The human body’s capability is mind-boggling. Countless conscious and unconscious functions keep you moving, thinking, and…well, living. Like a well-oiled machine, it performs the same critical processes day after day. But of course, your body is not a machine—it’s an organism.

As it turns out, humans are pretty complex. And to understand how the body works, we need to understand its parts—that’s where this article comes in.

What is the Human Body Made of?

The human body is made up of trillions of cells. As the smallest living part of the human body, they’re made up of molecules which, in turn, are composed of elements. About two hundred different types of cells are found in the body, and each fulfills a different function. Red blood cells, for example, transport oxygen, while white blood cells help protect the body from infection.

And though it may seem strange, your cells are not permanent fixtures. Billions of cells in your body die every day—and billions are regenerated. This cellular turnover is a natural part of life within the human organism. Some cells, like skin cells, die and regenerate very frequently. Others, such as some cells found in bones, have much longer life cycles.

Not all cells work independently—some work in groups to perform larger body functions. But more on that in the next section.

A Closer Look at the Human Organism

While it is important to study cells and understand their roles, sometimes we need to look at the bigger picture. The structure of the human body can be viewed in several levels, each slightly more zoomed out than the last. They include: cells, tissues, organs, organ systems, organism. Let’s break them down.

• Cells: Your body is made up of trillions of cells—with more than two hundred distinct cell types that perform a specific function. As mentioned above, cells are the smallest living piece of a human body. And billions of your cells die and regenerate every day.
• Tissues: Similar cells that group together to perform a joint function make up body tissue. The four main types include connective tissue, epithelial tissue, muscle tissue, and nervous tissue. Each performs a general function but can also be broken down into specific sub-tissues with more defined functions. Muscle tissue, for instance, contracts and expands to move parts of the body—this is a general function. While cardiac muscle is a tissue that performs a specific job: contracting and expanding to pump blood through the body.
• Organs: When two or more tissue types team up to perform a specific function, they form an organ. Although most of your body’s organs contain all four types of tissue. Organs perform specific tasks necessary for your body to function. The heart, for example, pumps oxygenated blood throughout your body. Your lungs bring oxygen into your body and expel carbon dioxide. And your liver helps remove toxins.
• Organ systems: Groups of organs that work together form organ systems. For example, the brain and spinal cord are two distinct organs that contain nerves that transfer information throughout your body—both belong to the nervous system. The human body has eleven organ systems: cardiovascular, digestive, endocrine, immune, integumentary, lymphatic, musculoskeletal, nervous, reproductive, respiratory, and urinary.
• Human organism: All together these smaller pieces create a single living entity—the human organism. From the cellular level to your organ systems, each part of your body performs functions with the same ultimate goal: keeping you living and healthy.

Five Organs You Can’t Live Without

While each part of the human organism fulfills an integral role, some are more important than others. The human body contains five vital organs you can’t survive without—your brain, heart, kidneys, liver, and lungs. Although its best to approach health and nutrition holistically, these key organs can help orient you as you study the intricacies of the human body.

More About Organ Systems

Now let’s take a closer look at your body’s eleven organ systems. As mentioned, each is made up of at least two organs as well as other tissues. Their unique body benefits are highlighted below:

• Cardiovascular system: Made up of three major components—the heart, blood vessels, and blood—this system transports oxygen, nutrients, and carbon dioxide throughout your body. It works closely with the respiratory system to bring oxygen in and eject carbon dioxide out.
• Digestive system: Your mouth, esophagus, stomach, and both your large and small intestines are the primary parts of this system, also called the gastrointestinal system. Certain nutrients are needed to survive, and thanks to your digestive system, your body can take in food and water, process it, and extract these vital nutrients.
• Endocrine system: Your hypothalamus, pituitary, and thyroid glands release hormones into your body to activate changes and guide biological processes. Think of this system as an inner-body communication network that helps to regulate growth and development, homeostasis (constant internal balance), metabolism, mood, reproduction, sexual function, and your sleep-wake cycle.
• Immune system: Not only does this proactive system defend your body against disease, it also plays a part in protecting it from pathogens. It includes your spleen, tonsils, thymus, and leukocytes (white blood cells).
• Integumentary system: Made up of your skin, hair, and fingernails, this system is responsible for protecting against external microorganisms and for keeping fluids within your body.
• Lymphatic system: Three organs—the lymph nodes, the lymph, and lymph vessels—help protect your body against diseases and infections.
• Musculoskeletal system: As the name suggests, this system’s primary function is to use muscles and the skeleton to support your body’s soft tissue and allow for movement. And it may surprise you it also includes cartilage, tendons, and ligaments, which help connect bones and muscles to each other and to other parts of the body.
• Nervous system: Composed of the brain, spinal cord, nerves, and sensory organs (skin, eyes, ears, tongue, and nose) this system’s primary function is to gather, transfer, and interpret the sensory information your body experiences. And, when necessary, it alerts other organs and systems. For example, if you see a car careening towards you (a visual stimulus), your nervous system may activate your fight-or-flight response, which tells your heart to beat more quickly, your lungs to breathe more rapidly, and your adrenal glands to release adrenaline.
• Reproductive system: This life-giving system is responsible for conceiving and birthing offspring. The associated organs include genitalia, fallopian tubes, the uterus, the prostate, and more. It’s also responsible for producing gametes (or sex cells) and sex hormones—estrogen and testosterone—which play vital roles in human development and growth.
• Respiratory system: The organs in this system—the lungs, larynx, pharynx, trachea, and diaphragm—are responsible for bringing oxygenated air into the body and expelling carbon dioxide. This vital system works closely with the cardiovascular system to provide your body with oxygen, which helps cells produce energy.
• Urinary system: Also called the excretory system, it utilizes your kidneys, bladder, ureters, and urethra to remove water and waste materials from your body and help to maintain pH.

Understanding the Human Body: An Ongoing Journey

The human organism is so complex that it might seem impossible to fully comprehend—especially with a single article. But learning its basic structure is your first step to understanding how your body works.

So don’t stop here! Use what you’ve just read as motivation to dive deeper into one of nature’s biggest marvels: the human body.

Cells are the building blocks of life, and they come in all shapes and sizes. Some cells are round and small, others are larger and web-like. No matter their look, the cells in your body are highly complex, elegant machines that make life possible.

The variety of cell shapes and sizes is essential. Your body always has a lot going on. Cells need to differentiate and specialize, channeling their energy toward specific tasks. Let’s explore the variety of cells in your body and learn more about their anatomy, function, and individual characteristics.

Go Deep Inside Your Cells

Diversity starts inside the cell— with cellular organelles. These teeny-tiny membrane assemblies fill your cell and help it perform its specific functions. There are many organelles, and you’ll read about a couple of the most important ones.

The organelles that operate inside of cells are necessary for completing designated assignments. Not every cell has every type of organelle. But all cells rely on these structures to work effectively.

Cell Membrane

The lipids that surround the cell and give it shape are organized into a two-layered barrier called a membrane. Cell membranes are made of fats and proteins. The fatty portions of the membrane keep water out of the cell, while the proteins allow nutrients and water to pass through.

Your cells need a membrane to stay organized, compact, and protect their contents from the surrounding body fluids. You can visualize a cell membrane by dropping a bit of oil into a cup of water. The micelles that form from mixing oil and water are a lot like the fatty membrane that encapsulates your cells.

Mitochondria

Mitochondria are the organelles that power the cell. It’s in and around the mitochondria that the food you eat is converted to cellular energy (or ATP as it’s known inside the cell).

Mitochondria are commonly referred to as the “powerhouses” of the cell. But these compact generators weren’t always stuck inside cells. Scientific research suggests that mitochondria were at one time their own cellular bodies. There is unique genetic information stored inside mitochondria. This material is called mitochondrial DNA.

As life evolved, it’s believed mitochondria were recruited by cells as an energy source. Now when cells divide, mitochondria replicate inside the cell along with the other organelles.

