Mechanics of Respiration

Mechanics of Respiration

Respiratory movements, respiratory pressures, compliance and work of breathing.

Respiratory Movements Muscles Thoracic Cage Movements of Lungs Respiratory Pressures Intrapleural Pressure Intra-alveolar Pressure Transpulmonary Pressure Compliance Work of Breathing

1 Respiratory Movements

Respiration occurs in two phases, inspiration and expiration.

During inspiration, thoracic cage enlarges and lungs expand so that air enters the lungs easily.

During expiration, thoracic cage and lungs decrease in size and attain the preinspiratory position so that the lungs empty easily.

During normal quiet breathing, inspiration is the active process and expiration is the passive process.

2 Muscles of Respiration

Muscles involved in respiratory movements are inspiratory muscles and expiratory muscles.

However, respiratory muscles are generally classified into two types.

  1. Primary or major respiratory muscles which are responsible for change in size of thoracic cage during normal quiet breathing.
  2. Accessory respiratory muscles that help primary respiratory muscles during forced respiration.

Inspiratory Muscles

Muscles involved in inspiratory movements are known as inspiratory muscles which are primary or accessory muscles.

Primary Inspiratory Muscles

Primary inspiratory muscles are diaphragm, which is supplied by phrenic nerve (C3 to C5) and external intercostal muscles, supplied by intercostal nerves (T1 to T11).

Accessory Inspiratory Muscles

Accessory inspiratory muscles are sternocleidomastoid, scalene, anterior serrati, elevators of scapulae and pectorals.

Expiratory Muscles

Muscles involved in expiratory movements are known as expiratory muscles which are primary or accessory muscles.

Primary Expiratory Muscles

Primary expiratory muscles are the internal intercostal muscles, which are innervated by intercostal nerves.

Accessory Expiratory Muscles

Accessory expiratory muscles are the abdominal muscles.

3 Movements of Thoracic Cage

During inspiration thoracic cage enlarges in all axes, viz. anteroposterior, transverse and vertical axis.

Increase in anteroposterior and transverse diameters occurs due to elevation of ribs.

Vertical diameter of thoracic cage is increased due to descent of diaphragm.

Change in size of thoracic cavity occurs because of movements of four units:
  1. Thoracic lid.
  2. Upper costal series.
  3. Lower costal series.
  4. Diaphragm.

1. Thoracic Lid

Thoracic lid is formed by manubrium sterni and the first pairs of ribs.

Movement of thoracic lid increases the anteroposterior diameter of thoracic cage.

2. Upper Costal Series

Upper costal series is constituted by second to sixth pairs of ribs.

Upper costal series increases the anteroposterior diameter and transverse diameter of the thoracic cage by pump handle movement and bucket handle movements.

Pump Handle Movement

During inspiration, there is elevation of upper costal series of ribs upwards and forward movement of sternum.

This movement is called pump handle movement.

It increases the anteroposterior diameter of the thoracic cage.

Bucket Handle Movement

Simultaneously, central portions of these ribs (arches of ribs) move upwards and outwards to a more horizontal position.

This movement is called bucket handle movement and it increases the transverse diameter of thoracic cage.

3. Lower Costal Series

It is formed by the seventh to tenth pairs of ribs.

Movement of lower costal series increases the transverse diameter of the thoracic cage.

These ribs also show bucket handle movement by swinging outward and upward.

Eleventh and twelfth pairs of ribs are the floating ribs, which are not involved in changing the size of thoracic cage.

4. Diaphragm

Movement of diaphragm increases vertical diameter of thoracic cage.

Normally, before inspiration diaphragm is dome shaped with convexity facing upwards.

During inspiration, due to the contraction of muscle fibers the central tendinous portion is drawn downwards so that the diaphragm is flattened and increases the vertical diameter of the thoracic cage.

4 Movements of Lungs

During Inspiration

During inspiration, due to enlargement of the thoracic cage, negative pressure is increased in the thoracic cavity.

It causes expansion of the lungs.

During Expiration

During expiration, thoracic cavity decreases in size to preinspiratory position.

Pressure in thoracic cage also comes back to the preinspiratory level.

It compresses lung tissues so that, air is expelled out of lungs.

Collapsing Tendency of Lungs

Lungs are under constant threat to collapse even under resting conditions because of certain factors.

Factors Causing Collapsing Tendency of Lungs

Two factors are responsible for collapsing tendency of lungs:

  1. Elastic property of lung tissues.
  2. Surface tension exerted on the surface of alveolar membrane by the fluid secreted from alveolar epithelium.

