Human respiratory system|types of respiratory system| Anatomy of respiratory system|Medical World

What is Respiratory system?

The term respiration means the exchange of gases between body cells and the environment. The involve two main process.

Types of Respiratory system

Breathing

Exchange of gases

Breathing

Breathing supplies oxygen to the alveoli, and elimination carbon dioxide. This is a movement of air into and out of the lungs.

Muscles of breathing

Expansion of the chest during inspiration occurs as a result of muscular activity, partly voluntary and partly involuntary. The main muscles used in normal quiet breathing are the external intercostal muscles and the diaphragm.

Intercostal muscles

there 11 pairs of intercostal muscles occupying the spaces between the 12 pairs of ribs. They are arranged in two layers, the external and internal intercostal muscles .

 
External intercostal Muscle
 
these extend downwards and forwards from the lower border of the rib above to the upper border of the rib below. They are involved in inspiration. 



       
The internal intercostal muscles- 
 
these extends downwards and backward from the lower border of the rib above to the upper border of the rib below, crossing the external intercostal muscle fibres at right angles. The internal intercostal are used when expiration becomes active, as in exercise.
The first rib fixed.therefore, when the external intercostal muscles contact they pull all the other ribs towards the first ribs. They ribcage moves as a unit, upwards and outwards, enlarging the thoracic cavity. The intercostal muscles are stimulated to contract by the intercostal nerves.
 

Diaphragm

The diaphragm is a dome-shaped muscular structure separating the thoracic and abdominal cavities. It forms the floor of the thoracic and the roof of the abdominal cavity and consists of a central tendon from which muscle fibres radiate to be attached to the lower ribs and sternum and to the vertebral column by two crura. When the diaphragm is relaxed, the central tendon is at the level of the 8th thoracic vertebra. When it contacts, its muscle fibres shorten and the central tendon is pulled downwards to the level of the 9th thoracic vertebra, lengthening the thoracic cavity. This decreases pressure in the thoracic cavity and increase it in the abdominal and pelvic. The diaphragm is supplied by the phrenic nerves. Quiet.restful breathing is sometimes called diaphragmatic breathing because 75% of the work is done by the diaphragm. During inspiration, the external intercostal muscles and the diaphragm contract simultaneously, enlarging the thoracic cavity in all direction, that is from back to front, side to side and top to bottom.

Accessary muscles of respiration

When extra respiratory effort is required, additional muscles are used. Forced inspiration is assisted by the sternocleidomastoid muscles and the scalene muscles, which link the cervical vertebrae to the first two ribs, and increase ribcage expansion. Forced expiration is helped by the activity of the internal intercostal muscles and sometimes the abdominal muscles, which increase the pressure in the thorax by squeezing the abdominal content.

Cycle of breathing

The average respiratory rate is 12-15 breaths per minute. Each breath consists of three phases;

Inspiration

Expiration

Pause

The visceral pleura is adherent to the lungs and the parietal pleura to the inner wall of the thorax and to the diaphragm. Between them is a thin firm of pleural fluid. Breathing depends upon changes in pressure and volume in the thoracic cavity. It follows the underlying physical principal that increasing the volume of a container decreases the pressure inside it, and that decreasing the volume of a container increases the pressure inside it. Since air flows from an area of high pressure to an area of low pressure, changing the pressure inside the lungs determines the direction of airflow.

Inspiration

Simultaneous contraction of the external intercostal muscles and the diaphragm expends the thorax. As the parietal Pleura is firmly adhered to the diaphragm and the inside of the ribcage ,it is pulled outward along with them. This pulls the visceral pleura outwards too, since the two pleura are held together by the thin film of pleural fluid. Because the visceral pleura is is firmly adherent to the lungs, the lung tissue is , therefore , also pulled up and out with the ribs, and downwards with the diaphragm. This expends the lungs, and the pressure within the alveoli and in the air passages falls, drawing air into the lungs in an attempt to equalise atmospheric and alveolar air pressures. The process of inspiration is active , as it needs energy for muscle contraction. The negative pressure created in the thoracic cavity aids venous return to the heart and is known as the respiratory pump. At rest, inspiration lasts about 2 seconds.

Expiration

Relaxation of the external intercostal muscles and the diaphragm results is downward and inward movement of the ribcage and elastic recoil of the lungs. As this occurs , pressure inside the lungs rises and expels air from the respiratory tract. At the end of expiration, the lungs still contain some sir, and are prevented from complete collapse by the intact pleura. This process is passive as it does not require the expenditure of energy. At rest, expiration lasts about 3 seconds, and after expiration there is a pause before the cycle begins.

Physiology variables affecting breathing

Elasticity

elasticity is the ability of the lung to return to its normal shape after each breath. Loss of elasticity , e.g. in emphysema, of the connective tissue in the lungs necessitates forced expiration and increased effort on inspiration.

