THE LUNG

The lung is the paired principal organ of respiration and it is composed of spongy wall of elastic tissue which feels like a rubber sponge being spongy in nature it floats in water and crepitates when handled, because of air resent in the alveoli.

Location

It is found within the thoracic cavity being separated from each other by the heart and other mediastinal content. It lies within the pleura cavity since it invaginates the pleura from the medial side, it is entirely enclosed by the pleura except at its hilum.

External Features

At birth the lungs is pinkish in colour but in adults it is dark grey in colour and mottled in appearance as a result of deposition of inhaled carbon particles in its surface underneath its loose connective tissue. The surface is shiny and smooth and finely marked into polyhedral surfaces by dark lines and these correspond to the lobules of the lungs.

The Weight

The right lung is about 625g while the left is about 565g. This is when the lung is filled with blood and other serious fluid, when it is empty of blood the right weighs about 240g while the left being smaller weighs even less. But in general the lung of a male weighs higher than the lung of the female.

Shape of the Lung

The lung take the shape of the cavity which it lies therefore it is conical in shape with an apex and a base, three surfaces; a coastal, diaphragmatic and medial surfaces, three borders; an anterior, inferior and posterior borders.

The Apex

It is the part of the lung that extends in to the root of the neck. It is lined by the cervical pleura, which is over lined by a tough membrane known as the suprapleural membrane. This membrane separates it from the subclavian artery and vein which grooves its anterior surface. The apex is about 3 – 4cm above the level of the 1^st intercostal cartilage and it levels with the neck of the 1^st rib posteriorly. It is about 2.5cm above the medial 1/3^rd of the clavicle. Posterior to the apex lies the cervicothoracic sympathetic ganglion.

The Base

It is concave and semilunar in shape. It over lies the superior surface of the diaphragm which separates it on the right from the lobe of the liver, and on the left separates the base of the left lung from the left hepatic lobe of the liver, the fundus of the stomach and the spleen. The doom of diaphragm being higher on the right side than on the left due to the presence of the liver beneath as a result of this the right lung is shorter with a deeper concavity of the base than that of the left lung.

Surfaces:

Costal Surface

It is smooth and convex in shape and overlies the thoracic cage and when fixed insitu (in anatomical position) it bears the marks of the ribs which it overlies. The costal surface is deeper behind than in front and it is lined by the costal pleura.

Diaphragmatic surface

It corresponds to the base of the lungs. It is that part that overlies the diaphragm.

It is concave in shape as it overlies the dome of the diaphragm.

Medial Surface

It is made up of two parts: the vertebral part and the mediastinal surface.

The vertebral part is full and rounded and lies in the paravertebral gutter. It is in contact with the posterior intercostal vessels and nerves and the splanchnic nerve. It corresponds to the posterior border of the lung.

The mediastinal surface is that part that overlies the heart and the pericardial sac and other structures of the mediastinum. It is deeply concave and bears the impress of those structures within the mediastinum which it overlies when fixed insitu. The most prominent of the structure which are found in the mediastinal surface is the hilum of the lung and the line of attachment of the pulmonary ligament.

The impression of the mediastinal surface of the right lung differs from that of the left. In the right lung, anteroinferior to the hilum and pulmonary ligament is a shallow depression known as the cardiac impression which continues superiorly with the groove for the superior vena cava and the terminal right brachiocephalic vein. The groove for superior vena cava is joined from behind by an arching groove above the hilum which lodges the azygous vein. Posterior to the hilum and pulmonary ligament are grooves for the oesophagus and azygous vein. Inferior to the cardiac impression is a shallow groove that lodges in the inferior vena cava.

On the left lung, the cardiac impression which also lies anteroinferior to the pulmonary ligament and hilum is more concave and deeper than that of the right side. Superior to the cardiac impression is a shallow groove that lodges the pulmonary trunk. It is joined by an arching groove which passes above and behind the hilum and the pulmonary ligament. This groove lodges in the aorta. Between the groove for the descending aorta and the pulmonary ligament is a smaller groove formed by the oesophagus, superior to the arching groove of the aorta is the groove for the left subclavian artery and the left brachiocephalic vein. On both lungs it should be noted that the 1^st rib and the subclavian vessels grooved the anterior surfaces.

BORDERS OF THE LUNGS

Anterior Border:

The anterior border are thin and sharp and over laps the pericardium and other mediastinal content on the right lungs the anterior border is more or less vertical and correspond closely to the costomediastinal line of pleura reflection. But on the left the anterior border is only vertical above the attachment of the 4^th costal cartilage. Below this level is present a variable size of cardiac notch which is as a result of the development of the heart at the area of the cardiac notch the pericardium is only covered by a double layer of parietal pleura. It marks the area of superficial cardiac dullness.

Inferior Border:

It is sharp and thin anteriorly where it separates the costal surface from the diaphragmatic surface. It is the part that descends into the costodiaphragmatic recess. Medially where it separates the diaphragmatic surface and mediastinal surface it becomes rounded.

