domingo, 14 de febrero de 2016

Respiratory System.

Respiratory System.
The cells of our body consume oxygen in order to obtain energy. In this process, called oxidation, the cells burn glucose using oxygen, releasing carbon dioxide, and obtaining the necessary energy to carry out several metabolic processes. The respiratory system is responsible for transporting and providing oxygen from the air to the blood and carbon dioxide from the blood to the air. This air will be exhaled after the exchange process. Anatomy of the Digestive System.
Respiration includes the whole exchange process. It has three different phases:
  • Pulmonary ventilation: Inhalation and exhalation of air: the air flows from the exterior of our body to our lungs and after that is expelled from the lungs to the exterior.
  • External respiration: Exchange of gases between the lungs and the blood.
  • Internal or tissue respiration: Exchange of gases between the blood and the cells or tissues.
Anatomy of the Respiratory System.
Respiratory Organs.
The respiratory system can be divided into the following parts:
  • Upper respiratory tract: made up of the nose, nasal cavity, pharynx and other structures.
  • Lower respiratory tract: made up of the larynx, trachea, bronchi, bronchioles and lungs.
The respiratory system can also be divided into two big divisions:
  • Respiratory tract.
  • Lungs.
Structure of the Respiratory System.
We are going to study, one by one, the most important anatomical structures of the respiratory system:
  • Nose: The nose has an external and a internal part. The external part is divided into two nasal channels called nostrils. The inner part is a large cavity located between the facial bones, just above the mouth. The floor of the nasal cavity is the hard palate, and the roof is a part of the ethmoid bone called cribriform plate. It is divided into two parts, the right and the left ones. It is connected to the pharynx through two openings called choanes. The nasal cavity is responsible for filtrating, heating and wetting the inhaled air. It also receives olfactory stimulus and modifies our voice.
  • Pharynx: This is a thirteen centimetre long duct similar to a funnel, that connects the nasal cavity with the cricoid cartilage, that it is the upper part of the larynx. It has three parts. The upper part is located just beneath the nasal cavity and is called nasopharynx. The nasopharynx is connected with the nasal cavity through the choanes, and with the mid ear through the Eustachian tube. The next part of the pharynx is the oropharynx, and it is located behind the oral cavity. The oropharynx connects the mouth with the respiratory and the digestive systems. The lower part of the pharynx is the hypopharynx, that connects the oropharynx with the oesophagus (digestive system) and the larynx (respiratory system).
  • Larynx: It is a short duct, made up of nine cartilaginous pieces, that connects the pharynx with the trachea (windpipe). The larger piece of the larynx is the thyroid cartilage, that it is also known as the Adam’s apple. This structure covers the thyroid gland. Another important piece is the epiglottis, a sort of flap that opens and closes the duct in order to prevent food from going into the trachea. The vocal folds are also located in the larynx.
  • Trachea: This is a twelve centimetre long and two and a half centimetre diameter duct that connects the larynx with the bronchi. It lies just in front of the oesophagus. The trachea is surrounded by fifteen incomplete cartilaginous rings, that protect and maintain the airway. This rings are C shaped, to allow the dilation of the oesophagus when the food is passing through it. At the level of the fifth dorsal vertebra the trachea bifurcates into two principal bronchi.
  • Bronchi: The trachea bifurcates into two principal bronchi, called right bronchus, that enters in the right lung and left bronchus, that enters in the left lung. After penetrating in the lung, each bronchus divides into two secondary bronchi. The secondary bronchi divides into tertiary bronchi. Each tertiary bronchi divides into two bronchioles. The bronchioles continue dividing successively, completing sixteen total divisions. The air that fills the bronchi and bronchioles (around 150ml) is not used to breathe, because there are not special structures to allow the exchange of gases, so it does not takes place. The structure where this exchange of gases takes place is called alveoli and these are located after the last bronchiolar division.
  • Lungs: the lungs are two large conic shaped organs, located in the thoracic cavity and separated by the heart and the mediastinum. Each lung is covered by two membranes. The outer one is attached to the thoracic wall and it is called parietal pleura. The inner one is attached to the lung surface and it is called visceral pleura. Between both layers there is an internal fluid that keeps both membranes together and that lubricates them the to avoid friction when they move. This liquid is called pleural effusion. The lower and broader part of the lung is called base of the lung. The upper part of the lung is called anterior border. The cavity from where blood vessels and bronchi enter is called hilum. The right lung is slightly larger than left one, because the left one must leave a space for the heart. The left one is, however, longer than right one, because it must leave a space for the liver. Both lungs have fissures that divide them into lobes. The left lung has one fissure that divides it into an upper and a lower lobe. The right lung has two fissures that  divide it into a lower, a middle and a upper lobe. The lungs are the organs where the bronchioles divide. After the last division, the alveolar sacs can be found. Each one of these sacs has two or three alveoli. The alveoli are covered by blood vessels, because these are the anatomical structures where the exchange of gases between the air and the blood takes place.
Respiratory System: Anatomy.

