The Wonderful World of Digestion


Components of the Digestive System

  • The mouth (tongue for moving food, teeth for grinding, salivary glands for moistening the passage of solids and digesting carbs)
  • Pharynx (directs food to the esophagus)
  • Epiglottis (to protect the windpipe from food entering)
  • Esophageal sphincters (doorways for protecting the upward travel of foods)
  • Esophagus (tunnel for food to travel down past the diaphragm)
  • Stomach (adds acids and enzymes in order to turn the mass of food into a semiliquid)
  • Pyloric sphincter(Allows passage from the stomach to the small intestine and prevents backflow from the small intestine)
  • Liver (Manufactures bile salts, detergent-like substances, to help digest fats)
  • Gallbladder (Stores bile until it’s needed)
  • Appendix( Stores lymph cells)
  • Small intestine (secretes enzymes that digest all energy-yielding nutrients to smaller nutrient particles; cells of wall absorb nutrients into blood and lymph)
  • Pancreas (Manufactures enzymes to digest carbs, fats, and proteins and releases bicarbonate to neutralize acidic chyme that enters the small intestine)
  • Large intestine (Reabsorbs water and minerals and passes waste along with water to the rectum)
  • Rectum (Stores waste before elimination)
  • Anus (Holds rectum closed and opens to allow elimination)

How does the Digestive System move food?

First, foods go into the mouth. There, they are chewed by the teeth and carbs are broken down by salivary glands. The tongue moves the foods to the back of the throat where the pharynx moves the food down the throat. The epiglottis protects the airway from the incoming food which at this point is now called a ‘bolus’. The bolus goes down into the esophagus after passing through the esophageal sphincter. It then travels down the esophagus, directed by gravity.

After this, the transport goes through the diaphragm to reach the stomach. Before reaching the stomach, the bolus has to go through the lower esophageal sphincter. When it comes into the stomach, which has a thick mucus lining in order to keep the cell walls from being damaged by the stomach acid, the bolus transforms into a semiliquid. This happens by being dissolved in a concoction of stomach acids and enzymes. The gastric juices digest proteins. The fats layer on top of these reactions while the carbohydrate digestion is discontinued since the enzymes from the salivary glands are deactivated. When the mass has been thoroughly turned into a semiliquid, the pyloric sphincter allows passage into the small intestine.

The food mass now changes names from bolus to chyme. The chyme is moved through the intestines, and stomach, by way of a specialized system of muscles. There are circular muscles that retract and longitudinal muscles that pinch the food downward. The chyme then bypasses the opening from the common bile duct. This duct drips fluids into the small intestine from the gallbladder and the pancreas. The chyme travels through the small intestine’s three main parts- the duodenum, the jejunum, and the ileum. while making this passage, the small intestine secretes enzymes that act on the energy-containing nutrients, the fats, carbohydrates, and proteins.

The pancreas contributes sodium bicarbonate which is alkaline- the opposite of the stomach acid. This combines with the chyme to make the pH neutral or slightly alkaline. Bile from the gallbladder goes down into the small intestine and acts as an emulsifier that brings fats into suspension in water. The fats are then broken down by enzymes in the aqueous solution.

The remaining chyme has little nutrient value, it is made up of water, undigested fibers, dissolved salts, and bodily secretions. Next, the chyme enters the large intestine, also called the colon. In the colon, the intestinal bacteria ferment some of the fiber. This produces gas and small fragments of fat that provide energy for the cells in the colon. The remaining materials that the body can recycle, like water and dissolved salts, are retrieved. the final product is held in the rectum until the anal sphincter opens and releases the waste.

Cellular Level

The walls of the intestines are wrinkled with an uncountable number of tiny hair-like villi. These villi are covered with two types of cells- the goblet cells that secrete mucus, and the cells with microvilli. The cells with microvilli are where the absorption of nutrients takes place. Each of these microvilli is covered with receptor proteins in a phospholipid bilayer (made of polar lipids with tails facing inside, making up the cell’s surface). The receptor proteins can function through one of three pathways; simple diffusion, active transport, and facilitated diffusion. These are the ways that ligands, in this case, the energy-containing nutrients, minerals, and vitamins, get into the cell. Through simple diffusion, the nutrients simply pass through the cell’s surface via protein channels. In active transport, the nutrients have to act as ligands that attach to transport proteins that use ATP for energy to be moved through the bilayer. Lastly, in facilitated diffusion, nutrients move through the phospholipid bilayer by attaching to the transport proteins and move along the surface.

The image below has more enzymes in the cell’s membrane then is necessary for understanding this concept, but it’s a great representation of what the bilayer consists of. The inside of the intestines is the extracellular fluid, while the large blue proteins act as different transport proteins.

Nutrients and the Circulatory System

Blood is taken to the digestive system through arteries. These arteries branch out into capillaries to reach every cell of the digestive system. These capillaries collect the nutrients that were gathered by the cell during absorption. This nutrient filled blood leaves the GI tract through veins. The veins then go to the liver through the hepatic portal vein where the blood circulates through fine capillaries throughout the liver while being filtered. The blood then collects into the hepatic vein to return to the heart.

Water-soluble nutrients enter the bloodstream directly. These are carbohydrates, some vitamins, minerals, and proteins. These nutrients go through the liver for filtration via the hepatic portal vein.


Nutrients and the Lymphatic System

The lymphatic system provides a route for fluids from tissues to enter the blood. Though it has no pump, the lymphatic system collects fluids from the cells and compartmentalizes them into vessels. The lymphatic system mainly moves fats and fat-soluble nutrients in a fluid solution. These fluids move by way of pressure built up while muscles near the area contract. The vessels collect behind the heart in the thoracic duct. The nutrients collected from the digestive system through the lymphatic system are ultimately added to the circulatory system (the blood). The biggest difference between these nutrients and the nutrients carried first by the blood is that they do not enter the liver to be filtered before entering the bloodstream.

Nerves and Hormones

In order to maintain homeostasis, the GI tract needs the means to control the functions of digestion and absorption. It does so by using nerves and hormones.

The endocrine and nervous system starts to react to food before it enters the body, when we smell foods it can trigger responses. When food enters the stomach, it stimulates cells to release the hormone gastrin that makes the stomach glands to secrete hydrochloric acid. This helps the stomach to achieve a level of acidity that digests food particles. hormones are also used to control the opening and closing of sphincter openings. The pyloric sphincter at the entrance to the small intestine is triggered by the pH level of the chyme (digested food substance) in order to open. The walls of the small intestine’s duodenum signals to the pancreas when chyme is present. This sends a signal to the pancreas to release basic pH bicarbonate-rich juices to help neutralize the pH of the chyme. These are just a few examples.

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