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Respiratory system

The respiratory system is the biological system of any organism that engages in gas exchange. Even trees have respiratory systems, taking in carbon dioxide and emitting oxygen.


Respiratory system in humans and animals

In humans and other mammals, the respiratory system consists of the lungs, the series of tubes leading to the lungs, and the chest structures responsible for bringing air into and out of the lungs during respiration.


The respiratory system consists of:

  • The nasal cavity in which air is moistened and in which the sense of smell operates
  • The nostril
  • The mouth
  • The pharynx, a common area in the back of the mouth through which air, food and drink passes
  • The larynx, or voicebox
  • The trachea, an air tube that connects with the bronchi
  • The right main bronchus and the left main bronchus tubes that carry air to and from the lungs
  • The bronchioles, branches of the bronchi which distribute air to the alveoli
  • The terminal bronchiole s
  • The respiratory bronchioles
  • The alveolar duct s
  • The alveoli, terminal sacs in which gas exchange occurs
  • The diaphragm

The right and left bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveoli form the right and left lungs respectively.

The trachea, bronchi, bronchiles, and terminal bronchioles make up the conducting zone. The respiratory bronchioles, alveolar ducts, and alveoli make up the respiratory zone.

The pulmonary blood vessels generally accompany the airways and also undergo numerous branchings. The pulmonary circulation has a very low resistance compared to the systemic circulation, and for this reason, all the pressures within the pulmonary blood vessels are low.

Air is moved in and out of the lungs by the movements of the rib cage and diaphragm, which expand the lungs to draw in air and compress the lungs to drive out air. A simple model of how the lungs are inflated can be built from a bell jar.


The major function of the respiratory system is gas exchange. In an average resting adult, the lungs take up about 250ml of oxygen every minute while excreting about 200ml of carbon dioxide.

The movement of gas through the larynx, pharynx and mouth allow us to speak, or phonate.

The large surface area the respiratory tract is constantly exposed to microbes, and the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.

Virtually all the body's blood travels through the lungs every minute. The lungs add and removes many chemical messengers from the blood as it flows through pulmonary capillary bed . The fine capillaries also traps blood clots that have formed in systemic viens.


There are various diseases of the respiratory system

Respiratory system in plants

Plant respiration is limited by the process of diffusion. Even a baobab tree is mostly dead because air can penetrate only skin deep. However, most plants are not involved in highly metabolic activities like hunting, i.e. they do not need the energy necessary for predators, and thus their breathing is limited.

Respiratory system in insects

Insects have no concentrated respiratory organs. (Perkins, 2003) Instead insects use a system of tracheae, thin channels, that run through their body, to improve on simple diffusion and let air flow more freely throughout the organism. Spiracles are small holes that open to the outside of the body and allow air in. Spiracles can be found along the abdomen and thorax of the insect body but never on the head. (Perkins, 2003) The spiracles can control the amount of air that is let into the insect. The spiracles lead to the trachea which act like "large distribution tubes" for the air being carried. (Perkins, 2003) The trachea lead to smaller tubes called tracheoles. The insects' cells can't be too far from these tracheoles because this is where oxygen and carbon dioxide diffuse in and out of the hemolymph. This simple system limits their size because insects are purely diffusive. No modern insect exists that is larger than a foot or so (in metric units, about half a metre). Many people fear big bugs, and they should be comforted by this fact.

Tissue engineering

In tissue engineering, respiration is an essential problem. The small depth of diffusion respiration sufficient to support the metabolism of an average human cell is less than a millimetre (0.04 in). Various substances can be used to enhance this depth, essentially having a haemoglobising role.



  • Perkins, M. 2003. Respiration Power Point Presentation. Biology 182 Course Handout. Orange Coast College, Costa Mesa, CA.

See also:

Respiratory system

Human organ systems

Cardiovascular system - Digestive system - Endocrine system - Immune system - Integumentary system - Lymphatic system - Muscular system - Nervous system - Skeletal system - Reproductive system - Respiratory system - Urinary system

Last updated: 10-24-2004 05:10:45