This section describes the lungs and its structures.

Lungs

• Primary organ of the respiratory system.

• Primarily responsible for gas exchange.

• Supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body.

• When we breathe, we inhale oxygen and exhale carbon dioxide. This exchange of gases is the respiratory tract's means of getting oxygen to the blood.  

RESPIRATION

• Oxygen enters through the mouth and nose ->larynx ->bronchi- -> bronchial tube -> releases oxygen into the alveoli.

• The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood.

• The waste-rich blood from the veins releases its carbon dioxide into the alveoli. Carbon dioxide follows the same path out of the lungs when we exhale.

ANATOMY OF THE LUNG

• There are 2 in number: a right lung and left lung.

• The lung is light, porous, and spongy in texture; it floats in water and crepitates when handled owing the presence of air in the alveoli.

• Highly elastic.

• Surface is smooth, shining, and marked out into numerous polyhedral areas, indicating the lobules of the organ.

• Shape: Each lung is conical in shape with its base resting on the diaphragm and its apex extending superiorly to a point approximately 2.5cm superior to each clavicle.

• Weight: An adult right lung usually weighs about 625g and left 565g but they may vary greatly. Their weight depends on the amount of blood or serous fluid. In proportion to body stature the lungs are heavier in men than women.

• Surfaces:

  • Costal surface (external or thoracic surface) is smooth and convex, its shape is adapted to that of the thoracic wall.

  • Medial surface (inner surface) has 2 parts:

    • Vertebral part lies in contact with the sides of the thoracic vertebral and interveretebral discs.

    • Mediastinal area is deeply concave, as its adapted to the heart at the cardiac impression.

• Borders:

  • Inferior border is thin and sharp where it separates the base from the costal surface and it extends into the costo diaphragmatic recess; medially, where it divides the base from the mediastinal surface, it is rounded.

  • Posterior border is broad and rounded, separates the costal surface from the mediastinal, corresponding to the leads of the ribs.

  • Anterior border is thin and sharp and it overlaps the front of the pericardium.

• Right lung is larger than the left lung.

• Each lobe is separated by deep prominent fissures on the surface of the lung.

• Each lung is divided into lobules that are separated from each other by connective tissues but the separations are not visible as surface fissures.

• Because major blood vessels and bronchi do not cross the connective tissues, individual diseased lobules can be surgically removed

• Root of the lung: the pulmonary root connects the medial surface of the lung to the heart and trachea. The root is formed by the bronchus, the pulmonary artery, the pulmonary veins, the bronchial arteries and veins, the pulmonary plexuses of nerves, lymphatic vessels, bronchial lymph glands and aerolar tissue, all of which are enclosed by a reflection of the pleura.

• Root of the right lung lies behind the superior vena cava and part of the right atrium and below the azygos vein.

• Root of the left lung passes beneath the aortic arch and in front of the descending aorta.

• One bronchial tube goes to the right lung and the other goes to the left lung.

• Once inside the lung, each bronchi subdivides into secondary bronchi, three on the right and two on the left, each of which enters and supplies one lung lobe.

  • Point of entry of the bronchi, vessels and nerves is called the hilus of the lung.

• There are about 30,000 bronchioles in each lung.

• The bronchioles other than the primary bronchi are supported by small cartilage plates embedded in their walls.

• As you move down the bronchial tree, the cartilage becomes more and more sparse and smaller and smooth muscles become abundant.

• The bronchioles devoid of cartilage in their walls, are very small tubes, one mm or less in diameter. Because of much smooth muscle and no cartilage in their walls, they can constrict if the smooth muscle contracts forcefully, which occurs during asthma attacks.

• Each respiratory bronchiole divides to form alveolar ducts that end as clusters of air sacs called alveoli.

• Cilia :

  • Are tiny hairs that line the bronchial tubes.

  • They have a continuous wave-like motion.

  • This motion carries mucous upwards and out into the throat, where it is either coughed up or swallowed.

