The Thorax

When looking at a chest X-ray it is important to remember that we are not only looking at the thorax but everything inside it too; the lungs, their markings and anatomy, the heart, boney anatomy (sternum, ribs) and soft tissue anatomy (liver). It is also important to remember that inspiration/expiration on taking a radiograph will have an affect on the appearance of it.

The Respiratory System

We should be able to see the apex/base of the lungs, the trachea and bification of such, the bronchi and the Hilum (the point at which the bronchi, blood vessels, nerves diverge from. Held together by pleura and connective tissue)

Lung markings are due to blood vessels and are important as absence of such can indicate pneumothorax and more prominent markings indicate other pathologies.

The Bronchial Tree - commences at the bification of the trachea (upper border of T5). The Right main bronchus is wider, shorter and more vertical than the left one, this means any foreign bodies may lodge in this one. The left Bronchus passes behind the arch of the aorta and in front of the oseophagus. The bronchi within  each lobe of the lung divide into smaller branches and lobules.

The right lung is separated into three lobes (Superior, medial and inferior lobe) whereas the left is only in two (<5% of people have an extra azygos lobe) Oblique (bottom) fissures and transverse (top) fissures seperate these lobes, fissures are infolding of pleural membrane which protect each lung (alongside the parietal pleura layer) These membranes contain a lubricating fluid (serous fluid) which reduces friction in respiration.  Each lung is further divided into bronchopulmonary segments composed of lobules which are wrapped in elastic connective tissue (Alveoli, Nerves, Lympathic vessels, branches of pulmonary and bronchiole arteries and terminal bronchiole).

The Heart

The heart lay inside the Mediastinum - a collection of tissues between the lungs, it is a broad partition medial to the lungs and extending from sternum and includes all contents of thoracic cavity (minus lungs).

Superior Mediastinum contains Arch of aorta
Anterior Mediastinum contains Right main pulmonary artery, left atrium, left atrial border, inferior vena cava and the right ventricle.
Middle Mediastinum contains Birfurcation of trachea and the main bronchi.
Posterior Mediastinum contains Thoracic part of decending aorta, Oesophagus, thoracic duct and lymph glands.

The ventricles of the heart have difference cardiac muscle thickness depending on how much pressure is generated (the left which supplies the whole body has a much thicker wall). Atria have comparatively little muscle wall as the pressure is a lot less there.

There are two coronary arteries which branch from the ascending aorta: the left divides into ventricular and circumflex branches. The anterior interventricular branch is in the anterior interventricular sulcus and supplies oxygenated blood to the walls of both ventricles. The circumflex lies in the coronary sulcus and distributes oxygenated blood to the walls of the left ventricle and atrium.

The right coronary artery supplies small branches to the right atrium and continues inferiorly to the right auricle dividing into posterior interventricular and marginal branches. The posterior interventricular branch supplies the walls of the two ventricles and septum with oxygenated blood. The marginal branch in the coronary sulcus transports oxygenated blood to the myocardium of the right ventricle.  

The Oseophagus

Extends from the laryngopharynx through mediastinum, diaphragm and to the superior portion of the stomach.

Composed of 4 layers 

1. Outer areolar layer (elastic fibres - attaches it to surrounding structures)
2. Muscular coat (muscle fibres - enables swallowing)
3. Submucous coat (loose areolar tissue - contains vessels, nerves and muscous glands)
4. Inner mucous coat (stratified squamous epithelium - folded rugae when empty)