Digestion may seem like a simple process, but little do you it is complicated and involves many different organs and steps. Digestion doesn't just start and end in the stomach. There are a list of organs that are involved in the digestion process. Not only is there digestion, but it also involves the process of metabolism. The digestive system includes many organs and the accessory alimentary organs that are related to digestion. The digestive organs that are the most obvious are the mouth, the pharynx, the esophagus, the stomach, the small intestine, and the large intestine. The accessory organs include the salivary glands, the teeth, the pancreas, the liver, and the gallbladder. All the organs listed play crucial parts in the digestion process.

The Mouth
This is where the first stages of digestion take place. The food placed in the mouth undergoes a process called mastication. The teeth are the main part of mastication. They chew the food breaking down all the small pieces while the tongue mixes the chew food around. The salivary glands secrete saliva, which binds the food together forming a bolus. When the bolus is ready to be further digested, it is pushed through the process of swallowing. The first steps of swallowing are the voluntary and involuntary swallowing. The first stage of swallowing in the voluntary buccal stage. The buccal stage takes place when the food had been thoroughly mixed with saliva and has formed a bolus. The bolus is pushed into the pharynx by the tongue and that is the voluntary part. The involuntary part of swallowing is called the pharyngeal-esophageal phase. In this phase the bolus is forced down the esophagus by peristalsis which is a contraction and relaxation of muscles. When the bolus reaches the end of the esophagus, it enters the stomach through the cardioesophageal sphincter.

The Esophagus
The esophagus is mad up of four basic layers; the mucosa, the submucosa, the muscularis externa, and the serosa. The esophagus is a tube that runs from the pharynx to the stomach. In the esophagus food propulsion happens through peristalsis. Peristaltic contractions force the bolus down the esophagus to the cardioesophageal sphincter.

The Stomach
The stomach is basically like a storage tank for the body. Many things happen in the stomach such as chemical and mechanical digestion and absorption. Food enters the stomach through the cardioesophageal sphincter. The bolus is now going to be chemically and mechanically digested. HCl (hydrochloric acid), pepsin, and gastric juices begin to chemically digest the proteins in the food. The stomach contracts and mixes the food in a process called segmentation until it forms a thick soupy liquid called chyme. The chyme then moves in small spurts through the pyloric sphincter into the duodenum of the small intestine.


(Watch the video on digestion up to the stomach)

The Small Intestine
The small intestine is where most of the body's digestive process takes place. The small intestine is split into three parts, the duodenum, the jejunum, and the ileum. The small intestine processes and absorbs many nutrients from the food that it takes in. Since it can only process small amounts at a time, the pyloric sphincter controls the amount of chyme that goes into the small intestine and when it goes. So much digestion happens in the small intestine because of all the ducts that are connected to it that let in chemicals that allow digestion to happen. Bile, pancreatic amylase, and other enzymes enter the small intestine through ducts like the bile duct, the common hepatic duct, and the accessory pancreatic duct. The small intestine is equip with features that allow for more efficient absorption such as microvilli, that increase the surface area of the small intestine, therefore increasing the amount of absorption. Also, finger-like projections called villi, that are rich with capillaries that carry absorbed nutrients to the blood stream. The food that is now left undigested travels to the large intestine through the ileocecal valve.

The Large Intestine
The main function of the large intestine include drying out indigestible food residue. the large intestine does this by absorbing water in the waste then expels the waste as feces. There are five basic subdivisions of the large intestine and they are the cecum, the appendix, the colon, the rectum, and the anal canal. The cecum starts the large intestine, it is where the ileocecal valve connects into the large intestine. The appendix hangs off the cecum. Looking at pictures of the large intestine, one can see that there are different sections of the colon. Ascending, transverse, descending, and sigmoid are the parts of the colon. The ascending colon carries the waste matter up then turns in the hepatic flexure and goes across the abdomen in the transverse colon. The waste then turns again in the splenic flexure and descends into the pelvis in the descending colon. When the waste reaches this point, it had entered the sigmoid colon which lies in the pelvis. After the sigmoid colon, the waste moves to the rectum, then the anal canal, and finally the anus which leads to the external anal sphincter and results in defecation.
Accessory Digestive Organs

