4.3 – Physiology (Function) of the Respiratory System
4.3 – Physiology (Function) of the Respiratory System
Blood Supply
The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation.
- This blood supply contains deoxygenated blood and travels to the lungs where erythrocytes pick up oxygen to be transported to tissues throughout the body.
- The pulmonary artery carries deoxygenated, arterial blood to the alveoli.
- The pulmonary artery branches multiple times as it follows the bronchi and each branch becomes progressively smaller in diameter.
- One arteriole and an accompanying venule supply and drain one pulmonary lobule. As they near the alveoli, the pulmonary arteries become the pulmonary capillary network.
- The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibres.
- The capillaries branch and follow the bronchioles and structure of the alveoli. It is at this point that the capillary wall meets the alveolar wall, creating the respiratory membrane.
- Once the blood is oxygenated, it drains from the alveoli by way of multiple pulmonary veins, which exit the lungs through the hilum.
Nervous Innervation
The blood supply of the lungs plays an important role in gas exchange and serves as a transport system for gases throughout the body. Innervation by both the parasympathetic and sympathetic nervous systems provides an important level of control through dilation and constriction of the airway.
- The parasympathetic system causes bronchoconstriction.
- The sympathetic nervous system stimulates bronchodilation.
Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from autonomic nervous system control. Sensory nerve fibres arise from the vagus nerve, and from the second to fifth thoracic ganglia. The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum. The nerves then follow the bronchi in the lungs and branch to innervate muscle fibres, glands, and blood vessels.
Pleura of the Lungs
Each lung is enclosed within a cavity that is surrounded by the pleura. The pleura (plural = pleurae) is a serous membrane that surrounds the lung. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum.
The pleurae consist of two layers:
- The visceral pleura is the layer that is superficial to the lungs, and extends into and lines the lung fissures (see Figure 4.11).
- The parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm.
The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers.
The pleurae perform two major functions:
- Produce pleural fluid that lubricates surfaces, reduces friction to prevent trauma during breathing, and creates surface tension that helps maintain the position of the lungs against the thoracic wall. T