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Area : Basic Biomedical Sciences

Unit: 1A Physiology

Physiology Of Respiration

Physiology of Respiration

Pulmonary Ventilation

Ventilation is the movement of air into and out of the lungs, as in breathing. There are two phases of ventilation.

1. Inhalation is the drawing of air into the lungs.

2. Exhalation is the expulsion of air from the lungs.

In inhalation, the active phase of breathing, the respiratory muscles contract to enlarge the thoracic cavity.

The diaphragm is a strong dome-shaped muscle attached around the base of the rib cage.

The rib cage is also moved upward and outward by contraction of the external intercostals muscles and, during exertion, by contraction of other muscles of the neck and chest.

During quiet breathing, the movement of the diaphragm accounts for most of the increase in thoracic volume.

As the thoracic cavity increases in size, gas pressure within the cavity decreases. When the pressure drops to slightly below atmospheric pressure, air is drawn into the lungs.

In exhalation, the passive phase of breathing, the muscles of respiration relax, allowing the ribs and diaphragm to return to their original positions. The tissues of the lung are elastic and recoil during exhalation. During forced exhalation, the internal intercoastals muscles and the muscles of the abdominal wall contracts, pulling the bottom of the rib cage in and down. The abdominal viscera are also pushed upward against the diaphragm.

Air Movement

Air enters the respiratory passages and flows through the ever-dividing tubes of the bronchial tree. As the air traverses this passage, it moves more and more slowly through the great number of bronchial tubes until there is virtually no forward flow as it reaches the alveoli.

Here the air moves by diffusion, which soon equalizes any differences in the amounts of gases present.

Each breath causes relatively little change in the gas composition of the alveoli, but normal

continuous breathing ensures the presence of adequate oxygen and the removal of carbon dioxide.

Tidal volume The amount of air moved into or out of the lungs in quiet, relaxed breathing.

Vital capacity The volume of air that can be expelled from the lungs by maximum exhalation following maximum inhalation.

Residual volume The volume of air that remains in the lungs after maximum exhalation.

Total lung capacity

The total volume of air that can be contained in the lungs after maximum inhalation

Functional residual capacity

The amount of air remaining in the lungs after normal exhalation.

Regulation of respiration

• Regulation of respiration is a complex process that must keep pace with moment-to-moment changes in cellular oxygen requirements and carbon dioxide production. Regulation depends primarily on the respiratory control centers located in the medulla and pons of the brain stem. Nerve impulses from the medulla are modified by the centers in the pons.
• Respiration is regulated so that the levels of oxygen, carbon dioxide, and acid are kept within certain limits.
• The control centers regulate the rate, depth, and rhythm of respiration. From the respiratory center in the medulla, motor nerve fibers extend into the spinal cord. From the cervical (neck) part of the cord, these nerve fibers continue through the phrenic nerve to the diaphragm. The diaphragm and the other muscles of respiration are voluntary in the sense that they can be regulated by messages from the higher brain centers, notably the cortex. It is possible for a person to deliberately breath more rapidly or more slowly or to hold his breath and not breath at all for a time.
• Usually we breath without thinking about it, while the respiratory centers in the medulla and pons do the controlling. Of vital importance in the control of respiration are the chemoreceptors. These receptors are found in structures called the carotid and aortic bodies, as well as out side the medulla of the brain stem.
• The carotid bodies are located near the bifurcation of the common carotid arteries, while the aortic bodies are located in the aortic arch. These bodies contain many small blood vessels and sensory neurons, which are vital importance in the control of respiration are the chemoreceptors.
• These receptors are found in structures called the carotid and aortic bodies, as well as out side the medulla of the brain stem.
• The carotid bodies are located near the bifurcation of the common carotid arteries, while the aortic bodies are located in the aortic arch. These bodies contain many small blood vessels and sensory neurons, which are sensitive to decreases in oxygen supply as well as to increases in carbon dioxide and acidity (H+).
• Impulses are sent to the brain from the receptors in the carotid and aortic bodies.
• The receptor cells outside the medulla are affected by the concentration of hydrogen ion in cerebrospinal fluid (CSF) mas determined by the concentrations of carbon dioxide in the blood.