Salivary Glands
The salivary glands are a very important part of the digestive system because they are the reason that the beginnings of starch digestion begins. There are three major salivary glands that empty their products into the mouth. The parotid glands, the submandibular glands, and the sublingual glands all contribute the saliva and to the mouth. Saliva is a clear fluid that contains mucus and serous fluids. The serous fluid contains the critical enzyme, salivary amylase, that begins starch digestion. The parotid glands are located up near the ears, the submandibular glands are located below the mandible, and the sublingual glands are under the tongue. The product of the salivary glands has many functions most importantly the salivary amylase begins the chemical break down of starch. Also the saliva chemically breaks down foods. This allows the taste receptors to taste the food that you are eating.

There are two different types of teeth that all humans have at some point in their lives. The teeth that grow in as a young child are called deciduous teeth. These are not permanent and they fall out when the adult or permanent teeth begin to grow in a push the baby teeth out. Teeth are very important in the process of mastication. They are used for cutting and breaking down the food into smaller pieces. While the process of mastication is happening, the food begins to form a bolus that will travel down the esophagus.

The pancreas produces enzymes that are key to digestion. The enzymes are secreted into the small intestine in a base fluid that will counter act the acidic chyme.

The liver is the largest gland in the body and it produces bile. Bile is needed for the emulsification of fat. The bile gets transported to the small intestine through the bile duct.

The gallbladders only purpose is the storage of bile. After the bile is made it goes through the cystic duct to the gallbladder. While the bile sits in the gallbladder the water is removed and the bile is concentrated.

Nutrition and Metabolism

The reason that we have to eat food is because of the ATP that the body produces when it breaks down the food. ATP is the energy source that fuels all bodily functions. The body needs nutrients to function properly. The way we get most nutrients is through food sources. The main sources of nutrients are carbohydrates, lipids, proteins, vitamins, and minerals.

Carbs are basically all the starches and sugars that we consume. Carbohydrates can be found in fruits, vegetables, bread, pasta, rice, and other grains.

Lipids are cholesterol. Lipids is also the category that fats fall under. They are classified in two different ways, unsaturated fats are in things like seeds and nuts, saturated fats are in things like meat and dairy products.

Proteins are built from amino acids, and there are essential amino acids that are needed to sustain life. Complete proteins like most meat products, eggs and milk are complete proteins. When a person is a vegetarian, they do not get all the essential amino acids from their foods. When this is the case, they must find alternate sources for protein.

Vitamins are required in small amounts for the body. Vitamins are no found completely in any food groups. Every food group must be consumed in order for the body to obtain a healthy balance of vitamins.

Minerals are inorganic substances, unlike vitamins. The body requires seven minerals to stay at top performance. Minerals can be found in foods such as milk, legumes, some meats, and mainly vegetables.

Metabolism- general term that describes any chemical process that is needed to maintain life.
There are two different kinds of metabolism, and they do two different things for the body. Catabolism breaks down complex molecules into simple ones and anabolism builds up small molecules into complex ones. In both types of metabolism ATP, cell fuel, is released and then captured to give energy to the cells.

Many processes take place during digestion some of them are:
Glucose is used to make ATP.
Cellular respiration occurs when any cell activity produces oxygen.
Glycolosis is the process that gives energy to glucose molecules so they will divide into pyruvic acids, a process that gives off ATP.
The Krebs cycle uses the oxygen given off during cellular respiration to make carbon dioxide and water.
ATP is produced on the electron transport chain.

Digestion is a process that must take place for life to be possible. Although the body cango periods without food, the body is still metabolizing whatever it can to keep the body running and keep producting ATP.

