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The human gas exchange system consists of the nasal passages, the pharynx or throat, the larynx or voice box, the trachea, the right and left bronchus and the lungs Larynx Trachea (with rings of cartilage) Left lung Ribs Diaphragm (a powerful sheet of muscle separating the thorax from the abdomen) Intercostal muscles Section through ribs Right bronchus Bronchioles The Human Gas Exchange System

The lungs are contained within the thoracic cavity, the sides of which are bounded by 12 pairs of ribs that articulate (join) with the vertebrae towards the back of the body, and the sternum or breast bone, towards the front The portions of the ribs that articulate with the breastbone are composed of cartilage rather than bone Cartilage Cartilage is softer and more pliable then bone and thus assists the movements of the rib cage during breathing Two sets of antagonistic muscles are located between the ribs – these are the intercostal muscles

This chest X-ray shows the gas-filled lungs within the thoracic cavity Air-filled lungs Ribs Position of the diaphragm Position of the heart

The trachea or windpipe is about 10 cm long and is supported by C-shaped rings of cartilage to prevent the tube from collapsing during breathing Trachea The trachea subdivides to give rise to the right and left bronchus – these tubes are also strengthened by cartilage The two bronchi subdivide to form an extensive network of bronchioles that deliver air to the gas exchange surfaces – the alveoli Right and Left bronchus Bronchioles Air enters the body through the nasal passages and mouth, and passes via the pharynx and larynx to the trachea Air is delivered to the alveoli as the trachea branches into bronchi and bronchioles The Trachea

This photomicrograph of a transverse section through the trachea shows the C-shaped ring of cartilage Magnify C-shaped cartilage ring

This magnified view of the wall of the trachea shows the cartilage cells together with the cells that line the lumen of the trachea – ciliated epithelium Cartilage cells Ciliated epithelium

Lumen of trachea This highly magnified view of the lining of the trachea shows the cilia and mucus-secreting goblet cells that make up the epithelium Goblet cell that secretes mucus to trap dust and other foreign material that may enter the respiratory system The wafting of these cilia removes the mucus and trapped foreign material from the respiratory system

The bronchioles divide many times forming respiratory bronchioles, which in turn divide to to form alveolar ducts that terminate in groups of sacs – the alveoli Respiratory bronchioles Alveolar duct Alveoli A single alveolus Each alveolus is a hollow, thin-walled sac that is surrounded by a dense network of capillaries and is the site of gas exchange in the lungs The Gas Exchange Surface

Gases are exchanged across the alveoli by diffusion According to Fick’s Law... Rate of diffusion = surface area x difference in concentration thickness of exchange surface Maximum rate of diffusion of respiratory gases is achieved by: the large surface area presented by the alveoli (there are about 350 million alveoli in the two lungs presenting an enormous surface area of approximately 90 square metres – about the area of a tennis court) the large differences in concentration of metabolites between the alveoli and the blood capillaries the thinness of the diffusion barrier (alveolar and capillary walls provide a total thickness of only mm) As deoxygenated blood from the body tissues flows through the network of capillaries surrounding each alveolus, oxygen diffuses into the blood and carbon dioxide diffuses from the blood into the alveolus; oxygenated blood travels from the lungs to the left of the heart for delivery to the body tissues The Gas Exchange Surface

Breathing in (inspiration) and breathing out (expiration) are mechanical processes involving the ribs, intercostal muscles and the diaphragm Two sets of antagonistic muscles are located between the ribs; these are the external and internal intercostal muscles External intercostal muscles Internal intercostal muscles The intercostal muscles are antagonistic in the sense that contraction of the external muscles raises the rib cage, whereas contraction of the internal muscles lowers the rib cage The diaphragm is a powerful sheet of muscle that separates the thorax from the abdomen; it is dome-shaped when relaxed and flattens on contraction Diaphragm The Mechanics of Breathing

The external and internal intercostal muscles are responsible for the movements of the rib cage during breathing Rib External intercostal muscles Internal intercostal muscles Contraction of the external intercostal muscles moves the ribs upwards and outwards during inspiration Relaxation of the external intercostal muscles causes the ribs to move downwards and inwards during expiration at rest During periods of increased activity such as during exercise, the internal intercostal muscles contract to bring out more forceful expirations Expiration at rest is a passive process; expiration during periods of exercise is an active process involving contraction of the internal intercostal muscles The Mechanics of Breathing

