The Lungs


Breathing and Exchanging of Gases

The lungs are situated on either side of the heart in the thorax or chest cavity surrounded by the ribs and the diaphragm at the base. Their function is to allow gaseous exchange between the air in the lungs and the blood in the capillaries. 

Air enters the airway through the nose or mouth. Membranes line much of the airways. These contain glands which secrete mucus and are lined with tiny hairs called cilia. 

Air then passes into the trachea, bronchi and bronchioles. Apart from the tiny bronchioles immediately before the alveoli, these tubes contain rings of cartilage. The bronchioles are able to constrict due to lack of cartilage to control flow of air into and out of the alveoli. 

The function of the cartilage is to support the trachea by preventing it from collapsing. This allows breathing and stops us from suffocating. 

The cartilage rings in the trachea are horseshoe-shaped rather than complete because the oesophagus is behind the trachea which allows us to swallow and breathe at the same time. 

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Gaseous Exchange

Gas exchange occurs in the alveoli of the lungs. As the blood flows through the capillaries surrounding the alveoli, carbon dioxide diffuses from the blood and oxygen diffuses the other way, from the alveoli to the blood along the concentration gradient. 

The blood vessel that brings deoxygenated blood from the heart into the capillaries surrounding the alveoli. The blood vessel that takes oxygenated blood away from the capillaries is the pulmonary vein. 

Two features of alveoli that aid the diffusion of gases are: 

  • Thin walls (one cell thick) 
  • Many alveoli for large surface area
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The capillaries also help in the exchange of gases between the blood and the air in the lungs:

There are lots of alveoli

Blood vessels surround alveoli and are tightly attached to them

There is a short diffusion pathway

The capillaries have thin walls

The blood comes from the heart under high pressure

Blood returns to the heart in the veins

Diffusion is aided by blood being constantly carried away from the alveoli by the circulation

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Inspiration is taking air into the lungs. 

Intercoastal muscles contract

Ribs move up and out

Diaphragm contracts which makes it move down and become flattened

Thorax gets bigger

Vacuum created in the lungs

Air rushes into the lungs down the trachea 

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Expiration is moving air out of the lungs. 

The intercoastal muscles relax

Ribs move down and in

Diaphragm relaxes and moves up becoming dome shaped

Thorax decreases in size

Air is forced out of the lungs 

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Increase in breathing rate in response to exercise

  • During exercise, the muscles respire more rapidly
  • This leads to an increased concentration of carbon dioxide in the blood (compared to normal)
  • This is detected by the chemoreceptors in the carotid artery
  • Impulses are sent to the medulla in the brain
  • An increased frequency of impulses from the medulla to the intercoastal muscles and diaphragm causes them to contract more frequently and more deeply
  • The rate and depth of breathing increases

If a person had reduced oxygen levels it would increase their breathing rate, their depth of breathing would get deeper and increase their heart rate. They may also faint to transport oxygen. 

Cellular respiration is when energy is released within cells

The role played by breathing in cellular respiration is that it provides the oxygen needed for the process of respiration.

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Lung Disease - Emphysema

Emphysema is characterised by chronic breathlessness. People who have been heavy smokers for many years often develop this condition. The airways are so damaged by the effects of smoke resulting in the need to breathe oxygen rich air from a machine in order to complete routine daily activities. 

The walls of alveoli break down, the number of alveoli decrease and the surface area also decreases. 

A patient with emphysema will have less oxygen in their blood because less oxygen will be able to diffuse into the blood from the alveoli. Oxygen is needed in the muscles for respiration because it releases energy for the muscles to contract. 

People with emphysema can only walk short distances because less oxygen has reached the tissues which means less energy for movement. 

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