The systems of the human body

When we say “system of the human body” we mean a set of organs that work together to perform the same function in the human organism. The systems of the human body are:

  1. the skeletal/muscular system
  2. the muscular system
  3. the respiratory system
  4. the circulatory system
  5. the lymphatic system
  6. the digestive system
  7. the nervous system
  8. the endocrine system
  9. the immune system
  10. the urinary system
  11. the reproductive system
  12. the integumentary system
  13. the sensory system


1. The skeletal/muscular system

Provides support, strength, and shape to the body (bones, joints, ligaments, tendons, cartilage). Bones support the body, protect internal organs, provide attachment sites for muscles and facilitate movement, and serve as storage for calcium and phosphorus salts. We recycle our entire skeleton approximately every 10 years.

[Bone marrow: Bone marrow is the fluid tissue found inside bones (mainly of the pelvis and sternum) and contains the primitive blood-forming cells, which, when mature, transform (give rise to) blood cells (red, white blood cells, and platelets). In other words, bone marrow is the blood-producing factory.]


2. The muscular system

Controls the body’s movements through the skeletal muscles, maintains the body’s posture and temperature. Movement is caused by the alternating contraction and relaxation of the muscle fibers. There are three types of muscles: skeletal, smooth, and cardiac muscle. Muscles account for approximately 2/5 of our total body weight, and most are organized in pairs. The largest muscle is the gluteus maximus, and the smallest is the muscle of the stapes (in the ear). Muscles can pull on a bone, but they cannot push it.

In summary:

[Skeletal Muscles Consist of long, solid, parallel muscle fibers that can contract intensely and rapidly but only for short periods of time. These muscles keep the body balanced, move it, and cover it.


Smooth muscles are found inside the eye, in the walls of the digestive system, the respiratory tract, the blood vessels, the internal organs, the reproductive organs, and the urinary tract. They are composed of short fibers, without transverse striations, with a spindle-shaped appearance, thinner than the muscle fibers of the skeletal muscles, and they are characterized by slow contractions but prolonged duration. Their size is smaller than skeletal muscles and therefore, it is easier to arrange them in different directions in an organ, providing contraction throughout the area. These muscles essentially perform movements without being subject to our will. Contraction is regulated through neural pathways and endocrine factors.

Cardiac muscles do not directly obey the will and resist fatigue. They are composed of short, branched, properly connected muscle fibers that form a network in the walls of the heart. The myocardium contracts continuously rhythmically and tirelessly. These muscles essentially send blood throughout the body.]


3. The respiratory system

Takes in air, brings it into the lungs, receives oxygen, which is then carried through the blood to the organs, and releases carbon dioxide. The process of taking in oxygen and expelling carbon dioxide is called “breathing”. The organs of the respiratory system are: the nasal cavities, pharynx, larynx, trachea, bronchial tree, and lungs.


Hiccup is caused by irritation of the diaphragm, which contracts more intensely (often due to distension of the stomach from fast ingestion).


Cough eliminates excess mucus and impurities that obstruct the respiratory tract, mainly the trachea, during breathing. It involves inhaling, holding the breath, and violently expelling it to sweep everything away.


The calm breathing during rest does not help to eliminate enough carbon dioxide that accumulates in the blood. The brain triggers a big breath to expel it.


Dust irritates the nasal cavity and, after a big inhale, it is expelled by sneezing (which reaches a speed of more than 160 km/h – some claim up to 450 km/h!).]


4. The circulatory system

The circulatory system or cardiovascular system spreads like a network throughout the body. It carries blood to all the tissues, supplying them with oxygen and nutrients, and removing waste products. The main components of the circulatory system are the heart, blood vessels (arteries, veins, and capillaries), and blood.


5. The lymphatic system

It is a network of vessels and glands (lymph capillaries, lymphatic vessels, lymphoid organs). It transports the excess fluid of tissues (lymph) towards the blood. Lymph is a colorless fluid that mediates the delivery of nutrients to cells and the removal of waste products. It contributes to the body’s defense by producing antibodies and destroying pathogenic microorganisms and cancer cells.


