SKELETAL SYSTEM:
Your skeleton includes bones, cartilage, joints, and ligaments. It provides shape and protection, while allowing your body to move. Bones protect and support body organs, provide levers for muscles to pull on, store calcium and other minerals, and are the site of blood cell production. The majority of the skeleton is bone. Cartilage is white semi-opaque connective tissue. It cushions bones during movement, absorbs stress, and provides strength and flexibility. Joints are the sites where two or more bones meet. You can move in many ways thanks to joints. Ligaments hold bones together. Ligaments help your body stand upright, and help the spine from bending too far back or sharply forward.
BONES:
Your bones are alive! Throughout your life, bones are forming and modifying. Bones are organs containing nervous tissue, cartilage, connective tissue, and blood vessels, and within the blood vessels are muscle and epithelial tissue. They perform several important functions including: (1) Support; (2) Protection; (3) Movement; (4) Mineral storage; and (5) Blood cell formation. Your body has 206 named bones, which come in many sizes: long, short, flat and irregular. For instance, the thigh bone, called the femur, is nearly 2 feet long in some people and has a large ball-shaped head. On the other hand, a bone in the wrist called the pisiform bone is pea sized. A bone’s size can be a good clue to its function. The femur supports body weight and so it needs to be large and strong. The femur, although strong, is a hollow bone and provides support without adding weight.
Even though bone looks solid, throughout your life bone is formed and modified, added to and subtracted from. It is a continual process. Bone remodeling, as the process is called, involves either creation or destruction of bone, and it occurs in response to hormonal and mechanical factors. Bone remodeling is accomplished through specialized bone cells called osteoblasts (bone-forming cells) and osteoclasts (bone-destroying cells). As you get older, the bone removal starts to exceed the bone building, which can lead to a condition called osteoporosis. Exercise and diet can help reduce this risk.
Your bones do more than support, protect, move, and store. They also are, in a sense, manufacturing plants where platelets, red and white blood cells are made. This making of blood cells is called hematopoiesis. Hematopoiesis takes place in the medullary cavity of the bone. Most making of blood cells happens in your body’s flat bones (e.g., the skull, sternum, and pelvis). A look at bone marrow shows a network of tiny and somewhat messy honeycombs. Inside are special cells called stem cells. A stem cell can divide itself and produce a daughter cell. This process of cell division is called mitosis. Through mitosis, a stem cell can keep on creating more and more daughter cells. The daughter cells then differentiate into one of the different blood cells (red cells, white cells or platelets). Numerous studies have identified the lines of differentiation that a stem cell follows to become the various types of blood cells. Although we still do not understand the process well.
INTEGUMENTARY SYSTEM:
Hair, or lack of it, is as much a part of a teens persona as the clothes they wear. Although the primary function of hair is protection, it does this in a limited capacity in humans- mostly protecting against the sun’s rays and light bumps to the head. It is a specialized skin extension and part of your integumentary system. Only the lips, palms and soles of humans are truly hairless.
NERVOUS SYSTEM:
Your brain sends and receives messages from your entire body through your nervous system. Your brain and spinal cord together are known as the central nervous system. Your spinal cord is connected to a huge network of nerves, called the peripheral nervous system, that reach every part of your body. Your brain and spinal cord contain neuron cell bodies in gray matter and neural axons in white matter. All along the cord, nerve fibers leave and enter the spinal cord, conveying information back and forth between your brain and other parts of your body.
Neurons can have specialized functions, according to what kind of information is being communicated, and can be organized into three different groups based on their function: sensory, motor, and interneural. Messages are transmitted through the neurons by electrical and chemical responses to stimuli. The sending and receiving of these messages results in complex behaviors such as thinking, making conversation, experiencing emotions, seeing, moving, and remembering.
Your brain is responsible, directly or indirectly, for nearly everything your body does, such as breathing, pumping blood through your circulatory system, making decisions, and sleeping. It also processes all of the information that comes from your sensory receptors, stores information in your memory and then allows you to use that information in new ways creating new thoughts and information.
Your brain is divided into several regions, each providing different functions. However, some functions are controlled by more than one region of the brain. Even so, these regions are adaptable, such that if one region is damaged, other regions will take over its functions. At the cellular level, your brain is composed of long nerve cells called neurons.
Your brain is composed of billions of neurons, the functional units of your brain and nervous system. Neurons communicate with each other by sending and receiving messages which are triggered by electrical impulses and sent via chemicals made in your brain called neurotransmitters.
