Reviewed By: Dr. Kurt Kloss, MD
Last Reviewed Date: Nov 20, 2019
Last Modified Date: Nov 20, 2019
Published Date: Mar 12, 2019
The human body is the most complicated organism on the planet. The organs, in a human body, are tissues that come together perform at least one purpose in the body. Many serve several functions. The largest of the human organs is also the easiest to see. The skin, along with the hair, fingernails, and toenails, covers most of the body. The skin is actually made up of three different layers. The epidermis is the outermost layer, which contains the pigments determining your skin color. It is also covered with tiny holes called pores from which sweat can escape the body.
The middle layer of skin is the dermis. It is so tightly connected to the epidermis that it is hard to tell when one starts and the other stops. The dermis contains the connective tissue, hair follicles, and sweat glands. The final layer is the hypodermis. This is technically not part of the skin; but, exists to connect the skin to the muscles underneath. This layer also supplies blood to the skin and contains most of the body's fat reserve.
The skin is more than just a covering for the blood connections underneath. The skin provides a waterproof layer of protection against invaders to the body, like viruses and bacteria. The skin also has the amazing ability to turn sunlight into Vitamin D, as well as regulating our body temperature. It also excretes waste through sweat.
Part of the reasons why the human body is so complicated is that all the parts are interrelated. When studying the different systems of the body, remember that organs in one system often perform functions in another. Keep this in mind while reading about the ten major organ systems in the body.
The Skeletal System
The skeletal system is composed primarily of 206 bones, and also the cartilage, tendons, and ligaments which connect the bones together. Without the connective tissue, the bones would float around in the body without much function. With these tissues, the bones will be able to both support the human body and give it its shape. It also facilitates a good body movement, mainly because of the joints.
Bones provide an important protective function in such a way that the skull protects the brain, and the ribs protect the heart, lungs, and other internal organs. From another angle, bones have a semi solid tissue within them that are located in the spongy part of the bone structure. This semi solid tissue is called the bone marrow that plays a vital role in generating red blood cells. Finally, your bones have also a big role in storing calcium for future use and are key players in regulating blood-sugar and fat deposits due to a hormone they release.
- Pictures of the Human Skeleton - Penn State University has photos of actual human bones.
- Bone Match Game - Students can test their knowledge about the human bones with the bone match game.
The Muscular System
The muscular system is made of three types of muscles: smooth, skeletal, and cardiac. These muscles work together to create the body’s movement in conjunction with the skeletal system. They also provide the strength and balance that the body needs to perform any basic function, like eating, breathing, or blinking. When your body moves, your muscles burn energy that provides much of the heat needed by your body.
Skeletal muscles are the muscles that are attached to your bones via tendons. These are the muscles that a person can control, such as the ones in the legs that you need to walk from room to room. This type of muscle is also called striated, because of their fibrous look. Smooth muscles are the ones controlled by the autonomic nervous system. In other words, they are the muscles used in involuntary movements, like the intestines when they digest food or the iris of the eyes when exposed to light.
Cardiac muscles are also part of the involuntary nervous system. They are distinct because they are attached to each other rather than to bones, which is the case of the skeletal muscles. Cardiac muscles are found solely in the heart.
- Test Your Knowledge of the Muscular System - This collection of links will explain about the muscles of the body.
- Introduction to the Muscular System - The muscular system does more than just help provide movement. Discover the structure and functions of the muscles in the body.
The Circulatory System
Many substances circulate throughout the human body, like hormones and chemicals. When scientists and doctors speak of the human circulatory system, however, they are almost always referring to the system which moves blood around the body. The circulatory system comprises the heart, arteries, capillaries, vein, and to some extent the lungs, trachea, ear, nose, and throat. Some even include the lymphatic system as part of the circulatory system.
The main purpose of the circulatory system is to provide oxygen and other needed nutrients to all parts of the body; and, to pick up waste material from the cells for elimination. Think of it as the system that offers food delivery and trash pick-up in one efficient movement. The heart is the engine of your system, as it pumps blood part of its journey through the body.
First, the blood must pick up the oxygen. After the blood is pumped through the first two chambers of the heart, it travels through the pulmonary artery to the lungs. There it deposits the waste picked up from cells in the body, primarily carbon dioxide, and absorbs oxygen from the lungs. The blood then re-enters the heart via the pulmonary vein and is pumped through the remaining two chambers of the heart. Then, most of it exits the heart through the aorta. The aorta that is the body's main artery begins to branch into progressively smaller arteries, bringing nutrient-rich blood to the body.
