Thoracic cavity & contents Thorax Unit Lecture 3 & 4 د. حيدر جليل الأعسم

Thoracic Cavity Chest (thoracic) cavity is bounded by chest wall and below by diaphragm. It extends upward into the root of the neck about one fingerbreadth above the clavicle on each side. The chest cavity can be divided into a median partition, called the mediastinum, and the laterally placed pleurae and lungs. Mediastinum: It is a movable median partition of chest cavity that extends superiorly to thoracic outlet and root of the neck and inferiorly to diaphragm. It extends anteriorly to sternum and posteriorly to vertebral column. Diaphragm: It is a very thin muscle that (with pleura and peritoneum) separates the chest from the abdominal viscera.

Thoracic Cavity Mediastinum is divided by an imaginary plane from anteriorly sternal angle to posteriorly lower border of body of 4th thoracic vertebra, into: A. Superior mediastinum: bounded by in front by manubrium sterni and behind by first four thoracic vertebrae. B. Inferior mediastinum: bounded in front by body of sternum and behind by lower eight thoracic vertebrae. It is further subdivided into: Middle mediastinum (pericardium and heart) Anterior mediastinum (a space between pericardium and sternum) Posterior mediastinum (between pericardium and vertebral column). Contents of Mediastinum: Thymus Heart and large blood vessels. Trachea Esophagus. Thoracic duct and lymph nodes. Vagus, phrenic nerves & sympathetic trunks.

The Diaphragm It is a thin muscular and tendinous septum that separates chest cavity above from abdominal cavity below. It is pierced by structures that pass between chest and abdomen. Diaphragm is the most important muscle of respiration. It is dome shaped & consists of a peripheral muscular part, which arises from margins of thoracic opening, and a centrally placed tendon. Origin: can be divided into three parts: A. Sternal part arising from posterior surface of xiphoid process B. Costal part arising from deep surfaces of lower six ribs and their costal cartilages C. Vertebral part arising by vertical columns or crura & from arcuate ligaments

The Diaphragm Right crus arises from the sides of bodies of first three lumbar vertebrae and the intervertebral discs. Left crus arises from the sides of bodies of first two lumbar vertebrae and the intervertebral disc. Medial borders of the two crura are connected by a median arcuate ligament, which crosses over the anterior surface of aorta. Lateral to crura the diaphragm arises from medial and lateral arcuate ligaments. Medial arcuate ligament extends from side of the body of 2nd lumbar vertebra to tip of transverse process of first lumbar vertebra. Lateral arcuate ligament extends from tip of transverse process of first lumbar vertebra to lower border of the 12th rib.

The Diaphragm Insertion: diaphragm is inserted into a central tendon, which is shaped like three leaves. The superior surface of the tendon is partially fused with the inferior surface of the fibrous pericardium. Some of the muscle fibers of the right crus pass up to the left and surround the esophageal orifice in a slinglike loop. These fibers appear to act as a sphincter and possibly assist in the prevention of regurgitation of the stomach contents into the thoracic part of the esophagus. Shape of Diaphragm Diaphragm curves up into right and left domes, or cupulae. Right dome: reaches as high as upper border of fifth rib. Left dome: may reach lower border of fifth rib. (right dome lies at a higher level?) Central tendon: at xiphisternal joint level

Openings in the Diaphragm 1. Aortic opening lies anterior to body of 12th thoracic vertebra between crura. It transmits aorta, thoracic duct & azygos vein 2. Esophageal opening lies at level of 10th thoracic vertebra in a sling of muscle fibers derived from right crus (It transmits esophagus, right and left vagus nerves, esophageal branches of left gastric vessels & lymphatics from lower 1/3 of esophagus 3. Caval opening lies at level of 8th thoracic vertebra in central tendon. It transmits inferior vena cava and terminal branches of right phrenic nerve. In addition to these openings: Sympathetic splanchnic nerves pierce crura Sympathetic trunks pass posterior to medial arcuate ligament on each side Superior epigastric vessels pass between sternal and costal origins of diaphragm.

Nerve Supply of the Diaphragm Motor nerve supply: Right and Left phrenic nerves (C3, 4, 5) Sensory nerve supply: Central surfaces of diaphragm: Periphery of diaphragm: Lower six intercostal nerves. Action and fucntions of the Diaphragm On contraction, diaphragm pulls down its central tendon and increases vertical diameter of thorax. Therefore, functions of the diaphragm are: 1. Muscle of inspiration. 2. Muscle of abdominal straining. 3. Weight-lifting muscle. 4. Thoracoabdominal pump for blood & lymph.

