lec 6 physiology

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Lec 6. Physiology Dr.Israa Harjan Mohsen
The respiratory system
Respiration
The process by which oxygen and carbon dioxide are exchanged between the atmosphere and body cells is called respiration. Respiration follows the following phases:
• Pulmonary ventilation – air gets in and out of the lungs
• External respiration – oxygen diffuses from the lungs to the bloodstream and how carbon dioxide diffuses from blood and to the lung
• Internal respiration – oxygen is delivered to and carbon dioxide collected from body cell.
Ventilation and perfusion
It is defined as gas exchange between the atmosphere and the lung. Ventilation, or breathing, is the process of moving air into and out of the lungs. There are two phases of ventilation: inspiration or inhalation is the movement of air into the lungs; expiration or exhalation is the movement of air out of the lungs.
Flow of air through the respiratory passages occurs because of pressure difference between the atmosphere and the alveoli. The air flows from the area of higher pressure toward the area of lower pressure during the inspiration and expiration. Changes in pressure within the alveoli result from changes in the volume of the thoracic cavity. Pressure in the lungs decreases as the volume of the lungs increases, and pressure increases as lung volume decreases.
The muscles of respiration cause the volume of the thoracic cavity to increase and decrease. Inspiration occurs when the diaphragm and the intercostal muscles contract, thus increasing the volume of the thoracic cavity. Expiration during quiet
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breathing occurs when the muscles relax, thus decreasing the volume of the thoracic cavity.
External respiration
External respiration is the diffusion of oxygen from the alveoli into pulmonary circulation (blood flow through the lungs) and the diffusion of carbon dioxide in the opposite direction. Diffusion occurs because gas molecule always move from areas of high concentration to low concentration. Each lobule of the lung has its own arterial blood supply; this blood supply originates from the pulmonary artery, which stems from the right ventricle of the heart. The blood present in the pulmonary artery has been collected from systemic circulation and is therefore low in oxygen and relatively
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high in carbon dioxide. The amount (and therefore pressure) of oxygen in the alveoli is far greater than in the passing arterial blood supply. Oxygen therefore moves passively out of the alveoli and into pulmonary circulation and on towards the left - hand side of the heart. Because there is less carbon dioxide in the alveoli than in pulmonary circulation carbon dioxide transfers into the alveoli ready to be exhaled.
Internal respiration
Internal respiration is the exchange of oxygen and carbon dioxide between blood and tissue cells, a phenomenon governed by the same principles as external respiration. Cells utilize oxygen when manufacturing the cells’ energy source, adenosine tri phosphate (ATP). In addition to ATP the cells produce water and carbon dioxide. Because cells are continually using oxygen, its concentration within tissues is always lower than within blood. So the continual use of oxygen ensures that the level of carbon dioxide within tissue is always higher than within blood. As blood flows through the capillaries oxygen and carbon dioxide follow their pressure gradients and continually diffuse between blood and tissue. The concentration of oxygen in blood flowing away from the tissues back towards the heart is described as being deoxygenated.
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External and internal respiration – oxygen and carbon dioxide follow their
pressure gradients
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Control of breathing
The rate and depth of breathing are controlled by the respiratory centres, which are found in the brain. The rate of breathing is set by the inspiratory centre of the medulla oblongata. The expiratory centre is thought to play a role in forced expiration. Also within the brain there are specialized chemoreceptors, which continually analyse carbon dioxide levels within cerebrospinal fluid (CSF).
As levels of carbon dioxide rise messages are sent to the diaphragm and intercostal muscles instructing them to contract. Another set of chemoreceptors analyse levels of oxygen as well as carbon dioxide. If oxygen falls or carbon dioxide rises, messages are sent to the respiratory centres stimulating further contraction.
Hypoxia and h ypoxaemia
Hypoxia is defined as a lack of oxygen within body tissues. Hypoxaemia is defined as a lack of oxygen within arterial blood. Naturally hypoxaemia will lead to hypoxia as the tissues are receiving less oxygen.
The major types of hypoxia and their causes
1- circulatory hypoxia\\ Heart failure, lack of cardiac output, leads to hypoxia
2-Haemic hypoxia\\ Lack of blood or hemoglobin (hemorrhage for example)
3-Histotoxic hypoxia\\ Poisoning (e.g. carbon monoxide inhalation)
4-Demand hypoxia\\ May occur when the demand for oxygen is high (e.g. during fever)