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Human Anatomy - Major Organ Systems

Reviewed By: Dr. Kurt Kloss, MD
Last Reviewed Date: Sep 25, 2018
Last Modified Date: Sep 25, 2018
Published Date: Aug 01, 2017

The human body is the most complicated organism on the planet. The organs in a human body are tissues that 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 which determine 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 is there to connect the skin to the musculature 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 bloody stuff underneath. The skin provides a waterproof layer of protection against invaders to the body, like viruses and bacteria. The skin also has an amazing capability to turn sunlight into Vitamin D as well as regulating body temperature. It also excretes waste through sweat.

Part of the reason 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.

Skeletal System

The skeletal system is composed primarily of the 206 bones of an adult human body, but 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 them, the bones both support the human body and give it its shape. It also helps allow for body movement, mainly with the joints.

The bones also provide an important protective function. The skull protects the brain. The ribs protect the heart, lungs, and other internal organs. As mentioned before, the bone marrow plays a vital role in creating red blood cells. The bones also store calcium for future use and play a role in regulating blood-sugar and fat deposits thanks to a hormone they release.

Muscular System

The muscular system is made up of three types of muscles: smooth, skeletal, and cardiac. These muscles work together to cause the body to move in conjunction with the skeletal system. They also provide the strength and balance the body needs to perform any basic function, like eating, breathing, or blinking. The energy the muscles burn while moving also provides much of the heat the body needs.

Skeletal muscles are the muscles which are attached to bones via tendons. These are the muscles a person can control, as in the leg muscles needed to walk from room to room. This type of muscle is also called striated because of the fibrous look to them. 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 as they digest food or the iris of the eyes.

Cardiac muscles are also controlled as part of the involuntary nervous system. They are distinct in the way they are attached to each other. They are attached to each other rather than to bones as skeletal muscles are. Cardiac muscles are found solely in the heart.

Circulatory System

There are many substances that are circulated 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 is made up of the heart, arteries, capillaries, veins, and to some extent the lungs, trachea, and the ear, nose, and throat area. Some even include the lymphatic system as part of the circulatory system.

The main purpose of the circulatory system is to get 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 food delivery and trash pick-up in one efficient system. The heart is the engine for the system, as it pumps the blood through its journey through the body. First, though, 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, the body's main artery, then 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. The heart is not designed to absorb the blood being pumped through the chambers. It needs its own supply that can 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 to 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 and larger veins until they unite into the largest vein, the interior vena cava, which brings the blood back into the heart for the process to begin all over.

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 to the nervous system. The nervous system consists of the brain, the spinal cord and other nerves, and the retina. The retina is the nerve connected to the eye which controls vision.

The brain and spinal cord are made up of a special kind of cell called a neuron. A neuron is specifically designed to allow for 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 where humans think, but it also controls all body functions, emotion, judgment, memory, the senses, and all the other systems of the body. Different parts of the brain control different aspects of the body or different areas of personality. The brain makes billions of decisions or connections every minute, far more than any computer. The signals are then sent along the spinal cord.

The spinal cord is a long collection of fibrous tissues called nerves. The signals are passed along these cords, like signals 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.

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, the pipe which connects 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 the heart has pumped there for that 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.

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 substance the stomach can digest. 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 and 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 like this, 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.

Whatever substances are left, then pass into the large intestines. Here the colon removes most of the water from the substances. The colon also has good bacteria which live there and make Vitamin K for the body. The waste material is then squeezed into the rectum. Finally, the waste exits the body.

Excretory System

The excretory system is made up of all the organs which help the body get rid of waste products. Waste products are sometimes created by cells; other times it is unneeded and unused substances which have entered the body and need to be removed. Some organs that are in 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, is there to clean out the blood, like a filtration system.The kidneys are the primary filter. Not only does the pair of organs pull out toxic material from the blood, they also regulate the amount of fluid in the body and maintain the right level of acidity of that fluid.

After waste and excess fluid are removed from the blood, the ureters carry the urine created in the kidneys 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 the outside of the body. In men, the urethra also acts as part of the reproductive system.

Endocrine System

The endocrine system is perhaps one of the lesser known and lesser understood of the body systems, but it is 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 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 others.

There are other glands found in the stomach, duodenum, liver, pancreas, and kidneys. The thyroid in the neck controls the body's metabolism. In females, the ovarian follicle produces several hormones which regulate the menstrual cycle and development of female sex characteristics. Males have testes, which not only cause men to develop male sexual organs, but also increase their muscle and bone mass.

Reproductive System

The reproductive system allows humans to produce offspring. Though there are differences in other systems between men and women, they are minor differences, whereas the reproductive systems are quite different as they have different roles. Interestingly, many of the sexual organs start off exactly the same in the developing fetus. Then at a certain point during the baby's development, the organs develop into the appropriate organs for the gender.

A female's main reproductive organs are the two ovaries. These two small organs hold all the eggs a woman has plus produces many of the hormones related to 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, the exterior opening of the body in the genital area.

The male equivalent of the ovaries is the two testes, which are housed in the scrotum. These organs produce the hormones related to male gender and reproduction as well as the sperm, which is 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.

Lymphatic/Immune System

The lymphatic and immune system also acts as a filter for the body. Unlike the urinary system, this system filters out invaders in the body, such as viruses and bacteria. The lymphatic system is made up 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 has to keep invaders, or antigens, out. The skin 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 describes the actions the body takes when a specific antigen is detected. This includes producing antibodies to fight the virus that has been inhaled into the lungs, for example. There are several tools the immune system uses, including killer T cells, which will kill infected cells and thus 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.