Biology - Unit 2 - Breathing and Gas Exchange

  • Created by: AmrAgamy
  • Created on: 04-01-19 20:01

Breathing and Gas Exchange 1


Cells get their energy through cellular respiration. This is breaking down food molecules to release the stored chemical energy they contain. Aerobic respiration is when oxygen is used to oxidise food, and Carbon Dioxide + Water are released as waste products. Anaerobic respiration is when cells respire without using oxygen. In anaerobic respiration, glucose is not completely broken down.

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Breathing and Gas Exchange 2

The Structure of the Gas Exchange System

The lungs are kept in the thorax by the ribcage and a muscular sheet of tissue called the diaphragm. The actions of these two bring about the movements of air in and out of the lungs. Intercostal muscles are several groups of muscles that run between the ribs, and help form and move the chest wall.

Image result for human gas exchange system diagram

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Structure of the Gas Exchange System (2)

The air passages of the lungs form a network called the bronchial tree. When we breathe in, air enters our nose/mouth and passes down the trachea. The trachea splits into two tubes called the bronchi, which each lead to a lung. These bronchi divide into smaller and smaller tubes called bronchioles, eventually ending at tiny air sacs called alveoli. This is where gas excahnge with the blood takes place. The walls of the trachea and bronchi contain cartilage. These support the airways and keep them open when we breathe in.

-The inside of the thorax is seperated from the lungs by two thin membranes called the pleural maembranes. They make up a continuous envelope around the lungs. Between them is a space called the pleural cavity, filled with a thin layer of pleural fluid. This acts as lube so that the surfaces of lungs dont stick to the chest wall when we breathe.

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Keeping The Airways Clean

The trachea and larger airways are lined with a layer of cells that have an important role in keeping the airways clean. Some cells in this lining secrete mucus, which traps dirt or bacteria that are breathed in. Other cells have tiny hairs called cilia, which beat back and forth, sweeping the mucus and trapped particles out towards the mouth. This way, dirt and bacteria dont enter the lungs where they might cause an infection.

Image result for mucus and cilia cells (

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Ventilation of the Lungs

Ventilation means moving air in and out of the lungs. This needs a difference in air pressure. Air moves from where the pressure is high to where it is low.  When we breathe, the pressure changes inside the thorax because it's volume changes.

The movements that cause ventilation are those of the ribs and the diaphragm. If you breathe in deep, you can feel your ribs move updwards and outwards. They are moved because the outer intercostal muscles contract, pulling them up. The muscles of the diaphragm also contract, pulling the diaphragm down into a flat shape. This increases the volume of the chest, decreasing the pressure, which makes air enter the lungs. The opposite happens when breathing out. Your inner intercostals contract pulling the ribs down and in. The diaphragm muscles relax and the diaphragm goes back to it's normal shape. The volume of the thorax decreases, increasing pressure, so air exits the lungs.

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Image result for diaphragm ventilation

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Gas Exchange in the Alveoli

During gas exchange, lungs are absorbing oxygen into the blood and removing carbon dioxide from it. This happens in the alveoli. For this to happen, the alveoli have a structure which brings the air and blood close together, over a large surface area. 

Deoxygenated blood is pumped from the heart to the lungs and passes through the capillaries surrounding the alveoli. This blood has come after giving up some of its oxygen to cells, and gained carbon dioxide. Around the lungs, the blood is seperated from the air inside each alveolus. Duo to the air in the alveolus having a higher concentration of oxygen than the blood entering the capillary network, diffusion happens and the oxygen enters the blood. This also happens with the carbon dioxide in the blood, which enters the lungs. The result is the blood leaving the capillaries and going back to the heart with more oxygen and less carbon dioxide. The heart then pumps that blood around the body again to supply respiring cells.

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Effects of Smoking

For the lungs to exchange gases properly, the air passages have to be clear, and the alveoli have to blean, with a big as possible surface area in contact with blood. Smoking upsests all of theses conditions.

Smoking can cause lung cancer, bronchitis and emphysema. It is also a major contributing factor to other conditions, such as coronary heart disease.

Effects of Smoke on the Lining of Air Passages

Remember, lungs are kept free of dirt and bacteria because of mucus and cilia. Chemicals in cigarette smoke destroy cilia, which means mucus is not swept away from the lungs and remains to block the air passages. On top of that, smoke irritates the lining of the airways, causing the cells to secrete more mucus. This causes the lung disease bronchitis.

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Effects Of Smoking


This is another lung disease which kills lots of people. Smoking causes one type of emphysema, because it damages the walls of the alveoli, which break down and fuse together again, forming irregular air spaces. This recuces the surface area for gas exchange, which makes it very inefficient. Therefore, the blood carries less oxygen, causing the sufferer to carry out as much excercise. There is no cure for Emphysema.

Lung Cancer

Cigarette smoke contains a strongly addictive drug - Nicotine. More than 60 of the chemicals in smoke are known to cause cancer, they are called carcinogens. They are contained in the tar that collects in a smoker's lungs. Cancer happens when cells mutate and start to divide uncontrollably, forming a tumour.

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Effects of Smoking

Carbon Monoxide in Smoke

One of the harmful chemicals in cigarretes is the poisonous gas, carbon monoxide. This gas enters the bloodstream and interferes with the ability of the blood to carry oxygen. Oxygen is carried around in the blood in the red blood cells, attached to a chemical called haemoglobin. Carbon monoxide can combine with the haemoglobin more tightly than oxygen can, forming a compound - Carboxyhaemoglobin. The haemoglobin will prefer carbon monoxide over oxygen. When this happens, the blood carries much less oxygen around the body.

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