Diaphragm Vertebral column Sternum (breastbone) Rib During an inspiration, the external intercostal muscles contract and raise the rib cage upwards and outwards; the diaphragm also contracts and flattens Inspiration - Breathing In

During an inspiration, the external intercostal muscles contract and raise the rib cage upwards and outwards; the diaphragm also contracts and flattens Sternum (breastbone) Rib Diaphragm The volume of the thorax increases, lowering the air pressure in the chest cavity to less than that of the atmosphere outside A pressure gradient is created between the atmosphere and the lungs, and air rushes in via the trachea to equalise the pressure difference Air moves from a higher to a lower pressure region and inflates the lungs as inspiration takes place Inspiration - Breathing In

During an expiration, the external intercostal muscles relax and lower the rib cage; the diaphragm relaxes and becomes dome-shaped The volume of the thorax decreases, raising the air pressure in the chest cavity to above that of the atmosphere outside A pressure gradient is created between the lungs and the atmosphere, and air rushes out via the trachea to equalise the pressure difference Expiration is assisted by the elastic recoil of the lungs following the stretching of elastic fibres during the process of inspiration Air moves from a higher to a lower pressure region and deflates the lungs as expiration takes place The mechanism described is that for breathing at rest At rest, inspiration is an active process involving contraction of the muscles of breathing Expiration is a purely passive process involving relaxation of the muscles of breathing together with elastic recoil of the lungs During forced breathing, as in exercise, expiration becomes an active process Expiration - Breathing Out

During periods of increased activity such as exercise, the rate and depth of breathing increases The more forceful, downward and inward movements of the rib cage during expiration of exercise are achieved through the contraction of the internal intercostal muscles At the same time, contraction of the abdominal muscles just below the thorax, push the diaphragm into a more domed position As the diaphragm pushes further into the thorax, the volume of the chest cavity decreases more significantly and the increased thoracic pressure aids expiration During periods of exercise, expiration becomes an active process

Expiration Inspiration External intercostal muscles contract and raise the ribs upwards and outwards External intercostal muscles relax and the ribs move downwards and inwards The diaphragm muscle contracts and flattens The diaphragm muscle relaxes and becomes dome-shaped The volume of the thorax increases The air pressure in the thoracic cavity falls below that of the atmospheric air Air rushes into the lungs along a pressure gradient The volume of the thorax decreases The air pressure in the thoracic cavity rises above that of the atmospheric air Air rushes out of the lungs along a pressure gradientSummary

The lungs are sealed in an airtight, fluid-filled, double- membrane sac called the pleura Two pleural membranes surround the lungs and the cavity between them is filled with pleural fluid to protect the lungs from the bony ribs The pressure within the pleural cavity is known as the intrapleural pressure and this pressure is always below that of the atmosphere (sub-atmospheric) The pressure within the airways of the respiratory system is known as the intrapulmonary pressure Intrapulmonary pressure within the airways The magnitude of these pressures varies during the breathing cycle Pressure Changes During the Breathing Cycle

The pressure and volume changes occurring during the breathing cycle can be represented as a graph where zero, on the pressure axis, represents atmospheric pressure As inspiration begins and the ribs move upwards and outwards, the pressure within the airways (the intra- pulmonary pressure) falls below that of atmospheric air (shown as 0 mm Hg) Air rushes into the lungs to equalise the pressures and intrapulmonary pressure increases to that of the atmosphere The intrapleural pressure falls even more below that of the atmosphere as the pleural cavity expands on inspiration As expiration begins and the ribs move downwards and inwards, the pressure within the airways (the intra- pulmonary pressure) rises above that of atmospheric air (shown as 0 mm Hg) Air rushes out of the lungs to equalise the pressures and intrapulmonary pressure falls to that of the atmosphere The intrapleural pressure rises as the pleural cavity decreases in size on expiration The volume of air inspired and expired during one breathing cycle is shown in the lower part of the graph Atmospheric pressure

The pressure within the pleural cavity (intrapleural pressure) is always below that of the atmosphere (sub-atmospheric) If a lung is pierced, then air rushes into the pleural cavity along a pressure gradient Atmospheric air pressure is GREATER than the fluid pressure within the pleural cavity As air rushes into the pleural cavity, the pressure difference across the lung wall is eliminated, and the stretched lung collapses The Collapsed Lung

Air enters the fluid-filled pleural cavity and this condition is known as a pneumothorax

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