An Introduction To Respiration

There are 3 different aspects involving gas exchange for cell metabolism; external respiration, internal respiration and cellular respiration.

External respiration External respiration is achieved by coordinated activities of respiratory muscles, airways, lung alveoli, pulmonary capillaries and neural control of breathing and sensory feedback. The function of the lungs is to act as a site for gas exchange between the atmosphere and the blood. This is their primary function. The airway is the pathway for gas entry into the lungs from the atmosphere and comprises an extensive conducting system and a respiratory zone. The conducting system includes the mouth and nasal cavities, then the trachea and its successive branches. Air flow in the conducting zone is by bulk flow, but forward movement slows in the respiratory zone due to the massive increase in surface area. Onward gas movement into the respiratory zone and in reverse from blood vessels and alveoli back to the conducting zone is by diffusion only. Gases also diffuse across the barrier between air and blood, this is at a rate determined by the physical properties of the barrier (ficks law of diffusion). Each of the million of alveoli us perfused by blood from many capillaries, keeping the concentration gradient high and the lining of these capillaries and the alveoli is in places only a single cell thick each, keeping the barrier thickness to a minimum.

Blood vessels and flow Output of the right heart into the pulmonary vessels (mixed venous blood) divides into right and left pulmonary arteries running into the lungs and dividing with the bronchioles to form a dense pulmonary circulation surrounding each alveolus. Each RBC spends up to a second in the capillary network, long enough to complete gas exchange and regain equilibrium. A large volume of blood can pool in the lungs, particularly during recumbency as the volume of blood in the lungs is equal to the volume of blood in the entire rest of the body. In keeping with the low pressure this blood is under, the pulmonary vessels have thin walls with less smooth muscle and so little opportunity for the redirection of blood, in contrast to the arterioles of the systemic circulation. The conducting airways are supplied by the bronchial circulation which is derived from the aorta (arterial blood) and is a minor amount compared to the pulmonary circulation to the lungs. The bronchial circulation plays no part in gas exchange, however as a consequence of some venous blood from this system draining into the pulmonary vein, some deoxygenated blood enters the pulmonary stream and continues into the left atrium. This is a small Right-Left shunt.

Lung ventilation The trachea is open to the atmosphere and remains at atmospheric pressure but the pressure within the lungs must be brought transiently subatmospheric to allow air to flow into the lungs. To achieve this, the lung tissue must be expanded by muscular effort. Each lung is invested in serous membrane, the visceral pleura, which is continuous with a similar membrane lining the thoracic cavity, the parietal pleura. The two pleurae are closely apposed, leaving a thin layer of fluid within the pleural space (a virtual body cavity with only 2-3 ml total space). There is a small vacuum in this space, the negative intrapleural pressure, which always exists due to the opposing effects of recoil by elastic lung tissue and the natural tendency of the chest wall to expand. Inspiration is achieved by expansion of the thoracic cavity, which acts on the adherent pleurae to expand the lung tissue. This in turn causes negative alveolar pressure and air is drawn down into the lungs. Thoracic expansion occurs due to the contraction of skeletal muscle in the diaphragm and the external intercostal muscles following stimulation by the respective phrenic and intercostal nerves. Rhythmic neural discharge in these nerves is controlled by the respiratory center in the medulla oblongata (there is no autonomic involvement in this process). Exhalation at rest is usually achieved passively by the relaxation of intercostals and the diaphragm, aided by the natural recoil of the lungs. Gravity helps in resting humans by returning the ribcage to its resting position, thus restoring thoracic cavity volume and forcing air from the lungs. During forceful breathing (at exercise) exhalation is aided by the internal intercostal muscles and possibly the abdominal muscles and viscera. In quadrupeds at rest, gravity helps expand the thoracic cavity but effort is required for exhalation. During exercise, inhalation and exhalation are linked to gait, because of the inertia of the organs. This can generate very forceful breathing but also limits the breathing frequency. Pleural integrity is vital for the breathing process, so a puncture of the external pleura allowing air into the pleural space, breaking the adhesion and preventing lung expansion can be fatal. In addition the lung may collapse away from the chest wall due to its own elastic recoil (pneumothorax). Lung inflation can be modelled with an excised lung in a sealed vessel suspended by its airway from an air inlet. The jar can be evacuated, allowing aur entry down the airway. Then lung volume can be measured at each negative pressure change. A typical pressure volume curve can then be obtained. Lung volume shown against jar vacuum pressure during both inflation and deflation.

