Basic breathing anatomy and physiology:
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Consist of the nose, nasal cavity and pharynx. They play an important role in filtering and humidifying the air we breathe in.
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Also known as the voice box. It is responsible for producing sound, and protecting the trachea from food aspiration.
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Also known as the windpipe, this connects the larynx to the left and right main bronchus.
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Is the lower end of the trachea and forms the ridge that divides the left and right main bronchi.
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Internal skeletal muscle that separates the contents of the thoracic cavity with the abdominal cavity. It plays an important role in ventilation.
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Pulmonary arteries take de-oxygenated blood from the right side of the heart to the lungs, where the blood is oxygenated through the capillaries, then returned to the left side of the heart via the pulmonary veins.
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Each lung is surrounded by two pleural membranes, the visceral pleura and the parietal pleura. These membranes are separated by pleural fluid. These membranes secrete lubricating fluid and allow free movement of the lungs against the chest wall as we breathe.
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Are the tiny air sacs attached to end of the bronchioles. This is where gas exchange takes place.
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The trachea divides into the left and right main bronchus which go on to subdivide throughout the lungs, into bronchi and then bronchioles.
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The right lung consists of 3 lobes, the left lung consists of two lobes. The lungs contain approximately 2,400 kilometres (1,500 mi) of airways and 300 to 500 million alveoli.
Breathing mechanics
Ventilation is the movement of gas in (inspiration) and out (expiration) of the lungs. Most of the time we breathe without conscious effort, rate and depth are controlled by the autonomic nervous system. Through different sensors and feedback systems, the body will detect changes in oxygen, carbon dioxide and acid levels and will respond accordingly. For example elevated carbon dioxide or acid levels in the blood will stimulate breathing.
The removal of carbon dioxide is dependent on minute ventilation (tidal volume of each breath x respiratory rate). If a patient has a low respiratory rate or low tidal volumes (hypoventilation), the minute ventilation will be reduced and blood carbon dioxide levels will rise.
Gas exchange
As mentioned, the main role of the respiratory system is the delivery of oxygen (O2) to the blood stream and the removal of carbon dioxide (CO2) from the body. In order for this gas exchange to occur the lungs need both adequate ventilation (V) and perfusion (Q). Gas exchange occurs between the alveolus and capillaries that surround the alveolus, as you can see in the diagram below. Capillaries cover 70% of the outside of alveoli, providing a large surface area for gases to diffuse across. The diffusion path is small as the capillaries and alveoli are one cell thick.
Gas exchange occurs through a process of diffusion. Gas moves from an area of high concentration to an area of low concentration. The air we breathe in has a partial pressure of oxygen (P02) of 21 Kpa, as air moves down to the alveolar it gets humidified and warmed and mixes with exhaled CO2, resulting in a PO2 of 13 Kpa in the alveolar. This concentration is higher than de-oxygenated blood in the capillaries (5-6 Kpa), allowing oxygen to diffuse from the alveolar into the capillaries.
How do we measure blood oxygen levels?
When oxygen enters the blood stream it attaches to haemoglobin. Each molecule of haemoglobin can hold up to four oxygen molecules. If one haemoglobin molecule has 3 oxygen molecules attached to it, it is 75% saturated. If it has 4 oxygen molecules attached to it, it is 100% saturated. When we use a pulse oximeter to check oxygen saturations, this is what it is measuring (SpO2).
About 98% of the oxygen in the blood is attached to haemoglobin, this leaves a small amount dissolved in plasma. The more oxygen dissolved in plasma the more likely every haemoglobin will be saturated. The oxygen molecules dissolved in the blood stream create a pressure, the more dissolved in the plasma the higher the pressure. This is what we are measuring when we take an arterial blood gas. We can measure the partial pressure of oxygen (PaO2) and carbon dioxide (PaCO2) in plasma.
Ventilation/ perfusion mismatch
We often describe causes of hypoxia with regard to ventilation/ perfusion mismatch or V/Q mismatch.
Normal V/Q matching – lung ventilation and blood flow are well matched resulting in normal gas exchange. V/Q = 1
Low V/Q ratio (shunt) – the alveolar are poorly ventilated compared to normal blood flow. There is a mismatch. V/Q = 0. This could be caused by pneumonia or airway obstruction.
High V/Q ratio – there is good ventilation but poor blood circulation. There is a mismatch. V/Q = infinity. This could be caused by cardiogenic shock, pulmonary embolism:
