respiratory system anatomy

• The lungs occupy almost the entire space within the thoracic cavity, leaving only a small gap between the visceral pleura and the parietal pleura.
• The enclosed space between the pleural membranes is known as the pleural cavity.
• The mediastinum is the space between the two lungs.
• The thoracic cavity is bounded posteriorly by the thoracic vertebral column, laterally by the ribs and anteriorly by the sternum.
• The thoracic cavity is also a closed space, bounded above by the superior thoracic aperture (thoracic inlet), capped by the suprapleural membrane, and below by the inferior thoracic aperture, closed off by the diaphragm.
• The thoracic inlet, or superior thoracic aperture, is defined as the bony cartilaginous ring to which the suprapleural membrane attaches.
• This ring is formed by the 1st thoracic vertebra, 1st rib, 1st costal cartilage and upper margin of the manubrium.
• The suprapleural membrane is suspended above by an attachment to the 7th cervical transverse process on each side.
• The suprapleural membrane rises upwards and extends 2.5 cm above the medial end of the clavicle.
• The suprapleural membrane closes the gap, but is penetrated by structures passing between the chest and the neck, such as the trachea and oesophagus.
• The thoracic outlet or inferior thoracic aperture is the space bounded by the costal margin in front, the tips of the 11th ribs, the 12th ribs and the inferior margin of the 12th thoracic vertebra.
• This aperture is closed off by the diaphragm.
• The diaphragm is a bi-domed muscular sheet inserting into a central tendon.
• Each side of the diaphragm is referred to as a hemidiaphragm.
• The diaphragm forms the principal muscle of breathing.
• Each hemidiaphragm descends to enable us to breathe in (inspiration) and relaxes to allow us to breathe out (expiration).
• There are several structures that need to pass through the thoracic inlet, and to do so they must pass through the suprapleural membrane. Actually, they take a sleeve of the fascial membrane as they do so. Therefore the trachea and oesophagus take pre-tracheal fascia, the vessels of the head and neck (carotid artery and internal jugular vein) take the carotid sheath, and the vessels of the upper limb (subclavian and axillary vessels) take the axillary sheath.
• The thoracic outlet transmits the thoracic structures destined for the abdomen (e.g. oesophagus, aorta), and also permits passage of venous blood from the lower half of the body back into the heart.
• Thoracic outlet syndrome (TOS) is a condition caused by the compression of the neurovasculature in the thoracic INLET.
• Some people have an extra cervical rib or an old fracture of a clavicle or an apical lung cancer, which limits space for the vessels.
• Symptoms of TOS include pain, weakness, numbness and tingling, swelling, fatigue or coldness in the forearm and hand.
• These symptoms mimic many other conditions, making diagnosis difficult.
• These structures consist of the brachial plexus, subclavian artery, and subclavian vein.
• Neurologic symptoms are the most common presentation of TOS occurring in 95% of cases.
• These include pain and paraesthesias (or tingling sensation).
• Later, these are accompanied by weakness and atrophy (or loss) of the muscles.
• Neurologic symptoms often arise secondary to anomalous scalene muscles, or inflamed, foreshortened scalene muscles, where the nerve roots of the brachial plexus arise.
• Venous compression of the subclavian vein in the costoclavicular space can lead to arm oedema (or fluid retention), and cyanosis (or bluish skin colour).
• Nerve compression including the phrenic nerves- diaphragm, vagus nerves- viscera, brachial plexuses and the sympathetic chain being impacted.
• The true thoracic outlet is the inferior thoracic aperture at the lower aspect of the chest. The thoracic outlet gives rise to the attachment of the diaphragm, a muscle which separates the thorax above from the abdomen below.  Structures which pass between these two regions will need to either pass through this muscle or go behind it.
• The periphery of the diaphragm is separated from the chest wall by a narrow gap – this is the costodiaphragmatic recess or phrenic angle. Lung tissue does not like to enter such narrow gaps, even in maximal inspiration when the lungs are drawn downwards. The costodiaphragmatic recesses may be partly encroached by lung tissue at this time, but never fully.
• A needle passed through the chest wall in the 9 th or 10th intercostal space will enter the pleural cavity at this recess. It is a way of accessing the pleural cavity to withdraw pleural fluid without risking injury to the lung tissue. Even so, it is a good idea to ask the patient to breath out before inserting the needle just to make sure that the lung is as far away as possible.
• The diaphragm is a sheet of skeletal muscle. It has a central tendinous portion (aponeurosis or central tendon) on which the heart sits, and a peripheral muscular part. The right dome sits higher than the left dome due to the presence of the liver below it. The precise position varies between individuals and depends on the phase of breathing and body position.
• When the domes of the diaphragm contract, the diaphragm flattens.
• Under normal circumstances, the descent of the diaphragm is only of the order of a couple of centimetres, which is referred to as 'quiet breathing', providing enough oxygen intake to satisfy the body's metabolic needs.
• If more oxygen intake is needed, the strength of contraction of the diaphragm can be increased so that it descends further, up to a maximum of 6-7cms, which is referred to as 'forced breathing'.
• Forced breathing involves expansion of the ribcage as well as descent of the diaphragm.
• The diaphragm attaches to the bony-cartilaginous rim of the thoracic outlet and its fibres ascend to the central tendon.
• Posteriorly, the diaphragm attaches to fascia covering the posterior abdominal wall muscles laterally and the vertebral column medially.
• The two muscles that the diaphragm crosses are the psoas major (medially) and quadratus lumborum (laterally).