HBS Digestive System Function Lecture Notes
The Oral Cavity, Pharynx, and Initial Digestion
The oral cavity serves as the primary entrance to the digestive system and contains several essential structures, including the salivary glands, the tongue, the teeth, the uvula, and the pharynx. Mechanical digestion is carried out by the teeth and tongue, which physically break down food. Chemical digestion begins simultaneously through the action of enzymes secreted by the salivary glands. Saliva is produced to help break down food as soon as it is placed in the mouth. The tongue plays a specific role in shaping the food to ensure it can be swallowed more easily.
A bolus is defined as the food that has been chewed and formed into a ball for transit. The oral cavity is also defined by the hard and soft palates. The hard palate is responsible for shaping food, while the soft palate is involved in the processes of swallowing, breathing, and speech. To ensure that food does not travel into the windpipe (go down the wrong tube), the epiglottis serves as a mechanism to block the entry to the trachea.
Physiology of the Esophagus and Stomach
Peristaltic movement is a muscular movement designed to make food go down through the digestive tract. In the esophagus, this movement functions to transport the bolus to the stomach. No actual digestion occurs within the esophagus itself; it is strictly defined as a pathway for food. Once food enters the stomach, the primary functions involve breaking down the ball of food into smaller components through both mechanical and chemical means. Mechanical digestion occurs as the stomach contracts to break down the food ball, while chemical digestion uses enzymes and acid to further decompose the material.
Chyme is the semi-liquid, highly acidic mixture of digested food and gastric juice that is created in the stomach. The stomach also plays a critical role in decontaminating incoming food matter by using acid to kill bacteria and break down food, thereby protecting the body. Specific cells within the stomach are responsible for forming enzymes and acids that break down proteins while also killing harmful substances. To prevent the acidic gastric juice from digesting the stomach itself, the organ is lined with a thick layer of mucus, and the cells of the stomach renew themselves frequently. Sphincters are valves that regulate the flow at the entries and exits of the stomach, preventing fluid from getting into the wrong areas.
Functions of the Pancreas, Liver, and Gallbladder
The pancreas is an organ measuring approximately in length, located behind the stomach between the small and large intestines. It performs dual functions: it produces digestive enzymes and regulates blood sugar by releasing insulin. It connects to the rest of the digestive system via the pancreatic duct, through which it delivers enzymes to mix with food. The specific enzymes produced by the pancreas include amylase (for breaking down carbohydrates), lipase (for digesting fat), and trypsin (for breaking down proteins). Insulin produced by the pancreas is related to the digestive system because it helps move sugar into the blood.
The liver measures approximately and is located in the upper right abdomen under the ribcage. In relation to digestion, the liver makes bile to break down fat and processes nutrients while filtering toxins out of the blood. Other functions of the liver include forming energy, producing proteins, and regulating cholesterol. There is a direct relationship between the liver and gallbladder: the liver produces bile, and the gallbladder stores and releases that bile. Bile enters the digestive tract at the duodenum, where it breaks down fat.
Processes of the Small and Large Intestines
The small intestine consists of three sections: the duodenum, the jejunum, and the ileum. These sections play roles in chemical digestion and nutrient absorption. The pH within the small intestine is maintained at a level of . This pH is regulated by bicarbonate delivered from the pancreas, which neutralizes the highly acidic chyme arriving from the stomach. Bile and pancreatic enzymes enter the small intestine at the duodenum via the common bile duct and the pancreatic duct. Food moves through the intestines via muscle contractions, and enzymes such as amylase, lipase, and proteases act within the small intestine to break down carbohydrates, fats, and proteins so the body can absorb nutrients.
The large intestine is comprised of the cecum, the colon, and the rectum. Its primary functions in relation to digestion are to receive water, absorb electrolytes, and form and eliminate waste. The large intestine is vital for maintaining water balance in the body; it absorbs water to keep the individual hydrated, preventing dehydration and keeping the overall fluid balance in check. The material remains in the large intestine for water absorption until it eventually leaves the body as waste.