How we know 3 - Digestion

How do we learn what happens in the small intestine?
How did we discover that the main role of the small intestine is to absorb carbohydrates, amino acids, and fats? When the human body was first studied, scientists recognized that the weight of the food that entered the small intestine was much greater than the weight of wastes that came out. They concluded that the small intestine must be absorbing some of the contents of the food. Scientists showed that the contents in the large vein (the portal vein) that collects all the blood leaving the small intestine has a high amount of carbohydrates and amino acids.

Why didn't they discover an increase in fats in the portal vein? Scientists knew that humans absorbed fats but they didn't know where. It was known that after eating a meal high in fat, there was a large increase in fatty acids in a large vein in the chest near the heart. So much fatty acid was present that when they drew blood from this vein from an animal that had eaten a high-fat meal, the blood had a white tinge! This discovery led scientists to figure out that fats were being absorbed through lymph vessels in the small intestine, since the lymph system empties into the blood stream into this vein near the heart.

How do we know about the functions of the large intestine?
We know that the large intestine absorbs only water and salt, and the bacteria that live in the large intestine survive by digesting anything edible that was left undigested from the small intestine. How did scientists find this out? Before scientists had the luxury of sophisticated testing techniques, they knew that materials entering the large intestine were soft and fluid, but excreted wastes were more dry and hard. From this, they concluded that the large intestine must be absorbing most of the fluid.

This was confirmed when they examined the blood that leaves the large intestine. This blood had more water and salt than did the blood entering the large intestine.
By culturing intestinal contents on growth medium in Petri dishes, scientists also discovered that the large intestine, but not the small intestine, has large numbers of bacteria. Researchers exactly determined the function of the bacteria by isolating them in specific cultures that contained different mixtures of carbohydrates, proteins, and fats. After incubating the bacteria, they tested each culture and found that not only can the bacteria break down certain foods that humans cannot (such as dietary fiber) but the bacteria also produce Vitamin K. Vitamin K is needed for normal blood clotting.