The main difference between ventilation and respiration is that ventilation is the provision of fresh air into the lungs while respiration is the gas exchange between the body and the external environment. Lungs are the organs involved in both ventilation and respiration of most vertebrates. Breathing occurs through inhalation in animals. Lungs provide close contact between blood and atmospheric air, facilitating respiration.
What is Ventilation — Definition, Process, Role 2. What is Respiration — Definition, Process, Role 3. Ventilation is the provision of fresh air into the lungs.
The conducting zones of the lungs are involved in ventilation. It includes nose, pharynx , larynx , trachea , primary bronchi, bronchial tree, and terminal bronchioles. Inspiration and expiration are the two events of ventilation. Inspiration and expiration are shown in figure 1. Figure 1: Inspiration and Expiration. Both inspiration and expiration occur based on the pressure differences between the atmosphere and the lungs.
Inspiration occurs when the pressure inside the lungs goes down. The atmospheric pressure is mm Hg. The increased volume of the lungs causes the reduction of pressure. It is achieved by the contraction of the diaphragm and intercostal muscles. When the pressure goes down to mm Hg, the atmospheric air comes into the lung. This is called inspiration. Two important aspects of gas exchange in the lung are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries.
For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible. However, factors such as regional gravity effects on blood, blocked alveolar ducts, or disease can cause ventilation and perfusion to be imbalanced.
The partial pressure of oxygen in alveolar air is about mm Hg, whereas the partial pressure of the oxygenated pulmonary venous blood is about mm Hg. When ventilation is sufficient, oxygen enters the alveoli at a high rate, and the partial pressure of oxygen in the alveoli remains high. In contrast, when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane.
The body has mechanisms that counteract this problem. In cases when ventilation is not sufficient for an alveolus, the body redirects blood flow to alveoli that are receiving sufficient ventilation. This is achieved by constricting the pulmonary arterioles that serves the dysfunctional alveolus, which redirects blood to other alveoli that have sufficient ventilation. At the same time, the pulmonary arterioles that serve alveoli receiving sufficient ventilation vasodilate, which brings in greater blood flow.
Factors such as carbon dioxide, oxygen, and pH levels can all serve as stimuli for adjusting blood flow in the capillary networks associated with the alveoli. Ventilation is regulated by the diameter of the airways, whereas perfusion is regulated by the diameter of the blood vessels. The diameter of the bronchioles is sensitive to the partial pressure of carbon dioxide in the alveoli. A greater partial pressure of carbon dioxide in the alveoli causes the bronchioles to increase their diameter as will a decreased level of oxygen in the blood supply, allowing carbon dioxide to be exhaled from the body at a greater rate.
As mentioned above, a greater partial pressure of oxygen in the alveoli causes the pulmonary arterioles to dilate, increasing blood flow. Gas exchange occurs at two sites in the body: in the lungs, where oxygen is picked up and carbon dioxide is released at the respiratory membrane, and at the tissues, where oxygen is released and carbon dioxide is picked up. External respiration is the exchange of gases with the external environment, and occurs in the alveoli of the lungs.
Internal respiration is the exchange of gases with the internal environment, and occurs in the tissues. The actual exchange of gases occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes. Instead, these gases follow pressure gradients that allow them to diffuse.
The anatomy of the lung maximizes the diffusion of gases: The respiratory membrane is highly permeable to gases; the respiratory and blood capillary membranes are very thin; and there is a large surface area throughout the lungs.
The pulmonary artery carries deoxygenated blood into the lungs from the heart, where it branches and eventually becomes the capillary network composed of pulmonary capillaries.
These pulmonary capillaries create the respiratory membrane with the alveoli. As the blood is pumped through this capillary network, gas exchange occurs. Although a small amount of the oxygen is able to dissolve directly into plasma from the alveoli, most of the oxygen is picked up by erythrocytes red blood cells and binds to a protein called hemoglobin, a process described later in this chapter.
Oxygenated hemoglobin is red, causing the overall appearance of bright red oxygenated blood, which returns to the heart through the pulmonary veins. Carbon dioxide is released in the opposite direction of oxygen, from the blood to the alveoli. Some of the carbon dioxide is returned on hemoglobin, but can also be dissolved in plasma or is present as a converted form, also explained in greater detail later in this chapter.
External respiration occurs as a function of partial pressure differences in oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries.