Posterior Border:

It corresponds to the vertebral part of the medial surface of the lungs. It separates the mediastinal surface from the costal surface and it is in contact with the thoracic vertebral and the intervertebral disc between them.

Fissures of the Lungs and Lobes

In the two lungs there is a complete oblique fissure which cut across the costal, diaphragmatic and medial surface. It follows the lines of the 6^th rib and when the arm is raised above the head level it overly the medial border of the scapular. This fissure is higher and more vertical on the left then on the right. But in general they commence 6cm below the apex of the lung, 2cm from the medial plane and at the level of the 3^rd and 4^th spine of the vertebra.

A transverse fissure is present on the right lung which passes transversely along the level of the 4^th rib to meet with the oblique fissure at the mid axillary line as a result of this there is the presence of three lobes in the right lungs which indicates the superior, the middle and inferior lobes.

And on the left lung there is the presence of two lobes the superior lobe and the inferior lobe. But it should be noted that on the anterior inferior part of the superior lobe of the left lung there is the presence of a tongue like process known as the lingular. This lingular process and the cardiac notch correspond to the middle lobe of the right lung.

Root of the Lung

It is formed by structures entering to and fro the hilum and they include the principal bronchus, the pulmonary artery and vein, the pulmonary autonomic plexus, the bronchial arteries and veins, the bronchial pulmonary lymph nodes and lymph vessels and some loose cutaneous tissues. The structures forming the root of the lung is arranged similarly horizontally but vertically the arrangement differs on the right.

The arrangement of the superior bronchus, the pulmonary artery, the pulmonary vein is as follows: On the right, the principal bronchus lies between the pulmonary artery and vein, somehow interposing between the two, but more or less posterior. On the left, it is the pulmonary artery that lies between the principal bronchus and the pulmonary vein.

The tracheal on bifurcation give rise to right and left principal bronchus for the corresponding right and left lung. The right principal bronchus which is more vertical and shorter than the left bronchus, it is about 2.5cm in length and it is the direct continuation of the trachea. As a result of this, foreign bodies that enter the respiratory tract always lodge into the right lung. The right principal bronchus before it enters the right lung, at the level of fifth thoracic vertebra gives off its first secondary bronchus for the superior lobe, as it enters the lung substance; it gives off two other secondary bronchi for the middle lobe and the inferior lobe of the right lung. The superior secondary bronchus gives rise to three segmental bronchi which are named; the anterior, the apical and the posterior segmental bronchus, the secondary bronchus to the middle lobe gives rise to medial and lateral segmental bronchi, the third bronchus for the inferior lobe gives off five segmental bronchi;  an apical, anterior basal, medial basal, posterior basal and lateral basal.

In the left lung, the left principal bronchus is more horizontally placed and longer about 5cm in length. It enters the substance of the lungs and there it divides into two secondary bronchi for the superior and inferior lobe.

The superior lobar bronchus will later divide into a superior division which will give off two segmental bronchi; the anterior and the apicoposterior. The apico-posterior will then divide into the apical and posterior segmental bronchus. The lower division will give off two segmental bronchi, the superior lingular bronchi and the inferior lingular bronchii. The inferior lobar bronchus will give two tertiary bronchi; superior and inferior lingular bronchus. The two lobar bronchi that supply the inferior lobe will give up an apical segmental bronchi and divide into two, one giving the anteromedial bronchus which will later divide into anterior and medial segmental bronchi.

The lower division will give up posterior and lateral segmental bronchus. Therefore there are ten segmental bronchi on the right and eight segmental tertiary bronchi on the left. Each tertiary bronchus and the particular area of lung tissue it supply constitutes the functional unit of the lung known as the bronchopulmonary segment which comprises of one segmental bronchus, a segmental artery which is a branch of pulmonary artery and inter segmental vein which lie in the septum.

THE VESSELS OF THE LUNG

1.                 The pulmonary arteries, they carry deoxygenated blood to the lung for oxygenation and they follow the bronchial tree.

2.                 Pulmonary veins: There are pairs of pulmonary veins on the right and left lung though sometimes in the left it can be three veins and they carry deoxygenated blood back to the left atrium of the heart.

3.                 The bronchial artery and vein are the nutrient supplying vessels of the lungs. They supply the bronchi and other related structures. Sometimes at the capillary level they anastomose with the pulmonary artery and in some rare cases they could replace the pulmonary artery.

LYMPHATIC DRAINAGE

The lymphatic vessels of the lung drain into the bronchopulmonary lymph nodes, which drain into the tracheobronchial group of lymph nodes.

Nerve Supply

The vagus nerve and the upper five sympathetic ganglions give up branches that form the pulmonary plexus which now supplies the lung.

Applied Anatomy

Tuberculosis of the Lung: The apex of the lung is the most prone part that is easily affected by tuberculosis. This is because it is the part that is less ventilated in the lung.