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The Alveoli

The alveolar sacs consist of many alveoli and are composed of a single layer of epithelial tissue.  There are about 700 million alveoli in the adult lung.  The average, healthy adult lungs hold about 13 to 13.5 pints of air (about six liters).  The alveoli are considered the functional unit of the lung.  

In the spaces between the alveoli of the lungs is elastic connective tissue which is important for exhalation.

The alveoli are surrounded by a network of pulmonary capillaries.  These capillaries are made of simple squamous epithelium, therefore there are only two cells between the air in the alveoli and the blood in the pulmonary capillaries which permits efficient diffusion of gases.  It is through the moist walls of both the alveoli and the capillaries that rapid exchange of CO2 and O2 occurs. Carbon dioxide diffuses from the red blood cells through the capillary walls, into the alveoli [Eastern Kentucky University].  CO2 leaves the alveoli, exhaled through the nose and mouth. The opposite process occurs with O2, which diffuses from the alveoli into the capillaries, and from there into the red blood cells.

Each alveolus is lined with a thin layer of tissue fluid, which is essential for the diffusion of gases, because a gas must dissolve in a liquid in order to enter or leave a cell.  This is called the “earthworm principle”.  An earthworm will breathe through it’s moist skin and suffocate if it’s skin dries out.

The inner walls of the alveoli are covered with a lipid material known as surfactant. This surfactant helps to stabilize the alveoli preventing their collapse.  

Absence of surfactant would be similar to a plastic bag that is wet on the inside. It’s walls would stick together and not allow for complete expansion.


The lungs are located on either side of the heart in the chest or thoracic cavity.  They are encircled and protected by the rib cage. They are separated from each other by the mediastinum and the heart.  The upper part of the lung near the collarbone or clavicle is called the apex and the broad lower part is called the base.  The base of each lung rests on the diaphragm below.  

Lung tissue is porous and spongy, due to the tremendous amount of air sacs that it contains.  

The right lung is larger and broader than the left lung.  This is due to the shape and location of the heart.  The right lung is also shorter due to the diaphragm’s upward displacement to accommodate the liver.  

The right lung is divided into three lobes or parts; superior, middle and inferior.  

The left lung is divided into two lobes; superior and inferior.  The left lung is smaller and narrower than the right lung.  The concave area occupied by the heart on the left side of the lung is called the cardiac impression.

The pleural membranes are the serous membranes of the thoracic cavity.  The parietal pleura lines the chest wall.  The visceral pleura is actually on the surface of the lungs.  Between the pleural membranes is a serous fluid which prevents friction and keeps the two membranes together during breathing.

Unfortunately, there may be an occasion where the pleural cavity may fill up with an enormous amount of serous fluid.  This would occur when there is an inflammation of the pleura of pleurisy.  The excess fluid can compress or even at times collapse the lung.  

This extra fluid can make it very difficult to breathe.  To alleviate such pressure, sometimes a procedure called a thoracentesis is performed.  This is where a hollow, tube like instrument is inserted into the thoracic and pleural cavities to drain the fluid.

A pneumothorax is another condition that may affect the pleural cavity.  This is a build up of air within the pleural cavity on one side of the chest.  The excess air increases pressure on the lung, causing it to collapse.  Breathing is not possible with a collapsed lung, but the unaffected lung can continue the breathing process.


The mediastinum is also called the interpleural space.  It is situated between the lungs. The mediastinum contains: the thymus gland, heart, aorta and it’s branches, pulmonary arteries and it’s veins, superior and inferior vena cava, esophagus, trachea, thoracic duct, lymph nodes and vessels.  Whew…that’s a bunch.


Pulmonary ventilation or breathing of the lungs is due to the changes in pressure which occur within the chest cavity.  This variation in pressure is brought about by cellular respiration and mechanical breathing movements.

The two aspects of ventilation are inhalation and exhalation.  These processes as we know are brought about by a variety of factors but let us not forget the nervous system and are respiratory muscles.  The brain structure that is responsible for our vital centers or functions is the __________.  OK… the brain stem; which consists of the medulla, pons and the midbrain.

With respect to breathing there are three important pressures:

atmospheric pressure–  the pressure of the air around us.  At sea level the pressure is around 760 mmHG.  At higher altitudes, the pressure is lower.

intrapleural pressure–  the pressure within the potential pleural space between the parietal and the visceral pleura.   Intrapleural pressure is always slightly below atmospheric pressure.  This is called “negative pressure” because the elastic lungs are always tending to collapse and pull the visceral pleura away from the parietal pleura.  The serous fluid prevents separation from the pleural membranes.

intrapulmonic pressure–  the pressure within the bronchial tree and the alveoli.  This pressure fluctuates below and above atmospheric pressure during each cycle.  


Inhalation is also called inspiration and is the movement of air into the lungs. Changes in the shape and size of the thoracic cavity result in changes in the air pressure within that cavity and in the lungs.  The difference in air pressure causes the movement of air into and out of the lungs.  Air moves from an area there pressure is high to area where pressure is lower.  Respiratory muscles are responsible for changes in the shape of the thoracic cavity that cause the air movements involved in breathing.

During inspiration, the intercostal muscles lift the ribs upward and outward.  This increases the volume of the thoracic cavity.  At the same time the sternum rises along with the ribs and the diaphragm contracts and becomes flattened downward.  As the diaphragm moves downward, pressure is exerted on the abdominal viscera This causes the anterior muscles to protrude slightly, increasing the space within the chest cavity in a vertical direction. As a result, there is a decrease in pressure. Since atmospheric pressure is now greater, air rushes all the way down to the alveoli, resulting in inhalation.

Exhalation is also called expiration, wherein the opposite of inhalation takes place. Expiration is a passive process; the intercostal muscles and diaphragm that were once contracted, now relax. This causes the ribs to move down and diaphragm to move up. The surface tension of the fluid lining the alveoli diminishes the elasticity of the lung tissue and causes the alveoli to relax. The aforementioned action, coupled with the relaxation of the contracted respiratory muscles, relaxes the lungs. This decreases the space within the thoracic cavity, increasing pressure internally. The pressure increase forces air from the lungs, causing exhalation.

As the size of the thoracic cavity size is altered, lungs can adapt their capacity to accommodate–this ability is known as compliance. The lung’s compliance decreases and ventilation decreases as the lung’s tissue becomes diseased and fibrotic.