The Respiratory System

The air inside the lungs is never replaced completely. It is always a mixture of the air left inside and the air inspired. In other words, the air in the lungs is oNy becoming better and worse with each inspiration and expiration.

Qualitywise, the expired air differs from inspired air in the following respects :

2.

It contains more carbon dioxide.

3.

It contains more water vapour.

4.

It is warmer (or at the same temperature as

that of the body).

5.

It may contain some bacteria.

What is Respiration?

Respiration is a vital process in all living organisms. It goes on non-stop throughout life.

This chapter explains the various aspects related to respiration — the raw material used, the end products formed and the amount of energy liberated, etc.

Some experiments to demonstrate the mechanism of breathing are very interesting.

Need for Respiration:

There are five important points to remember about this chemical reaction in respiration.

This part of respiration, yielding energy, occurs inside the living cells and hence, it is better known as cellular or tissue respiration.

The breakdown of glucose to carbon dioxide and water does not occur in a single step but in a series of chemical steps.

Some of these steps occur in the cytoplasm of the cell and some inside the mitochondria.

Each breakdown step is due to a particular enzyme.

The energy liberated in the breakdown of the glucose molecule is not all in the form of heat, but a large part of it is converted into chemical energy in the form of ATP - a chemical substance called adenosine triphosphate.

The essential steps of cellular respiration are the same in plants and animals.

Animals need more Energy:

The need for the production of energy is greater in animals than in plants.

This is because animals consume more energy in doing physical work.

They have to move about for obtaining food or run away to escape enemies.

They have to chew their food and have to take after their eggs or young ones, and so on.

Birds and mammals including ourselves have also to produce a lot of heat for keeping their body warm. This heat comes through respiration in the cells.

The amount of heat to keep the body warm is quite large.

Think about the cold winter days when the outside temperature is far below our body temperature.

We are constantly losing heat to the outside air, and more of it has to be continuously produced to make up for the loss.

Liver cells in particular produce much heat, and the muscle cells also contribute to it.

The energy used in all cellular activities is obtained from the oxidation of glucose (C6H 20 6), a carbohydrate.

Glucose:

If the simple carbohydrate (glucose) is not available directly, the cells may break down the proteins or fats to produce glucose for respiratory needs.

Think for a while about the wild animals which are totally flesh-eaters.

The main constituent of their diet is a protein with very few carbohydrates.

The excess amino acids absorbed through protein- digestion are broken down in the liver to produce sugar (glucose) and the nitrogenous part is converted into urea which gets excreted out.

The glucose thus produced may be used immediately or may get stored in the liver cells as glycogen for future needs.

A similar process takes place in humans if they take excessively protein-rich food.

Aerobic and Anaerobic Respiration:

In animals, there is normally aerobic respiration using oxygen.

Anaerobic respiration (in the absence of oxygen) is only exceptional in some cases as in the tapeworms living inside the human intestines.

Anaerobic respiration may occur even in our own body in the fast-working skeletal muscles temporarily.

During continuous physical exercise as fast running, walking over long distances, swimming, wrestling, weight-lifting, etc., our muscles work too fast but not getting enough oxygen.

In this situation, the muscles are working in the absence of oxygen (anaerobic respiration) to provide extra energy.

The product of anaerobic respiration in such muscles is lactic acid.

Accumulation of lactic acid gives the feeling of fatigue.

This is a condition that may be called oxygen debt.

When you rest after such exercise, the lactic acid gets slowly oxidized by the oxygen later available and then the “debt is cleared” producing carbon dioxide in the process.

Aerobic respiration in animals:

The chemical changes taking place in aerobic respiration in animals are the same as in aerobic respiration in plants.

Thus by taking 180 g of glucose the energy released is 686 kilocalories, or if expressed in kJ (kilojoules) the energy released is about 2890 (686 x 4 2) kJ.

Anaerobic respiration in animals:

In animal cells, particularly in the skeletal muscle cells, anaerobic respiration may occur when they have to work very fast with insufficient oxygen.

It is a slow process.