Why did 40% of the human population evolve to tolerate lactose?

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Explanation:

Primary lactose intolerance is the most common. It is caused by a decrease in lactase production with age, so that lactose becomes poorly absorbed ( 5 ). This form of lactose intolerance may be partially caused by genes, because it's more common in some populations than others

Explanation:

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the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

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