The Endocrine System

The endocrine system is unlike any other system in the body. Although it is extrememly important, it is not large in area like other systems. The endocrine organs are spread out thoroughout the body. The endocrine organs are considered some of the most important in the body.

Hormone Chemistry
Hormones are very complex substances. They can be defined as chemical substances that are produced and secreted into the extracellular fluid of cells. They also regulate the other metabolic processes of cells in the body. Hormones are classified as either amino acid based or steriod based. The two different types of hormones are built of different substances. Amino-acid based are made up of proteins and peptides. Steroid based are made up of cholesterol. There are also local hormones known as prostagladins.

Mechanisms of Hormone Action
Hormones operate throughout the body with what is called target cells and targe organs. This means that in order for specific hormones to affect specific organs, there must be a receptor on the membrane of the cells. After the hormones bind with the cell, that is when they become effective. Hormones basically increaase or decrease the metabolic processes tht take place in the body. When a hormone binds with the target cell there are a few things that can happen: 1. the plasma membrane changes in permeability or electrical state. 2. Protein synthesis and certain regulatory molecules in the cells. 3. Activation or inactivation of enzymes. 4. Mitosis stimulation. In the case of amino acid hormones, they cannot enter the target cells that they are affecting, so instead they bind to the receptor and then use a second messenger system using different steps to utalize the hormone. In this process, catalysts are used to stimulate the second messenger molecule such as cAMP to activate the hormone in the target cell.

Control of Hormone Release
Negative feedback is the cheif means of regulating hormone levels in the blood. Negative feedback uses sensors to know when a certain hormone concentration is too high, and in turn sets off the release of a different hormone to regulate the level until the body has reached homeostasis.
There are three differnt types of stimuli thatactivate endocrine organs, they are hormonal, humoral, and neural. A hormonal stimulus is when an endorine organ is prodded into action by another endocrine organ releasing a hormone. Humoral stumili is when a hormone is release due to changing levels of ions or nutrients in the blood. A neural stimulus is when nerve fibers stimulate a hormone release.

Major Endocrine Organs
There are a handful of small organs that make up the endocrine system. These organs are the pituitary, the thyroid, the parathyroid, the adrenal, the penial, and the thymus glands. Also the pancreas and the gonads which are the ovaries and the testes. Also the hypothalmus in the brain is considered an endocrine organ. Endocrine glands are ductless, meaning that they secrete their hormones directy into the blood.

Pituitary Gland
The pituiary gland is located in the brain, hanging, snuggled by Turk's Saddle of the sphenoid bone. There are two parts of the pituitary gland, the anterior and the posterior pituitary.

The Anterior Pituitary
The anterior pituitary is full of hormones that are called tropic hormones. Tropic hormones the the hormones that are secreted and that causes their target organs that they stimulate to release their hormones, triggering a full body reaction. Some hormones that the the anterior pituitary are Growth Hormone, Prolactin, Adrenocorticotropic hormone, Thyroid-stimulating hormone, ganodotropic hormones, follicle-stimulationg hormone, Luteinzing hormone and interstitial cell-stimulating hormone. The anterior pitutary and the hypothalamus have a special relationship, unlike any other in the body. Although the A.P. secretes many different hormones, the hypothalmus secretes the hormones that control when the A.P. will secrete it's hormones. These hormones are called releasing and inhibiting hormones. The hypothalamus is also where ADH is made.

The Posterior Pituitary
The posterior pituitary is not considered an endocrine organ because it doesn't release the hormones it makes, it just stores the hormones made by the hypothalamic neurons. There are two hormones that the posterior pituitary secretes. Oxytocin and ADH are secreted by the posterior pituitary.

Thyroid Gland
The thyroid gland is one easily recognized and easily located. It's located at the base of the neck. The thyroid gland produces and secretes two differen hormones. Thyroid hormone which is the body's major metabolic hormone, which is actually two different hormones (thyroxine and triiodothyronine) and calcitonin which reduces calcium levels in the blood by creating calcium deposits in the bones.

