Descriptions of Respiration

January 19, 2016

Respiration supplies the body with energy. The circulatory system takes oxygen and glucose to the cells and removes waste products. We can improve our fitness by taking exercise. Anaerobic respiration occurs when there is too little oxygen present.


What is respiration?

Enzymes in cells catalyse photosynthesis, protein synthesis - joining amino acids together, and aerobic respiration.

Aerobic respiration

Respiration is not the same thing as breathing. That is more properly called ventilation. Instead, respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the process happen in tiny objects inside the cell cytoplasm, called mitochondria.

This is the equation for aerobic respiration:

glucose + oxygen → carbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. In plants, for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids can then join together to make proteins. The energy is also used:

  • To allow muscles to contract in animals
  • To maintain a constant body temperature in birds and mammals

The circulatory system

Blood carries oxygen and nutrients to the body'scells,and waste products away from them. The circulatory system consists of:

  • The heart, which is the muscular pump that keeps the blood moving
  • The arteries, which carry blood away from the heart
  • The veins, which return blood to the heart
  • The capillaries, which are tiny blood vessels that are close to the body's cells

The diagram outlines the circulatory system. Oxygenated blood is shown in red, and deoxygenated blood in blue. A process called diffusion takes place in the capillaries. Diffusion is where particles of a high concentration move to an area of low concentration. Glucose and oxygen diffuse into the cells from the capillaries. Carbon dioxide diffuses out of the cells into the blood in the capillaries.

Effect of exercise on breathing

During exercise, the muscle cells respire more than they do at rest. This means:

  • Oxygen and glucose must be delivered to them more quickly
  • Waste carbon dioxide must be removed more quickly

This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected by measuring the pulse rate. The stroke volume also increases – this is the volume of blood pumped each beat. The total cardiac output can be calculated using the equation:

Cardiac output = stroke volume x heart rate

During hard exercise, the oxygen supply may not be enough for the needs of the muscle cells. When this happens, anaerobic respiration takes place, as well as aerobic respiration.

Fitness versus health

Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate is likely to return to normal more quickly after exercise.

But being fit is not the same as being healthy. Healthy people are free from disease and infection: they may or may not be fit as well. It is possible to be fit but unhealthy, or healthy but unfit.

Anaerobic respiration

When exercising very hard, the heart cannot get enough oxygen to the muscles. Anaerobic respiration does not need oxygen. It releases energy from glucose but the amount is much lower. It happens when there is not enough oxygen for aerobic respiration. Here is the word equation:

glucose → lactic acid (+ energy)

Much less energy is released by anaerobic respiration than by aerobic respiration. The lactic acid that forms causes muscle fatigue and pain.

The after effect of exercise

During hard exercise when anaerobic respiration occurs with aerobic respiration, anoxygen debt builds up. This is now known as Excess Post-exercise Oxygen Debtor EPOC. This is because glucose is not broken down completely to form carbon dioxide and water. Some of it is broken down to form lactic acid. Panting after exercise provides oxygen to break down lactic acid. The increased heart rate also allows lactic acid to be carried away by the blood to the liver, where it is broken down.

Blood pressure

Arteries carry blood away from the heart.

The blood in the arteries is under pressure because of the contractions of the heart muscles. This allows the blood to reach all parts of the body.

Blood pressure is measured in millimetres of mercury, mmHg. There are two measurements:

  • Systolic pressure - the higher measurement when the heart beats, pushing blood through the arteries.
  • Diastolic pressure - the lower measurement when the heart rests between beats.

A young, fit person may have a blood pressure of about 120 over 70, which means their systolic pressure is 120 mmHg, and their diastolic pressure 70 mmHg.

Blood pressure varies with age. It also varies with lifestyle factors such as:

  • Diet
  • Stress
  • Exercise
  • Body mass
  • Alcohol consumption



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