Compliance

this is the stretchability of the lungs, I.e. the effort required to inflate the alveoli. The healthy lung is very compliant, and inflates with very little effort. When compliance is low the effort needed to inflate the lungs is greater than normal, e.g. when insufficient surfactant is present. Note that compliance and elasticity are opposing forces.

Airway resistance

when this is increased, e.g. in bronchoconstriction more respiratory effort is required to inflate the lungs.

Lungs volume and capacities

In normal quiet breathing there are about 15 complete respiratory cycles per minute. The lungs and the air passage are never empty and, as the exchange of gases takes place only across the walls of the alveolar ducts and alveoli, the remaining capacity of the respiratory passage is called the anatomical dead space.

Tidal volume

this is the amount of air passage into and out of the lungs during each cycle of breathing

Inspiratory reserve volume

this is the extra volume of air that can be inhaled into the lungs during maximal inspiration. I.g. over and above normal Tidal Volume.

Inspiratory capacity

this is the amount of air that can be inspired with maximal effort. It consists of the tidal volume plus the inspiratory reserve volume.

Functional residual capacity

this is the amount of air remaining in the air passages and alveoli at the end of quiet expiration. Tidal air maxes with with air, causing relatively small changes in the composition of alveolar air.

Expiratory reserve volume

this is the largest volume of air which can be expelled from lungs during maximal expiration.

Residual volume

this cannot be directly measured but is the volume of air remaining in the lungs after forced expiration.

Vital capacity

this is the maximum volume of air which can be moved into and out of the lungs; VC= Tidal volume +IRV+ ERV

Total lung capacity

this is the maximum amount of the air lungs can hold. In an adults of average built, it is normally around 6 litres. Total lung capacity represents the sum of the vital capacity and the residual volume. It cannot be directly measured in clinical tests because even after forced expiration, the residual volume of air still remains in the lungs.

Alveolar ventilation

this is the volume of air that moves into and out of the alveoli per minute. It is equal to the tidal volume minus the anatomical dead space, Multiplied by the respiratory rate:

Alveolar ventilation= TV-anatomical dead space × respiratory rate = (500-150)ml × 15 per minute = 5.25 liters per minute

Exchange of gases

Although breathing involved the alternating processes of inspiration and expiration, gas exchange at the respiratory membrane and in the tissue is a continuous and ongoing process. Diffusion of oxygen and carbon dioxide depends on pressure difference ,e.g. between atmospheric air and the blood and the tissues.

Composition of air

Atmospheric pressure at sea level is 101.3 kilopascals or 760mmHg. With increasing height above sea level, atmospheric pressure is progressively reduced and at 5500m, about two-thirds the height of Mount Everest , it is about half at sea level. Under water, pressure increases by approximately 1 atmospheric per 10 m below sea level. Air is a mixture of gases: nitrogen, oxygen , carbon dioxide, water vapour and small quantities of inert gases. The percentage of each in inspired and expired air is listed in table-

...	Inspired air	Expired air
oxygen	21	16
Carbon dioxide	0.04	4
Nitrogen	78	78
Water vapour	Variable	Saturated

Alveolar air

The composition of alveolar air remains fairly constant and is different from atmospheric air. It is saturated with water vapour, and contains more carbon dioxide and less oxygen. Saturation with water vapour provide 6.3 Kpa thus reducing the partial pressure of all the gases present. Gaseous exchange between the alveoli and the bloodstream is a continuous process, as the alveoli are never empty, so it is independent of the respiratory cycle. During each inspiration only some of the alveolar gases are exchanged.

Diffusion of gases

Exchange of gases occurs when a difference in partial pressure exists across a semipermeable membrane. Gases move by diffusion from the higher concentration to the lower until equilibrium is established. Atmospheric nitrogen is not used by the body so its partial pressure remains unchanged and is the same in inspired and expired air, alveolar air and in the blood

External respiration

This is exchange of gases by diffusion between the alveoli and the blood in the alveolar capillaries, across the respiratory membrane. Each alveolar wall is one cell thick and is surrounded by a network of tiny capillaries. The total area of respiratory membrane for gas exchange in the lungs is about equivalent to the area of a tennis court. Venous blood arriving at the lungs in the pulmonary artery has travelled from all the tissue of the body and contains high level of CO2 and low level of O2.

Internal inspiration

This is exchange of gases by diffusion between blood in the capillaries and the body cells. Gas exchange does not occur across the wall of the arteries carrying blood from the heart to the tissues, because their walls are too thick. PO2 of blood arriving at the capillary bed is therefore the same as blood arriving at the capillary bed is therefore the same as blood leaving the lungs.

Transport of gases in the bloodstream

They are follow as -

Oxygen

Oxygen is carried in the blood in:

chemical combination with haemoglobin

solution in plasma water

Carbon dioxide

Carbon dioxide is one of the waste products of metabolism-

as bicarbonate ions in the plasma

some is carried in erythrocytes, loosely combined with haemoglobin

some is dissolved in the plasma