Parathyroid Glands
These glands are small collections of gland tissues that are normally found on the posterior surface of the thyroid gland. The parathyroid gland secretes PTH which is the most important hormone in the regulation of calcuim ions in the blood keeping homeostasis.

Adrenal Glands
The adrenal glands are located onthe superior surface of the kidneys. They look like little hats sitting ontop of the kidneys. The adrenal glands have two different parts, much like the pituitary gland. The adrenal cortex and the adrenal medulla are the two seperate parts of the adrenal glands.

The Adrenal Cortex
The Adrenal cortex produces and secretes steroid hormones called corticosteriods. There are three differen groups of corticosteriods that the adrenal cortex secretes and they are mineralocorticoids, glucocorticoids, and sex hormones. Of the mineralcorticoids, aldosterone is one of the most important. It regulates the soduim and potassuim ion levels in the blood. Cortisone and cortisol are the major glucocorticoids. The hormones of this category help keep up normal cell metabolism and help to resist long term stressors. Glucocorticoids have anit-inflammitory properties and also seem to inhibit pain. The sex hormones that are produced there are androgens and some estrogens.

The Adrenal Medulla
The adrenal medulla releases two hormones when it is stimulated by sympathetic hormones. The two hormones are very similar but do different things. Epinephrine (adrenaline) and norepinphrine (noradrenaline). Because of the secretion of these hormones, the fight or flight response happens when the body is introduced to a stressor.

Pancreatic Islets
The pancreatic islets are one of the few mixed glands in the body. The exocrine parts of the islets are primarily digestive system related but the endocrine parts produce insulin and glucagon. Insulin is released from beta cells in the pancreatic islets.

Pineal Gland
The only hormone that is known to be secreted by the pineal gland is substancial amounts is melatonin. Melatonin is a hormone that is linked with sleep and feeling drowsy.

Thymus Gland
The thymus gland is one that is only truly useful and needed in infancy and childhood. The thymus gland houses a special group of white blood cells called T cells when a person is young, and slowly shrinks in size and is made up of connective tissue and fat when adulthood is reached.

The Gonads
Hormones of the Ovaries
The ovaries are loaced in the pelvic cavity of females. There are two almond sized organs. Not only do the ovaries produce the female reporductive cells (eggs) but they also produce two different groups of steroid hormones. Progesterone and estrogens, the estrogens mainly estrone and estradiol that stimulate the development of the secondary sex characteristics in females. What this does is promote the growth and maruration of perpoductive organs. Also the estrogens and the progesterone work together to stimulate the menstural cycle, which readies the uterus to care for a fertilized egg. During pregnany, both estrogen and progesterone play important roles. Estrogen helps to maintain the pregnancy and prepare the breasts to produce milk. Although the ovaries were the site of the estrogen production before pregnancy, it is now the placenta that is responsible for producing estrogen. Progesterone helps to quiet the menstral cycle so the embryo implanted on the wall of the uterus won't be aborted. Also, progesterone also helps prepare the breast tissue for lactation. The ovaries are stimulated to release their hormones by gonadotropichormones of the anterior pituitary gland.
Hormones of the Testes
Not unlike in females, males also have two hormone producing sex hormonees, the testes. The testes are located in the scrotum outside of the pelvic cavity.. Not only do the testes produce androgens, but they also produce sperm. the androgens, mainly testosterone, stimulates the deveolpment of adult male sex characteristics. Testosterone is needed for the maturation of males.

