Respiratory System

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The Larynx

Also known as the voice box or 

Adam’s Apple.

It is about 5cm long.

It allows air to reach the lungs via the trachea and protects the trachea when we eat.

Air passes in and out of the larynx each time the body inhales or exhales.  The larynx filters bacteria and warms and moistens the air as it passes down to the lungs.

When you swallow, a part of the larynx called the epiglottis closes tightly over the trachea. This flap of cartilage stops food and saliva going into your lungs. 

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The Trachea

The trachea (also known as the wind pipe) is approx 10cm long and comes down the front of the neck from the larynx.

It is the passageway for air into & out of the lungs between the larynx and the bronchi.

The trachea divides into 2 bronchi – the left bronchus goes into the left lung & the right bronchus goes into the right lung.

The trachea is strengthened by incomplete rings of cartilage.

The trachea has a ciliated lining which contains mucus secreting goblet cells.  The mucus that these cells secret collects any foreign matter or bacteria and the cilia pushes this back up to the larynx. This is good because it stops bacteria & foreign matter reaching the lungs.

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The Lungs

We have 2 lungs which are either side of the heart.

The left lung is divided into 2 lobes – the upper & lower lobes

The right lung is divided into 3 lobes – the upper, middle & lower lobes.  Lobes are then sub-divided into smaller lobules.

The function of the lungs is to allow the exchange of gases into and out of the blood.  

The lungs are covered in a special membrane called the pleural membrane

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Pleural Membrane

The lungs are surrounded by two membranes, the pleura. The outer pleura is attached to the chest wall, the inner one is attached to the lung. In between the two is a thin space known as the pleural cavity or pleural space. It is filled with pleural fluid produced by the pleura.  This can become infected or inflamed and cause a variety of problems e.g. pleurisy.

The pleural fluid lubricates the pleural surfaces and allows the layers to slide against each other easily during respiration. It also provides the surface tension that keeps the lung surface in contact with the chest wall.

The pleural membrane also helps to keep the two lungs away from each other and air tight, so if one lung is punctured and collapses due to an accident, the other pleural cavity will still be air tight, and the other lung will work normally.

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Bronchi and Bronchioles

Singular =  bronchus 

Plural = bronchi

The bronchi connect the trachea to the lungs.

Like the trachea the bronchi are also strengthened with incomplete rings of cartilage.

The bronchi sub-divide into smaller and smaller branches.  The smallest braches are called bronchioles.  The bronchioles become smaller and smaller as they spread further into the lungs until they are just one cell thick.  Like the trachea they are lined with ciliated epithelium

The bronchioles at the end of the line (called terminal bronchioles) have clusters of alveoli at the end of the them.

The bronchioles job is to deliver air to the alveoli.

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Plural = alveoli

Singular = alveolus

The alveoli are in clusters at the end of the terminal bronchioles & are surrounded by a network of capillaries.

Oxygen diffuses from the alveoli into the capillaries and carbon dioxide diffuses from the capillaries into the alveoli.

The function of the alveoli is gaseous exchange (we will look at this in more detail later)

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Ciliated linings

Cilia are microscopic hairs and they line the trachea and bronchi.  They collect foreign matter and bacteria and push it up & out of the trachea and bronchi.

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Intercostal Muscles

Intercostal muscles are several groups of muscles that run between the ribs, and help form and move the chest wall. The intercostal muscles are mainly involved in the mechanical aspect of inspiration & expiration. These muscles help the diaphragm to expand and shrink the size of the chest cavity when you breathe.

During inspiration the intercostal muscles contract & during expiration they relax.

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The Diaphragm

The diaphragm is a band of muscle which separates the chest from the abdomen.

It plays a lead role in inspiration & exhalation. It moves downwards and flattens when we inspire, enlarging the chest cavity and pulling air in through the nose or mouth. When we expire, the diaphragm moves upward, becoming dome shaped, forcing the chest cavity to get smaller and pushing the gases in the lungs up and out of the trachea, then up through the larynx to the nose and mouth.

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Gas exchange

Remember that gases move from high pressure to low pressure.

Oxygen in the alveoli is under more pressure than it is in the deoxygenated blood in the capillaries so it moves from the alveoli to the capillaries.

Carbon dioxide in the deoxygenated blood in the capillaries is under more pressure that it is in the alveoli so it moves from the capillaries to the alveoli.  This carbon dioxide is expelled from the lungs through expiration.

Once the pressure in the capillaries and alveoli are the same this gaseous exchange stops.

So – all over the lungs the alveoli exchange oxygen and carbon dioxide (which are both gases) with the capillaries that surround them – this is why the process is called gaseous exchange.

After the blood in the capillaries has been oxygenated in the lungs it then returns to the heart via the pulmonary vein to be pumped around the body delivering oxygen to the cells and collecting carbon dioxide. 

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Inspiration and expiration

Inspiration is when we breathe air into our lungs.

During inspiration the diaphragm contracts – it flattens out and lowers.  The intercostal muscles contract & raise the ribs.

This increases the size of the chest cavity which reduces the pressure in the chest cavity.

This reduced pressure causes air to be sucked into the lungs and the lungs expand.

Expiration is when we breathe air out of our lungs.

During expiration the diaphragm relaxes – it domes up. The intercostal muscles relax and lower the ribs.

This reduces the size of the chest cavity which increases the pressure in the chest cavity.

This increased pressure causes air to be pushed out of the lungs and the lungs deflate.

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How does the body know how to inhale?

Nerve cells called chemoreceptors in the aorta and carotid artery send impulses to the respiratory centre in the medulla oblongata in the brain when they detect low oxygen and high carbon dioxide levels.

The medulla oblongata then sends a nerve impulse to the diaphragm telling it to contract.

The pons varolii in the brain stops inspiration when it receives impulses from stretch receptors in the lung tissue.  The pons varolii then sends a nerve impulse to the diaphragm telling it to relax.

Air entering our body contains 21% oxygen and 0.0% carbon dioxide. Air leaving our body contains 15% oxygen and 4% carbon dioxide.  

This means that the air we inhale has 100 x more carbon dioxide and 6% less oxygen that the air we inhale.

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