The human body is an intricate and complex network of interrelated systems. It is perfectly synchronized to be born, grow, and reproduce. It is important to stay as healthy as possible, by eating the right foods, staying active, and making the right choices, as these systems work hard for you to continue to live and to extend your life.

Integumentary System

Also known as the dermal system, it consisted of the skin, hair, nails, and associated glands. This system has multiple roles in homeostasis, including protection, temperature regulation, biochemical synthesis, sensory reception, and absorption. There are two main layers in the skin, which are the dermis and epidermis. The dermis is the thicker interior layer, while the epidermis is the thinner, exterior layer. The skin is 12%-15% of the body weight making it the heaviest organ in the body. The skin forms a barrier; keeps bacteria, viruses, and other microbes out, and prevents fluid loss. The skin also regulates body temperature by releasing sweat, which helps cool your body. If body temperature drops, the skin reduces sweat production and engages in thermiogenesis, or heat generation, by increasing metabolic rate and by shivering. The skin has over two million pores with sweat glands. The pores in the skin releases waste products, which includes perspiration. If pores did not exist, waste would accumulate in the body and we will all die out. Hair and nails are structures that emerge from the skin. Except on the palm of the hands, the soles of the feet, and on lips, the skin is covered with dead hair. The hair is only alive when it is in the dermis, where it grows. The nails are formed with the same substances as a bird’s claw and a horse’s hoof. Healthy nails usually grow one millimeter per week, slightly faster than toenails.

Skeletal System

The skeletal system includes all the bones in the body. There are about 300 bones in a newborn baby and about 206 bones in a full-grown adult. Bones provide the structure of the body, give support, and protect the internal organs. Bones allow us to move with the help of the muscular system. They allow us to walk, run, swim, and do any activity or movement. The area where bones meet is called a joint. Muscles are connected to bones by tendons, and bones connect each other by ligaments. Joints allow us to swing our arms and bend our knees and elbows. Fibrous, cartilaginous, and synovial joints are the types of joints. Fibrous joints are also called immovable joints. They are attached by a thin layer of strong connective tissue. There is no movement between these joints, such as the sutures of the skull. Cartilaginous joints are where two bones are connected by a layer of white fibro cartilaginous, providing a limited degree of movement, such as the joints in the vertebrae. Synovial joints are freely movable joints, which most of the joints of the body are of this type. Under this category, there are the four main synovial joints. The ball-and-socket joint is formed when the bone with the round head fits into the cup-shaped socket, such as the shoulder joint and hip joint, giving a movement in all directions. The hinge joint is where the convex surface of the bone fits into the concave surface of the other bone, such as the elbow and knee, giving a movement in one plane. The gliding joint gives gliding movement between flat surfaces as the surface slides over one another, such as the tarsals and the carpals. The pivot joint allows for the bones to rotate around one another, such as the radius and the ulna. Inside a bone is the bone marrow, where 2.6 million red blood cells are produced every second. The strongest and heaviest bone in the body is the hollow femur, or the thigh bone, and it is stronger than steel. The smallest bones in the body are the stirrup, hammer, and anvil, all located in the ear.

Muscular System

The system includes all the muscles in the body, having the ability of converting chemical energy into mechanical energy. There are three types of muscles: cardiac muscle, smooth muscle, and skeletal muscle. Cardiac muscles are involuntary and are only found in the heart. The heart must pump the blood continuously in order to stay alive, so the medulla oblongata, which is a part of the brainstem, controls the cardiac muscles so that you do not have to tell your brain to make your heart pump blood every second. Smooth muscles are also involuntary, and are located in all of the internal organs, except for the heart, such as the stomach and the intestines. Skeletal muscles are the only muscles that are voluntary, and are attached to a bone by a tendon. They control movement, such as walking, running, swimming, punching, kicking, talking, etc. The skeletal muscles alone make up 50% of the body weight.

Circulatory System

The circulatory system includes the heart, blood, and the blood vessels. The heart has four chambers, with two atria and two ventricles. The right atrium receives deoxygenated blood from the body and pumps the blood to the right ventricle. The right ventricle then pushes the blood to the lungs, where blood becomes oxygenated blood. The left atrium receives the blood and pushes it to the right ventricle. The right ventricle then pumps the blood to the Aorta, the largest artery, and to the body. In one drop of blood, there are five million red blood cells carrying oxygen and carbon dioxide; 5,000-10,000 white blood cells fighting infections; plasma, which is the liquid component of blood; 200,000-300,000 platelets, which help form blood clots; fibrinogen, which weave around platelets at a cut or wound to prevent the flow of blood out of the body; and germs, trying to infect the body. There are three types of blood vessels, which are arteries, veins, and capillaries. Arteries convey blood away from the heart. Most of the time, arteries carry oxygenated blood; the only exception is the pulmonary artery, which carries deoxygenated blood. Veins convey blood to the heart. All veins, with the exception of the pulmonary vein, carry deoxygenated blood. Veins have valves similar to those of the heart to prevent backflow. Blood in veins flows in one direction, and in most cases, against gravity. Being one cell thick, capillaries are the smallest blood vessels in the body. Gas exchange between oxygenated and deoxygenated blood occurs through diffusion in capillaries. There is 60,000 miles of blood vessels in the body, so if the blood vessels were laid end to end, they would wrap around the Earth more than two and half times, or halfway to the moon.