Lung compliance The slope of the pressure-volume curve, or the volume change per unit of pressure change, is known as lung compliance. In the normal range of expanding pressure the lung is very compliant but compliance is reduced at high lung volume as collagen limits further stretch. The lung volume at any point during deflation is higher than during inflation, this is mainly due to surface tension and effects of the elastic/collagen in the lung being slow to recoil. The difference in volumes is called an hysteresis loop. Compliance is reduced (less inflation for each unit pressure) if pulmonary venous pressure increases and the lung becomes engorged, if an alveolar oedema occurs or if unventilated alveoli collapse causing the alveolar walls to adhere. compliance increases (more inflation for each unit pressure) with age as lung elasticity is lost and so the tissue recoils less, this makes exhalation difficult and is associated with emphysema. The higher surface area to volume ratio in the smaller alveoli means that they create larger surface tension (P) and tend to inflate larger alveoli, creating lung instability. Surface tension forces also contribute to elastic recoil resisting inflation. This can be demonstrated if a lung is filled with saline rather than air. The filling pressure required is small as the liquid fill abolishes surface tension forces. To combat surface tension, alveolar epithelial cells secrete surfactants (phosphatidyl glycerol and dipalmitoyl phosphatidylcholine) which spread in lamellar sheets over the liquid surface of the alveoli. Emboli interfere with blood supply and will block surfactant synthesis, causing a section of the affected lung to collapse. Surfactant synthesis is required before birth to allow the lungs to expand with the first breath. Surfactant production only begins close to normal term, explaining why animals born prematurely will have breathing difficulties.

Gas Exchange Gas Laws Components of a gas mixture exert a partial pressure (P) proportional to their concentration. Total atmospheric gas pressure (barometric, measured in mmHg), denoted Pb , is normally given as 760 mmHg at sea level although this can vary with atmospheric conditions. Air O2 concentration is 21%, so the partial pressure of O2 (PO2) in air at sea level is (21x760)/100= 160 mmHg. The amount of a gas in solution is its tension, defined as the pressure of that gas above the liquid sufficient to keep it in solution but neither add any more gas now allow any evaporation. So if dissolved gas tension is less than the partial pressure above it, gas will dissolve in solution (and vice versa). Gas tension in body fluid such as blood plasma is also denoted P in mmHg abd depends in partial pressure and gas solubility. Partial pressure and tension for a given gas in the respiratory system tend to be equal, explaining why partial pressure and tension are used interchangeably.

Conventional notation in respiratory physiology Subscri pt is used to describe the sample measured, for example PA is the alveolar sample, PT the tissue, Pa in arterial blood and Pv in mixed venous blood. Ppv refers to the pulmonary venous tension. V is for volume so VE is the volume expired per breath or the tidal volume. V can therefore also be used to denote the rate of breathing in l/min or gas consumption/transfer/production, so VAO2 is the pulmonary oxygen uptake, the same for carbon dioxide is the delivery rate to the alveolar air. Q is used to denote blood flow rate and C is used to denote concentration.

Internal respiration internal respiration refers to the ability of the respiratory systems to deliver O2 from the air to a cell's mitochondria for cellular respiration and also to transport CO2 back to the lungs for removal from the body. A pressure head is required for each gas, so for O2 this is atmospheric pressure, while for CO2 the pressure head is tissue gas tension. PatmO2 is set by atmospheric pressure x the fraction of dry gas molecules which are O2 (160 mmHg). So either lowering barometric pressure (increasing altitude) or breathing a lower proportion of O2 (like in a nitrogen mixture) will lower the pressure head, this reducing alveolar Po2 and the maximum possible oxygen uptake by the tissue.

Oxygen pressure gradient In the atmosphere the oxygen pressure is approx 160 mmHg, however once in the trachea, the P drops to 149 mmHg due to the atmospheric air being humidified in the respiratory tract. Water vapour at body temperature is 47 mmHg, and so the total pressure exerted by the atmospheric gas portion becomes 713 mmHg, meaning the O2 fraction is 149 mmHg. In the alveoli, compared to the atmosphere, there is a higher Pco2 and lower Po2 as the RBC's always flowing through in the pulmonary blood continually deplete the oxygen and increase the carbon dioxide. However with each breath the alveolar ventilation brings in fresh air to replenish the Po2 and reduce the Pco2. The nature of alveolar gas exchange has an important impact on blood gas tension. During hyperventilation, respiration increases (both depth and rate), and so alveolar air is refreshed more vigorously, allowing the alveolar partial pressures to approach that of tracheal air. Capillary Po2 is never completely depleted as there must be a diffusion gradient to allow the oxygen to move into tissue fluid and cells. The systemic arterial blood never has as high a tension as the alveoli as some systemic blood supplying the airway and some cardiac venous blood mixing into the pulmonary venous system, lowering its Po2, an example of a right to left shunt.