Our intestine is an essential organ of the digestive system and extends from the stomach outlet to the anus. Strictly speaking, it actually begins at the mouth and ends at the anus. It is anatomically divided into several sections that differ in their structure and function. The total length of the intestine in an adult is approximately 5 to 7 meters .

The intestine is divided into two main sections:

The small intestine is the longest section of the intestine (approximately 4–6 meters) and the main site of digestion and nutrient absorption. It has a relatively low bacterial density compared to the large intestine (1,000–10,000 bacteria/ml of intestinal content). It consists of three sections:

Duodenum :

Length: approx. 25 cm.

Begins at the stomach outlet (pylorus).

Main task: Neutralization of the
Acidic stomach contents with the help of secretions from the pancreas and bile. Small ducts from both the pancreas and the bile drain into the duodenum.

Site of fat digestion and initial absorption of nutrients.

Jejunum :

• Length: approx. 2.5 meters.


• Responsible for the absorption of carbohydrates, proteins and vitamins, such as vitamin C, A, D, E, K or minerals, e.g. magnesium, iron or zinc

Ileum :

• Length: approx. 3 meters.


• Task: Absorption of vitamin B12, bile acids and other nutrients.


• Transition into the large intestine through the Bauhin valve (ileocecal valve).

The large intestine is approximately 1.5 meters long and serves to thicken and store stool, as well as absorb water and electrolytes. It has a high bacterial density, which functions to ferment fiber and produce short-chain fatty acids (100,000,000,000 - 1,000,000,000,000 bacteria/ml of intestinal contents). It is divided into the following sections:

Appendix (caecum):

• The beginning of the large intestine.
• Carries the appendix, which plays a role in the immune system.

Large intestine (colon):
Is further divided into four sections:

• Ascending colon (ascending branch).
• Colon transversum (transverse branch).
• Descending colon (descending branch).
• Sigmoid colon (S-shaped section).
• Tasks: Recovery of water and minerals, fermentation
  indigestible substances by intestinal bacteria.

Rectum:

• Last section of the intestine, which serves to store stool.
• Ends at the anus, where stool is released in a controlled manner by two sphincter muscles.

The intestinal wall consists of several layers that are specialized for digestion, absorption and protection of the body:

Mucous membrane (mucosa):

• Inner layer with specialized cells for nutrient absorption.
• Contains intestinal villi and microvilli, which greatly increase the surface area for nutrient absorption.

Submucosa:

• Contains blood vessels, nerves and lymphatic vessels to supply the mucosa.

Muscle layer (muscularis):

• Consists of two muscle layers:
• Inner circular muscle layer.
• Outer longitudinal muscle layer.
• Responsible for the peristaltic movements that transport the food pulp.

Connective tissue layer (serosa or adventitia):

• Outer layer that covers the intestine and connects it to the abdominal cavity.

The blood supply to the intestine is ensured by branches of the abdominal aorta:

Small intestine:

• Supplied by the superior mesenteric artery.

Colon:

• Upper section: superior mesenteric artery.
• Lower section: inferior mesenteric artery.

The venous blood flows via the portal vein (vena portae) to the liver, where it is cleaned and further processed.

The lymphatic system of the intestine plays an important role in the immune system and in the transport of fats:

• Lymph nodes along the intestine filter pathogens.


• Fats are transported into the bloodstream via the lymphatic vessels (chyle vessels).

The intestine has its own nervous system, the so-called enteric
Nervous system (ENS) , often referred to as the “gut brain”:

• It autonomously controls intestinal movement and digestive processes.
• Is closely connected to the central nervous system via the gut-brain axis .

The anatomy of the intestine is impressively complex and finely tuned to fulfill the functions of digestion, nutrient absorption, and immune defense. The division into the small and large intestine, as well as the microscopic adaptations (e.g., intestinal villi), illustrate how efficiently the intestine functions as the central organ of the digestive system.