middle meatus in the hiatus semilunaris .
• This opening, unfortunately, is inefficiently placed as an adequate drainage point.
• The infra-orbital nerve lies in a groove which bulges down into the roof of the sinus.
• Its floor bears the impressions of the upper premolar and molar roots. 
• These roots are separated only by a thin layer of bone which may, in fact, be deficient so that uncovered dental roots project into the sinus.

Functions of sinus:

• They make the head light
• Vocal resonance

The infection of sinus is called as sinusitis.

Pharynx

Pharynx is a muscular tube like structure, that extends from the end of soft palate to the beginning of the esophagus.
It has three parts:

• Nasopharyx
• Oropharynx
• Laryngopharynx or hypopharynx

Nasopharynx;

• It lies behind the nasal cavity
• It has the openings of two eustachean tubes
• It has some lymphoid organ called as adenoid.
• It is lined by cilliated columnar epithelium.
• It serves as passage of air.

Oropharynx:

• It lies behind the oral cavity, behind the uvula.
• It is lined by the stratified squamos epithelium.
• It serves as the passage of both the food and the air.
• The palatine tonsils (tonsils) lie here in the oropharynx.
• Waldeyer’s ring.

Hypopharynx:

• It is the part of pharyns that lies in front of the larynx.
• It is lined by stratified squamos epithelium.
• It serves as passage of the food only.

Larynx 

• It is the part of the respiratory system that begins from the nasopharynx and ends in the trachea.
• It is also called as voicebox.
• It lies in front of cervical vertebrae, c3-c6.
• It is lined by cilliated columnar epithelium.

It has the following cartilages: (9 total)

• Thyriod
• Cricoid
• Epiglottis (elastic cartilage)
• Arytenoid (2)
• Corniculate 2 and 
• Cueniform. 2

Muscles of larynx 

Extrinsic laryngeal muscles

• Thyrohyoid muscles
• Sternothyroid muscles
• Omohyoid muscles
• Inferior constrictor muscles
• Digastric
• Stylohyoid
• Mylohyoid
• Geniohyoid
• Hyoglossus

Intrinsic Muscles

Cricothyroid muscles 

• lengthen and stretch the vocal folds.

Posterior cricoarytenoid muscles 

• abduct and externally rotate the arytenoid cartilages, resulting in abducted vocal folds.

Lateral cricoarytenoid muscles 

• adduct and internally rotate the arytenoid cartilages, which can result in adducted vocal folds.

Transverse arytenoid muscle 

• adducts the arytenoid cartilages, resulting in adducted vocal folds.

Oblique arytenoid muscles 

• narrow the laryngeal inlet by constricting the distance between the arytenoid cartilages.

Vocalis muscles 

• increase the thickness of the vocal folds, lowering the tone.

Thyroarytenoid muscles –

•  sphincter of vestibule, narrowing the laryngeal inlet, shortening the vocal folds, and lowering voice pitch.

Inner surface of larynx:

It has two vocal cords, they are

• False vocal cords (upper)
• True vocal cords (lower).
• The space between these folds is called as the ventricle of the larynx.

Blood supply

laryngeal branch of ascending pharyngeal artery

Nerve supply (from the branches of vagus nerve)- 

• Superior laryngeal nerve, that supplies the mucosa above vocal folds, and a muscle, cricothyroid.
• Recurrent laryngeal nerve, it supplies the mucosa below the vocal folds, and all muscles of the larynx except the cricothyroid.

Functions of larynx:

• Air passage
• Protection of airway
• Humidification and warming of air
• Voice production

Trachea

• Trachea begins from the lower part of larynx.
• It is made of 16-20 incomplete rings of hyaline cartilage.
• The posterior part has a muscle called trachealis muscle.
• It is lined by cilliated columnar epithelium.
• It bifurcates to become two bronchi, at the level of T4.

Function:

• Air passage
• Mucocilliary clearance
• Cough reflex
• Humidification of air

Bronchi 

• They are the branches from the trachea, and has the same tissue as the trachea.
• They divide 23 times to reach the alveoli.
• They are lined by cilliated columnar epithelium.
• The division goes as bronchi, bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts and alveoli.
• The part after respiratory bronchioles is there for gas exchange.

Lungs 

• They are a pair of the lungs in the thoracic cavity, on either side of midline.
• They are enclosed in the thin sac like structure called as pleura.
• The pleura has two layers. The parietal pleura is the outer covering, which is tough and is pain sensitive, attached to the thorax wall.
• The inner layer is the visceral layer, it is attached to the the lungs, and is pain insensitive.
• The space between the two is called as the pleural space.
• The right lungs is the larger one and has three lobes.
• The left lung is the smaller one, with the two lobes only, due to relative position of the heart.
• The medial surface of the lung is called hilum, from where there enters the bronchi, pulmonary arteries, pulmonary veins and the lymphatics.
• Each lungs has small multiple respiratory units called as alveoli, where the gas exchange occurs.
• The total surface of the lungs is around 120 m2, the size of a tennis court.

