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Gaseous Exchange and Respiration in Mammals

14 min read

The respiratory system

Air enters through the nose, passes down the trachea, then the bronchi and bronchioles, and reaches the alveoli. Gaseous exchange takes place across the thin walls of the alveoli and the capillaries around them by diffusion.

The human breathing systemThe nose and mouth lead to the trachea or windpipe, which splits into two bronchi, one to each lung. Below the lungs a sheet of muscle called the diaphragm separates the chest from the abdomen.nose and mouthtrachea (windpipe)bronchuslungdiaphragm
Gas exchange in an alveolusAn alveolus, a tiny air sac in the lung, surrounded by a capillary. Oxygen passes from the air in the alveolus into the blood, and carbon dioxide passes from the blood into the alveolus to be breathed out. The walls are one cell thick and moist.air(oxygen in)oxygen → blood← carbon dioxidealveolus wall: one cell thickcapillary with blood

Transport of gases

Oxygen is carried by haemoglobin in red blood cells as oxyhaemoglobin. In the lungs, where the oxygen level is high, haemoglobin takes up oxygen. In the tissues, where the oxygen level is low, it releases oxygen. Carbon dioxide is carried mostly as hydrogencarbonate ions in the plasma, some as carbaminohaemoglobin and a small amount dissolved.

The oxygen dissociation curve of haemoglobinAn S-shaped curve of the percentage saturation of haemoglobin with oxygen against the oxygen pressure. At the high pressure in the lungs haemoglobin is nearly full. At the low pressure in active tissues it releases oxygen easily. The curve shifts to the right at high carbon dioxide levels, the Bohr effect, so more oxygen is released to working muscles.oxygen pressure (kPa)% saturation of haemoglobinlungs: nearly fulltissues: oxygen releasedright shift: high CO₂

Respiration

Respiration is the release of energy from food in the cells. Aerobic respiration uses oxygen and makes a lot of energy. Anaerobic respiration takes place without oxygen and releases little.

Aerobic respiration: glycolysis in the cytoplasm, then the Krebs cycle and electron transport in the mitochondria.
FeatureAerobicAnaerobic in muscleAnaerobic in yeast
OxygenNeededNot neededNot needed
ProductsCO₂ and H₂OLactic acidEthanol and CO₂
ATP per glucoseAbout 3822

During hard exercise the muscles run short of oxygen and make lactic acid. This builds an oxygen debt, which is repaid by deep breathing after exercise.

Aerobic respiration releases far more energy than anaerobic respiration.
Try itGas exchange in the alveoli

Oxygen diffuses across the alveolus wall faster when the surface area is bigger and the wall is thinner. Choose the values.

Check yourself Where is most carbon dioxide carried in the blood?
Check yourself What does anaerobic respiration in muscle produce?
Check yourself How much ATP does aerobic respiration make per glucose?

Key terms

Haemoglobin
The red pigment that carries oxygen.
Aerobic respiration
Respiration with oxygen.
Anaerobic respiration
Respiration without oxygen.
Oxygen debt
The oxygen needed after exercise to remove lactic acid.

Summary

  • Gas exchange takes place by diffusion across the alveolus wall.
  • Haemoglobin carries oxygen; CO₂ is carried mostly as hydrogencarbonate.
  • Aerobic respiration makes about 38 ATP; anaerobic makes 2.
  • Lactic acid builds an oxygen debt in hard exercise.
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