Respiration is a characteristic of all living organisms. Respiration is a process by which food molecules are broken down to release energy for vital activities for all living cells.  . It involves the oxidation of food substances with the release of energy in living cells Oxidation is a chemical reaction involving the loss of electrons, addition of oxygen, or the loss of hydrogen There are 2 forms of respiration: aerobic and anaerobic

Aerobic respiration

Aerobic respiration Aerobic respiration is the breakdown of food substances in the presence of oxygen with the release of a large amount of energy Carbon dioxide and water are released as waste products Many reactions are involved and each reaction is catalyzed by an enzyme system Mitochondria are importance in aerobic respiration Most animals, including man, and green plants respire aerobically.


Anaerobic respiration

In anaerobic respiration oxygen is not needed although it can take place in the presence of oxygen. In anaerobic respiration glycolysis takes place. The 3-crbon molecules are then converted into other products. Anaerobic respiration is known as fermentation. The glucose molecule is only partially broken down. The ethanol produced still contains much energy. This explains why a small amount of energy is set free.


Comparison of aerobic and anaerobic respiration


·         Both processes begin with the breakdown of sugar (glucose)

·         Both processes involve pyruvate

·         Both processes are involved in cellular respiration



Aerobic respiration Anaerobic respiration
Oxygen requirements Oxygen is required for the process to occur Oxygen is not used in this type of respiration
Reactants and Products Reactants: Pyruvate NAD, CoA

Products: Acetyl CoA, NADH, CO2 (waste)

Reactants: Pyruvate and NADH

Products: NAD+ or ethanol

Energy Output 36 ATP molecules are produced through this process 2 ATP molecules are released through this process

Gaseous exchange in man

Gaseous exchange occurs in the lungs where oxygen from the atmospheric air is absorbed by blood and carbon dioxide produced during respiration is released into the environment.

Stages of Gas Exchange

  1. Breathing- movement of air into and out of lungs
    1. Inhalation – draws air in; diaphragm expands
    2. Exhalation – pushes air out; diaphragm contracts
  2. External respiration- exchange of oxygen and carbon dioxide between air and blood in lungs
  3. Internal respiration – exchange of oxygen and carbon dioxide between blood and body cells
  4. Oxygen transport- moving oxygen and carbon dioxide in the blood.

Human Respiratory System

The respiratory system also helps the body maintain homeostasis, or balance among the many elements of the body’s internal environment


 Nasal passage: inside the nose, the sticky mucous membrane lining the nasal cavity traps dust particles, and tiny hairs called cilia help move them to the nose to be sneezed or blown out.

Larynx: The larynx is essential to human speech.

Trachea: Located just below the larynx, the trachea is the main airway to the lungs

Bronchi: The bronchi branch from the trachea into each lung and create the network of intricate passages that supply the lungs with air.

 Lungs: Together the lungs form one of the body’s largest organs. They’re responsible for providing oxygen to capillaries and exhaling carbon dioxide.

Diaphragm: The diaphragm is the main respiratory muscle that contracts and relaxes to allow air into the lungs.


  • Even though alveoli are so small there are huge numbers of them which results in a large surface area for gas exchange.
  • Also the wall of the alveoli is made up of a single layer of thin cells and so are the capillaries, this creates a short diffusion distance for the gases. Therefore this allows rapid gas exchange.
  • The alveoli are covered by a dense network of blood capillaries which have low oxygen and high carbon dioxide concentrations.
  • This allows oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood.
  • Finally, there are cells in the alveolar walls which secrete a fluid that keeps the inner surface of the alveoli moist, allowing gases to dissolve.
  • This fluid also contains a natural detergent that prevents the sides of the alveoli from sticking together.


The process of breathing pumps in oxygen into our body (and removes CO2).Breathing occurs involuntarily but the rate of breathing is controlled by the respiratory system of brain.

Comparison of inspiration and expiration

Feature Inspiration Expiration
1 Diaphragm Relaxes and arches upwards Contracts and flattens
2 Ribs Moves downwards and inwards Move upwards and outwards
3 Volume of thoracic cavity Decreases Increases
4 Lungs Compressed by decrease in thoracic cavity volume Expand in respond to air pressure to fill up thoracic cavity
5 Air pressure of lungs Increases as lungs are compressed Decreases due to lung expansion
6 Air movement Lung pressure higher than atmospheric pressure. Air forced out of lungs Atmospheric pressure > lung pressure. Air enters lung

Rate of breathing

The average breathing rate in an adult man at rest is about 15 to 18 times per minute. This breathing rate increases with increased physical activity.

  1. Oxygen required for breathing and respiration is carried by hemoglobin pigment present in our blood. The normal range of hemoglobin in the blood of a healthy adult person is from 12 to 18 grams per deciliter of blood.
  2. The deficiency of hemoglobin in the blood of a person reduces the oxygen carrying capacity of blood resulting in breathing problems, tiredness and lack of energy.