Why are there zero hours of daylight at the North Pole and South Pole in the winter?

“During the northern hemisphere winter solstice, the Sun's incoming rays are perpendicular to the Tropic of Capricorn at 23.5 degrees south latitude. The Sun's path is the lowest above the horizon in locations north of the equator, and these regions experience the shortest day of the year.”

the glands which are associated with each description is given below:

pancreas - is an endocrine and exocrine gland that secretes juices with enzymes for digestion. salivary glands- with protection and digestion in the mouth. liver- secretes bile for digestion.   lachrymal glands - secrete tears to moisten eyes and protect them from particles and infection.   sweat gland -regulates water, salt, and temperature. oil glands - to form a protective barrier on the skin.   goblet cells - secrete mucus for protection.