A portion of the oxygen-rich blood is pumped into a special artery that brings blood to the heart itself. The heart is a strong muscle and needs a constant supply of blood, just as the rest of the body does. The heart is not designed to absorb the blood being pumped through the chambers. It needs its own supply that would be capable to feed the heart muscle cells.
As the blood from the arteries reaches its destination, the arteries become very small and transition into capillaries. Capillaries are special blood vessels that allow the cells surrounding them to absorb the oxygen from the blood in them and dump their waste. The capillaries, then, transition into veins. The blood in veins is much darker as is has little oxygen in it. The veins slowly begin to combine into larger veins until they unite to form the largest vein, the interior vena cava, which brings the blood back into the heart. That’s when the process begins all over.
- Human Circulatory System - This site gives an overview of the entire circulatory system.
The Nervous System
With the systems already discussed, there are already many complex operations taking place. Something needs to control all the functions. That duty falls on the nervous system. The nervous system consists of the brain, the spinal cord and other nerves, as well as the retina. The retina is the nerve connected to the eye, which controls vision.
The brain and spinal cord are made of a special kind of cell called the neuron. A neuron is specifically designed to allow for a rapid communication between cells. This is done either via chemical or electric communication. The brain has billions of synapses where neurons are interconnected and constantly communicating.
The brain isn't just the organ that is used for humans to think; but, it also controls all body functions, emotion, judgment, memory, the senses, as well as all the other systems of the body. Different parts of the brain control different aspects of the body or different areas of one’s personality. The brain makes billions of decisions or connections every minute, far more than any computer. Signals are then sent along the spinal cord.
The spinal cord is a long collection of fibrous tissues called nerves. Signals are passed along these cords, like the ones sent along a telephone wire, to the appropriate muscle or organ. In return, the nerves send input signals back to the brain, like pain sensations.
- Neuroscience for Kids - In spite of the name, this is a really cool look at the brain and nervous system.
The Respiratory System
The respiratory system is highly intertwined with the circulatory system. The lungs expand pulling air in through the nose or mouth. The diaphragm, located underneath the lungs, is the organ actually responsible for causing inhalation. The air, then, rushes through the throat into the trachea, which is the pipe connecting the lungs to the throat. The trachea then splits into two, each branch leading into a different half of the lungs.
As air fills the lungs, the oxygen from the air is absorbed into the blood that the heart has pumped there for that specific purpose. Once the lungs are mostly filled, the lungs begin to recoil, pushing air back out the trachea, throat, mouth and nose. The difference this time is that the blood which absorbed the oxygen has dumped carbon dioxide into the lungs. So the air from an exhalation has a much higher level of carbon dioxide than normal air.
The Digestive System
The digestive system might be the most complicated system in the body. It certainly has a large number of organs. The digestive system's purpose is to remove nutrients from the foods and beverages a person consumes so that they can be used by the cells in the body as fuel. As most humans consume a variety of foods and often many that are unhealthy, the digestive system has a big job.
The mouth, teeth, tongue, and salivary glands are the first organs of the digestive system. They start the digestive process by mechanically reducing the food into a paste that can be digested by the stomach. The food, or beverage, is then swallowed down the esophagus and carried into the stomach. Here, stomach acids begin the chemical breakdown of the food.
Once the stomach is done with the food, it passes into the small intestines. The small intestines play a vital part in digestion. Bile from the liver and gall bladder as well as digestive enzymes from the pancreas enter the small intestines to break down the proteins from the food; and, to emulsify the fat. Once the nutrients are broken down, the villi, small hair-like projection which line the lower end of the small intestines, absorb the soluble nutrients so they can enter the bloodstream.
Then, whatever substances are left move on to the large intestines. Here, the colon removes most of the water from the substances. The colon also is populated by good bacteria which live in it and produce Vitamin K for the body. As a final part of the digestion, the waste material is then squeezed into the rectum and the waste exits the body.
- Your Digestive System and How It Works - This page explains how food is digested as it travels through the digestive system.
The Excretory System
The excretory system is composed of all the organs, which help the body get rid of waste products. Waste products are sometimes created by cells; other times they are the unneeded and unused substances, which have entered the body that need to be removed. Some organs of other systems are also excretory organs. For instance, the skin excretes sweat and the lungs excrete carbon dioxide.
The urinary system is also an important piece of the puzzle. The urinary system, the kidneys, ureter, bladder, and urethra, exists to clean out the blood, like a filtration system. The kidneys are the primary filter. Not only does this pair of organs pull out toxic material from the blood; but, they also regulate the amount of fluid in the body and maintain its right level of acidity.
