The right answer is vein (it contains blood and has valves).
The infra-cardiac veins of small and medium caliber possess this system of fight against gravity. The valves consist of a dense connective tissue, surmounted by endothelial cells, corresponding to a fold of the intima. They allow blood circulation in the sense organ / heart but prevent venous reflux.
Ok, think about it. Does your skin prevent germs from getting inside you?
Does your skin prevent water from leaving the body too quickly? Does your skin protect your insides from getting damaged when you fall? Do you put food on your skin to absorb nutrients, or do you put food in your mouth like a normal person?
<span>Alleles are each of two or more alternative forms of a gene that are found at the same position in a chromosome. Traits on the other hand, are the characteristics that are defined by genes. So the combination of alleles for a gene is what manifests itself into a trait.</span>
The cramps can be caused due to anaerobic respiration in the muscles and lactic acid formation.
During intense physical activity the muscles become deprived of oxygen, in this condition, anaerobic respiration takes place in the muscle cells causes the formation of the lactic acid.
The physical training increases the strength of the muscles, which can help the muscles not to get tired easily. that is why, physical training is required before any marathon.