  • The mucous catches and gets rid of much of the duct, germs and other unwanted matter that has invaded the lung.

• The alveolar epithelium and interposed fused basal laming from a very thin barrier called the blood-air barrier. The squamous alveolar cells make up 97% of alveolar.

• Type II cells are also present in the alveoli.

  • Type II cells continually make pulmonary surfactant .

• Pleural membranes: the lung is surrounded by a membrane having two closely opposed layers known as the pleura:

  • Visceral pleura is the inside layer of fine connective tissue that is somewhat continuous with the lung tissue.

  • Parietal layer is separated from the visceral layer by a thin film of lubricating fluid that facilitates the smooth sliding of one surface over another during movement of the lung.

  • The pleura secretes a small amount of fluid that lubricates the surface so that they slide past one another as the lungs expand and contract

• Diaphragm: is the strong wall of muscle that separates the chest cavity from the abdominal wall. By moving up and downward, it creates suction in the chest to draw in air and expand the lung.

• The diaphragm's job is to help pump the carbon dioxide out of the lungs and pull the oxygen into the lung.

• Blood supply: the lung receives blood flow from both the bronchial circulation and the pulmonary circulation.

  • Bronchial Circulation: consists of a small portion of the output of the left ventricle and supplies part of the tracheobronchial tree with systemic arterial blood.

    • Arise from the aorta or from the intercostals arteries.

    • They supply blood to the tracheobronchial tree down to the level of the terminal bronchioles as well as the pulmonary blood vessels, the visceral pleura, the hilar lymph nodes, and branches of nerves, including the vagus.

    • The bronchial circulation consist of about 2% of the output of the left ventricle. You can survive without this circulation.

    • The venous drainage of the bronchial circulation is unusual. Some of the bronchial venous blood enters the azygos and hemiazygos veins, a substantial portion enters the pulmonary vein.

    • The blood in the pulmonary veins have undergone gas exchange with the alveolar air, that is why the pulmonary veins contain "arterial blood."

  • Pulmonary Circulation: consists of the entire output of the right ventricle and supplies the lung with the mixed venous blood draining all the tissues of the body.

    • This blood undergoes gas exchange with the alveolar air in the pulmonary capillaries.

    • Pulmonary blood flow is approximately equal to 100% of the output of the left ventricle.

    • There is about 250-300cc of blood per square meter of body surface area in pulmonary circulation.

    • It takes a red blood cell about 4-5 seconds to travel through the pulmonary circulation at resting cardiac output.

    • Capillaries in the pulmonary circulation have an average diameter of around 6um; they are slightly smaller than the average red blood cell (8um). Red blood cells must change their shape slightly to pass through pulmonary circulation.

    • Pulmonary artery tree subdivides into approximately 280 billion pulmonary capillaries, where gas exchange occurs.

    • Pulmonary capillaries wall are thin and have small amount of smooth muscle found in them. The pulmonary vessels offer much less resistance to blood flow than the systemic arterial vessels do.

    • They are much more distensible and compressible than systemic arterial blood vessels.

    • Pulmonary artery carries unoxygenated blood to the lungs. Pulmonary artery receives mixed venous blood from the right ventricle. Form branches similar to the airways. Pulmonary artery follows the bronchi to the terminal bronchioles then forms a capillary network in the walls of alveoli.

• Autonomic Nervous System (ANS) and the Lung:

  • Bronchi and bronchioles walls are innervated by the ANS

  • There are abundant muscarinic receptors and cholinergic receptors causing bronchoconstriction is stimulated.

  • There are beta adrenergic receptors in the bronchial epithelium and smooth muscle. These receptors will mediate bronchodilation.

  • Stimulation of the sympathetic division of the ANS causes the smooth muscles of the bronchial branches causes them to relax. Dilating the bronchial branches allows more air to pass through the lungs.

  • Stimulation of the parasympathetic division of the ANS cause, the smooth muscles of the bronchial branches to contract, causing constriction of the bronchial branches allowing less air through the lungs.

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