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This is a background article for other aspects see respiration In human and animal physiology, respiration is the transport of oxygen from the ambient air to the tissue cells and the transport of carbon dioxide in the opposite direction. This is in contrast to the biochemical definition of respiration, which refers to cellular respiration: the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and ATP (energy). Although physiologic respiration is necessary to sustain cellular respiration and thus life in animals, the processes are distinct: cellular respiration takes place in individual cells of the animal, while physiologic respiration concerns the bulk flow and transport of metabolites between the organism and external environment.
In unicellular organisms, simple diffusion is sufficient for gas exchange: every cell is constantly bathed in the external environment, with only a short distance for gases to flow across. In contrast, complex multicellular organisms such as humans have a much greater distance between the environment and their innermost cells, thus, a respiratory system is needed for effective gas exchange. The respiratory system works in concert with a circulatory system to carry gases to and from the tissues.
In air-breathing vertebrates such as humans, respiration of oxygen includes four stages:
- Ventilation from the ambient air into the alveoli of the lung.
- Pulmonary gas exchange from the alveoli into the pulmonary capillaries.
- Gas transport from the pulmonary capillaries through the circulation to the peripheral capillaries in the organs.
- Peripheral gas exchange from the tissue capillaries into the cells and mitochondria.
Control of respiration
The control unit of ventilation consists of a processor (the breathing centre in the brain) which integrates inputs (emotional, chemical and physical stimuli) and controls an effector (the lungs) via motor nerves arising from the spinal cord. In humans, quiet breathing occurs by the cyclical contraction of the inspiratory muscles, particularly the diaphragm. Inhalation is normally an active process, and exhalation is passive. However, when ventilation is increased (over 40 litres per minute), such as during heavy exercise, muscle activity becomes involved in exhalation. Under these circumstances, the work of breathing over time can exceed the metabolic rate of the rest of the body.
The pattern neuronal firing when breathing can be divided into inspiratory and expiratory phases. Inspiration shows a sudden ramp increase in motor discharge to the inspiratory muscles (including pharyngeal dilator muscles). Before the end of inspiration, there is a decline in motor discharge. Exhalation is usually silent, except at high minute ventilation rates.
The mechanism of generation of the ventilatory pattern is not completely understood, but involves the integration of neural signals by respiratory control centres in the medulla and pons. The nuclei known to be involved are divided into regions known as the following:
- medulla (reticular formation)
- ventral respiratory group (nucleus retroambigualis, nucleus ambiguus, nucleus parambigualis and the pre-Botzinger complex). The ventral respiratory group controls voluntary forced exhalation and acts to increase the force of inspiration.
- dorsal respiratory group (nucleus tractus solitarius). The dorsal respiratory group controls mostly inspiratory movements and their timing.
• Coordinates transition between inhalation and exhalation • Sends inhibitory impulses to the inspiratory area • The pneumotaxic center is involved in fine tuning of respiration rate.
• Coordinates transition between inhalation and exhalation • Sends stimulatory impulses to the inspiratory area – activates and prolongs inhalation (long deep breaths) • Pnemotaxic control overrides signals from the apneustic area to end inspiration There is further integration in the anterior horn cells of the spinal cord.
Determinants of Ventilatory Rate
Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate. Blood levels of oxygen become important in hypoxia. These levels are sensed by chemoreceptors in the medulla oblongata for pH, and the carotid and aortic bodies for oxygen and carbon dioxide. Afferent neurons from the carotid bodies and aortic bodies are via the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X), respectively.
Levels of CO2 rise in the blood when the metabolic use of O2 is increased beyond the capacity of the lungs to expel CO2. CO2 is stored largely in the blood as bicarbonate (HCO3-) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H+ and HCO3-. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood.
During moderate exercise, ventilation increases in proportion to metabolic production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. Lactic acid produced during anaerobic metabolism lowers pH and thus increases breathing. In aerobic metabolism, one molecule of acid (CO2) is produced in order to produce 6 molecules of the energy carrier ATP, whereas in anaerobic metabolism, 6 molecules of lactic acid are produced to provide the same amount of energy.
Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by muscle spindles via a reflex arc involving the spinal cord.
Drugs for example respiration stimulating drugs can greatly influence the control of respiration. Opioids and anaesthetic drugs tend to depress ventilation, especially with regards to Carbon Dioxide response. Stimulants such as Amphetamines can cause hyperventilation.
Pregnancy tends to increase ventilation (lowering plasma carbon dioxide tension below normal values). This is due to increased progesterone levels and results in enhanced gas exchange in the placenta. Ventilation is temporarily modified by voluntary acts and complex reflexes such as sneezing, coughing and vomiting.
- Central chemoreceptors of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the pH of their environment .
- Peripheral chemoreceptors act most importantly to detect variation of the oxygen in the arterial blood, in addition to detecting arterial carbon dioxide and pH.
- Mechanoreceptors are located in the airways and parenchyma, and are responsible for a variety of reflex responses. These include:
- The Hering-Breuer reflex that terminates inspiration to prevent over inflation of the lungs, and the reflex responses of coughing, airway constriction, and hyperventilation.
- The upper airway receptors are responsible for reflex responses such as, sneezing, coughing, closure of glottis, and hiccups.
- The spinal cord reflex responses include the activation of additional respiratory muscles as compensation, gasping response, hypoventilation, and an increase in breathing frequency and volume.
In addition to involuntary control of respiration by the respiratory center, respiration can be affected by conditions such as emotional state, via input from the limbic system, or temperature, via the hypothalamus. Voluntary control of respiration is provided via the cerebral cortex, although chemoreceptor reflex is capable of overriding conscious control.
Classifications of respiration
There are several ways to classify the physiology of respiration:
- Sudden Infant Death Syndrome
- Myasthenia gravis
- Severe acute respiratory syndrome
- Aspiration (medicine) - Pulmonary edema
By intensive care and emergency medicine
- Mechanical ventilation
- Iron lung
- Intensive care medicine
- Liquid breathing
- Oxygen toxicity
- Medical ventilator
- Life support
- General anaesthesianik
By other medical topics
- Respiratory therapy
- Breathing gases
- Hyperbaric oxygen therapy
- Gas embolism
- Decompression sickness
- Oxygen toxicity
- Nitrogen narcosis
- Carbon dioxide poisoning
- Carbon monoxide poisoning
- Salt water aspiration syndrome
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