Small Intestine

Small Intestine

The small intestine is, I believe, by far the most important organ of the digestive system. The small intestine, in its whole function, is really about absorption. The stomach secretes many substances. The small intestine takes that slurry and absorbs it into the hepatic portal system. This system then leads to the liverA large organ that produces bile, detoxifies blood, and stores nutrients..

The small intestine has three areas: duodenumThe first section of the small intestine where most digestion occurs; it receives chyme, bile, and p, jejunumThe middle section of the small intestine, primarily responsible for nutrient absorption., and ileumThe last section of the small intestine, responsible for absorbing vitamin B12 and bile salts.. These are all connected to a mesenteryA fold of peritoneum that supports and connects the intestines to the abdominal wall. that suspends them in position. If we had done the fetal pig dissection, you would have seen a large glob of intestines. By cutting the mesentery, you could spread the intestine out fully. Each part—the duodenum, jejunum, and ileum—has very different histology, though they all perform absorption in distinct ways.

Small Intestine Mucosa

The small intestine aims to expand its surface area to enhance nutrient absorption. A cell with more surface area relative to its volume absorbs more nutrients. The small intestine achieves this first by forming large circular folds (Plicae Circulares) – Permanent folds in the small intestine that increase surface area for nut. These folds are called plicae circulares. They circle the lumenThe inside space of a hollow organ or structure. of the intestine. These circular folds are sometimes called plicae circulare.  These are macroscopic features able to be seen with the naked eye. 

Each circular fold has smaller finger-like projections called villiFinger-like projections in the small intestine that increase surface area for absorption., and on these villi are even smaller projections called microvilliTiny projections on the surface of epithelial cells that increase surface area for absorption..  The microvilli are incredibly small. They form a dense forest along the villi. You can see them like eyeliner around the top of this vilus. This area is called the brush borderA dense layer of microvilli on the surface of certain cells, increasing surface area for absorpti. Many enzymesProteins that speed up chemical reactions in the body. remain here to perform chemical digestion on the slurry. This multi-layered structure greatly increases surface area. If the small intestine becomes inflamed, it occurs in conditions like Crohn’s or Celiac disease. The plicae circulares swell and fuse. This swelling reduces surface area, and consequently, nutrient absorption decreases. Over time, this can lead to malabsorption and nutrient deficiencies. Food moves too quickly through the small intestine for proper absorption.

What’s in a Villus?

Each villus contains an artery, vein, and lymphatic vessel. Small substances are absorbed into the vein, while larger substances like fats are absorbed into the lymphatic vessel. The microvilli on individual columnar cellsThe basic structural and functional units of life. form that brush border, which hosts numerous enzymes to complete digestion before nutrient absorption. The small intestine, like the stomach, is lined with simple columnar epithelial cells and contains mucus cells for lubrication. This is what you get when you use an open resource. However, this picture’s mucosaThe innermost lining of the digestive tract that contains mucus-secreting cells for protection and a does not show you the many mucus cells that would be scattered in the mucosa. I guess I will have to add them in!!

Duodenal Mucosa

The most superiorAbove or toward the upper part of the body. part of the small intestine, called the duodenum connects to the stomach, separated by the pyloricRelating to the pylorus, the region of the stomach that connects to the small intestine. sphincter. The duodenum also receives input from the accessory organs: pancreasA gland that produces digestive enzymes and hormones like insulin and glucagon., liver, and gall bladderA muscular organ that stores urine before excretion..  The pancreas is spewing really acidicA solution with a pH below 7, having a higher concentration of H⁺ ions. substances into the duodenum to add to the already acidic slurry from the stomach. There are lots of submucosal glandsGlands in the airway walls that secrete mucus. in the duodenum that secrete a very bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide rich solutionA homogeneous mixture of two or more substances.. This bicarbonate rich solution raises the pHA measure of hydrogen ion concentration in a solution. of the slurry.

Jejunum Mucosa

The jejenum is all about absorption.  It has the surface area increases features as we described.  In this histological picture on the right, you can see scattered mucus cells. They appear like little white bubbles in the epithelium.  These open areas between the villi or finger-like projections are called intestinal crypts (Crypts of Lieberkühn) – Glandular structures in the small intestine that produce digestive en.  Much like the gastric glands of the stomach, these crypts have secretory cells in them.

Ileum Mucosa

The iliumBroad, flaring upper portion of the hip bone; forms the “wings” of the pelvis. ss really the last place that nutrients could be absorbed by the small intestine. In the ilium the small intestine is also getting the remnants of food ready for the large intestine. Chemical digestion is continuing to extract the last bit of those nutrients.

At this point, the chymeA semi-fluid mixture of partially digested food and digestive juices that moves from the stomach int or food slurry is kind of turning back into a bolusA rounded mass of food mixed with saliva that is ready for swallowing. and becoming less liquid-y.  There are more mucus cells scattered among the epithelium. These cells try to provide lubrication and buffering The process of stabilizing pH by binding or releasing H⁺ ions. for the food as it enters into the large intestine. The small intestine and large intestine are separated by the ileocecal valve. Unfortunately, when the ileocecal valve opens, a little bit of the contents of the large intestine spews back. It enters into the ilium. These contents could put the ilium at risk of infection or invasion. There are these structures called Peyer’s patches that are strategically placed just before the ileocecal valve.

Peyer’s Patches

Peyer’s Patches are found in the submucosa of the ilium just before the ileocecal valve.. These patches, similar to tonsils, protect against pathogens and are part of the immune system’s MALT (mucosa-associated lymphoid tissue). These patches are kind of like little lymph nodes that are monitoring the backwash coming from the large intestine

Celiac’s and Crohn’s

As mentioned before both of these diseases Celiacs disease and Crohn’s disease cause inflammation of the small intestine’s mucosa.  This strips the small intestine of its carefully created surface area and reduces the amount of absorption of nutrients. There are different types of Crohn’s disease one of my relatives has jejunitis.  She spent a lot of time in her childhood with flare ups and suffered many issues associated with malnutrition.