Functions of lungs:

• Respiration
• Angiotensin converting enzyme production
• Protection
• Metabolism of certain chemicals and drugs
• Excretion of volatile chemicals like ethanol

Muscles of respiration 

Primary muscles:

• Diaphragm
• Intercoastal muscles

Secondary muscles:

• Sternomastoid
• Scalanei 
• Trapezius
• Abdominal muscles

Respiration 

It has two phases:

Inspiration

• Active process, needs energy.
• The pulling of diaphragm down to abdominal cavity increases the thoracic volume, and hence reducing the pressure in the lungs. This causes the air to move from the atmosphere into the lungs (pump handle movement).
• Similarly the pulling of the ribcage anterior by the intercoastal muscles also has the same effect (bucket handle movement).

Expiration:

• It is the passive process.
• The diaphragm recoils back into the thoracic cavity, and hence decreasing the thoracic volume, and increasing the pressure. And hence the air rushes out of the lungs into the air.
• Forced expiration is active process. The abdominal muscles are forced inside, and creating the increased intrathoracic  and intraabdominal pressure.

Lung volumes 

• Tidal volume- 500 ml
• Inspiratory reserve volume- 2.5 L
• Inspiratory capacity- 3 L
• Expiratory reserve volume- 1.5 L
• Residual volume- 1.2 L
• Vital capacity- 5 L
• Total lung capacity

• Minute ventillation
• Dead space
• Alvelar ventillation 

Forces and Pressures during respiration 

• Intrapleural pressure: it is always negaive pressure. It is around -5 cm H2O during equlibrium and it becomes more negative -7.5 during inspiration. This negative pressure is created by the two opposite forces, the chest recoil and the recoil of the lungs.
• Intra alveolar pressure: it is the pressure inside the alveoli. It is 0 (atmospheric) during equilibrium, -1 during inspiration and +1 during expiration. 

Compliance of lungs:

• The change is volume with change in pressure is called as lung compliance.
• It is calculated as dV/dP.
• It is around 0.02 L/cmH2O.

Gas transport in blood 
Oxygen:

• As oxyhemoglobin 99%
• Dissolved oxygen 

Carbon dioxide:

• As bicarbonate 70%
• Carbaminohemoglobin 23%
• Dissolved form 7%

Gas exchange and oxygen transport 

A. Partial pressure equals the total pressure times the fractional gas concentration.
B. Assuming that total pressure is atmospheric (760 mm Hg) and the fractional concentration of O2 is 0.21, then
Po2 = 0.21 × 760 = 160 mm Hg
C. The partial pressure of humidified inspired air is calculated as follows:
where
Patm = atmospheric pressure
PI gas = partial pressure of inspired gas
PH2O = partial pressure of H2O vapor
F gas = concentration of gas
The partial pressure of H2O at 37° is 47 mm Hg. Thus,

PI gas = F gas (Patm − PH2O

Saturation is the percentage of Hb-binding sites occupied by O2.

• 1. Each gram of Hb has an oxygen capacity of 1.34 mL O2, and because 100 mL of blood contains 15 g Hb, completely oxygenated blood contains approximately 20 mL O2 (1.34 mL O2 × 15 g Hb/100 mL).
• 2. Thus, the oxygen capacity of Hb in blood is approximately 20 mL O2/100 mL of blood or 20 vol%.
• 3. Each Hb molecule contains four subunits: two have α chains and two have β chains.

Concentration gradients of the gases. 

• The PO2 is about 104 mmHg in the alveolar air and 40 mmHg in the blood arriving at an alveolus.  
• Oxygen therefore diffuses from the air into the blood, where it reaches a PO2 of 104 mmHg. 
• Before the blood leaves the lung, however, this drops to about 95 mmHg because blood in the pulmonary veins receives some oxygen-poor blood from the bronchial veins by way of anastomoses.
• The PCO2 is about 46 mmHg in the blood arriving at the alveolus and 40 mmHg in the alveolar air. Carbon dioxide therefore diffuses from the blood to the alveoli

The Bohr effect. 

• Active tissues also generate extra CO2, which raises the H concentration and lowers the pH of the blood. Like elevated temperatures, a drop in pH shifts the oxygen-hemoglobin dissociation curve to the right and promotes oxygen unloading. 
• The increase in HbO2 dissociation in response to low pH is called the Bohr effect.

Haldane effect 

• Low level of HbO2 or O2 favors the systhesis of carbaminohemoglobin.
• This effect is called Haldane effect.

Control of respiration
Respiratory center:

• Primary: medulla
• Secondary: pons

Chemoreceptors:

• Central- medulla, they detect increased CO2 level
• Peripheral-
• Aortic arch and carotid bodies
• They detect decreased O2 level.