EXTERNAL RESPIRATION– It involves “breathing” in which oxygen is taken up by the capillaries of the lungs” alveoli and eliminates carbon dioxide.
INTERNAL – During which oxygen is released into the tissue and carbon dioxide is absorbed by the blood.

Blood provides the transport medium for the gases between the lungs and tissue cells. In addition, it contains a pigment, hemoglobin, with a particular affinity for oxygen. Once oxygen is utilized in the metabolic process resulting in the production of energy, water, and waste material.

BREATHING THE RESPIRATORY SYSTEM

The alternation of active inhalation (or inspiration) of air into the lungs through the mouth or nose with the passive exhalation (or expiration) of the air. During inhalation, the diaphragm and intercostal muscles contract, which enlarges the chest cavity and draws air into the lungs. Relaxation of these muscles forces air out of the lungs at exhalation. Breathing is part of respiration and is sometimes called external respiration.

Breathing supplies oxygen to the alveoli and eliminates carbon dioxide. There are many types of breathing in which the rhythm, rate, or character is abnormal.

BREATH SOUND– The breath sounds can be heard through a stethoscope placed over the lungs during breathing. Regular breath sounds are soft and called vesicular, they may be increased or decreased in disease cases.

MUSCLES OF BREATHING FOR RESPIRATION

Expansion of the chest during inspiration, ours as a result of muscular activity, partly voluntary and partly involuntary. The intercostal muscles and the diaphragm are the main muscles in normal quiet breathing. During difficult or deep breathing they are assisted by muscles of the neck, shoulders, and abdomen.

INTERCOSTAL MUSCLES

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

The external intercostal muscle fibers.- These extend downwards and forwards from the lower border of the rib above to the upper border below.

The internal intercostal muscle fibers.- These extend downwards and backward from the lower border of the rib above to the upper border below, crossing the external intercostal muscle fibers at right angles.

The first rib is fixed. Therefore, when the intercostal muscle muscles contract they pull all the other ribs toward the first rib. Because of the shape and size of the ribs, they move when pulled upwards, 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 cavity and the roof of the abdominal cavity. It consists of a central tendon from which muscle fibers radiate to be attached to the lower ribs sternum and vertebral column by two crura.

The intercostal muscles and the diaphragm contract simultaneously, enlarging the thoracic cavity in all directions that is from back to front side to side, and top to bottom.

CYCLE OF BREATHING

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

INSPIRATION
EXPIRATION
PAUSE

The described previously, the visceral pleura is adherent to the lungs, and the partial pleura to the inner wall of the thorax and the diaphragm. Between them is a thin film of serous fluid.

INSPIRATION

When the capacity of the thoracic cavity is increased by simultaneous contraction of the intercostal muscles and the diaphragm. The partial pleura moves with the walls of the thorax and the diaphragm. This reduces the pressure in the pleural cavity to a level considerably lower than atmospheric pressure. The visceral pleura follows the parietal pleura, pulling the lungs with it. This expands the lungs and the pressure within the alveoli and in the air passages fall, drawing air into the lungs in an attempt to equalize the 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 intercostal muscles and the diaphragm results in downward and inward movement of the rib cage and elastic recoil of the lungs. As this occurs, the pressure inside the lungs exceeds that in the atmosphere, so air is expelled from the respiratory tract. The lungs still contain some air 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 next cycle begins.

PHYSIOLOGICAL VARIABLES AFFECTING BREATHING

1- ELASTICITY: The term elasticity describes the ability of the lung to return to its normal shape after each breath. Loss of elasticity of the connective tissue in the lungs necessitates forced expiration and increased effort on inspiration.

2-COMPLIANCE: This is a measure of the distensibility of the lungs I.e. the effort required to inflate the alveoli. The healthy lung is very compliant and compliments with a very small effort. When compliance is low the effort needed to inflate the lungs is more eminent than standard ex- in some diseases where elasticity is reduced or when an insufficient surfactant is present. It should be noted that compliance and elasticity are opposing forces.

AIRWAY RESISTANCE

When this is increased, ex-in bronchoconstriction, more respiratory effort is required to inflate the lungs.

LUNG VOLUMES AND CAPACITIES

In normal quiet breathing, there are about 15 complete respiratory cycles per minute. The lungs and the air passages are never empty, as the exchange of gases occurs only across the walls of the alveolar ducts and alveoli. The remaining capacity of the respiratory passages is called the anatomical dead space(about 150 ml).

TIDAL VOLUME(TV). This is the amount of air passing into and out of the lungs during each breathing cycle (about 500ml at rest).

INSPIRATORY RESERVE VOLUME(IRV). This is the extra volume of air that can be inhaled into the lungs during maximal inspiration, i.e. over and above typical TV.

INSPIRATORY CAPACITY(IC). This is the amount of air that can be inspired with maximum effort. It consists of the tidal volume ( 500ml ) plus the inspiratory reserve volume (IRV).

FUNCTIONAL RESIDUAL CAPACITY(FRC). This is the amount of air remaining in the air passages and alveoli at the end of a quiet expiration. Tidal air mixes with this air, causing relatively small changes in the composition of alveolar air in respiration.

EXPIRATORY RESERVE VOLUME (ERV). This is the immense volume of air that can be expelled from the lungs during maximal expiration.