Respiratory System

The major organ in this system is the lungs. The left lung only has two lobes, while the right lung has three. Each lobe has its own blood supply, so if one lobe is damaged, the other four keep working. When inhaling, fresh oxygen fill the lungs, while when exhaling, carbon dioxide escapes the body. Air is entered first into the nasal cavity or mouth, through the pharynx and larynx, passing the epiglottis and into the trachea, which then branches out into bronchi and bronchioles, and air finally reaches the alveoli. Capillaries wrap around the alveoli, where the actual gas exchange occurs in which oxygenated blood from the alveoli diffuses with deoxygenated blood in the capillaries and vise versa.

Digestive System

This system is responsible for the absorption of nutrients and conversion of energy from foods in the digestive tract, which is the series of the organs linked together. When food enters the mouth, mechanical digestion begins with the mouth chewing the food. Saliva is released through the salivary glands and helps with digestion in the mouth. Once the food is swallowed, it goes past the epiglottis and down the esophagus, beginning involuntary control. The epiglottis is a flap of tissue at the base of the tongue. When breathing, the epiglottis covers the epiglottis to allow air to pass through the trachea, while swallowing food, the epiglottis covers the trachea to allow food to pass through the esophagus and prevent choking. At the end of the esophagus, there is a valve called the sphincter, which allows food to enter the stomach and closes to prevent the backflow of food or acid. In the stomach, further digestion occurs, with the stomach mixing and churning the food, as it releases hydrochloric acid and digestive enzymes. At this point, the stomach is now filled with a thick liquid, also known as chyme, and passes through pylorus, a valve at the end of the stomach, and enters the small intestine. Much of absorption takes place in the small intestine, in which its inner walls are covered with villi, microscopic finger-like projections. The liver, gallbladder, and pancreas are not part of the digestive tract, but they are essential to digestion. The liver produces bile, which helps store fat. The liver also plays a major role in the handling and processing of nutrients from the small intestine. Bile is cached in the gallbladder until it is needed. The pancreas produces enzymes that help digest proteins, carbohydrates, and fats. It also produces a substance that helps neutralize the hydrochloric acid. The enzymes and bile flow through ducts and in the small intestine to help break down food. After nutrients are absorbed from the small intestine, the undigested food goes through the large intestine, also known as the colon, where water is absorbed from the undigested matter, forming solid waste. The appendix is a small pouch that hangs at the end of the large intestine. It is a vestigial structure, meaning it has no purpose in this system. The rectum holds the solid waste and then pushes it through the anus as feces.

Nervous System

The nervous system includes the spinal cord, brain, and 47 miles of nerves. Made up of billions of neurons, the brain is the command center of the entire body. It controls every bit of movement and processes thoughts. The brain also helps respond to information it receives from senses. It is made up of three parts: the cerebrum, cerebellum, and brain stem. The largest part of the brain is the cerebrum, which is 85% of the whole brain. The cerebrum is the thinking part of the brain, which helps move voluntary muscles, helps solve problems, and helps with reasoning. The memory lives in the cerebrum, including short-term memory, and long-term memory. The cerebrum has two halves, in which the right half controls the left half of the body and the left half controls the right half of the body. Located at the back of the brain, the cerebrum controls balance, movement, and coordination. The brain stem controls all involuntary actions, such as respiration, heart beating, digestion, and circulation. The brain stem is attached to the spinal cord, which runs down the vertebrae and some information received from senses is processed in the spinal cord, but most is sent to the brain. Branching out the spinal cords are millions of nerves, with billions of neurons in nerves. There are three types of neurons: motor neurons, sensory neurons, and interneurons. Motor neurons are stimulated by interneurons and sensory neurons and transmit impulses to muscles and glands, which cause muscle to move. Sensory neurons are stimulated by the senses, such as touch, taste, smell, vision, and hearing, and send the information they receive to the spinal cord and brain. Interneurons, also known as associated neurons, are only found in the spinal cord and brain. They are stimulated by sensory neurons or other interneurons. A reflex is an instant and involuntary movement that is controlled by the spinal cord only. For example, if a person looks away and places their hand on a hot stove, their hand will jerk away automatically because sensory neurons stimulate the interneurons in the spinal cord, which then stimulate the motor neurons to move the hand away. Without reflexes, Humans would suffer more injuries from everyday objects. A reaction, however, uses sensory neurons to stimulate interneurons in the brain, which then stimulate the motor neurons to react. For example, a ball is thrown at a high speed, and a person sees it coming and dodges it; they used their sight to react and dodge the ball. Therefore, a reflex is much faster than a reaction, because in a reflex, the information from the stimulus is sent to the spinal cord and back, whereas in a reaction, the information is sent to the brain and back.

