10.6.20 Lesson #6 Small Intestine: Jejunum and Ileum

I have a striking challenge today of describing to you what happens in the next 9 feet of our 1 inch in diameter small intestines! This process takes 3-5 hours and is responsible for 90% of the absorption of our nutrients. We rely on this special 9 feet for completing the breaking down (digestion) and absorption of our fat and water soluble vitamins, minerals such as iron and calcium, and the molecules from proteins, fats, and carbohydrates.

The jejunum is about 3 feet long and is mainly in the upper left quadrant of our abdominal cavity. The ileum is about 6 feet long and twists and turns in the lower right quadrant. At the end of the ileum is the ileocecal sphincter which is the hatch to the large intestine. You can find it just to the inside of your right hip bone.

Structure: First, we have circular folds that cause the chyme to spiral and cover more surface area which increases absorption as the chyme moves. Secondly, we have villi which are like tiny fingers that also increase surface area and provide a home for a special network of blood and lymph capillaries. Nutrients absorbed through these are transported out of the area into blood vessels and the lymph system. Thirdly, we also have microvilli which are so tiny that even under a microscope they form a fuzzy line and are appropriately named the brush border! These are formed from plasma membranes to increase the absorptive surface area.

Intestinal and pancreatic juices combine to make a watery clear/yellow fluid with a pH on the alkaline side at 7.6. This liquid cocktail aids in the absorption of substances from the chyme as it comes in contact with microvilli. Absorptive cells in the area also release enzymes to help digestion and absorption. These all further break down and make molecules ready to be absorbed. Absorption is the movement of tiny molecules from inside the small intestine through the lining to the blood and lymphatic systems.

Besides the chemical digestion going on, there is some movement happening. “Segmentation are localized contractions that slosh chyme back and forth, mixing it with digestive juices and bringing food particles into contact with the mucosa for absorption,” says Tortrora and Derrickson 10th edition. Picture squeezing the middle and end of a tube. The contents sloshes back and forth but can’t go further. Peristalsis is what moves the chyme north to south. This contraction begins in the lower portion of the stomach and travels a short ways into the small intestine. This wave slowly travels the length of the small intestine, with each wave taking from 1.5 to 2 hours and repeating until the small intestine is empty again.

The absorption process is amazing in and of itself! We have several ways this happens: simple diffusion, facilitated diffusion, osmosis and active transport. This goes on for all the types of carbohydrate molecules, amino acids (from proteins), lipids broken down by bile salts and holding onto important fat soluble vitamins, as well as water and water soluble vitamins and ions! Over 2 gallons of water is absorbed daily! Left behind are any undigested or unabsorbed materials. These are passed into the large intestine where we will finish our north to south journey next week!


Tortora and Derrickson, Introduction to the Human Body, 10th Edition

NTA slides 2019, Digestion and Elimination