A message is transmitted in this way. The cell body sends an electrical impulse down its axon. The axon terminal at the end of the axon is stimulated by the impulse and releases a chemical neurotransmitter into a gap, called a synapse, between the axon terminal and a dendrite of the next neuron. The neurotransmitter crosses the gap, is received by the next cell’s dendrite, and travels to the cell body of that cell. If enough neurotransmitter messages are received, the receiving neuron will generate an electrical signal, which will travel down its axon and continue sending the message on to more cells. This describes the journey of only one message. Your neurons can convey and receive hundreds of messages at a time.
DIGESTIVE SYSTEM:
The digestive system is the combination of parts that allows you to eat food and convert that food into usable energy for your body. It also removes the waste products from the consumption and conversion of the food. Below is a general description of how food passes through your body. It is followed by a brief description of the esophagus, stomach, intestine, colon, liver, pancreas, and gall bladder.
THE JOURNEY OF FOOD:
When food is put into the mouth, the salivary glands start the flow of saliva. Enzymes in saliva start the process of digesting complex carbohydrates into simple sugars. Thirty-two teeth tear up the food with the assistance of jaw muscles. The tongue guides the food into a sphere called a bolus that is easy to swallow.
As you begin to swallow, the throat, or pharynx, helps transport food toward the stomach. A flap called the epiglottis covers the windpipe so that food won’t go in the wrong direction. The bolus moves down a narrow, 10-inch long tube called the esophagus. The esophagus links the throat with the stomach. Food moves down the narrow esophagus with the aid of electrical signals from the parasympathetic nervous system which causes esophageal muscles to contract. Actually they alternate between expansion (relaxing) and contraction (tightening). This action, called peristalsis, occurs not only in the esophagus but throughout the entire digestive system.
When food reaches the end of the esophagus, nerves signal a strong ring-like muscle called the cardiac sphincter to relax and open. When it does the slimy food bolus is propelled out the esophagus and into the stomach.
From the esophagus to the anal canal, the walls of the intestines are made up of four basic layers. These layers house the nerves, glands, ducts, and blood vessels (veins and arteries) that perform activities required for digestion, including secretion of mucus, digestive enzymes and hormones; absorption of the end products of digestion into the blood, and protection against infectious disease.
In the stomach food is mixed by muscle action, while nerve messages stimulate the production of gastrin, which stimulates the stomach lining cells to pour out hydrochloric acid. Sometimes just smelling, tasting, or even thinking about food is enough to start the process going. Many gastric juices, including gastrin, hydrochloric acid, pepsinogen, pepsin, rennin (an enzyme which children produce to digest milk), and mucus help break down food into the basic nutrients.
The presence of gastrin and the action of the vagus nerve signals the pancreas that digestion is occurring, and the pancreas secretes digestive juices which are delivered to the stomach to help break down the food material. About six hours after eating, the food matter, which has been churned by muscular actions and chemically transformed by gastric juices, is now a liquid called chyme. Water is the most abundant substance in chyme. Your body absorbs, though capillaries, any nutrients that are fully digested and available. Most of the food matter, though, will not be ready for absorption until it has been in the small intestine.
Absorption of nutrients resulting from the digestion of food primarily occurs in the small intestine. The small intestine is 20-some feet of coiled fleshy tubing and includes the duodenum, jujunum, and ileum. Due to its intricate design, the small intestine offers a vast surface area for the absorption of nutrients. Chyme is propelled by peristalsis through its spirals and folds.
The liver, gallbladder, and pancreas also provide digestive juices. Fats, proteins, and carbohydrates are broken down into absorbable molecules in the small intestine. Bile acids dissolve fat into the watery contents of the intestine, much like detergents dissolve grease from a frying pan. After fat is dissolved, it is digested by the enzymes from the pancreas and the lining of the intestine. Water plays an important role; approximately 9 liters of water, most of it from the digestive system’s secretions, enter the small intestine daily. Besides peristalsis, the small intestine also uses another kind of muscle contraction, segmentation, which allows the walls to contract to form separate compartments, to give the contents a thorough mixing.
After being in the duodenum, food matter not yet absorbed gets a chance in the jejunum where enzymes are produced to complete digestion. The journey for the food materials through the small intestines ends in the ileum, although it plays only a small role. Mainly the ileum absorbs nutrients that have been digested earlier in the stomach, duodenum, and jejunum. The ileum ends at the cecum, a small pouch.
Digestion comes to the end of the journey within the small intestines. Nutrients exit the walls of the small intestine and are absorbed into the bloodstream in about 12 hours. Through digestion, food is transformed into simple nutrients that the body can absorb. With a high-fat meal, digestion time may be longer.
