Video: Structural organization of the human body

Our bodies are like wonderful machines. They help us run, play, create ideas, solve problems, enjoy food and music. This is all thanks to our organs that work together in harmony. Like everything in the universe, we are all made up of small particles called atoms. But how do we go from tiny atoms to fully functioning human beings?

That's what we're going to learn in this tutorial on the structural organization of the human body.

Structurally, the human body can be organized into six levels. Atoms and molecules form the smallest level, the chemical level of organization. Molecules organize to form organelles and cells. Cells unite to form tissues. Tissues combine to form organs. And organs work with each other to form an organ system. The highest level of organization is the organism, involving all these organ systems working together to ensure the machine runs smoothly.

Let's take these levels two at a time, starting with the chemical and cellular levels of organization.

Chemically, our bodies are made up of matter which consists of elements. Remember the periodic table from chemistry class? Elements from that table make up the chemical constitution of our bodies. Most of the body is formed by four major elements: oxygen, carbon, hydrogen, and nitrogen. Other elements are present in smaller proportions but are not less important, like calcium in bones, sodium and potassium for neurons to conduct impulses, as well as phosphorus and magnesium, and so on.

Elements are made up of atoms, which in turn consists of subatomic particles such as protons, neutrons, and electrons. The atom, however, is the smallest unit of the element that still retains its unique properties. Atoms come together to form molecules. This combination could involve atoms of the same element, like two hydrogen atoms forming a molecule of hydrogen, or could involve atoms of different elements like hydrogen and oxygen forming water.

The body includes inorganic compounds such as water, salts like sodium chloride, acids and bases, and organic compounds like carbohydrates, lipids, proteins, and nucleic acids. These macromolecules organize themselves to form the organelles of a cell.

The cell is the structural and functional unit of all living things. It works like a little factory with all the organelles performing specific functions in their respective departments. Not all cells of the body look or function the same, but in general, most have a nucleus, plasma membrane, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and cytoskeletal structures.

The structure and appearance of a cell usually depends on its function. For example, red blood cells that carry oxygen to tissues don't have a nucleus. Skeletal muscle cells that need lots of energy for contraction have a large number of mitochondria. Neurons that carry information around the body have long processes known as axons.

When similar cells are combined to perform their specific function, they form tissues. That brings us to the next two levels: the tissue and organ levels of organization.

There are four main types of tissues in the body: epithelial, connective, muscle, and nervous tissue.

Epithelial tissue includes epithelial cells that have a variety of functions. Some of them include protection from the outside environment by covering the surface of the body in our skin; secretion of sweat, mucus, and hormones by lining glands; and absorption of nutrients and water in organs such as the stomach and small intestine.

The second type of tissue is connective tissue, which connects cells and tissues. This is important for support. Special types of connective tissue such as bone, tendons, and cartilage are essential for locomotion. Others like adipose tissue serve as shock absorbers and store energy. Blood cells, like we saw earlier, are also part of connective tissue, transporting oxygen to tissues.

Muscle tissue is what makes our limbs move. That's thanks to the contraction of muscle cells which belong to skeletal muscle tissue. But that's not all. Muscle tissue also includes cardiac muscle tissue for contraction of the heart, pumping blood into blood vessels; and smooth muscle tissue in the walls of hollow organs like the digestive tract, which propels food through the tract by a process known as peristalsis.

The last type of tissue is nervous tissue, found in the brain, spinal cord, and nerves. This includes neurons, which carry information rapidly around the body, and glial cells, which support the neurons and maintain the environment outside the cells.

When two or more tissue types combine together, they form organs like the heart, kidney, lungs, and stomach. Let's look at an example.

In this image of the stomach wall, we can see epithelial tissue lining the surface with glands, connective tissue providing support underneath, and muscle tissue with smooth muscles needed for peristalsis. All together they form an organ -- the stomach.

Organs don't work alone. They work in teams -- the organ systems.

There are 11 organ systems in the human body. Let's learn a little about each of them, starting with the biggest one, the integumentary system. This includes the skin and its appendages, such as hair, nails, sweat, and sebaceous glands. By lining the surface of the body, this system is essential for protection, forming a barrier and our first line of defense against the germs of the outside world.

Our skin is necessary for the regulation of body water and temperature using our sweat glands. It also plays an important role in vitamin D synthesis using the sun's ultraviolet rays. Underneath skin, we have muscles and bones.

Bones and joints form the skeletal system. Aside from the obvious requirement for support and locomotion, bones also store calcium and participate in red blood cell synthesis. Skeletal muscles work with bones to enable movement. They form the muscular system.

Movement and many other functions are governed by our nervous system, the control center, which includes the brain, spinal cord, and nerves. This system perceives sensory information, processes it, and generates motor and glandular responses. It regulates organs in other systems, such as the heart and lungs. Our brains are also essential for higher cognitive abilities such as learning, language, and memory.

Another system that regulates body functions is the endocrine system, which includes organs such as the pituitary gland, thyroid and parathyroid glands, the suprarenal glands, and so on. These organs use chemical messages known as hormones to bring about their effects all over the body. They regulate metabolism, growth, and reproduction.

Our sixth system is the respiratory system, including the nose, pharynx, larynx, tracheobronchial tree, and lungs. This system performs the important function of gas exchange, allowing oxygen from air to enter the bloodstream and remove carbon dioxide. It works with the cardiovascular system, which includes the heart and blood vessels.

The heart is the muscular pump that propels blood into vessels. Blood vessels transport blood all around the body, not only supplying tissues with oxygen and nutrients, but removing carbon dioxide and waste as well.

Another set of vessels in the body are the lymphatic vessels, which along with organs like the thymus, lymph nodes, and spleen form the lymphatic system. This system returns extra fluid from tissues back to circulation and also houses important cells needed for immunity -- our white blood cells or leukocytes.

Our ninth system is the digestive system. Starting from the mouth, this system includes the esophagus, stomach, small and large intestines, and anal canal. It also includes accessory organs like the liver, gallbladder, and pancreas which assist in the process of digesting food. This system ensures water and nutrients are absorbed, fluid and electrolyte balance is maintained, and waste products are removed from the body.

Another pair of organs that participate in waste management are the kidneys, which form the urinary system along with the ureters, urinary bladder, and urethra. They remove metabolic waste from blood in the form of urine and are important for water, electrolyte, and acid and base balance in the body.

And finally, we've reached the male and female reproductive systems. The male reproductive system includes organs like the penis, prostate, seminal glands, and testes, while the female system includes the vagina, uterus, uterine tubes, and ovaries. These systems synthesize hormones and form gametes, the sperm cells and ova for reproduction. The embryo and fetus grow, develop, and receive nourishment in the uterine cavity.

The interaction of all these 11 systems results in a smoothly functioning machine, the organism. Thus, step by step, we've built an organism from an atom, a process that involves six progressively larger levels.

The next time you're out running or playing, notice how your breathing changes, your heart beats faster, and your muscles contract. They're all reminders of how these systems work seamlessly to help us live our lives.

Keep building your knowledge of the human body with our study units and quizzes at Kenhub!