Here’s a fun fact: every cell in your body contains mitochondria except red blood cells.

Ribosomes

Cells need to be able to produce proteins. Ribosomes are the center of protein production inside the cell. They use specialized codes to read the information stored in RNA molecules (genetic directions for building proteins). Ribosomes create nucleic acids and proteins from the instructions found in RNA.

Nucleus

In the nucleus, you’ll find all the blueprints for a cell. The nucleus is the cell’s “brain.” It sends out instructions about how the cell should function, and it houses the DNA needed to replicate the cell.

This organelle is often depicted in images as the dark center of a cell. It’s densely packed with DNA that is sensitive to degradation and damage from the environment around it. A double-layered membrane surrounds the nucleus to protect the DNA stored inside.

Cell Types Explained

Now that you understand some of the crucial cellular operation centers, let’s zoom out to discuss different cell types. Hundreds of specialized cells are inside your body right now. A student of science like yourself could spend a lifetime learning about each and every cell type.

Instead of debating the minutia of each type, here are the basics about some of the most common varieties of cells in your body.

1. Skin and Epithelial Cells

The cells you can most easily see are your skin cells. That’s because the largest and outermost organ of your body is made entirely of special skin cells—also called epithelial cells. These epithelial cells that create the meshwork of your skin are the same type of cells that line your digestive tract, blood vessels, and hollow organs.

Skin cells have unique properties. These special attributes help explain the function of your epithelial cells. Here are a few ways skin cells can work in your body:

• Skin cells come together to create tissues that can secrete mucus, sweat, and oil.
• Epithelial cells harden through a process called keratinization to protect your body from invading pathogens and injuries.
• They also pigment the skin. A protein (melanin) in your skin cells that influences your skin color, and whether or not you have freckles.
• Skin cells also keep you hydrated. Those outermost skin cells protecting your softer insides are great at trapping water beneath your skin.
• Inside your body epithelial cells secrete mucus. The lining of your esophagus, nasal passages, and intestines are all made of epithelial cells that lubricate these surfaces.

2. Blood Cells

Red and white blood cells circulate all over your body to deliver oxygen, carry away carbon dioxide waste, and play the starring role in your immune system. Their ubiquitous nature might make them seem simple, but there’s more to these blood cells than you might think.

Red blood cells (RBCs) are also called erythrocytes. They are unique because they do not have a nucleus (unlike most other cells). Because they lack a nucleus, RBCs are hollow in the center—kind of like a donut. Their unique shape makes them more efficient at exchanging and transporting oxygen molecules—their primary role in your body.

Without a nucleus and other organelle structures, RBCs cannot replicate on their own. Instead, your body generates new red blood cells in bone marrow tissue.

RBCs use a protein called hemoglobin to carry oxygen throughout your body. The hemoglobin inside RBCs gives blood its signature red color.

White blood cells (WBCs), or leukocytes, are agents of the immune system. They search for invading pathogens and initiate and complete your body’s immune responses.

There are two main classes of WBCs—granulocytes and mononuclear leukocytes:

• As the name suggests, granulocytes are WBCs filled with granules. Inside each granule are proteins and enzymes that can digest and destroy pathogens. Granulocytes are responsible for the creation of pus, and they play an important role in allergies.
• Mononuclear leukocytes don’t have granules. Instead, they have one large nucleus and special organelles called lysosomes. These lysosomes act like holding cells for microbes and other potential pathogens. Mononuclear leukocytes can use these lysosomes to trap and destroy invaders through a process called endocytosis.

3. Nerve and Brain Cells

The brain is full of spidery cells that allow you to think, read, move, and remember. Brain cells are the main components of your central nervous system. They use chemical messengers called neurotransmitters to communicate between other body cells.

Two kinds of brain cells exist in your body—neurons and glia. Both are necessary for efficient electro-chemical signaling throughout the body.