Factors Preventing Collapsing Tendency of Lungs

In spite of elastic property of lungs and surface tension in alveoli of lungs, collapsing tendency of lungs is prevented by two factors.

1. Intrapleural Pressure

Intrapleural pressure which is always negative keeps the lungs expanded and prevents the collapsing tendency of lungs.

2. Surfactant

Surfactant is a surface acting material or agent that is responsible for lowering the surface tension of a fluid and thereby prevents the collapsing tendency of lungs.

Surfactant that lines the epithelium of alveoli in lungs is known as pulmonary surfactant and it decreases the surface tension on the alveolar membrane.

Cells in Lungs Secreting Pulmonary Surfactant

  1. Type II alveolar epithelial cells in lungs.
  2. Clara cells in bronchi.

Chemistry of Surfactant

Surfactant is a lipoprotein complex formed by lipids, especially phospholipids, proteins and ions.

  1. Phospholipids: Phospholipids form about 75% of the surfactant. The major dipalmitoylphosphatidylcholine (DPPC) is the major phospholipid present in the surfactant.
  2. Other lipids: Triglycerides and phosphatidylglycerol (PG).
  3. Proteins: Called specific surfactant proteins which are of four types, SP-A, SP-B, SP-C and SP-D. SP-A and SP-D are hydrophilic, while SP-B and SP-C are hydrophobic. Surfactant proteins are vital components of surfactant and surfactant becomes inactive in the absence of proteins.
  4. Ions: Mainly calcium ions.

Functions of Surfactant

  1. Surfactant reduces surface tension in the alveoli of lungs and prevents collapsing tendency of lungs. The phospholipid molecule in the surfactant is responsible for this.
  2. Surfactant is responsible for stabilization of alveoli, which is necessary to withstand the collapsing tendency.
  3. It plays an important role in inflation of lungs at birth. In fetus, lungs are solid and not expanded. First respiratory movements are attempted by the infant, the lungs tend to collapse repeatedly. And, the presence of surfactant in alveoli prevents lungs from collapsing.
  4. Hydrophilic proteins of surfactant play a role in defense in the lungs by destroying bacteria and viruses.

Effect of Deficiency of Surfactant: Respiratory Distress Syndrome

Deficiency or absence of surfactant in infants causes collapse of lungs.

This condition is called respiratory distress syndrome or hyaline membrane disease.

Deficiency of surfactant occurs in adults also and it is called adult respiratory distress syndrome (ARDS).

5 Respiratory Pressures

Pressures are exerted in thoracic cavity and lungs during respiration are of two types.

  1. Intrapleural pressure.
  2. Intra-alveolar pressure.

6 Intrapleural Pressure

Definition

Intrapleural pressure is the pressure existing in pleural cavity, that is, in between visceral and parietal layers of pleura.

It is exerted by suction of that lines the pleural cavity.

It is also called intrathoracic pressure since it is exerted in the whole of thoracic cavity.

Normal Values of Intrapleural Pressure

Respiratory pressures are always expressed in relation to atmospheric pressure, which is 760 mm Hg.

Under physiological conditions, the intrapleural pressure is always negative.

Condition Intrapleural Pressure
At the end of normal inspiration −6 mm Hg (760 − 6 = 754 mm Hg)
At the end of normal expiration −2 mm Hg (760 − 2 = 758 mm Hg)
At the end of forced inspiration −30 mm Hg
During forced inspiration with closed glottis: Müller maneuver −70 mm Hg
During forced expiration with closed glottis: Valsalva maneuver +50 mm Hg

Cause for Negativity of Intrapleural Pressure

Pleural cavity is always lined by a thin layer of fluid that is secreted by the visceral layer of pleura.

This fluid is constantly pumped from the pleural cavity into lymphatic vessels.

Pumping of fluid creates the negative pressure in pleural cavity.

Measurement of Intrapleural Pressure

Intrapleural pressure is measured by direct method and indirect method.

In the direct method, intrapleural pressure is determined by introducing a needle into the pleural cavity and connecting the needle to a mercury manometer.

In indirect method, intrapleural pressure is measured by introducing the esophageal balloon, which is connected to a manometer.

Intrapleural pressure is considered as equivalent to the pressure existing in the esophagus.

Significance of Intrapleural Pressure

  1. Throughout the respiratory cycle intrapleural pressure remains lower than intra-alveolar pressure. This keeps the lungs always inflated.
  2. It prevents collapsing tendency of lungs.
  3. It causes dilation of vena cava and larger veins in thorax. Also, negative pressure acts like suction pump and pulls venous blood from lower part of body towards heart against gravity. Thus, the intrapleural pressure is responsible for venous return. So, it is called the respiratory pump for venous return.

