(IGP) IAS Pre: GS - Science & Technology : Biology - The Respiratory System

The Respiratory System


Cellular respiration involves the breakdown of organic molecules to produce ATP. A sufficient supply of oxygen is required for the aerobic respiratory machinery of Kreb’s Cycle and the Electron Transport System to efficiently convert stored organic energy into energy trapped in ATP. Carbon dioxide is also generated by cellular metabolismand must be removed from the cell. There must be an exchange of gases: carbon dioxide leaving the cell, oxygen entering. Animals have organ systems involved in facilitating this exchange as well as the transport of gases to and from exchange areas.


Single-celled organisms exchange gases directly across their cell membrane. However, the slow diffusion rate of oxygen relative to carbon dioxide limits the size of singlecelled organisms. Simple animals that lack specialized exchange surfaces have flattened, tubular, or thin shaped body plans, which are the most efficient for gas exchange. However, these simple animals are rather small in size.


Large animals cannot maintain gas exchange by diffusion across their outer surface. They developed a variety of respiratory surfaces that all increase the surface area for exchange, thus allowing for larger bodies. A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cellmembranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist.


(a) Sponges and jellyfish lack specialized organs for gas exchange, so they take gases directly from the surrounding water.

(b) Flatworms and annelids use their outer surfaces as gas exchange surfaces. Earthworms have a series of thin-walled blood vessels known as capillaries. Gas exchange occurs at capillaries located throughout the body as well as those in the respiratory surface

(c) Amphibians use their skin as a respiratory surface. Frogs eliminate carbon dioxide 2.5 times as fast through their skin as they do through their lungs. Eels (a fish) obtain 60% of their oxygen through their skin. Humans exchange only 1% of their carbon dioxide through their skin. Constraints of water loss dictate that terrestrial animals must develop more efficient lungs.

(d) Arthropods, annelids, and fish use gills: Gills greatly increase the surface area for gas exchange. They occur in a variety of animal groups including arthropods (including some terrestrial crustaceans), annelids, fish, and amphibians.Gills typically are convoluted outgrowths containing blood vessels covered by a thin epithelial layer. Typically gills are organized into a series of plates and may be internal (as in crabs and fish) or external to the body (as in some amphibians).Gills are very efficient at removing oxygen fromwater: there is only 1/20 the amount of oxygen present in water as in the same volume of air.Water flows over gills in one direction while blood flows in  the opposite direction through gill capillaries. This countercurrent flow maximizes oxygen transfer. Terrestrial vertebrates utilize internal lungs:

(e) Tracheal Systems: Many terrestrial animals have their respiratory surfaces inside the body and connected to the outside by a series of tubes.Tracheae are these tubes that carry air directly to cells for gas exchange. Spiracles are openings at the body surface that lead to tracheae that branch into smaller tubes known as tracheoles. Body movements or contractions speed up the rate of diffusion of gases from tracheae into body cells. However, tracheae will not function well in animals whose body is longer than 5 cm.

(f) Lungs: Lungs are ingrowths of the body wall and connect to the outside by as series of tubes and small openings. Lung breathing probably evolved about 400 million years ago. Lungs are not entirely the sole property of vertebrates, some terrestrial snails have a gas exchange structures similar to those in frogs.


1. Movement of an oxygen-containing medium so it contacts a moist membrane overlying blood vessels.

2. Diffusion of oxygen from the medium into the blood.

3. Transport of oxygen to the tissues and cells of the body.

4. Diffusion of oxygen from the blood into cells.

5. Carbon dioxide follows a reverse path.


(a) The Pathway
(b) Breathing
(c) Central Control of Breathing
(d) Local Control of Breathing
(e) Diseases of the Lungs

  • Pneumonia
  • Asthma
  • Emphysema
  • Chronic Bronchitis
  • Chronic Obstructive Pulmonary Disease
  • Lung Cancer

(a) The Pathway

  • Air enters the nostrils
  • passes through the nasopharynx,
  • the oral pharynx
  • through the glottis
  • into the trachea
  • into the right and left bronchi, which branches and rebranches into
  • bronchioles, each of which terminates in a cluster of alveloi

Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).

(b) Breathing

In mammals, the diaphragm divides the body cavity into the

  • abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the
  • thoracic cavity, which contains the heart and lungs.

