Anatomy and physiology of the respiratory system

The respiratory system consists of the upper respiratory system, which includes the nose, throat and associated structures, and the lower respiratory system referring to the trachea, bronchi and lungs. The cardiovascular system transports the gases in the blood between the lungs and the cells. The overall exchange of gases between atmosphere, the blood and the cells is respiration. Three processes are involved

· pulmonary ventilation (breathing) the inflow and outflow of air between the atmosphere and the lungs

· external respiration the exchange of gases between the lungs and the blood

· internal respiration the exchange of gases between blood and cells

The respiratory and cardiovascular systems participate equally in respiration. Failure of either system disrupts homeostasis and rapid death of cells from oxygen starvation occurs. It is the primary function of the respiratory system to supply oxygen and rid the body of carbon dioxide, the product of metabolism.

Control of ventilation

The act of breathing is spontaneous in that it is accomplished without conscious effort or awareness, but unlike automatic functions such as myocardial contractions or peristalsis, it involves skeletal muscles and is susceptible to voluntary interference.

The respiratory Centre

There is no well-defined respiratory nerve centre. Respiration is controlled by the nerve cells in the reticular formation of the brain stem, particularly in the medulla, while the Pons is said to contain a “ Pneumotaxic centre” that helps co-ordinate medullary activity. These centres send impulses down the spinal cord and then via the phrenic nerve to the diaphragm and via the intercostal nerves to the intercostal muscles. The reticular formation has an inbuilt rhythmical pattern of activity that maintains the rhythm of breathing. It is possible to divide the respiratory centre into three areas

Inspiratory area, located in the medulla oblongata, which transmits nerve signals to activate the muscles of inspiration (inactive during expiration, the inspiratory neurons oscillate on and off in rhythm to the respiratory cycle). The expiratory area located in the medulla oblongata (normally inactive, although during strenuous activity the neural signals activate the muscles of expiration). The pneumotaxic centre

located in the pons, which controls the rate of breathing, by inhibiting the inspiratory area’s nerve signals to the muscles of inspiration.

Respiratory muscles, like other skeletal muscles possess skeletal receptors which sensing length, form part of a reflex loop (via Vagus) that ensures the muscle contraction is appropriate to the anticipated respiratory load and required effort.

Inspiratory muscles

· the sternodomastoid, which lifts the sternum upwards

· the anterior serrati which lifts many of the ribs

· the scaleni that lift the first two ribs

· the external intercostals, which pull the rib cage out

Expiratory muscles

· the abdominal recti which pulls downward on the lower ribs, also compresses the abdominal content toward the diaphragm

· the internal intercostals that pull the rib cage down

Chemical control

Changes in the Pa02 and the pH have a profound effect on the central chemoreceptors located in the medulla. A falling Pa02 (known as the Hypoxic Drive) triggers peripheral receptors located in the carotid and aortic bodies. Under normal circumstances, PaC02 plays the primary role in chemical control of ventilation. Pain and nerve impulses from exercising limbs cause an increase in the rate and depth of breathing.

Airways

Air is filtered, warmed and humidified during its passage through the nasal cavity. The mucous membrane that lines the nasal cavity is richly supplied with blood vessels and serous glands that secrete watery mucous. The hair in the nares filters out coarse particles. Fine particles are trapped and entrained in the muco-cilary stream toward the pharynx. The mucous and serous secretions add as much as 1,000ml of water per day to inspired air.

Lungs

The lungs are a conical shaped, spongy mass generally blue grey in colour. They are enclosed in the thoracic cage, which consist of the sternum, vertebral column, and ribs with the intercostal muscles and the diaphragm below. The right lung has three lobes, (upper, middle and lower), while the left lung is divided into two lobes, (upper and lower). The right and left lungs are separated from each other by the mediastinum, which contains the heart, large vessels, trachea and oesophagus. The lobes are functionally independent of each other. The functional unit is the alveoli or air sac. Gas exchange between blood and air occur at this level. The lungs provide a large surface area for gas exchange to take place. The total surface area of the lungs in a normal adult is approximately 80m2, about the size of a tennis court.

Gases and blood enter and leave the lungs at the hilum, bronchi and pulmonary veins and arteries. Two layers of pleura surround each lung and lie adjacent to the chest wall. As the volume of the thoracic cage increases through the action of the inspiratory muscles, the chest wall expands taking the lung along with it.