Urinary System

This system includes the kidneys, bladder, and the ureters. The urinary system concentrates on the formation and elimination of urine. In an adult, 2,500 pints of blood passes through the kidneys each day. Blood enters the kidneys through the renal arteries and is filtered by the kidneys. In the kidneys, waste combines with water in the blood and the nephrons filters the waste, which then converts into urine. Purified blood exits the kidneys and continues through blood circulation by the renal veins. The urine then runs down the ureters and enters the bladder. The bladder is a storage sack that holds the urine until the urine fills the bladder halfway. At that point, the sphincter would open and urine is release out the body through the urethra.

Endocrine System

This system is all about the glands and hormones in the body. Hormones transfer information from one set of cells to another. Many different hormones circulate throughout the bloodstream, as each one only affects the cells that are genetically program to receive and respond to the message. A group of cells that produce and secrete chemicals is called a gland. The gland selects and removes the chemical from the bloodstream and uses it somewhere else in the body. Major glands that make up this system are the pituitary, hypothalamus, thyroid, parathyroid, adrenals, , and the reproductive glands, which include the ovaries and testes. The pancreas is also part of the endocrine system, even though it associates with the digestive system. The pituitary gland, being the size of a pea and located below the brain, releases hormones that affect growth, metabolism, and the system of reproduction. The hypothalamus, just above the pituitary gland, is a tiny group of brain cells that transmit messages from the body to the brain. The butterfly-shaped thyroid gland controls the rate of production of energy from nutrients. The parathyroid controls the level of calcium in the blood. The adrenal glands secrete hormones that are responsible for method in which energy is stored and food being used, giving important effects on chemicals in the body, and on characteristics, such as body shape and hairiness. The reproductive glands secrete hormones that affect reproductive activity.

Immune System

This system includes many types of cells, mainly white blood cells that protect the body from bacterial, viral, and fungal infections. In a viral attack, the virus enters the body and attacks body cells, where it uses its DNA and reproduces in the host cell. The macrophage determines whether this virus is really the enemy. Then it devours the virus and rips it apart. Afterwards, it displays the fragments, or antigens, on its surface. At this point, the macrophage is able to destroy all the viruses, but the virus is reproducing faster than the macrophage can destroy them. The macrophage must find certain helper T-cells that are able to recognize the virus fragments, so that they can unite. Once united, the helper T-cell and macrophage produce chemicals that stimulate the production of killer T-cells. At this point, the virus has infected many of the body cells, so it is the killer T-cell’s job to search and destroy those cells already infected with the virus. The helper T-cells alone produce chemicals that stimulate the production of B-cells, which convert into plasma and flood the body with millions of antibodies. The antibodies can only destroy certain virus cells, but they can flag any virus so that the macrophage can destroy. As the virus begins to decrease, so does the number of antibodies, macrophages, B-cells, helper and killer T-cells. What only remains are the memory B-cells and memory T-cells. If the same virus returns to invade the body again, the cells are able to destroy the virus much quicker.

Lymphatic System

In this system, lymph nodes, the spleen, thymus, bone marrow, adenoids and tonsils play an important role in the system. The lymphatic system removes lymph from tissues and returns it to the bloodstream. It also stores cells needed by the body to fight infections, such as cancer. The system is important in the immune system as it fights infections that enter the body. Lymph nodes are located in various places in the body, which filters fluids and traps viruses and bacteria. The spleen is an organ that is responsible for the destruction and recycling of the old red blood cells and the production of white blood cells. The thymus produces T lymphocytes during fetal development. Bone marrow produces all kinds of blood cells for the body. Adenoids and tonsils remove harmful germs when respiring or swallowing foods.

Reproductive System

The reproductive system’s purpose is to continue the existence of humans (any animal species for that matter). The difference is humans require sexual or artificial (i.e. sperm bank) means to reproduce while some other animals or plants can reproduce asexually. Major organs include the scrotum, testes, epididymides, vasa deferentia, seminal vesicles, prostate gland, bulb urethral glands, urethra, and penis in males; major organs in females include the ovaries, uterine tubes, uterus, vagina, clitoris, and vulva. For sexual reproduction to occur, the subject’s body uses hormones to make them carry out this activity. Unfortunately, diseases such as HIV (Human Immunodeficiency Virus that later leads to AIDS), HPV (Human Papillomavirus), Hepatitis C, Chlamydia, Pubic Lice, and others are spread through sexual reproduction and can lead to illness, pain, and in most cases, death. HIV is the most well known sexually transmitted disease, or STD, but it still is prevalent in many third-world nations and is still present here in America. Some STD’s like Pubic Lice can be cured (with an over-the-counter cream, much like head lice) and most can be treated to some extent. Abstinence is the only way to guarantee an individual does not receive an STD, but they are less likely to get it if they take the correct precautionary measures like getting tested.