Figure 2. In external respiration, oxygen diffuses across the respiratory membrane from the alveolus to the capillary, whereas carbon dioxide diffuses out of the capillary into the alveolus. Although the solubility of oxygen in blood is not high, there is a drastic difference in the partial pressure of oxygen in the alveoli versus in the blood of the pulmonary capillaries. This difference is about 64 mm Hg: The partial pressure of oxygen in the alveoli is about mm Hg, whereas its partial pressure in the blood of the capillary is about 40 mm Hg.
This large difference in partial pressure creates a very strong pressure gradient that causes oxygen to rapidly cross the respiratory membrane from the alveoli into the blood. The partial pressure of carbon dioxide is also different between the alveolar air and the blood of the capillary. However, the partial pressure difference is less than that of oxygen, about 5 mm Hg.
The partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg. However, the solubility of carbon dioxide is much greater than that of oxygen—by a factor of about 20—in both blood and alveolar fluids. As a result, the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar.
Internal respiration is gas exchange that occurs at the level of body tissues Figure 3. Air movement in a reverse pathway from alveoli to mouth and nose, is exhalation.
Inhalation, followed by exhalation, equals one ventilation. This is what you observe chest rise and fall when determining the breathing rate. A ventilation can only take place if the brainstem, cranial and associated peripheral nerves, the diaphragm, intercostal musculature and lungs are all functional. Combining the function of all these structures, the pulmonary ventilation mechanism establishes two gas pressure gradients. One, in which the pressure within the alveoli is lower than atmospheric pressure — this produces inhalation.
The other, in which the pressure in the alveoli is higher than atmospheric pressure — this produces exhalation. These necessary changes in intrapulmonary pressure occur because of changes in lung volume. So, how does the lung volume change? Quite simply, it is a combination of muscle contractions stimulated by the central nervous system , and the movement of a serous membrane within the thorax called the pleura.
The pleura is made of two layers: a parietal layer that lines the inside of the thorax and a visceral layer that covers the lungs and adjoining structures blood vessels, bronchi, and nerves. Between the visceral and parietal layers is a small, fluid-filled space, called the pleural cavity. The initiation of ventilation begins with the brainstem, where impulses action potentials generate within the medulla oblongata, then travel distally within the spinal cord.
The impulse traverses individually through cervical nerves three, four and five until just above the clavicle. Here, the three cervical nerves merge into one large nerve called the phrenic nerve, which attaches distally to the diaphragm.
Imagine these two nerves resembling a pair of suspenders on the anterior chest. The delivered impulse from the phrenic nerve initiates diaphragm contraction. The intercostal muscles are a group of intrinsic chest wall muscles occupying the intercostal spaces. They are arranged separately in three distinct layers external intercostal muscles, internal intercostal muscles, and innermost intercostal muscles.
The intercostal nerves that stimulate these muscles originate from the spinal cord thoracic nerves Inhalation is initiated as the dome-shaped diaphragm is stimulated.
As it contracts and flattens, the thorax expands inferiorly. The internal and innermost intercostal muscles relax, while the external intercostal muscles contract from stimulus by the thoracic nerves. This produces an upward and outward movement of the ribs similar to the movement of a bucket handle , and the sternum similar to when pulling upward on a handle of a water pump. The fluid in the pleural cavity acts like glue, adhering the thorax to the lungs. Hence, as the thorax expands vertically and laterally, the parietal layer drags the visceral layer along with it, causing the lungs to expand.
Adequate expansion of the lungs results in a decreased pressure within the alveoli. Therefore, when the alveolar pressure drops below atmospheric pressure, air rushes into the lungs. Remember, inhalation requires a stimulus initiated from the central nervous system. Think of it like turning on a light. The light stays unlit until you flip a switch CNS , releasing electricity and stimulating the components of the light bulb.
As long as the switch is on and there is an impulse, the light stays lit. However, if you turn off the switch, the stimulus ceases, and the light shuts down.
Exhalation is akin to turning off the switch, so to speak. Thoracic stretch receptors constantly monitor chest expansion. Consequently, the diaphragm and the external intercostal muscles relax, decreasing the thoracic volume — like letting air out of a balloon. Assisting with this passive process, the internal and innermost intercostal muscles are stimulated.
Their contraction pulls the ribcage and attached pleura further downward and inward, compressing the lungs and increasing the air pressure within the alveoli.
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