Lungs and pulmonary ventilation

The respiratory and cardiovascular systems combine to provide an effective delivery system that carries oxygen to and removes carbon dioxide from tissues. This transportation involves four separate processes:

1.       pulmonary ventilation(breathing): movement of air into and out of the lungs

2.       pulmonary diffusion: the exchange of oxygen and carbon dioxide between the lungs and the blood

3.       transport of oxygen and carbon dioxide via the blood

4.       capillary diffusion: the exchange of oxygen and carbon dioxide between the capillary blood and the metaboically active tissues

The first two processes are referred to as external respiration because they involve moving gases from outside of the body into the lungs and then the blood. Once the gases are in the blood, they must be transported to the tissues. When blood arrives at the tissues, the fourth step of respiration occurs. This gas exchange between the blood and the tissues is called internal respiration. Thus, external  and internal respiration are linked by the circulatory system.

Pulmonary  ventilation 

Pulmonary ventilation, commonly reffered to as breathing, is the process by which we move air into and  out of the lungs. The anatomy of the respiratory system is illustrated below. Air is typically drawn into the lungs through the nose, although the mouth must also be used when the demand for air exceeds the amount that can comfortably be brought in through the nose.  Nasal breathing is advantageous because the air is warmed and humidified as it swirls through the bony irregular sinus surfaces(turbinates or conchae). Of equal importance, the turbinates churn the inhaled air, causing dust and other particles to contact and adhere to the nasal mucosa. This filters out all but the tiniest particles, minimizing irritation and the threat of respiratory infections. From the nose and mouth, the air travels through the pharynx, larynx, trachea and bronchial tree. These anatomical structures serve as the transport zone of the lungs because gas exchange occurs when air finally reaches the smallest respiratory units: the respiratory are bronchioles and the alveoli. The respiratory bronchioles are primarily transport tubes also, but are included in this region because they contain clusters of alveoli. This is known as the respiratory zone because it is the site of gas exchange in the lungs. 

The lungs are not directly attached to the ribs. Rather, they are suspended by the pleural saes. The pleural sacs have a double wall: the parietal pleura, which lines are thoracic wall, and the visceral or pulmonary pleura, which lines the outer aspects of the lung. These pleural walls envelop the lungs and have a thin film of fluid between them that reduces friction during respiratory movements. In addition, these sacs are connected to the lungs and to the inner surface of the thoracic cage, causing the lungs to take the shape and size of the rib or thoracic cage as the chest expands and contracts.

The anatomy of the lungs, the pleural sacs, and the thoracic cage determines airflow into and out of the lungs, that is, inspiration and expiration.

Inspiration

Inspiration is an active process involving the diaphragm and the external intercostales muscles. Figure a shows the resting positions of the diaphragm and the thoracic cage, or thorax.  With inspiration, the ribs and sternum are moved by the external intercostal muscles. The ribs swings up and forward. At the same time, the diaphragm contracts, flattening down toward the abdomen. 

These actions, illustrated in figure b, expand all three dimensions of the thoracic cage, increasing the volume inside the lungs. When the lungs are expanded they have a greater volume, so the air within them has more space to fill. According to Boyle's gas law, which states that pressure x volume is constant(at a constant temperature), the pressure within the lungs decreases. As a result, the pressure in the lungs(intrapulmonary pressure) is less than the air pressure outside the body. Because the respiratory tract is open to the outside, air rushes into the lungs to reduce this pressure difference. This is how air moves into the lungs during inspiration.

During forced or labored breathing, as during heavy exercise, inspiration is further assisted by the action of other muscles, such as the scaleni(anterior, middle, and posterior) and sternocleidomastoid in the neck and the pectoralis in the chest. These muscles help raise the ribs even more than during regular breathing.

Expiration

At rest, expiration is usually a passive process involving relaxation of the inspiratory muscles and elastic recoil of the lung tissue. As the diaphragm relaxes, it returns to its normal upward, arched position. As the external intercostal muscles relax, the ribs and sternum move back into their resting positions(c). While this happens, the elastic nature of the lung tissue causes it to recoil to its resting size. This increases the pressure in the lungs and causes a proportional decrease in volume in the thorax, and therefore air is forced out of the lungs.

During forced breathing, expiration becomes a more active process. The internal intercostales muscle actively pulls down the rib. This action can be assisted by latissimus dorsi and quadratus lumborum. Contracting the abdominal muscles increases the intraabdominal pressure, forcing the abdominal viscera upward against the diaphragm and accelerating its return to the domed  position. These muscles also pull the rib cage down and inward.

The changes in intra-abdominal and intrathoracic pressure that accompany forced breathing also help return venous blood back to the heart; this is similar to the action of the muscle pump in the legs in assisting the return of venous volume. As intra-abdominal and intrathoracic pressure increases, it is transmitted to the great veins - the pulmonary veins and superior and inferior venae cavae - that transport blood back to the heart. When the pressure decreases, the veins return to their original size and fill with blood. The changing pressures within the abdomen and thorax squeeze the blood in veins, assisting in return through a milking action. This phenomenon is known as the respiratory pump and is essential in maintaining adequate venous return.