Normal digestive physiology is essential for breaking down food into its basic components so that nutrients can enter the blood stream. Digestion begins by chewing food, which starts to get broken down by saliva. Food then goes down the esophagus into the stomach, and then to the large intestine. The liver and pancreas are the only solid organs in the digestive system, and both supply fluids to the intestine to further digest food. Sugars, amino acids, and fatty acids are absorbed through the lining of the small intestine into the blood, while all muscle contractions in the digestive system are controlled by nerves both outside and within each organ.
Digestion begins as soon as food enters the mouth. The salivary glands secrete fluid filled with enzymes, mucus, electrolytes, and water. Potassium and bicarbonates are released in the saliva ducts, which help to regulate the acid produced in the stomach. Chewing breaks down and softens food to make it easier for enzymes to work. Swallowing is aided by the tongue and the peristaltic contractions of the esophagus, which are controlled by muscle structures called sphincters.
The stomach is the component of digestive physiology where food is liquefied. It can contract and expand depending on consumption, while enzymes and muscle contractions aid digestion. The lower part of the stomach allows liquefied food to pass into the small intestine where fluids from the pancreas and liver mix in, such as bile, which serves to dissolve fat. Fluids from the pancreas break down fat, protein, and carbohydrates.
What is left in the small intestine is either absorbed into the blood or passes through as waste. Just about all nutrients are passed into the blood here, including electrolytes such as sodium, potassium, chloride, and organic molecules. What is absorbed by the small intestine travels through the blood to the liver via the portal vein, where vitamins are stored and the release of glucose into the blood is controlled. The liver also metabolizes fat and protein and is responsible for the storage and distribution of fat, so it is a crucial part of digestive physiology.
In the large intestine, any water and electrolytes that are left are absorbed. What remains in this part of digestive physiology is dehydrated, while bacteria and mucus are mixed in to form feces. Microbial organisms break down cellulose and carbohydrates, and any fatty acids and vitamin K left are absorbed and utilized for metabolism.