Your Digestive System
The story we’re about to tell is of stormy seas, acid rains, and dry, desert-like conditions. It’s an arduous journey that traverses long distances and can take several days. It’s one in which nothing comes through unchanged. It’s the story of your digestive system whose purpose is turn the food you eat into something useful — for your body!
Down the Hatch
It all starts with that first bite of pizza. Your teeth tear off that big piece of crust. Your saliva glands start spewing out spit like fountains. Your molars grind your pizza crust, pepperoni, and cheese into a big wet ball. Chemicals in your saliva start chemical reactions. Seemingly like magic, starch in your pizza crust begins to turn to sugar! A couple of more chews and, then, your tongue pushes the ball of chewed food to the back of your throat. A trap door opens, and there it goes, down your gullet!
Next, your muscles squeeze the wet mass of food down, down, down a tube, or oesophagus, the way you would squeeze a tube of toothpaste. It’s not something you tell your muscles to do — they just do it — in a muscle action called peristalsis. Then, the valve to the stomach opens and pizza mush lands in your stomach!
Inside your stomach
Imagine being inside a big pink muscular bag — sloshing back and forth in a sea of half-digested mush and being mixed with digestive chemicals. Acid rains down from the pink walls which drip with mucus to keep them from being eroded.
Sound a little like an amusement ride gone crazy? Every time you think you’ve got your equilibrium back, the walls of muscle contract and fold in on themselves again. Over and over again, you get crushed under another wave of slop. Every wave mixes and churns the food and chemicals together more–breaking the food into even smaller and smaller bits. Then another valve opens. Is the end in sight you ask, as the slop gets pushed into the small intestine.
Inside the small intestine, chemicals and liquids from places like your kidneys and pancreas break down and mix up the leftovers. The small intestine looks like a strange underwater world filled with things that resemble small finger-like cactus. But they’re not cactus, they’re villi. Like sponges, they’re able to absorb tremendous amounts of nutrients from the food you eat. From the villi, the nutrients will flow into your bloodstream.
But hold on! The story’s still not over yet — the leftovers that your body can’t use still have more traveling to do! Next, they’re pushed into the large intestine. It’s much wider and much drier. You find that the leftovers getting smaller, harder and drier as they’re pushed through the tube. After all, this is the place where water is extracted and recycled back into your body. In fact, the leftovers that leave your body are about 1/3 the size of what first arrived in your intestines!
Where Food Turns Into Poop
Finally, the end of the large intestine is in sight! Now the drier leftovers are various handsome shades of brown. They sit, at the end of their journey, waiting for you to expel them — out your anus. Of course, you know the rest! A glorious,if slightly stinky, journey, don’t you think?
• An adult’s intestines are at least 25 feet. Be glad you’re not a full-grown horse … their coiled-up intestines are 89 feet long!
• Chewing food takes from 5-30 seconds
• Swallowing takes about 10 seconds
• Food sloshing in the stomach can last 3-4 hours
• It takes 3 hours for food to move through the intestine
• Food drying up and hanging out in the large intestine can last 18 hours to 2 days!
• In your lifetime, your digestive system may handle about 50 tons!!
Digestive System – Accessory Organs
There are three pairs of salivary glands that secret saliva into the oral cavity:
1. Parotid salivary glands – located anterior to the outer ear. These glands produce secretions that empty by way of the parotid duct into the vestibule near the second upper molar.
2. Sublingual salivary glands – are under the floor of the mouth and are drained by numerous sublingual ducts.
3. Submandibular salivary glands – located on the medial side of the mandible under the mylohyoid line. Submandibular ducts drain secretions through an opening on either side of the lingual frenulum.
The saliva contains salivary amylase which begins digestion of complex carbohydrates and mucins which are glycoproteins that enhance the lubricating qualities of saliva. Saliva also helps to control oral bacterial populations.
Mastication, or chewing, is performed by the teeth.
The bulk of the tooth is formed by a bony substance called dentin. Cytoplasmic processes extend into the dentin from cells in the pulp cavity. Highly vascular connective tissue within the pulp cavity receives blood and sensation through blood vessels and nerves that enter the root at the apical foramen and travel through the root canal.
The tooth is anchored to the bony socket of the alveolar process by collagen fibers of the periodontal ligament. A bony substance called cementum covers the dentin of the root and the fibers of the periodontal ligament are anchored in cementum.
The crown is the visible portion of the tooth above the gingivae. The dentin of the crown is covered by enamel, the hardest material in the body. The neck is the boundary between the crown and the root.
The liver is the largest visceral organ and has more than 200 different functions that fall in one of three categories:
1. Metabolic regulation – for example, regulation of circulating levels of carbohydrates, lipids and amino acids.
2. Hematological regulation – liver cells synthesize plasma proteins and phagocytic cells remove old or damaged red blood cells.
3. Synthesis and secretion of bile – bile helps neutralize acidic chyme from the stomach and enables digestion of lipids in the small intestines.
Blood supply to the liver
The liver has two sources of blood: hepatic artery proper which delivers oxygenated blood and hepatic portal vein which delivers blood containing nutrients from the intestines. The stomach, spleen, pancreas and large intestines also drain blood into the hepatic portal vein. The hepatic veins drain blood from the liver and delivers it to the inferior vena cava.
The right and left ducts collect the bile secreted by their respective liver lobes. These ducts combine to form the common hepatic duct. The common hepatic duct fuses with the cystic duct to form the common bile duct.
The gall bladder is a hollow pear-shaped, muscular organ that stores and concentrates bile. Between meals, bile secreted by the liver enters the gall bladder through the cystic duct. Under the stimulation that occurs during a meal, bile is ejected from the gall bladder into the cystic duct which fuses with the common hepatic duct to form the common bile duct which opens into the duodenum at the duodenal papilla.
The pancreas is primarily an exocrine organ producing digestive enzymes and buffers and is secondarily an endocrine organ. The pancreatic exocrine secretions are delivered to the duodenum by a large pancreatic duct which joins the common bile duct at the duodenal ampulla. A small accessory pancreatic duct may branch from it and empty its secretion separately at the lesser duodenal papilla.