Have you ever heard the phrase, “I know this like the back of my hand!” Well, it may make you question…do you really know the back of your hand? You can see skin but not all the individual skin cells there, at least not without a microscope. You can’t see the cells underneath the skin like red blood cells or muscle cells or bone cells. But your body is made of trillions of cells. Cells that work together. Cells that are very organized with specific functions. Cells that carry your whole DNA code but use specific parts of your DNA code depending on the cell’s function. Your cells can make up body tissue. Tissues can make up an organ, like a lung for example. And organs are part of an organ system like your respiratory system. One thing to keep in mind is that organ systems do not work in isolation. They work together and are part of a big team. We’re going to intro the 11 major human body systems. Keep in mind this is just a quick intro so we’re doing the bare bones (no pun intended) of the human body systems and our focus is on each system’s general function with a small mention of some structure examples. Remember, there are entire textbooks that can focus on just one of these systems where you can explore the details of structure and additional functions. Because these systems are all important, it’s hard to know where to start! So, we’re going to go in alphabetical order, because we don’t want you to think that one is way more important than another. First, circulatory system. You think of blood, and you should because blood carries gases like oxygen, which your body needs, and helps remove carbon dioxide, which your body needs to expel. Your blood also transports nutrients that your body needs. Your heart is included in this system. It is a pumping machine that transports the blood around. The heart has four chambers: right atrium, right ventricle, left atrium, and left ventricle. The heart also has valves to prevent a backflow of blood. Arteries are vessels that typically carry blood away from the heart (think "a" for away), veins typically carry blood back to the heart, and capillaries are tiny blood vessels throughout your body. Did you ever hear the misconception that your blood is blue and turns red when it reaches oxygen? That’s not true, at least, not in humans. Now, diagrams may use red and blue coloring to represent oxygen-rich and oxygen-poor blood but that’s not representing blood color. Your blood is red---and always red even inside your body---though the shade of red can vary slightly due to the amount of oxygen present. If veins are visible under the skin, they might look blue or green but that’s not their actual color either---more about that in our video description. #2- Digestive System. It’s important for breaking down and absorbing food for your body to get nutrients. Digestion actually starts in the mouth----you have some awesome enzymes in your saliva that get the process going. Your stomach contains acid and more enzymes to further this process. Your small intestine absorbs most of the nutrients with these amazing structures called villi and microvilli. Your large intestine has a major function of reabsorbing water from this process. Waste then passes through the anus. In addition to major structures in the digestive system, the digestive system includes some accessory structures. #3- Endocrine System. You know how you're bigger than you were when you were six years old? Growth hormone is a hormone that's made a big impact on you. Notice how your heart starts to race when you have a big test that you haven't studied for? That's another hormone----adrenaline. The endocrine system includes many glands that secrete hormones. Hormones have the ability to act as messengers on the targets they bind to in order for some action to occur. #4- Excretory System. It’s another system that starts with e! This system has two major functions: (1) maintaining an osmotic balance by managing water and solutes in the body and (2) ridding the body of metabolic wastes. So, with “excretory,” I like to think “exit” because metabolic wastes will be exiting. What is metabolic waste? We’re not talking digestive wastes like feces here. Metabolic wastes can include carbon dioxide or nitrogenous wastes like urea (which will exit in the urine). The excretory system can include the liver (highly involved in detoxification and produces urea), skin (think about sweating), and lungs (think about exhaling). But a big focus tends to be on the urinary system, which includes the kidneys, the bladder, the ureters, and the urethra. Kidneys assist in removing wastes from the blood. You know they’re important because an individual with impaired kidneys may need to go on a machine called dialysis to replace that function. The kidneys also have functional units called nephrons which are often explored in advanced biology courses. #5- Integumentary System A long, fancy word is appropriate for your largest organ----your skin. Your skin helps protect your organs from outside damage, helps with temperature regulation, and from losing precious water. It's got layers and then layers within the layers. What do we mean by that? Well, we have the epidermis, dermis, and hypodermis. Within the epidermis alone, you have several layers of different types of cells. Your most outer layer consists of cornified cells. They're dead cells that are continuously shed off, with replacement cells coming from layers below. Your integumentary system is also more than just your skin! It includes hair, nails, and sebaceous glands which secrete oil. #6- Lymphatic and Immune System Has anyone ever checked your lymph nodes on your neck when you feel sick? You have many lymph nodes, and they tend to swell during some illnesses. Lymph is this clear fluid from blood plasma that surrounds cells. This system collects, filters, and returns the lymph to the blood and one major function of this is that it aids in immune function: protecting your body from pathogens like viruses and bacteria. Structures like lymph nodes, the thymus, spleen, tonsils, and bone marrow play significant roles in your immune system. The immune system includes specialized white blood cells that will target pathogens in a variety of ways. Some white blood cells make antibodies that will bind to pathogens and make them a target to be ingested by cells called macrophages. Ok more than halfway done. Stick with us here! #7- Muscular System. No bones about it---your bones can’t do much without muscle to move them. Skeletal muscle tissue for that example specifically, but you actually have three major types of muscle tissue: skeletal, smooth, and cardiac and they all function in many different areas of the body. #8- Nervous System. Your body would be a nervous wreck without something to coordinate it! The nervous system is responsible for coordinating all kinds of actions in your body: voluntary and involuntary. It consists of two general regions: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord, and the peripheral nervous system includes nerves throughout the body. The peripheral nervous system can provide sensory information for the central nervous system. Then the central nervous system can process that info and execute motor responses or regulate body mechanisms. With this system, it's definitely worth mentioning the specialized cells known as neurons and glia. These cells are essential for communicating messages in the brain and throughout the body. #9- Reproductive System. Like it sounds, this system’s major function is that it allows for reproduction: think – babies! There are both external and internal structures that are part of the female and male reproductive system. Some structure examples: gonads, which are reproductive organs, produce the gametes: egg cells and sperm cells. Specifically, egg cells are produced by the ovaries, and sperm cells are produced by the testes. #10- Respiratory System. Got oxygen? You need it so your cells can make ATP, that energy currency we’ve mentioned before. The respiratory system allows for the intake of oxygen into the body and exhaling carbon dioxide out of the body. This system includes structures like the trachea and the lungs. Alveolar sacs in the lungs contain alveoli where gas exchange actually occurs. Gas exchange happens in the alveoli because the alveoli have thin walls and lots of surface area in direct contact with capillaries. And finally #11, the skeletal system. Bones. Adult humans have 206 bones---you have more at birth but some bones fuse together----and these bones support you, protect your organs (think: ribcage), and even produce blood cells from the bone marrow that is inside the bone. The human skeleton can be divided into the axial skeleton and the appendicular skeleton. Understanding how the human body works is fascinating – and it’s necessary so that we can figure out what to do when things go wrong. There are medical careers that specialize specifically with each of these systems. The most amazing part to us though is their beauty in how they work together – working every second to let you do the things that you do. Want an example? Let’s say that--- at the end of the day--- you race your friend down the hall. Think of all the body systems interacting for that to happen. Your respiratory system is going to increase its breathing rate---and that extra oxygen is going to be delivered through the circulatory system to your body tissues. Your muscles are interacting with the skeletal system for movement. These are just some interactions of the systems working together for that one example. The human body - is just truly magnificent. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.