The inner surface of the thoracic cavity and the outer surface of the lungs are lined with pleural membranes which adhere to each other. If air is introduced between them, the adhesion is broken and the natural elasticity of the lung causes it to collapse. This can occur from trauma. And it is sometimes induced deliberately to allow the lung to rest. In either case, reinflation occurs as the air is gradually absorbed by the tissues. Because of this adhesion, any action that increases the volume of the thoracic cavity causes the lungs to expand, drawing air into them. During inspiration (inhaling), The external intercostal muscles contract, lifting the ribs up and out. The diaphragm contracts, drawing it down . During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breath 15-18 times a minute exchanging about 500 ml of air. In more vigorous expiration, The internal intercostal muscles draw the ribs down and inward. The wall of the abdomen contracts pushing the stomach and liver upward.

Under these conditions, an average adult male can flush his lungs with about 4 liters of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain.

Component Atmospheric Air (%) ExpiredAir (%)
(plus inert gases)
78.62 74.9
O2 20.85 15.3
CO2 0.03 3.6
H2O 0.5 6.2
  100.0% 100.0%

The above table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epitheliumof the alveoli. Fromhere it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the hemoglobin therein.

At the same time, some of the carbon dioxide in the blood diffuses into the alveoli fromwhich it can be exhaled. Composition of atmospheric air and expired air in a typical subject.Note that only a fraction of the oxygen inhaled is taken up by the lungs.

(c) Central Control of Breathing

The rate of cellular respiration (oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20-25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing.

It is a rising concentration of carbon dioxide— not a declining concentration of oxygen — that plays the major role in regulating the ventilation of the lungs. Certain cells in the medulla oblongata are very sensitive to a drop in pH. As the CO2  content of the blood rises above normal levels, the pH drops [CO2  + H2O – HCO3 – + H+], and the medulla oblongata responds by increasing the number and rate of nerve impulses that control the action of the intercostal muscles and diaphragm. This produces an increase in the rate of lung ventilation, which quickly brings the CO2  concentration of the alveolar air, and then of the blood, back to normal levels.

However, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate but there has been no increase in the production of CO2 .

(d) Local Control of Breathing

The smooth muscle in the walls of the bronchioles is very sensitive to the concentration of carbon dioxide.Arising level of CO2 causes the bronchioles to dilate. This lowers the resistance in the airways and thus increases the flow of air in and out.


  • In the cells O2 helps in the break down of food.

  • The process of breakdown of food in the cell with the release of energy is called cellular respiration.

  • When breakdown of glucose occurs with the use of O2 it is called aerobic respiration.

  • Food can also be break down without using O2.  This is called anrobic respiration.

  • Yeast can survive in the absence of air. They respire anaerobically and during this process yield alcohol. They are therefore used to make wine and beer.

  • Anaerobic respiration takes place in the muscle cells to fulfill the demand of energy in this process due to the partial break down of glucose, the glucose produce lactic acid, and the accumulation of this lactic acid causes muscle cramp.

  • Average human being at rest breaths in and out 15-18 times a minute.

  • In the plant cells O2 is used to break down glucose into CO2 and water as in other organisms.

  • Break down of glucose with the use of O2 give – CO2 + water + energy.

  • Break down of glucose with out the use of O2 give – alcohol + CO2 + energy.

  • In plants each part can independent take in o2 from the air and give out CO2 .

  • Organism use O2 to breakdown glucose in the first step 6 carbon molecules is break down into 3 carbon molecules. This is called private. This process takes place in cytoplasm.

  • Break down of pyruvate using O2 takes place in the mitochondria. This process is called aerobic.

  • The energy released during cellular respiration is immediately used to synthesis a molecules called ATP. This is used to fuel all other activities in the cell.

  • In human being the respiratory pigment is hemoglobin which has a very high affinity for O2.

  • CO2  is more soluble in water then O2 is, hence CO2 is mostly transported in the dissolved from in our body.

  • Cigarette smoking leads to disease emphysema. In this, bronchioles get obstructs. Many alveoli coalesce to form large chamber due to destruction of area across which respiratory gases are exchanged. All these changes seriously reduce both oxygen uptake and CO2  elimination.

  • At depth much nitrogen diffuses and dissolves in blood and body fats. This makes the driver lose his strength and work capacity. If he is lifted rapidly to sea surface, nitrogen is evolved and forms gas bubbles, which may block pulmonary vessel producing serious shortness of breath. Dizziness, paralysis and mental derangement may  be caused by bubbles in the vessels of brain and spinal cord.

  • Carbon mono oxide from carboxyhae-moglobin, which is relatively stable compound and can not bind any oxygen. So, amount of hemoglobin available for oxygen transport is reduced, which causes  headache, dizziness, nausea and even death.

  • The barometric pressure falls progressively with rise in altitude, so the partial pressure of O2 decrease. This lowers the alveolar PO2 and consequently oxygenation decrease which lead to mountain sickness.

  • The more active tissue, the more it is affected by shortage of O2. So skeletal muscles, heart and brain are much more affected than skin, intestine and bone.

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