Respiratory Movements
Introduction
Respiration
occurs in two phases namely
During
inspiration, thoracic cage enlarges and lungs expand so that air enters the
lungs easily. During expiration, the thoracic cage and lungs decrease in size and
attain the preinspiratory position so that air leaves the lungs easily.
During
normal quiet breathing-
-inspiration
is the active process
-expiration
is the passive process
MUSCLES
OF RESPIRATION
Anatomically
respiratory muscles are of two types:
However,
functionally 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.
Primary
inspiratory muscles
Primary
inspiratory muscles are the diaphragm, which is supplied by phrenic nerve (C3
to C5) and external intercostal muscles, supplied by intercostal nerves (T1 to
T11).
Pump
handle movement
Contraction
of external intercostal muscles causes elevation of these ribs and upward and
forward movement of sternum. This movement is called pump handle movement. It
increases anteroposterior diameter of the thoracic cage.
Bucket
handle movement
Simultaneously,
the 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.
Lower
Costal Series
Lower
costal series includes seventh to tenth pair of ribs. Movement of lower costal
series increases the transverse diameter of thoracic cage by bucket handle movement.
Bucket
handle movement
Lower
costal series of ribs also show bucket handle movement by swinging outward and
upward. This movement increases the transverse diameter of the thoracic cage.
Eleventh
and twelfth pairs of ribs are the floating ribs. These ribs are not involved in
changing the size of thoracic cage.
Movement
of diaphragm increases the vertical diameter of thoracic cage. Normally, before
inspiration the diaphragm is dome shaped with convexity facing upwards. During
inspiration, due to the contraction, muscle fibers are shortened. But the
central tendinous portion is drawn downwards so the diaphragm is flattened. Flattening
of diaphragm increases the vertical diameter of the thoracic cage.
Movements
of Lungs
During
inspiration, due to the enlargement of thoracic cage, the negative pressure is
increased in the thoracic cavity. It causes expansion of the lungs. During expiration,
the
thoracic cavity decreases in size to the preinspiratory position. Pressure
in the thoracic cage also comes back to the preinspiratory level. It compresses
the lung tissues so that, the air is expelled out of lungs.
Accessory
inspiratory muscles
Sternocleidomastoid,
scalene, anterior serrati, elevators of scapulae and pectorals are the
accessory inspiratory muscles.
Expiratory
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.
Movements
of Thoracic Cage
Inspiration
causes enlargement of thoracic cage. Thoracic cage enlarges because of increase
in all diameters, viz. anteroposterior, transverse and vertical
diameters.
Anteroposterior
and transverse diameters of thoracic cage are increased by the elevation of
ribs. Vertical diameter is increased by the descent of diaphragm.
In
general, change in the size of thoracic cavity occurs because of the movements
of four units of structures:
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 pair of ribs. It is also
called thoracic operculum. Movement of thoracic lid increases the anteroposterior
diameter of thoracic cage. Due to the contraction of scalene muscles,
the first ribs move upwards to a more horizontal position. This increases the
anteroposterior diameter of upper thoracic cage.
2. Upper
Costal Series
Upper
costal series is constituted by second to sixth pair of ribs. Movement of upper
costal series increases the anteroposterior and transverse diameter of the
thoracic
cage.
Movement
of upper costal series is of two types:
Collapsing
Tendency of Lungs
Lungs
are under constant threat to collapse even in resting conditions because of
certain factors.
Factors Causing Collapsing Tendency of Lungs
Two
factors are responsible for the collapsing tendency of lungs:
1. Elastic
property of lung tissues: Elastic tissues of lungs show constant
recoiling tendency and try to collapse the lungs
2. Surface
tension: It is the tension exerted by the fluid secreted from
alveolar epithelium on the surface of alveolar membrane. Fortunately, there are
some factors, which save the lungs from collapsing.
Factors
Preventing Collapsing Tendency of Lungs
In
spite of elastic property of lungs and surface tension in the alveoli of lungs,
the collapsing tendency of lungs is prevented by two factors:
1. Intrapleural
pressure: It is the pressure in the pleural cavity, which is
always negative (see below). Because of negativity, it keeps the lungs expanded
and prevents the collapsing tendency of lungs produced by the elastic tissues.
2. Surfactant:
It is a substance secreted in alveolar epithelium. It reduces surface
tension and prevents the collapsing tendency produced by surface tension.