The Skeletal System

The bones are by far the most important structures in the body. Without them, we would not stand or walk and all of our organs would not have the protection they needed. Bones serve five major functions. First they are used as support. They are the frame work for the body. Next, protection. Bones create safe places for organs that need solid protection such as the skull for the brain and the ribs for the lungs. Without the bones there would be no movement in the body. Skeletal muscle connects to the bones by tendons and without that there is no possible way of moving. Also the bones are used for storage. There are spaces within the bones in which fat is stored. The bones also serve has storage places for minerals such as calcium and phosphorus. Lastly, the bones are a place for blood cell formation. Hematopoiesis occurs in the marrow cavities of certain bones, usually long bones.

Classification of Bones
There are four different categories that bones can fall into, and there are two different types of bones in the body. The first is compact bones that is made of dense material and it looks very smooth. The other is spongy bone that is composed of small needle like projections which leaves a lot of open space. The four categories of bones are as follows:
Long Bones:
These bones are typically longer than they are wide, they have a shaft, with a head at each end. Long bones are comprised of mostly compact bone.
Short Bones:
These bones are generally cube-shaped and mostly comprised of spongy bone. These bones are very small and make up the wrist and the ankle. There is a special kind of spongy bone, sesamoid bones, which form within tendons.
Flat Bones:
These bones are exactly ho0w they sound, they are thin and flat and usually have a curve to them. They are comprised of two layers of compact bone with a layer of spongy bone in between. The skull and ribs are made up of flat bones.
Irregular Bones:
These bones are the rejects of the group. If they do not fit into one of the preceding categories, they are irregular bones. The vertebrae and the pelvis are the two examples of hip bones.

Long bones have a very complex structure; they are not as simple as they look. There are many terms to describe the bones and many different microscopic parts that are involved in the anatomy of long bones. Some of the terms are:
  • Diaphysis: also means shaft, makes up the length of the bone
  • Periosteum: fibrous connective tissue membrane that covers the bone
  • Perforating Fibers: also called Sharpey's fibers; they secure the periosteum to the bone
  • Epiphyses: ends of long bones; thin layer of compact bone enclosed over spongy bone
  • Articular Cartilage: covers the end of the bones instead of the periosteum. The cartilage provides a slippery smooth surface for the bones to glide over while moving.
  • Yellow Marrow Cavity: the space within the long bones that as an adult is filled with fatty tissues. In young children, these cavities house red marrow, when blood cells are formed
  • Bone markings: projections, processes, depressions, or cavities that scar the outside surface of the bone causing them to look bumpy and appear to have holes and dents.
  • Tuberoisty: large rounded projection
  • Crest: Narrow ridge of the bone
  • Trochanter: very large blunt, irregularly shaped process
  • line: Narrow ridge of bone
  • Tubercle: small, rounded projection
  • Epicondyle: Raised area above a condyle
  • Spine: sharp, slender, often pointed projection
  • Process: any bony prominence
In young children, the skeleton is not yet made out of bone, it is made of cartilage. The process of cartilage being replaced by bone is called ossification. This process starts with a cartilage model of the bone. Then bone building cells called osteoblasts cover the model in bone matrix. The only parts of cartilage left in the body are the joints, the growth plates, and some places like the bridge of the nose, connections of the ribs, and ears.

Types of Fractures
Although bones are very hard, they still break with pressure. There are six basic types of fractures, and they are:
  • Comminuted: Bone breaks into many fragments, this fracture is more common with elderly people whose bones are more brittle.
  • Compression: The bone is crushed. This type is common in bones that are porous.
  • Depression: The portion of he broken bone is pushed inwards. This type is typical of a skull fracture
  • Impacted: The broken ends of the bone are forced into each other, and this commonly occurs when a person tries to break their fall by outstretching their arms.
  • Spiral: These fracture happen when there is an excessive twisting force applied to the bone and these are common in sports.
  • Greenstick: The bone breaks incompletely, these are common in children because of the flexibility of their bones.

Bone Remodeling
After a fracture happens, the body takes steps to heal itself. Te first thing that happens after a bone is broken is the formation of a hematoma because of the broken blood vessels that accompany a fracture.