7 Intra-alveolar Pressure

Definition

Intra-alveolar pressure is the pressure existing in alveoli of lungs.

It is also known as intrapulmonary pressure.

Normal Values of Intra-alveolar Pressure

Normally, intra-alveolar pressure is equal to atmospheric pressure, which is 760 mm Hg.

It becomes negative during inspiration and positive during expiration.

Condition Intra-alveolar Pressure
During normal inspiration −1 mm Hg (760 − 1 = 759 mm Hg)
During normal expiration +1 mm Hg (760 + 1 = 761 mm Hg)
At the end of inspiration and expiration Equal to atmospheric pressure: 760 mm Hg
During forced inspiration with closed glottis: Müller maneuver −80 mm Hg
During forced expiration with closed glottis: Valsalva maneuver +100 mm Hg

Measurement of Intra-alveolar Pressure

Intra-alveolar pressure is measured by using plethysmograph.

Significance of Intra-alveolar Pressure

  1. It causes flow of air in and out of alveoli. During inspiration, the intra-alveolar pressure becomes negative, so the atmospheric air enters the alveoli. And, during expiration, the air is expelled out of alveoli.
  2. It also helps in exchange of gases between alveolar air and blood.

8 Transpulmonary Pressure

Transpulmonary pressure is the difference between intra-alveolar pressure and intrapleural pressure.

It is the measure of elastic forces in lungs, which is responsible for collapsing tendency of lungs.

9 Compliance

Definition and Significance

Compliance is the ability of lungs and thorax to expand or it is the expansibility of lungs and thorax.

It is defined as the change in volume that occurs per unit change in pressure.

Determination of compliance is useful as it is the measure of stiffness of lungs.

Stiffer the lungs, less is the compliance.

Normal Values

Compliance is expressed in relation to respiratory pressures.

Compliance in Relation to Intra-alveolar Pressure

Compliance is the volume increase in lungs per unit decrease in intra-alveolar pressure.

  1. Compliance of lungs and thorax together: 130 mL/1 cm H2O pressure.
  2. Compliance of lungs alone: 220 mL/1 cm H2O pressure.

Compliance in Relation to Intrapleural Pressure

Compliance is the volume increase in lungs per unit decrease in intrapleural pressure.

  1. Compliance of lungs and thorax together: 100 mL/1 cm H2O pressure.
  2. Compliance of lungs alone: 200 mL/1 cm H2O pressure.

Thus, if lungs are removed from thorax, the expansibility (compliance) of lungs alone is doubled.

It is because of absence of the inertia and the restriction exerted by structures of thoracic cage, which interfere with expansion of lungs.

Applied Physiology: Variations

Increase in Compliance

Compliance increases in physiological and pathological conditions.

  1. In elders, lung compliance increases due to loss of elastic property of lung tissues.
  2. In emphysema (obstructive respiratory disease), lung compliance increases because of damage of alveolar membrane.

Decrease in Compliance

Compliance decreases in the following pathological conditions:

  1. Deformities of thorax like kyphosis and scoliosis.
  2. Paralysis of respiratory muscles.
  3. Pleural effusion (accumulation of fluid in pleural cavity).
  4. Fibrotic pleurisy (inflammation of pleura resulting in fibrosis).
  5. Abnormal thorax due to presence of air (pneumothorax), fluid (hydrothorax), blood (hemothorax) and pus (pyothorax) in pleural space.

10 Work of Breathing

Work done by respiratory muscles during breathing to overcome the resistance in thorax and respiratory tract is known as work of breathing.

Work Done by Respiratory Muscles

During the respiratory processes, inspiration is an active process and expiration is a passive process.

So, during quiet breathing, respiratory muscles perform the work only during inspiration and not during expiration.

Utilization of Energy

During work of breathing, energy is utilized to overcome three types of resistance.

  1. Airway resistance.
  2. Elastic resistance of lungs and thorax.
  3. Nonelastic viscous resistance.

1. Airway Resistance

Airway resistance is the resistance offered to passage of air through respiratory tract.

Work done to overcome this is called airway resistance work.

2. Elastic Resistance of Lungs and Thorax

Work done to overcome this elastic resistance is called compliance work.

3. Nonelastic Viscous Resistance

Work done to overcome this viscous resistance is called tissue resistance work.