Surfactant
Surfactant
is a surface acting material or agent that is responsible for lowering the
surface tension of a fluid. Surfactant that lines the epithelium of the alveoli
in lungs
is
known as pulmonary surfactant and it decreases the surface tension on
the alveolar membrane.
Source
of secretion of pulmonary surfactant
Pulmonary
surfactant is secreted by two types of cells:
1. Type
II alveolar epithelial cells in the lungs, which are called surfactant
secreting alveolar cells or pneumocytes. Characteristic feature of these cells
is the presence of microvilli on their alveolar surface.
2. Clara
cells, which are situated in the bronchioles. These cells are also called
bronchiolar exocrine cells.
Chemical
Composition of surfactant
Surfactant
is a lipoprotein complex formed by lipids especially phospholipids, proteins
and ions.
1. Phospholipids:
Phospholipids form about 75% of the surfactant. Major phospholipid present
in the surfactant is dipalmitoylphosphatidylcholine (DPPC).
2. Other
lipids: Other lipid substances of surfactant are triglycerides
and phosphatidylglycerol (PG).
3. Proteins:
Proteins of the surfactant are called specific surfactant proteins. There
are four main surfactant proteins, called SPA, SPB, SPC and SPD.
SPA and
SPD are hydrophilic, while SPB and SPC are hydrophobic. Surfactant proteins are
vital components of surfactant and the surfactant becomes inactive in the
absence of proteins.
4. Ions:
Ions present in the surfactant are mostly calcium ions.
Formation of surfactant
Type
II alveolar epithelial cells and Clara cells have a special type of membrane
bound organelles called lamellar bodies, which form the intracellular
source of surfactant. Laminar bodies contain surfactant phospholipids and
surfactant proteins. These materials are synthesized in endoplasmic reticulum
and stored in laminar bodies. By means of exocytosis, lipids and proteins of lamellar
bodies are released into surface fluid lining the alveoli. Here, in the
presence of surfactant proteins and
calcium,
the phospholipids are arranged into a lattice (meshwork) structure called tubular
myelin. Tubular myelin is in turn converted into surfactant in the form
of a film that spreads over the entire surface of alveoli. Most of the
surfactant is absorbed into the type II alveolar cells, catabolized and the
products are loaded into lamellar bodies for recycling.
Factors necessary for the formation and spreading of surfactant
Formation
of surfactant requires many substances. Formation of tubular myelin requires
DPPC, PG and the hydrophobic proteins, SPB and SPC. Formation of surfactant
film requires SPB, SPC and PG. Type II alveolar epithelial cells occupy only
about 5% of alveolar surface. However, the surfactant must spread over the
entire alveolar surface. It is facilitated by PG and calcium ions. Glucocorticoids
play important role in the formation of surfactant.
Functions of surfactant
1. Surfactant
reduces the surface tension in the alveoli of lungs and prevents collapsing
tendency of lungs.
Surfactant
acts by the following mechanism:
1. Phospholipid
molecule in the surfactant has two portions. One portion of the molecule is hydrophilic.
This portion dissolves in water and lines the alveoli. Other portion is hydrophobic
and it is directed towards the alveolar air. This surface of the phospholipid
along with other portion spreads over the alveoli and reduces the surface
tension. SPB and SPC play active role in this process.
2.
Surfactant is responsible for stabilization of the alveoli, which is necessary
to withstand the collapsing tendency.
3.
It plays an important role in the inflation of lungs after birth. In fetus, the
secretion of surfactant begins after the 3rd month. Until birth, the lungs are
solid and not expanded. Soon after birth, the first breath starts because of
the stimulation of
respiratory
centers by hypoxia and hypercapnea. Although the respiratory movements are
attempted by the infant, the lungs tend to collapse repeatedly. And, the
presence of surfactant in the alveoli prevents the lungs from collapsing.
4.
Another important function of surfactant is its role in defense within the
lungs against infection and inflammation. Hydrophilic proteins SPA and SPD
destroy
the bacteria and viruses by means of opsonization. These two proteins also
control the formation of inflammatory mediators.
Effect
of deficiency of surfactant – respiratory distress syndrome
Absence
of surfactant in infants, causes collapse of lungs and the 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). In addition, the deficiency of surfactant increases the
susceptibility for bacterial and viral infections.
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