Physiology of Respiration.
First, we will study the process called pulmonary ventilation, that explains how the air flows from the exterior of our body to the lungs and from the lungs to the exterior of our body. Then, we will study the exchange process between the air and the blood, that takes place in the pulmonary alveoli and it is called external respiration. Finally we will study the exchange of gases between the blood and the internal tissues, that is called internal or tissue respiration.
Pulmonary Ventilation.
Pulmonary ventilation, also called breathing, is the movement of the air between the exterior of the body and the lungs. The air enters the lungs from the environment to in a process called inhalation. The air exits the lungs to the environment in a process called exhalation. These movements of air are a consequence of the changes of pressure in the lung and of the special properties of this organ, that is capable of increasing its volume by distension and to return to its original size by elasticity.
The process of entrance of air into the lungs is called inhalation. It takes place when the lungs expands, increasing their volume. The expansion results from the contraction of the respiratory muscles: the diaphragm and the internal intercostal muscles. The most important muscle is, by far, the diaphragm. When it contracts, its convex morphology changes, becoming flatter. This movement pulls the lung down, enlarging its lower part. The internal intercostal muscles raise the thoracic cage, causing the expansion of the lungs because they are closely attached to the ribs. These two processes increase the volume of the lungs. The higher volume leads to a drop in the internal pressure, so that the air moves from the environment to the lung.
The release of air is called exhalation. It is a passive process, no muscular contraction is required. The elastic fibres of the lungs and the weight of the thoracic cage decrease the volume of the lungs when the respiratory muscles relax. The reduction of volume leads to an increment of the internal pressure, so that the air moves from the interior to the exterior.
Although this is a passive process, the contraction external intercostal and the abdominal muscles can accelerate the release of air. This is called forced exhalation, and is carried out when the body needs to improve the exchange of air.
Ventilation Volumes.
During normal breathing around 500ml of air is exchanged between the lungs and the environment. This amount of gas that enters and afterwards exits from the lungs is called Tidal Volume (VT).
Not all this gas is available to exchange oxygen and carbon dioxide. Around 150ml of air never reaches the alveoli and stand in the outer respiratory ducts: nasal cavity, pharynx, larynx, trachea, bronchi and bronchioles. This volume that is not directly used in the pulmonary respiration is called Dead Space (DS).
The Respiratory Minute Volume (MV) is the amount of air exchanged between the lungs and the environment per minute. An adult human being breathes approximately  twelve times per minute, exchanging 500ml per breathing (Tidal Volume), so it is easy to calculate that the MV is 6000ml/min. 
We can breathe more deeply, inhaling more than 500ml. We can reach between 3000ml and 3500ml more in a forced inhalation. This is called Inspiratory Reserve Volume (IRV). We can even take more air if, just before the forced inhalation, we exhale as much air as we can. This air that we can release through forced exhalation, around 1200ml, is called Expiratory Reserve Volume (ERV).
After exhaling all the air that forced expiration allows, there is a volume of air that remains in the respiratory system. This amount of gas that we cannot release is very important, because it prevents the duct and alveolar sacs from collapsing. It is 1200ml more or less, and it is called Residual Volume (RV).
If we add the Tidal Volume to the Inspiratory Reserve Volume we obtain the Inspiratory Capacity (IC). It is around 3600ml. If we add the Residual Volume to the Expiratory  Reserve Volume we obtain the Functional Residual Capacity (FRC). It is around 2400ml.
The Inspiratory Reserve Volume added to the Tidal Volume and to the Inspiratory Reserve Volume is called Vital Capacity. It is around 4800ml. If we add all the volumes (IRV+VT+ERV+RV) we obtain the Total Lung Capacity (TLC). It is around 6000ml. 
Ventilation Volumes

