Biology Chapter 4

  • Created by: Emily
  • Created on: 17-04-14 10:33

Structure of the Human Gas-exchange System

Oxygen is required for aerobic respiration and CO2 produced during respiration needs to be removed.

Large organisms have lots of cells which respire.
We have high metabolic and respiratory rates due to our high body temp. As a result, lots of oxygen is needed and a lot of waste CO2 is made. 

Lungs- a pair of lobed structures consisting of a series of highly branched tubules (bronchioles) ending in alveoli

Trachea - flexible area supported by cartilage rings which prevents it from collapsing.

Trachea walls- muscles lined with ciliated epithelium and goblet cells (produces mucus and traps dirt and bacteria)

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Mechanism of Breathing

Ventilation - process of breathing (consists of inspiration and expiration)

Inspiration - Breathing in. Air pressure in the atmosphere is greater than air pressure in the lungs - forces air into the alveoli.

Expiration - Breathing out. Air pressure of lungs is greater than the air pressure of the atmosphere. Air is forced out of the lungs.

Main Components:

Diaphragm - Sheet of muscle between the thorax and the abdomen.

Internal Intercostal Muscles, whose contractions lead to expiration.
External Intercostal Muscles , whose contractions lead to inspiration.


Inspiration - External intercostal muscles contract, internal intercostal muscles relax, Diaphragm contracts and moves down, ribcage moves up and out, thorax volume increases, pressure in the lungs decreases and air comes in.

Expiration - External intercostal muscles relax, internal intercostal muscles contract, Diaphragm relaxes and moves upwards, ribcage moves down and in, thorax volume decreases, pressure in lungs increases and air goes out.

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Pulmonary Ventilation

Pulmonary Ventilation - Volume of air taken in one minute / dm3min-1

Tidal Volume - volume of air in each breath / dm3

Ventilation Rate - how many breaths per minute / min-1

PV = tidal volume x ventilation rate

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Exchange of Gases in the Lungs

Essential features:

  • Large SA:vol - speeds up rate of exchange
  • Very thin - to keep the diffusion pathway short and so allow materials to cross rapidly
  • Partially permeable - to allow selected materials to diffuse easily
  • Movement of the environmental medium - e.g. air, to maintain a diffusion gradient.
  • Movement of the internal medium - e.g. blood, to maintain a diffusion gradient

Q - Summarise why diffusion of gases between the alveoli and the blood will be very rapid

  • Red blood cells slow down as they pass through capillaries allowing more time for diffusion
  • Distance between alveolar air and RBCs reduced as RBCs flatten against capillary walls.
  • Alveolar & capillary walls are thin -  have very short distance over which diffusion happens
  • Breathing constantly ventilates lungs, and heart constantly circulates blood to alveoli ensuring a steep concentration gradient.
  • conc. gradient maintained by blood flow through pulmonary capillaries.
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Exchange of Gases in the Lungs

Alveoli function and adaptation

  • thin surface area - alveolar epithelium one cell thick - short diff. pathway - increased diff. rate
  • many alveoli - large SA - increased diff. rate
  • Steep conc. gradient between alveoli and capillaries - increased diff. rate 
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  • Caused by one of two rod-shaped bacteria:
    • Mycobacterium tuberculosis
    • Mycobacterium bovis

Persistent cough, tiredness, loss of appetite - weight loss. Later, fever and coughing up blood.

 Course of infection - 
Bacteria is inhaled and then grows and divides at the top of the lugs (with lots of oxygen). The immune system is stimulated and WBCs collect at the site of infection. WBCs ingest bacteria. The lymph node then enflames and enlarges, draining the area (primary infection). In healthy people, the infection is usually controlled and there are few symptoms, however some bacteria survive. Years later, the bacteria re-emerges, again in the upper regions of the lungs (post-primary tuberculosis). This is not as easily controlled. Bacteria destroys the lung tissue, causing cavities. The sufferer coughs up damaged lung tissues and blood. Without treatment, bacteria spreads around the body, which can be fatal.

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  • Spreads via droplets 
    • cough, sneeze, laugh, talk
  • usually need close contact with the infected person over a period of time.
  • Can be spread from cows to humans as M.bovis infects cattle. Can be in milk

Risk Groups:

  • families
  • people from countries where TB is common
  • have reduced immunity (HIV/Diabetes patients)
  • those in contact with an infected person for long periods e.g. in overcrowded conditions
  • those who work/stay in long-term care facilities e.g. hospitals, prisons
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Pulmonary Fibrosis

Formation of scar tissue, causing thickening of lung epithelium. Probably caused by microscopic lung injury. Increases diff. pathway and decreases lung elasticity.


  • Shortness of breath - scar tissue reduces lung vol, thickens epithelium and reduces elasticity - lungs hold less air, harder for oxygen to diffuse across thickened surface. Ventilation is difficult so conc gradient can't easily be maintained
  • Chronic, dry cough - obstructs airways - reflex action causes coughing but nothing expelled, hence dry.
  • Chest pain - causes damae to fibrous tissues - leading to further damage and scarring
  • Weakness and Fatigue - reduces oxygen intake into blood, causing a low rate of respiration = tiredness.
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Allergic reaction - pollen, dust mites, fur
WBCs release histamine - lining of bronchi and broncioles inflame - epithelial cells secrete excess mucus - fluid enters airways from capillaries - muscles contract and constrict the bronchioles - ventilation becomes difficult.


Difficulty breathing - as said above
Wheezing - caused by air passing through constricted bronchi - shortness of breath
Tightness of the chest -  constricted bronchi & bronchioles - lungs unable to ventilate
Coughing - reflex response to clear 
bronchi & bronchioles

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  • caused by smoking 
  • elastin protein in lungs = permanently stretched/damaged
  • less elastin = alveoli can't recoil and expell air
  • SA of alveoli reduces/sometimes burst
  • reduction in rate of respiration

Q - How does emphysema reduce gas exchange?

Alveoli break down and the surface area is reduced so less diffusion can take place. Elastin is permanently stretched, and as a result, less air can be expelled. Therefore there is a small concentration gradient and reduced diffusion.

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