RESIDUAL VOLUME (RV). This cannot be directly measured but is the volume of air remaining in the lungs after forced expiration in respiration.

VITAL CAPACITY (VC). This is the maximum volume of air that can be moved into and out of the lungs.

VC = Tidal volume + IRV +ERV

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) x respiratory rate = (500-150) ml x 15 per minute = 5.25 liters per minute.

EXCHANGE OF GASES

Although breathing involves the alternating processes of inspiration and expiration, gas exchange in the respiratory membrane and the tissues is a continuous and ongoing process. The diffusion of oxygen and carbon dioxide depends on pressure differences, ex- between atmospheric air and the blood, or between blood and tissue.

Composition of air

Atmospheric pressure at sea level is 101.3 kilopascals (kpa) or 760 mmHg. With an increasing height above sea level, atmospheric pressure is progressively reduced, and at 5500m, it is about half that at sea level. Underwater, pressure increases by approximately 1 atmosphere per 10 m below sea level.

Air is a mixture of gases: nitrogen, oxygen, carbon dioxide, water vapor, and small quantities of inert gases. Each gas in the mixture exerts a part of the total pressure pro-potential to its concentration, i.e. the partial pressure. This is denoted as, ex. PO2, PCO2.

ALVEOLAR AIR

The composition of alveolar air remains equitably constant and is different from atmospheric air. It is saturated with water vapor and contains more carbon dioxide and less oxygen. Saturation with water vapor provides 6.3 kpa (47 mmHg) thus reducing the partial pressure of all the other gases present. Gaseous exchange between the alveoli and the bloodstream (external respiration) 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.

EXPIRED AIR

This is a mixture of alveolar air and atmospheric air in dead space.

DIFFUSION OF GASES

The exchange of gases occurs When a difference in partial pressure exists across a semipermeable membrane. Gases move by diffusion from higher to lower concentrations 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 blood.

EXTERNAL RESPIRATION

This is an 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 surrounded by a network of tiny capillaries. The total area of the respiratory membrane for gas exchange in the lungs is about equivalent to the area of a tennis court. Venous blood arriving at the lungs has traveled from all the tissues of the body and contains high levels of CO2 and low levels of O2.

Carbon dioxide diffuses from venus blood down its concentration gradient into the alveoli until equilibrium with alveolar air is reached. Through the same process, oxygen diffuses from the alveoli into the blood. The slow flow of blood through the capillaries increases the time available for gas exchange to occur.

INTERNAL RESPIRATION

This is an exchange of gases by diffusion between blood in the capillaries and the body cells. Gaseous exchange does not occur across the walls 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, accordingly, the same as blood leaving the lungs. Blood arriving at the tissues has been cleansed of its CO2 and saturated with O2 during its passage through the lungs and therefore has a higher PO2 and a lower PCO2 than the tissues. This creates concentration gradients between capillary blood and the tissues and gaseous exchange occurs.

TRANSPORT OF GASES IN THE BLOODSTREAM

The transport of blood oxygen and carbon dioxide is essential for internal respiration to occur.

OXYGEN

Oxygen is maintained in the blood in the:

Chemical combination with the hemoglobin.
A solution in plasma water.

Oxyhemoglobin is an unstable compound that under certain conditions readily dissociated extricating oxygen. Components that increase dissociation include low O2 levels, low PH, and raised temperature. There is an increased exposition of carbon dioxide and heat in active tissues, which leads to an increased release of oxygen. In this way, oxygen is available to tissues in greatest need. when oxygen leaves the erythrocyte, the deoxygenated hemoglobin turns purplish.

CARBON DIOXIDE

Carbon dioxide is one of the waste products of metabolism. It is excreted by the lungs and is transported by three mechanisms:

Firstly, as bicarbonate ions in plasma (70%)
Secondly, some carry erythrocytes, loosely combined with hemoglobin as carbaminohemoglobin (23%).
Thirdly, some are dissolved in the plasma.

CONTROL OF RESPIRATION

Control of respiration is usually involuntary. Settlement power is exercised during activities such as speaking and singing but is overridden if blood CO2 increases (hypercapnia).

THE RESPIRATORY CENTRE

This is developed by groups of nerves in the medulla, the respiratory rhythmicity center, which controls the respiratory pattern, ex- the rate and depth of breathing. Shared discharge of respiratory neurons within this center fixed the speed and depth of breathing. The activity of the respiratory rhythmicity center is adjusted by nerves in the pons ( the pneumatic center and the apneustic center), in response to input from other parts of the brain.

CHEMORECEPTORS

These are receptors that respond to changes in the partial pressures of oxygen and carbon dioxide in the blood and cerebrospinal fluid. They are located centrally and peripherally.

Firstly, central chemoreceptors. These are located on the surface of the medulla oblongata and stand washed in cerebrospinal fluid.

Secondly, peripheral chemoreceptors. These stand situated in the arch of the aorta and the carotid bodies.

OTHER FACTORS THAT INFLUENCE RESPIRATION

Breathing may be revised by the more heightened centers in the brain by:

Speech, singing
Emotional displays, ex. crying, laughing, fear
Drugs, ex. sedatives, alcohol
sleep

Similarly, temperature also influences breathing. In fever, respiration is increased due to metabolic rate, while in hypothermia it is depressed, as is metabolism.