Pulmonary Respiration Physiology.
The physiology of pulmonary respiration is based on the concentration gradients or differences in partial pressure. The internal membrane of the lungs is extremely thin (around 10.5μm), so that the gases are easily exchanged. And the internal surface of the lung is really broad, around 70m2 if we count all the alveolar surface.
The air that reaches the alveoli is very rich in oxygen, between 100-105mmHg. The concentration of oxygen in the blood in the capillaries of the lung, however, is quite low, around 40mmHg. Due to this, the oxygen tends to flow from the air to the blood, until both concentrations become equivalent. When the blood exits the capillaries of the lung, its concentration of oxygen is approximately 110mmHg.
To improve the movement of oxygen in the blood, it is transported linked to a special protein called hemoglobin.
Carbon dioxide concentration in the air is around 40mmHg. When the blood arrives in the alveoli, its concentration of carbon dioxide is around 45mmHg. Due to this, the carbon dioxide tends to flow from the blood to the air until both concentrations become equivalent. When the blood exits the capillaries of the alveoli the concentration of carbon dioxide is 40mmHg.
The carbon dioxide is not transported by any protein, but is transformed into a different substance called bicarbonate.
Tissue Respiration Physiology.
The situation in the tissues is just the opposite to in the lungs. The extracellular fluid that surrounds the cells is very poor in oxygen, because it has been consumed by the cells. Its concentration is 40mmHg. As we studied, the concentration of oxygen in the blood that comes from the lungs is 100mmHg. Due to this, the oxygen tends to flow from the blood to the tissues until both concentrations become equivalent.
Carbon dioxide, however, is more concentrated in the extracellular matrix, because the cells produce and release this substance during their metabolic activity. Its concentration is 45mmHg, whereas in the blood the concentration is around 40mmHg. So the carbon dioxide flows from the extracellular matrix to the blood until both concentrations become equivalent.
A part of this carbon dioxide is transported by the blood linked to hemoglobin, but only a low quantity, around the 23%. A small amount, the 7%, is transported dissolved in the plasma. The rest of the carbon dioxide, around the 70%, is transformed into bicarbonate by an enzyme called carbonic anhydrase. It is so how it is transported, because this substance can be easily dissolved in the plasma.
Respiratory System: Anatomy.

Control of Breathing. 
Breathing is an extremely controlled process, because it must be finely adjusted to the requirements of the body. An ordinary human being consumes around 200ml of oxygen per minute. While intense exercising, however 30 times this amount can be consumed. To increase the amount of taken oxygen the body increases the respiratory rate and depth.
The respiratory rate at rest is controlled by some areas of the nervous system located in the bulb and the pons. The Bulb Rhythmic Area controls the basic system of respiration and the respiratory rate at rest. The Pneomotaxis Centre controls the coordination between inhalation and exhalation. The Apneustic Centre controls the inhalation.
Other zones of the brain have connections to these respiratory centres and they can raise or decrease the respiratory rate when it is necessary. When the pH of the blood decreases, for instance, it is related to an increment of the bicarbonate dissolved in the plasma, so the respiratory rate must be increased to release the excess of carbon dioxide. When some receptors detect that the amount of oxygen drops, they promote an increment of the respiratory rate. There are many different chemical receptors in out body, such us the carotid and aortic receptors.

Some hormones can also have different effects on the respiratory system. Adrenalin, for instance, affects not only to the respiratory rate, but also to the amount of air inhaled by changing the diameter of the bronchioles and increasing the air flow to the alveoli. Other hormones have just the opposite effects.

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