Pleura (Pleurae) Pleurae and lungs lie on either side of mediastinum within chest cavity. Each pleura has two parts: Parietal layer: lines thoracic wall, covers thoracic surface of diaphragm and lateral aspect of mediastinum, and extends into the root of neck to line undersurface of suprapleural membrane at thoracic outlet; Visceral layer: completely covers outer surfaces of lungs and extends into depths of interlobar fissures. The two layers become continuous with one another by means of a cuff of pleura that surrounds the structures entering and leaving lung at hilum of each lung. To allow for movement of pulmonary vessels and large bronchi during respiration, pleural cuff hangs down as a loose fold pulmonary ligament.

Parietal Pleura Cervical pleura: lining undersurface of suprapleural membrane. Costal pleura: lines inner surfaces of ribs, costal cartilages, intercostal spaces, sides of vertebral bodies, and back of sternum. Diaphragmatic pleura: covers upper thoracic surface of diaphragm. Costodiaphragmatic recess is slit-like spaces between costal and diaphragmatic parietal pleurae that are separated only by a capillary layer of pleural fluid. In quiet respiration, costal & diaphragmatic pleurae are in apposition to each other below lung border. In deep inspiration, margins of lung base descend and expand with in this recess. Mediastinal pleura: covers and forms lateral boundary of mediastinum. Costomediastinal recess is a slit-like space situated along anterior margins of pleura between mediastinal and costal pleura which are separated by a capillary layer of pleural fluid.

Pleural cavity It is a slit-like space that separate parietal and visceral layers of pleura. Pleural cavity normally contains a small amount of pleural fluid, which covers the surfaces of pleura as a thin film and permits two layers to move on each other with the minimum of friction. During full inspiration, lungs expand and fill the pleural cavities. However, during quiet inspiration the lungs do not fully occupy pleural cavities at four sites: right & left costodiaphragmatic recesses right & left costomediastinal recesses.

Nerve Supply of Pleura Parietal pleura is sensitive to pain, temperature, touch, and pressure: Costal pleura is segmentally supplied by intercostal nerves. Mediastinal pleura is supplied by phrenic nerve. Diaphragmatic pleura is supplied over the domes by phrenic nerve and around periphery by lower six intercostal nerves. Visceral pleura is sensitive to stretch but is insensitive to common sensations such as pain and touch. It receives an autonomic nerve supply from the pulmonary plexus.

Lungs Lungs are soft and spongy and very elastic organs. If the thoracic cavity were opened, lungs would immediately shrink to one third or less in volume. In children, they are pink, but with age, they become dark and mottled because of inhalation of dust particles that become trapped in phagocytes of the lung. Lungs are situated on each side of mediastinum & separated from each other by heart and great vessels and other structures in mediastinum. Each lung is conical, covered with visceral pleura, and suspended free only by its root in its own pleural cavity.

Lungs Apex : projects upward into the neck above the clavicle Concave base: (sits on diaphragm) Convex costal surface: corresponds to the concave chest wall Concave mediastinal surface: is molded to pericardium and other mediastinal structures. At about middle of this surface is Hilum, a depression in which bronchi, vessels, and nerves enter and leave the lung forming the root of the lung. Anterior border is thin & overlaps heart and on left lung, cardiac notch. Posterior border is thick and rounded & lies beside vertebral column.

Lobes and Fissures of Lungs Right Lung Right lung is slightly larger than the left and is divided by oblique and horizontal fissures into 3 lobes: upper, middle, and lower lobes. Oblique fissure runs from inferior border upward & backward across medial & costal surfaces until it cuts posterior border 2.5 inches below apex. Horizontal fissure runs horizontally across costal surface at the level of 4th costal cartilage to meet oblique fissure in midaxillary line. Middle lobe is thus a small triangular lobe bounded by horizontal and oblique fissures. Left Lung Left lung is divided by a similar oblique fissure into 2 lobes: upper and lower lobes. No horizontal fissure in left lung.

Bronchopulmonary Segments Bronchopulmonary segments: They are structurally and functionally independent unit of a lung lobe and are surrounded by connective tissue. They are, anatomic, functional, and surgical units of lungs. Lobar (secondary) bronchus: passes to a lobe of lung, gives off branches Segmental (tertiary) bronchi to bronchopulmonary segment. Segmental bronchus is accompanied by a branch of pulmonary artery, but the tributaries of pulmonary veins run in the connective tissue between adjacent bronchopulmonary segments. Each segment has its own lymphatic vessels and autonomic nerve supply.

Blood Supply of Bronchi, connective tissue of lung & visceral pleura: Bronchial arteries: branches of descending aorta). Bronchial veins drain into azygos and hemiazygos veins. Alveoli receive deoxygenated blood from terminal branches of pulmonary arteries. Oxygenated blood leaving alveolar capillaries drains into tributaries of pulmonary veins, which follow intersegmental connective tissue septa to the lung root. Two pulmonary veins leave each lung root to empty into left atrium of the heart. Lymph Drainage of Lungs All lymph from lungs leaves hilum And drains into tracheobronchial nodes and then into bronchomediastinal lymph trunks. Nerve Supply: Sympathetic trunk & Vagus nerve (parasympathetic)

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