Lungs
Lungs
How do you tell if someone is alive?
Check they are breathing!
Thus brings us to our next main respiratory organ – the lungs.
We have two lungs – RIGHT and LEFT. They are located in the lateral aspects of the mediastinum within the thorax. They are surrounded by the pleura.
Unlike other paired organs in the body the right and left lungs are NOT mirror images of each other.
First let’s look at the structures that both lungs have. You may be given specimen of the lung in your MRCS exam and asked to identify its’ structures, orientate it as in-situ. These are the basics of how you could describe the structures and help position the lung in the body
This is the blunt tip of the lung at its most superior aspect. You need to always remember it projects upwards beyond the first rib so can be injured easily during procedures such as central line insertion.
The inferior surface of the lung, sitting on the diaphragm
The right and left lungs do not have an identical lobular structure.
The right lung has three lobes; superior, middle and inferior. The lobes are divided from each other by two fissures:
- Oblique fissure – Runs from the inferior border of the lung in a superoposterior direction, until it meets the posterior lung border.
- Horizontal fissure– Runs horizontally from the sternum, at the level of the 4th rib, to meet the oblique fissure.
The left lung contains superior and inferior lobes, which are separated by a similar oblique fissure. The lingula (little tongue) may be seen as homologous to the middle lobe.
These are the edges of the lungs corresponding to its anatomical position – anterior, inferior and posterior borders.
This tells us which area the lung is facing or rather what their adjacent neighbouring structures are.
- Mediastinal surface of the lung is facing the lateral aspect of the middle mediastinum. When given a specimen of the lung – this is where you find the hilum.
- The diaphragmatic surface makes up the base of the lung. This surface sits on the doem of the diaphragm and is concave in shape.
- The costal surface is smooth and convex. It is situated facing the internal surface of the ribs.
Blood Supply
Pulmonary Circulation
The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation.
The pulmonary artery arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli.
As they near the alveoli, the pulmonary arteries become the pulmonary capillary plexus. The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibers.
The capillaries branch and follow the bronchioles where they meet the alveolar wall, creating the respiratory membrane.
Once the blood is oxygenated, it drains from the alveoli by way of two pulmonary veins (superior and inferior) which arise from each lung, carrying the blood to the left atrium of the heart. From there, the heart pumps out oxygenated blood to the rest of the body.
Bronchial Circulation
The lung parenchyma is supplied by the bronchial arteries.
Bronchial arteries are variable, but there are usually two for the left lung, which typically arise directly from the descending aorta. There is a single right artery which may arise from the aorta, from a common trunk with the left bronchial artery, or from one of the upper intercostals, the subclavian or the internal thoracic artery.
These arteries run along the posterior aspects of their respective bronchi as far as the small bronchioles. However, the alveoli, and thus the respiratory epithelium is entirely supplied by the pulmonary arterial tree.
Because the lung stroma receives its blood supply from the bronchial arteries it remains viable following a pulmonary embolism (which is a blockage in the pulmonary arteries therefore affecting gas exchange).
The bronchial veins – right and left (usually two each side) empty into into the azygos and hemiazygos veins, respectively.
Innervation
At the root of the lung lies a posterior pulmonary nerve plexus which comprises sympathetic (T2–4), parasympathetic (vagal) and visceral afferent fibres. Fibres pass from thence around the lung root to form an anterior pulmonary plexus. Fibres pass from these plexuses into the lung along the bronchi and blood vessels.
The postganglionic sympathetic fibres causes bronchodilation of the bronchial muscles and also weakly vasoconstrict the bronchial vessels. This also causes inhibition of glandular secretion.
The parasympathetic system primarily via vagus nerve supplies secretomotor fibres to the pulmonary mucous glands. It also causes bronchoconstriction and vasodilation.
Finally, the visceral afferent fibers transmits information that is required in the coughing reflex, stretch reception, blood pressure, chemoreception and nociception.
Lymphatic Drainage
The lymph formed within the lungs drains into the pulmonary lymph nodes which are found along the lobar bronchi and the bronchopulmonary lymph nodes which sit on the left and right major bronchi.
Finally the superior and inferior tracheobronchial lymph nodes which conjugate around the superior and inferior aspects of the bifurcation of the trachea.