The Digestive System
Starting with the skeletal system, we have been steadily building our human body. Once we had the support frame, we added the muscular system, then the circulatory system, nervous system (brain, spinal cord, peripheral), and respiratory system. One thing that all these systems need is a steady supply of nutrients and energy. To supply those needs, we turn to the digestive system.
The main function of the digestive system is to provide energy and nutrients to the entire body. It accomplishes that by taking in food and liquids and breaking them down into its hundreds of compounds, vitamins and minerals. These are then absorbed into the blood stream throughout some of the digestive organs.
The digestive system is composed of the mouth, pharynx, esophagus, stomach, pancreas, liver, gall bladder, small intestine and large intestine.
I. Mouth – The mouth, also known as the buccal cavity is the entrance point for food and drink to enter into the digestive tract. It is a complex structure composed of the upper and lower mandibles, a host of muscles and nerves, along with the teeth, tongue and salivary glands.
A. Teeth – Normally humans have 32 teeth (8 incisors, 4 canines, 8 premolars and 12 molars).
1. Incisors – Consist of the 4 teeth in the front of the mouth. They are chisel shaped and designed for biting and tearing food away from its source.
2. Canines – Consists of the tooth on each side of the incisors. They are also designed for biting and tearing food away from its source.
3. Premolars – Consists of the two teeth between the incisors and molars. They are not sharp, but not exactly flat either. They have small ridges called cusps. Premolars are designed for chewing and grinding of food particles into smaller pieces before swallowing.
4. Molars – Consists of the last 3 teeth in each side of the jaw. Like the premolars, molars have cusps and are designed for chewing and grinding.
B. Tongue – The tongue is a unique muscle as it only has one end attached where most of the muscles in the body attach at each end. The tongue is a powerful muscle that is used as part of the digestive system as well speech and sensory functions. The parts of the tongue include the papillae, sulcus terminalis, tonsils, adenoids, and the frenulum linguae.
1. Papillae – Papillae are the small bumps on the tongue that most people refer to as the taste buds. There are about 3,000 taste buds on the tongue. The taste buds at the tip and front of the tongue are used to detect salt and sweet tastes. Those on the side of the tongue detect sour tastes and those on the back of the tongue detect bitter tastes. There are 4 types of papillae: filiform, fungiform, foliate, and vallate.
a. Filiform Papillae– Named filiform because they are thread-like is shape. Their main function is to hold onto food, such as when you lick an ice cream cone.
b. Fungiform Papillae – Named fungiform because they look like tiny mushrooms. Their main function is taste.
c. Foliate Papillae – Named foliate because they look like leaves. Their main function is taste.
d. Vallate Papillae – Named vallate because they are ring shaped. Their main function is also for taste.
2. Sulcus terminalis – The sulcus terminalis, also known as the terminal sulcus is a v-shaped groove on the upper surface of the tongue about two-thirds back from the tip. It is considered to the marker that separated the oral and pharyngeal parts of tongue.
3. Tonsils – Tonsils are located near the rear of the tongue and since they are considered part of the lymphatic system, they will be discussed in more detail at that time.
4. Adenoids – Adenoids are located in the upper back of the mouth and since they are considered part of the lymphatic system, they will be discussed in more detail at that time.
5. Frenulum linguae – A frenulum is a structure in the body that helps to restrict the movement of another structure. In the case of the tongue, the frenulum linguae is the flap of tissue located in the middle of the rear underside of the tongue that extends part way along undersurface of the tongue. It serves to restrict movement of the tongue.
C. Salivary Glands – The salivary glands secrete saliva that helps to start the digestion of the food and to help lubricate the food particles to make them easier to swallow. There are 3 major pairs of salivary glands: parotid, submaxillary, sublingual.
1. Parotid Salivary Glands – The parotid glands are the located at the sides of the back of the mouth. They are the largest of the salivary glands. They produce a watery serum part of the saliva.
2. Submaxillary Glands – Also known as the mandibular glands, the submaxillary glands are located in the rear floor of the mouth. The submaxillary glands are less than half the size of the parotid glands and produce a mixture of a serum and mucous saliva which accounts for nearly 70% of the total saliva produced by all three glands combined.
3. Sublingual Glands – The sublingual glands are located in the floor of the mouth under the tongue. They secrete saliva that is predominantly mucous based.
II. Pharynx – The role of the pharynx in digestion is to direct food towards the stomach and away from the lungs.
III. Esophagus – The primary function of the esophagus is the transportation of food and liquids from the mouth to the stomach. The walls of the esophagus contain muscles that rhythmically contract in a pattern to help push food downward.
