Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Concentration Gradient
What is Concentration Gradient?
A difference in the concentration of a substance between two regions. Particles move randomly, and the net result of that random movement is a flow of particles from the region where they are more concentrated to the region where they are less concentrated: that net movement is diffusion, and it continues until the concentrations are equal. At the alveolus, oxygen is at a higher concentration in the alveolar air than in the blood arriving in the capillary, so oxygen diffuses into the blood; carbon dioxide is at a higher concentration in that blood than in the alveolar air, so carbon dioxide diffuses out. Both gradients are maintained by ventilation on one side and by blood flow on the other, and no energy from respiration is used to move either gas.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Questions students ask about Concentration Gradient
Why are the alveoli so good at gas exchange?
There are very many of them, so the total surface area is very large. The alveolar wall and the capillary wall are each one cell thick, so the distance a gas must travel is extremely short. The lining is moist, so gases dissolve before they diffuse. A dense network of capillaries carries absorbed oxygen away and delivers carbon dioxide, and continuous ventilation refreshes the air on the other side. The last two matter most, because they are what keep the concentration gradients steep.
Why do you breathe faster when you exercise?
Because your muscle cells are respiring faster to release the energy needed for contraction. Faster respiration uses more oxygen and produces more carbon dioxide. Breathing faster and more deeply increases the volume of air moved per minute, which supplies the extra oxygen, removes the extra carbon dioxide, and keeps the concentration gradients at the alveoli steep so that diffusion continues rapidly. The energy is released in the muscle cells, not in the lungs.