Trachea

The trachea is a fairly rigid “pipe” about 2cm in diameter and 10cm long in the adult, lying in front of the oesophagus. The rigidity of the trachea and the bronchi is due to the cartilage in their walls. At the carina, the trachea divides into right and left main bronchus and subsequently divides into lobar, segmental and small bronchi.

The position and direction of these airways are of practical importance in a bronchoscopy or during postural drainage. The right main bronchus is larger than the left and bifurcates at a less acute angle from the trachea, and therefore endotracheal tubes can slide down the right main bronchus causing the left lung not to inflate and collapse (right main bronchial intubation). The lining of all these airways are made up of columnar ciliated epithelium, which produces mucous to trap foreign material and the cilia propel this layer up the airways to be coughed out. (The muco-ciliary escalator)

The bronchi

The bronchus divides into smaller airways, bronchi and bronchioles, which are approximately 1mm in diameter and are chiefly for conducting gases. Gaseous exchange takes place at the level of the alveoli, which bud off the bronchioles. The pulmonary circulation nourishes the conducting airways. If there is an obstruction to the pulmonary circulation, e.g. pulmonary embolism, part of the lung dies (pulmonary infarction).

The alveoli

Half the total lung alveoli are located in the alveolar ducts and are responsible for 35% gas exchange. The alveoli sac exists in clusters of 15-20 with common walls between them (increasing the surface area of the lungs). Alveolar sacs are responsible for 65% gas exchange. Alveoli are little “bubbles” about 1/3 mm in diameter. There are 300 million of them and the total surface area is 40 times the surface area of the body.

The gas in the alveoli is separated from the blood in the pulmonary capillaries by a very thin layer. This is the alveolar-capillary membrane consisting of the alveolar lining cells, the endothelial cells of the capillaries and the interstitial space (the alveolar – capillary membrane). There are many other cells in and around the alveoli, the macrophages which engulf “foreign invaders” and cells which produce surfactant.

Surfactant

Surfactant is a lipoprotein that is produced by surfactant secreting cells in the alveolar epithelium. The function of surfactant is to reduce the surface tension of the fluids lining the alveoli. When it is absent lung expansion is extremely difficult, requiring intrapleural pressures of 20-30mmHg to overcome the tendency of the alveoli to collapse. Surfactant produces most of the elastic recoil of the lung necessary for expiration as well as preventing the collapse of alveoli.

Pulmonary circulation

Although both the systemic and the pulmonary circulation accept the total cardiac output of the heart, there are many differences between the two.

The pulmonary artery carries mixed venous blood and the pulmonary vein carries oxygenated blood, opposite to what occurs in the systemic circulation. The pressure in the pulmonary artery is 1/6th of the aorta. The resistance in the pulmonary circulation is 1/10th of the systemic.

The physiology of respiration

Oxygen and carbon dioxide exist in dissolved form in the blood and as such exert pressure. The measurement of the partial pressures exerted by these gases may be measured by blood gas analysis. Pulmonary function studies are concerned with ventilation (the mass movement of air into and out of the lungs). Blood gas studies are concerned with respiration (the exchange of gases between air and blood and between blood and tissue).

Respiratory function monitoring

Respiratory function monitoring can be divided into four major categories

· the adequacy and efficiency of oxygen exchange.

· the adequacy of ventilation.

· respiratory mechanics and ventilatory reserve.

· ancillary diagnostic techniques.

Distribution of ventilation

Ventilation is the movement of gases in and out of the pulmonary system. To respire, inspired gases must reach the alveolar epithelium. The distribution of inspired gases throughout the pulmonary tree and lung parenchyma is normally dependent upon two major factors, airway resistance and alveolar physics.

Airway resistance

Gas flows from a region of higher pressure to one of lower pressure across a pressure gradient. The rate at which this gas is displaced (flow) is a function of the pressure gradient and resistances to that gas flow.

Laminar flow

Laminar gas flow refers to a streamlined molecular flow in which there is little friction primarily between the molecules and sides of the tube. Laminar flow follows the general principle of Poiseuille, which demonstrates that at a constant driving pressure the flow rate of a gas will vary directly with the fourth power of the radius of the airway. Thus small changes in airway calibre may greatly affect the delivery of air to the alveoli.

Turbulent flow

Turbulent flow refers to gas molecules interacting in a random manner. The resistances are not only at the sides of the tube but are also caused by molecular collision of the gases. All other things being equal, resistance to the flow is considerably greater when the flow is turbulent as opposed to laminar.