ESOPHAGUS: The esophagus is a muscular tube that connects your throat to your stomach. When food is swallowed it travels down this narrow tube. Food moves through the esophagus rather quickly, reaching your stomach in about 10 seconds.
STOMACH: Your stomach is a J-shaped organ located between the end of your esophagus and the beginning of your small intestine. At each end, your stomach has muscular valves which control the movement of food in and out of your stomach. Your stomach provides mechanical digestion and movement of food through the digestive tract by peristalsis. While peristalsis occurs, your stomach is mixing the food with gastric juices. This is the beginning stage of digestion of proteins caused by the enzyme pepsin, which is produced in your stomach. Your stomach also absorbs a few fat-soluble substances, such as aspirin and alcohol, conveying them into the circulatory system. Most absorption occurs further along in the digestive tract.
INTESTINE and COLON: Your small intestine is the site of nearly all of digestion and absorption of nutrients. It is the longest part of the digestive tract and is divided into three subsections – the duodenum, followed by the jejunum and the ileum. There are two ducts in the duodenum originating in the pancreas and the liver. The duct from the pancreas secretes pancreatic juice and the liver duct provides bile, both of which are types of digestive juices. Inside the small intestine are many deep folds, which increase the surface area and causes the chyme, a paste-like mixture of food and gastric juices, to spiral through the small intestine to mix further with intestinal juice and slow its journey to give more time for absorption. Furthermore, these folds are covered with tiny finger-like projections called villi. These villi have absorptive cells on the surface and a dense capillary bed inside and are larger and more abundant at the beginning of your small intestine. As chyme moves through your small intestine, it is rubbed back and forth over the intestinal wall. This rubbing action helps the chyme move through the digestive tract, while mixing and absorption through the wall into the blood occur. When chyme leaves your small intestine, passing through the ileocecal valve and entering the large intestine, it is primarily composed of indigestible matter and water. The function of your large intestine is to absorb water from this matter and eliminate the remainder through the end of the large intestine, the rectum and the anus, as feces. No more digestion occurs in this organ, just absorption. Your large intestine wraps around the small intestine and is divided into the cecum, at its beginning, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and at its end the rectum and the anal canal leading out of the body. A small appendage extruding from the cecum is known as the appendix and does not seem to have a digestive function, but has something of an immune function.
LIVER: Your liver is the largest gland in your body and rich in blood. The only organ in your body that receives more blood than the liver is your brain. Your liver has more than 500 functions in your body. Some of those functions include filtering and purifying your blood, storing glucose as glycogen, secreting a number of plasma proteins, and producing bile. The production of bile is your liver’s digestive function. Bile is a yellowish-green alkaline solution composed of bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes. Bile salts are responsible for the digestive function of breaking down globs of fats into droplets that are more easily acted on by enzymes and digested. Bile passes through the hepatic duct to the storage site of the gallbladder, then is secreted through the common bile duct to the duodenum of the small intestine. Bile is constantly produced in the liver, but it is only released into the small intestine when chyme (partially digested food mixed with gastric juices) is present.
PANCREAS: Your pancreas is considered an accessory digestive organ, but this term can be misleading because the pancreas is critical to digestion. Its function is to produce a number of enzymes that break down many types of food—proteins, fats, and carbohydrates. Complete digestion could not occur without the addition of these enzymes. Pancreatic juices also contain bicarbonate, which neutralizes the acid in chyme that has come from the stomach. Your pancreas also produces hormones important to the regulation of some related bodily functions, such as the metabolism of carbohydrates. Two important hormones made in the pancreas are insulin and glucagon, both of which play a part in your body’s ability to use glucose. When the body doesn’t produce enough insulin, a person develops a type of diabetes.
GALL BLADDER: Your gallbladder is located in a recess on the surface of your liver. It is a green, thinly walled sac of muscle about 10 centimeters long. Like the liver, it is enclosed in a serosa layer. It serves as a storage place for bile that isn’t immediately required for digestion. While in your gallbladder, bile is concentrated, sometimes becoming 10 times more concentrated than when it entered the gallbladder. When your gallbladder is empty, its inner layer forms folds, like the inner layers of the stomach. When the sac is full and there is chyme in the small intestine, bile is squeezed out by the muscles in your gallbladder, moves through the cystic duct to the bile duct and into the duodenum. This occurs because the gallbladder muscle contracts when it is stimulated by an intestinal hormone that is released when chyme reaches the duodenum.
