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Ventilation
Ventilation is the process wherey air is passed in and out of the lungs by movements of the intercostals muscles and the diaphragm which alter the volume of the thoracic cavity. The 2 types of intercostals muscles are external intercostals that slant forwards and downwards and the internal intercostals that slant backwards and downwards.
Inspiration is as such. It is an active process, whereby the external intercostals contract and the internal intercostals relax( both sets are innervated by the thoracic nerve ). Then, the rib cage move forward and outward. The muscles of the diaphragm contract(innervated by phrenic nerve). So, the volume of the thorax increases. The pressure in the thorax drops to less than atmospheric pressure, so air rushes in to inflate the alveoli. The insurge of atmospheric air causes the lung atmosphere to return to atmospheric pressure.
Expiration however is a mainly passive process. The external intercostals relax and the internal intercostals contract. The rib cage thus, moves backward and downward. The muscles of the diaphragm relax and the volume of the thorax decreases. The pressure in the thorax rises to greater than the atmospheric pressure and air is forced out of the alveoli by the elastic recoil of the lungs.
There are 3 factors that affect the energy required to breathe- Airway Resistance, Alveolar Surface Tension and Lung Compliance. Airway resistance is mainly caused by mid-sized bronchi diameter controlled by autonomous nervous system, and locally influenced by immune system. The parasympathetic stimulation causes bronchoconstriction which causes resistance to increase. The sympathetic simulation causes bronchodilation which causes resistance to decrease. If histamine is released by inflammatory processes, it will constricts the airways. Alveolar Surface Tension occurs when alveolar surface is coated with a watery film. At air-water interface, water molecules have stronger attraction to each other than to air.
This unequal attraction produces a force called surface tension. Pressure in alveolus is directly proportional to surface tension, and inversely proportional to radius of alveolus. The smaller the radius, the greater the inwardly-directed pressure (with same T), hence the pressure gradient is created. So, air flows from region of higher pressure to lower pressure. The smaller alveolus tends to empty air into the larger one and collapse. The lung Surfactant is a mixture of phospholipids & proteins produced by Type II alveolar cells which prevents lung collapse. It decreases alveolar surface tension and increases lung compliance.
The ability of the lung to stretch is measured as: Compliance = Dvolume/ Dpressure . Thus, with lower compliance, more energy required to breathe but with higher compliance, less energy required.
Lung surfactant can only be produced by fetus at late stage of pregnancy. Therefore, premature babies do
not have enough surfactant to reduce alveolar surface tension, thus poor lung compliance. So, alveoli tend to collapse at end-expiration. Each breath is like blowing up a new balloon and newborn muscles still weak. The treatment is to give surfactant replacement to prevent lunge from collapsing. The body si thus conformed such that less energy required to breathe by having low airway resistance and alveolar surface tension with high lung compliance. In fact only 3% of total energy expenditure of the body is used for quiet breathing!
One respiratory cycle is equals to inhalation followed by exhalation. The normal breathing rate for a resting adult is 16 to 20 respiratory cycles per minute. The respiratory centre is located in the medulla oblongata of the brain. It consists of the inspiratory centre that is located in the ventral portion of the breathing centre. It generates rhythmic cycle of breathing. The expiratory centre is located in the dorsal and ateral portions of the breathing centre. The Pons control autonomic breathing control can be irregular if inputs from Pons are disrupted. There are 2 forms of control, the involuntary and voluntary control. They act through the chemical or nervous means.
The chemical control is as such the partial pressure of oxygen is monitored by carotid & aortic bodies . When the arterial partial pressure of oxygen is less than 60 mmHg, stimulation of peripheral chemoreceptors occurs. The amount of H+ is monitored by carotid & aortic bodies. The partial pressure of carbon dioxide is the main respiratory regulator. It mainly affects on central chemoreceptor. Carbon dioxide pass through the blood-brain barrier, but H+ could not. All of which affects mainly, the inspiration centre. This stimulates the thoracic/intercostals nerves and phrenic nerves causing external intercostals and disphragm to contract and the internal intercostals to relax. Thus, the voume of thorax will increase and decreased pressure in the thoracic cavity. Air rushes in and causes inflation of alveoli. Stretch receptors in the alveolar wall is stimulated and the vagus nerve
The body clears its airway in the following ways. One of which is coughing. It begins with a deep inspiration followed by forced expiration against a closed glottis. The glottis is then suddenly opened, producing an explosive outflow. One yawns because of deep inspiration with mouth open, often accompanying with stretching (involuntary). Like hiccup, all humans and animal are capable of yawning- even unborn babies! It may aid the spreading of surfactant, prevent the collapse of the alveoli. A common theory can be to bring more oxygen in & take more carbon dioxide out of the blood. Somehow, yawning seems to be contagious, it can be due to the many benefits.
There are many diseases or conditions that influence the Mechanics of Breathing like Pneumothorax, Pulmonary fibrosis, Emphysema, Bronchitis and Asthma. Others serve to minimize or prevent Gas Exchange like Pulmonary edema, Smoke inhalation and Carbon monoxide poisoning.
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