Biology Unit 1 - The respiratory system


Lung structure

Air breathed in, enters the trachea. The trachea splits into two bronchi - one leading to each lung. Each bronchus then branches off into smalled tubes called bronchioles. The bronchioles end in small air sacs called alveoli. Gas exchange takes place in the alveoli.

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Inspiration (breathing in)

  • the intercostal and diaphragm muscles contract
  • the ribcage moves upwards and outwards and the diaphragm to flatten. Increasing the volume of the thorax
  • as the thorax volume increases, the pressure decreases
  • this causes air to flow into the lungs
  • inspiration requires energy so is an active process

Expiration (breathing out)

  • intercostal and diaphragm muscles relax
  • ribcage moves downwards and inwards and diaphragm becomes curved again
  • volume of the thorax decreases, and the air pressure increases
  • air is forced out of the lungs
  • it does not require energy so is a passive process
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Alveoli and Gas Exchange

Alveoli Structure

The alveoli are surrounded by a network of capillaires. Each alveolus is made of a single layer of thin, flat cells called alveolar epithelium. The walls of capillaries are made from capillary endothelium. The walls of the alveoli contain a protein called elastin, which helps the alveoli return to their normal shapeafter inspiration and expiration.

Gas exchange in the alveoli

Oxygen diffuses out of the alveoli, across the alveolar epithelium and the capillary endothelium and into haemoglobin in the blood. Carbon dioxide diffuses into the alveoli from the blood and is then breathed out.

Factors affecting rate of diffusion

  • a thin exchange surface - walls are one cell thick meaning there is a short diffusion pathway
  • a large surface area - lots of alveoli means theres a large surface area for gas exchange

There is also a steep concentration gradient of oxygen and carbon dioxide between the alveoli and capillaries, which increases rate of diffusion. It is maintained by the flow of blood and ventilation.

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Measuring Lung function

Pulmonary Ventilation

pulmonary ventilation is the volume of air taken into the lungs in one minute, measured in dm3 min-1

Pulmonary ventilation = tidal volume X ventilation rate

Tidal colume - is the volume of air in each breath                 Ventilation rate - is the number of breaths per minute

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Lung disease - Pulmonary Turberculosis (TB)

Infection - when someone becomes infected by the bacteria, immune system cells build a wall around the bacteria in the lungs. This forms small, hard lumps called tubercles. Infected tissue inside the tubercles, dies and the gas exchange surface is damaged, so tidal volume is decreased. TB also cause fibrosis that can further reduce tidal volume.

Symptoms - persistant cough, coughing up blood and mucus, chest pains, shortness of breath and fatigue. Sufferers may also have a fever and many lose weight due to loss of appetite. Many people can be infected but not show any symptoms because the infection is in an inactive form (asymptomatic). They are unable to pass the infection on but if they become weakened by another infection or malnutrition, then it may become active and they will show syptoms and be able to pass on infection.

Transmission - TB is tranmitted by droplet infection - this is when an infected person coughs or sneezes and tiny drops of saliva and mucus containing bacteriaare released from their mouth and nose and if an uninfected person breathes in these droplets, the bacteria are passed on. TB tends to be more widespread in ares where hygine levels are poor and in crowded conditions.

TB can be prevented with the BCG vaccine, and is treated with antibiotics.

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Lung diease - Fibrosis

Fibrosis is the formation of scar tissue in the lungs which can be because of infection or exposure to asbestos or dust. The scar tissue is thicker and less elastic than normal lung tissue meaning that the lungs are less able to expand and cant hold as much air as normal so tidal volume is reduced. It is also harder to force air out of the lungs due to the loss of elasticity.

The rate of gas exchange is reduced in the alveoli because diffusion is slower across the thicker, scarred membrane. As less oxygen can diffuse into the bloodstream, the body cells recieve less oxygen and the rate of aerobic respiration is reduced meaning less energy is released and sufferers often feel tired and weak. Fibrosis sufferers also have a faster breathing rate in order to get enough air into their lungs to oxygenate their blood.

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Lung disease - Asthma

Asthma is a respiratory condition where airways become inflamed and irritated. Causes may vary but it is usually because of an allergic reaction to substances like dust or pollen.

During an attack, the smooth muscle lining of the bronchioles contract and large amounts of mucus is produced. This causes constriction of the airways, making it difficult for the person to breath properly. Air flow into the lungs is severly reduced so less oxygen enters the alveoli and moves into the blood. This means the body cells recieve less oxygen and therefore the rate of aerobic respiration in cells is reduced.

Symptoms include wheezing, tight chest and shortness of breath, these symptoms come on very quickly during an attack. They can be relieved by drugs (inhaler) which cause the muscle in the bronchioles to relax, opening up the airways.

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Lung disease - Emphysema

Emphysema is caused by smoking or long term exposure to air pollution - foreign particles in the smoke or air become trapped in the alveoli. This causes inflammation, which attracts phagocytes to the area. The phagocytes produce an enzyme that breaks down the protein elastin. The loss of elastin means the alveoli can't recoil or expel air as well so it remains trapped in the alveoli. It also leads to the destruction of the alveoli walls, which reduces the surface area of the alveoli meaning rate of gas exchange is reduced. The lack of oxygen delivered to cells, leaves sufferers feeling tired and weak.

Symptoms include shortness of breath and wheezing. People with emphysema have an increased breathing rate as they try to increase the amount of air reaching their lungs.

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