Functions and divisions, Mechanics of respiration, Pulmonary Circulation

Tissues in the body utilize Oxygen and produce Carbondioxide due to metabolism.

The main function of the respiratory system is to take in Oxygen from the atmosphere to deliver it to the tissues and to take out Carbondioxide from the tissues.

The entire respiration can be divided into three main divisions:

1. External respiration/ventilation/breathing

• Absorption of Oxygen and removal of Carbondioxide from the body.

2. Transport of gases (Oxygen & Carbondioxide) in the blood.

3. Internal respiration or cellular respiration

• Utilization of Oxygen and production of Carbondioxide by the cells.

Air passages

Air passages form the communication between the atmosphere and the lung

It consists of the following parts:

• Nose and nasal cavity
• Pharynx
• Larynx
• Trachea (windpipe)
• Bronchi (divide extensively)
• Alveoli

Tracheo-bronchial Tree

Trachea (windpipe)

• 10-11 cm in length
• Lies in front of oesophagus
• Descends from the 6th cervical vertebra till the 5th thoracic vertebrae
• Divides into the left and right bronchi

Large bronchi

• Divide and re-divide into ever smaller branches, the smallest branches, called bronchioles.

Bronchioles

• Give rise to alveolar ducts which lead to tiny sac-like structures called pulmonary alveoli.
• The two lungs contain approx. 300 million alveoli giving a surface area of more than 70 square meters.

Between the trachea and the alveolar sac, the air passage divides 23 times.

The trachea is labeled as generation `zero‘. The two major divisions of the trachea form the 'first' generation and so on…

The alveolar sac is the 23rd and the last generation.

Conducting zone:

• 0 to 16th generation where no exchange of gases is possible.
• Called dead space (150 ml).

Respiratory zone:

• 17th generation onwards
• Volume is 4 litres.

Respiratory Membrane

Alveoli is separated from the blood by a wall called the respiratory membrane .

It consists of the following:

• Alveolar wall
• Capillary wall
• It has a thickness of 0.5 mm. Due to its thinness, the gaseous exchange between the alveoli and blood capillaries is completed within a fraction of a second.

The lungs are covered by 'pleura', which has two layers:

1. Parietal pleura
2. Visceral pleura

Potential space called the pleural cavity is filled with lubricating serous fluid called the pleural fluid (2ml).

Functions of Lungs

Main function: gas exchange

Non-respiratory functions of lungs

1. Defence mechanism

• Humidifies the inspired air.
• Bronchial secretions contain  immuno­globulins (IgA) that help resist infection.
• Prevents foreign bodies from reaching the alveoli.

2. Functions of pulmonary circulation

• Reservoir for left ventricle
• Pulmonary circulation acts as a filter for blood clots
• Forms fluid exchange medium and drug absorption

3. Metabolic functions

• Synthesis of surfactant

Application and implication in nursing

Asthma:

• It is wheezing and breathlessness caused by a narrowing of the airways

Bronchitis:

• It is inflammation of the lungs (larger airways)

Emphysema:

• It is a disease of the alveoli (air sacs) of the lungs

Mechanism of Respiration

Pulmonary ventilation

• consists of inspiration and expiration.

Inspiration

• Active process.
• During inspiration, the size of the thoracic cavity is increased by contraction of inspiratory muscles.
• Expansion of lungs: drawing in of air into the lungs.

Expiration

• Passive process.
• Elastic recoil of the thoracic wall and lungs.

At rest, an adult breathes at a respiratory rate of 12-14 breaths/min.

Amount of air inspired or expired per breath is approx. 500 ml.

6-7 litres of air is breathed in or out of the lungs per minute and this is called pulmonary ventilation.

Mechanism of Inspiration

Inspiration is an 'active' process.

During inspiration, the thorax is enlarged by the following movements:

1. Rib Movements
2. Diaphragmatic Movements

Rib movements

On inspiration

• Upper ribs move upwards to assume a more horizontal position due to contraction of external intercostal muscles to cause increase in the antero-posterior diameter of the chest.
• The lower ribs also move outwards and upwards to cause increase in the transverse diameter of the thorax.

Movement of Diaphragm

• During inspiration, the diaphragm muscle contracts causing an increase in the vertical diameter of the thoracic cage.
• Diaphragmatic movements account for 75% of inspiration at rest.

Accessory Muscles of Inspiration (Acts during deep inspiration)

Scalene and sternocleiclomastoid muscles:

• These muscles help to elevate the thoracic cage during deep inspiration.

Intrinsic muscles of the larynx:

• Abductor muscles of the vocal cords contract early in the inspiratory phase, pulling the vocal cords apart and opening the glottis.

Mechanism of Expiration

In resting state, expiration is a passive process

During forced expiration, muscles of expiration contract, which are

• Anterior abdominal wall muscles
• Internal intercostal muscles
• Adductor muscles of vocal cords

Contraction of (acts during deep inspiration)

Anterior abdominal wall muscles

• Increases the intra-abdominal pressure and draws the lower ribs down and medially thus helping in expiration.

Internal intercostal muscles:

• Pull the upper ribs down thus helping in expiration

Adductor muscles of vocal cords

• Its function is protective, i.e. to prevent the entry of food and fluid into the trachea.

Pulmonary circulation

Characteristic Features of Pulmonary Circulation

• Thin walled vessels.
• Highly elastic and more distensible.
• Larger Pulmonary capillaries – forms capillary basket.
• Less vascular resistance.
• Low pressure system.

Functions of pulmonary circulation

• Reservoir for left ventricle.
• Pulmonary circulation as a filter for blood clots\Fluid exchange and drug absorption.