After the waste and excess fluid are removed from the blood, the ureters carry the urine created in the kidneys, all the way to the bladder. The bladder stores the urine until it can be eliminated. When the body is ready to eliminate the urine, the urine passes through the urethra to be excreted. In men, the urethra also acts as part of the reproductive system.
- The Urogenital System - This explanation of the excretory system includes photographs of actual organs in the urogenital system.
- Excretory System Model - Make a model of the excretory system. This would be a great class or science fair project.
The Endocrine System
The endocrine system is, perhaps, one of the less famous and understood part of all the body systems; but, a very important one. The endocrine system is made up of glands which produce chemicals called hormones which control body development, puberty, metabolism, and even mood. The endocrine glands are different from other glands in the body, like sweat or salivary glands which have ducts. Endocrine glands are ductless.
In the brain, the hypothalamus, the pineal, and the pituitary gland secrete a wide array of hormones. The hypothalamus controls growth, along with many other functions. The pineal gland produces melatonin, which influences how well a person sleeps. The pituitary gland, also, helps control growth, blood-sugar levels, egg production in females, as well as other function.
There are other glands found in the stomach, duodenum, liver, pancreas, and kidneys. The thyroid in the neck controls the body's metabolism. In women, the ovarian follicle produces several hormones, which regulate the menstrual cycle and the development of female sex characteristics. Males have testes, that help men develop male sexual organs as well as increase their muscle and bone mass.
- Endocrine System Diseases - When something goes wrong with one of the glands in the endocrine system, the effect can be devastating to a person's health.
The Reproductive System
The reproductive system allows humans to produce offspring. Though there are dissimilarities in bodily systems between men and women, these are just minor differences. But, when it comes to the reproductive system, they are quite different as they have completely different roles. Interestingly, sexual organs start developing similarly for a fetus, in both genders. Then, at a certain point during the baby's development, the organs begin developing into the appropriate gender-appropriate organs.
A female's main reproductive organs are the two ovaries. These two small organs hold all the eggs a woman has in addition to producing many of the hormones associated with reproduction and the menstrual cycle. These are connected to the uterus by the fallopian tubes. The uterus serves as the home for a developing fetus during pregnancy. The cervix connects the uterus to the vagina that is the external opening of the body for the genitals.
The male’s equivalents of the ovaries are the two testes, which are housed in the scrotum. These organs produce the hormones associated with the male gender characteristics as well as reproduction in the form of sperm (these latter are the father's contribution to a baby's genetics). The sperm is then stored in the epididymis until it passes into the vas deferens, picking up seminal fluid from the accessory glands. The vas deferens, then, takes the sperm mixture to the penis during ejaculation.
- The Reproductive System Development - Organs in a developing baby's reproductive system start off the same, regardless of the baby’s gender. This article explains how sexual organs develop differently in males and females.
- Anatomy Bowl - This free Jeopardy-like game tests knowledge about anatomy. This link connects to the questions about the reproductive system.
The Lymphatic/Immune System
The lymphatic and immune system act as a filter for the body. Unlike the urinary system, this system filters out invaders to the body, such as viruses and bacteria. The lymphatic system is made of the tonsils, thymus, spleen, and many lymph nodes found all over the body. This system circulates a liquid called lymph throughout the body, via the bloodstream.
This fluid circulation is important in maintaining the health of the body. It removes excess fluid from around tissues, transports white blood cells to the bones, and absorbs fatty acids and fat from the intestines. It also plays a part in the immune system as it carries antigen-producing cells to the lymph node when an infection has been discovered. As lymph passes through the lymph nodes, they act as filters, catching the intruders in their tissues.
When describing the immune system, scientists describe two different types of protections. The first is innate immunity. They are the natural protectors the body owns to keep invaders, or antigens, out. The skin, for instance, acts as a barrier for most of the body. Saliva, tears, mucus and sweat all have properties that kill many antigens. Stomach acid destroys most antigens that are consumed.
The adaptive immune system, which is the second type, describes the actions taken by the body when a specific antigen is detected. This includes the production of antibodies to fight the virus that has been inhaled into the lungs, for example. There are several tools that the immune system uses, including killer T cells, which will exterminate infected cells and kill the virus. The immune system also has an amazing ability to remember antigens which have invaded from the past. This is why a person, who has previously had the chicken pox, for example, won't get sick with it again. If the virus enters the body, the immune system already has the tools needed to fight it on hand.
- The Immune System - This introduction to the immune system goes into greater detail about how the system fights the invaders in the body.
- Immune System Basics - The immune system is a complicated network of tissues which work together to fight antigens and pathogens. This article breaks it all down.