Lungs - The organs to breathe

Structure of the Lungs
The lungs are located in the chest inside a lubricated membrane called the pleural membrane. - This allows the lungs to move freely inside the pleural cavity. 
The lungs are connected to the outside via the trachea (windpipe). 
The trachea is a tube kept in a rigid shape due to rings of cartilage. 
The larynx or voice box is located at the top of the trachea while at the bottom end it branches into two bronchi. These lead into the lungs.

The bronchi in turn branch off into smaller and smaller bronchioles. These end in tiny air sacs called alveoli. It is here that gaseous exchange takes place. The surface area of all these alveoli is very large so as to be able to absorb oxygen very quickly.

The lungs are very delicate and can easily be damaged. The cells lining the airways have very tiny hair like structures called cilia on them. These cilia are coated in a sticky mucus. The beating cilia force the mucus and any particles of dirt up out of the lungs. It eventually drops down into the oesophagus so the mucus is attacked by the stomach acid, destroying any pathogens.

How We Breathe
When we breathe, air is drawn into the lungs so that gaseous exchange can take place. The lungs are unable to draw in air on their own. The chest cavity where the lungs are positioned, is an air tight container.


When we breathe in the diaphragm muscle contracts, pulling the sheet down. The intercostal muscles in between the ribs also contract which pulls the whole ribcage upwards and outwards. These together increase the volume of the chest. Air is drawn into the lungs because the the pressure inside them is lowered as the chest volume is increased.
 
When we breathe out the diaphragm relaxes as does the intercostal muscles.This decreases the volume of the chest, increasing the pressure. This forces air out of the lungs.
So it is the changing volume of the chest which causes air to enter and leave the lungs. The lungs themselves are just like balloons which are inflated and deflated.

Gaseous Exchange
The alveoli are the tiny air sacs at the ends of the bronchioles and the site of gaseous exchange. It is here that oxygen is absorbed into the blood while carbon dioxide is put into the air.

How We Breathe?
When we breathe, air is drawn into the lungs so that gaseous exchange can take place. The lungs are unable to draw in air on their own. The chest cavity where the lungs are positioned, is an air tight container.

When we breathe in the diaphragm muscle contracts, pulling the sheet down. The intercostal muscles in between the ribs also contract which pulls the whole ribcage upwards and outwards. These together increase the volume of the chest. Air is drawn into the lungs because the the pressure inside them is lowered as the chest volume is increased.
 
When we breathe out the diaphragm relaxes as does the intercostal muscles.This decreases the volume of the chest, increasing the pressure. This forces air out of the lungs.

So it is the changing volume of the chest which causes air to enter and leave the lungs. The lungs themselves are just like balloons which are inflated and deflated.

Gaseous Exchange
The alveoli are the tiny air sacs at the ends of the bronchioles and the site of gaseous exchange. It is here that oxygen is absorbed into the blood while carbon dioxide is put into the air.

Deoxygenated blood arrives at the alveoli in tiny blood capillaries. These have very thin walls, as does the alveoli itself. This makes it easier for the gases to pass from the air into the blood or vice versa. The deoxygenated blood has red blood cells low in oxygen and blood plasma high in carbon dioxide. The carbon dioxide diffuses from the blood plasma into the air. The oxygen diffuses from the air into the red blood cells. Blood constantly moves through the capillaries picking up O2 and giving up its CO2.

Adaptations of Alveoli:

• Gas exchange happens because of several factors in the alveolus and the blood capillaries that control the rate of gas exchange:
  
• Very thin wall of both the alveolus and the capillary, they are one cell thick which makes the diffusion distance shorter, increasing the rate.
  
• The difference in concentration of gases between the alveolus and the capillary is very large, increasing the diffusion rate of gases.
  
• The alveolus are balloon shaped which gives it a very large surface area for faster diffusion.
  
• The walls of the alveolus are lined by a thin film of water in which gases dissolve in during diffusion, this makes it faster.
  

 
Smoking
Smoking causes a number of diseases, some of them life threatening. To understand the effects of smoking you need to look at the components of cigarette smoke.
Short Term Effects:
Cilia can’t vibrate anymore, the air inhaled isn’t clean. Goblet cells release more mucus which makes the trachea narrower.
Nicotine increases heart beat rate and blood pressure.
Carbon monoxide combines with haemoglobin instead of oxygen combining with it.
Carboxyhaemoglobin is formed which is stable.
Less oxygen transported to cells.

Diseases Caused By Tar:
Chronic Bronchitis:

• Tar makes goblet cells in trachea produce excess mucus
  
• Mucus falls into lungs
  
• Bacteria in mucus breed causing infections like bronchitis
  
• The layer of excess mucus lining the walls of the alveoli increase the diffusion distance of gases making gas exchange slower
  

Emphysema:

• The excess mucus lining the alveoli irritates it, causing strong coughs which damage the alveoli.
  
• The alveoli lose its shape and surface area making gas exchange much slower.
  
• This cause short breathes and sounds while breathing.

Lung Cancer:

• When tar reaches the lungs, it is absorbed by cells of the bronchi, bronchioles and the lungs.
  
• The tar causes excessive division and reproduction of these cells which develops into cancer
  
• The cancer can be spread to other organs too (malignant cancer)

Diseases Caused By Nicotine:
This is the substance which makes smoking addictive. Nicotine is a stimulant which can make the heart beat faster and increase the amount of adrenaline released. It also makes the smoker more shaky. Increasing the normal heart beat rate can cause stress for the heart which can lead to heart disease.

Coronary Heart Disease:
Nicotine helps cholesterol deposition on walls of coronary arteries. This causes atheroma/atherosclerosis. Carbon monoxide also increases risk of blood clots forming which might results in blocking the artery. Less oxygen is delivered to cardiomyocytes (heart cells), a heart attack or failure can take place leading to death.

Carbon Monoxide
This is created due to incomplete burning of the tobacco. This gas binds irreversibly to the haemoglobin in red blood cells preventing them from carrying oxygen. This will make the smoker more out of breathe. If the smoker is pregnant then the amount of oxygen which is being passed on to the developing foetus is reduced. This slows down the growth of the foetus as it develops.

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