IV. Stomach – The stomach is located just below the diaphragm just left of center. There is a sphincter muscle where the esophagus connects to the stomach. When food is being passed down the esophagus, the sphincter opens to allow the food to pass through and then squeezes closed to force the food into the stomach. Once in the stomach, the sphincter keeps any food and liquid from passing back up into the esophagus. Now the stomach begins to contract and churn the food as it secretes enzymes, gastric juices and hydrochloric acid to breakdown the food. A unique design feature of the stomach is the inner wall of the stomach lining. It secretes mucous and peritoneal fluid that protects the stomach lining from being eaten away by the digestives juices and acid. When this system fails, often due to a bacterium known as H. pylori, ulcers form in stomach lining.
V. Pancreas – The pancreas is a small gland located just below the stomach. The pancreas produces insulin, glucagon and hormones that are used by the stomach to help digest carbohydrates and fats. The insulin also functions to maintain the blood’s glucose levels. When the pancreas’s production of insulin becomes compromised, it can lead to diabetes.
VI. Liver – Many people are surprised to learn that the liver is actually a very large gland. It is located just under the diaphragm on the right side of the body. The liver produces bile which helps in the breakdown of fatty foods. It also produces most of the body’s cholesterol. Among the other functions of the liver is the production of amino acids, conversion of glucose to glycogen, the regulation of glucose in the blood, the production of urea, filter harmful substances from the blood and the storage of vitamins A, B12, D and K.
VII. Gall Bladder – The gall bladder is a small organ located along the edge of the liver. It stores and concentrates the bile produced by the liver.
VIII. Small Intestine – The small intestine connects the stomach to the large intestine and measures approximately 16 to 19 feet in length. It is only referred to as the small intestine because it is smaller in diameter than the large intestine. The main absorption of food into the blood stream takes place in the small intestine. The small intestine is divided into three sections: duodenum, jejunum and ileum.
A. Duodenum – The duodenum is the first and shortest of the three sections of the small intestine. I measures approximately 12 to 16 inches in length. The duodenum regulates the emptying of food from the stomach. The walls of the duodenum contain glands that secrete hormones, enzymes and a mucous that aids in the further digestion of food and passage into the jejunum.
B. Jejunum – The jejunum is about 8 to 10 feet long. The jejunum uses bile from the bile duct to in raise the pH level of the contents to a neutral or slightly alkaline lever. At this pH, the enzymes in the jejunum work more optimally in digestion of food.
C. Ileum – The ileum connects the rest of the small intestine to the large intestine and is the longest section of the small intestine. Whatever nutrients are still remaining in the food are now absorbed by the ileum along with bile salts and vitamin B12.
IX. Large Intestine – The large intestine connects to the small intestine and ends at the anal opening. The main function of the large intestine is the absorption of water and removal of solid waste from the body. The large intestine is divided into 6 sections: caecum, ascending, colon, transverse colon, descending colon, sigmoid colon and the rectum.
A. Caecum – The caecum is a pouch shaped structure that connects the ileum to the large intestine. It contains the Bauhin or ileocecal valve that helps to regulate the amount of food passing into the large intestine and to prevent any food from passing back into the small intestine. The absorption of fluids and salts begin here.
B. Ascending Colon – The ascending colon runs from the caecum upward to where it bends, forming the transverse colon. In addition to the absorption of fluids, the ascending colon absorbs additionally nutrients including potassium and vitamin K.
C. Transverse Colon – The transverse colon is so named because it runs transverse (across) the abdomen to where it bends downward becoming the descending colon. The transverse colon continues to extract fluids, vitamins and nutrients from the food.
D. Descending Colon – The descending colon, located on the left side of the abdomen, runs down and empties into the sigmoid colon. The main function of the descending colon is the storage of solid waste before it passes out of the body.
E. Sigmoid Colon – The sigmoid colon is so named because of its S-shaped structure. It is lined with powerful muscles used to push the solid waste to the rectum and out of the body.
F. Rectum – The rectum is the last section of the large intestine and used as a storage area of the solid waste material before expulsion. The posterior end of the rectum has a sphincter muscle used to control when the waste material is to be expelled from the body.
Digestion starts with the mouth and ends with the rectum. The entire process can take up to 48 to 72 hours under normal conditions.
We think little of the complexity of the digestive process when we sink our teeth into a juicy steak and baked potato. If everything is working the way our Creator God designed it work, there is little need to think about it until it becomes time eliminate what is left from the same steak and potato.
The intricacies of the digestive system are so exquisite that the probability of it all having evolved together is virtually zero. On the other hand,
For you formed my inward parts;
you knitted me together in my mother’s womb.
I praise you, for I am fearfully and wonderfully made.
Wonderful are your works;
my soul knows it very well.
My frame was not hidden from you,
when I was being made in secret,
intricately woven in the depths of the earth. Psalm 139:13-15
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