Neurons are web-shaped brain cells with a central body called a soma. All neurons have branched appendages called dendrites that can receive electro-chemical messages from neighboring neurons. A neuron can transmit signals along the longest portion of its cell body, called the axon.

Glial cells look a lot like neurons, but they differ in one important way: Glia cannot transmit electrical signals like neurons can. Their purpose is to support the transmission of electro-chemical signals from neurons by acting as insulation. Glial cells make it possible for tiny electro-chemical messages to travel the entire length of the body. Their insulating role speeds up signaling across long distances.

4. Muscle Cells

Your heart, hamstrings, and every other muscle in your body are composed of muscle cells—also known as muscle fibers. These fibers wrap tightly around each other like bundles of strong, stretchy cords to create your muscles.

Individual muscle fibers contain filamentous proteins that allow the fiber to lengthen and contract. These proteins are called actin, myosin, and titin. Each has a role in the contraction-relaxation cycle of a muscle fiber.

Nerve cells from the central and peripheral nervous system send messages to muscle fibers to coordinate your movements. Some muscle movements are voluntary, like lifting your hand to wave hello. Other muscle fiber contractions are unconscious or involuntary, such as the constriction of your pupils in bright light.

There are three main types of muscle fibers and muscle tissue, and each kind of tissue utilizes muscle fibers differently:

• Skeletal muscle cells are under conscious control. These muscle fibers attach directly to bones via tendons. Skeletal muscle fibers are long and cylindrical, like tubes that bundle together. These muscle cells are also multi-nucleate, which means they have more than one nucleus.
• Smooth muscles are made up of smooth muscle fibers. You can find smooth muscle fibers inside of the organs of your body. Your eyes, stomach, bladder, intestines, and blood vessels are all built with smooth muscle tissue. Unlike skeletal muscle, you do not have voluntary control over smooth muscle fibers.
• What makes cardiac muscle cells unique is their location. These muscle fibers can only be found in one place—your heart. Cardiac muscle cells are ultra-strong, elastic cells that allow your heart to pump blood in one coordinated and efficient heartbeat.

5. Fat Cells

Fat storage is a sometimes-taboo topic. But the cells that house fat are extremely valuable to your body. Adipocytes are fat cells, and when they assemble together they make up fat tissue.

Try to break out of the “fat = bad” mindset for a minute. Your body stores fat just like a bank stores money. Having fat on hand is essential when your body needs energy. And adipocytes house the fat your body wants or needs to save for later use.

Brown adipocytes are sometimes called “baby fat.” These fat cells are so named because you have lots of brown fat cells during infancy. The primary role of brown fat is thermogenesis (heat) and these adipocytes maintain body heat because they are full of mitochondria. Infants depend on brown fat reserves because they lack the ability to shiver or use other means to warm themselves.

As you age, your supply of brown fat cells shrinks, but doesn’t go away entirely. Current scientific research suggests that the mitochondria that fill brown fat cells disappear as you get older, causing the brown fat you stored in infancy to resemble white fat cells.

The main function of white fat cells is energy storage. When glucose isn’t available from your diet, a process called gluconeogenesis kicks in. Through gluconeogenesis, fat can be broken down and converted to usable glucose molecules to power the rest of the cells in your body. Fat available for gluconeogenesis comes from the white adipocytes you may be trying to burn during exercise.

Cellular Health Comes First

You are the cells in your body, and it’s important to take good care of them. No matter the cellular type, complete nutrition is the best way to help your cells thrive. Focus your eating on foods that will deliver nourishing vitamins, minerals, and macronutrients to your cells. Be picky about what you put into your body. Steer clear of over-processed and nutrient-poor foods. Instead, build your meals with the quality nutrition of whole foods—lean proteins, plant-based fats, fiber, vegetables, and fruit.

Prioritizing the health of your cells will pay dividends in your overall wellness. When your cellular health is soaring, you feel great, too. Channel your energy into supporting your cells with a diet rich in essential vitamins and minerals. Whole nutrition from quality foods can optimize the health and wellbeing of the cells that make you.