6. The digestive system

Ensures the passage, digestion, absorption of nutrients, and elimination of waste through the rectum in the form of feces. The organs of the digestive system are the oral cavity, the salivary glands, the pharynx and esophagus, the stomach, the pancreas, the liver, the gallbladder, the small and large intestine.


7. The nervous system

Controls the function and harmonious cooperation of all organs, contributes to thinking, perception, communication, and emotion. The organs of the nervous system are the brain, nerves, and senses (nervous and sensory system).


8. The endocrine system

Produces hormones, regulates the function of tissues, and transmits chemical messages throughout the body. Glands create hormones, which are chemical substances that control complex and important organic processes such as reproduction, metabolism, etc. The endocrine glands include the hypothalamus, pituitary gland, pineal gland, thyroid gland, thymus gland, adrenal glands, pancreas, ovaries, and testes.


9. The immune system

The immune system is responsible for the defense of the organism. It fights diseases and infections. It consists of many organs and tissues, with the most important being the bone marrow and the thymus gland. Secondary organs include the tonsils, spleen, lymph nodes, etc.


10. The urinary system

Produces and eliminates urine and waste products from metabolism. The kidneys filter blood and convert waste into urine, which is stored in the bladder. It maintains the balance of water and electrolytes. It consists of the kidneys, ureters, bladder, and urethra.


11. The reproductive system

Reproduces living beings. The male sexual organs produce sperm and the female organs produce eggs and receive the embryo until birth. The male reproductive system consists of the two testes with their scrotum (the sac that surrounds the testes), the epididymis (organs of maturation and storage of sperm), the spermatic ducts, the seminal vesicles, the prostate gland, and the penis with the urethra. The female reproductive system consists of the two ovaries, the two fallopian tubes, the uterus and cervix, the vagina, and the vulva.


12. The integumentary system

Protects all the systems. It protects against diseases. It is an important sensory interface with the environment. It consists of the skin, hair, nails, sweat glands, and sebaceous glands. The skin protects against sunlight, retains fluids in the body, and plays an active role in the sense of touch. Together with the hair, it controls the body’s temperature. The skin is the largest organ in the human body.


13. The system of sensory organs

It helps the organism to perceive and communicate with the environment. The sensory organs belong to the nervous system because they use specific nerves to transmit stimuli from the environment, converting them into electrical signals, and transforming them into sensations. The senses are five: smell, taste, vision, touch, and hearing. The sensory organs that serve the five senses are respectively: the nose, the tongue, the eyes, the skin, and the ears.


Types of Tissues

There are four types of tissues: epithelial, connective, muscular, and nervous.

1. Epithelial: The epithelial tissue is made up of polyhedral cells that are closely connected to each other and have minimal intercellular substance. They form surfaces that cover the outside of the body or line various internal cavities. The role of epithelial tissue is mainly protective. It also removes mucus and dust, allows for diffusion and absorption of substances, and contributes to the production and secretion of products.


2. Connective Tissue: The connective tissue consists of cells that are located within an abundant extracellular matrix. The extracellular matrix may contain two types of protein fibers, collagen, which provides strength and elasticity, and elastin, which provides more elasticity. The connective tissue connects structures together, provides support, and protection. It is classified into connective tissue proper, cartilage tissue, and bone tissue.

2a. Connective Tissue Proper: Cells with abundant extracellular matrix. The connective tissue proper is classified into loose and dense. Loose connective tissue is mainly found in the dermis. Its extracellular matrix contains fibers of collagen and elastin. The extracellular matrix of dense connective tissue is mainly composed of collagen fibers in bundles. It is found at the joints and in tendons that connect the skeletal muscles to bones. Adipose tissue is a special type of loose connective tissue, whose cells (adipocytes) store fat.


2b. Cartilage: Cartilage tissue is solid and at the same time flexible. Its cells, the chondroblasts, are located within cavities of the extracellular matrix. This tissue is found in articular cartilage, the pinna of the ear, intervertebral discs, etc.


2c. Bone: Bone tissue, which is found in bones, consists of an extremely hard extracellular matrix, which contains salts and collagen fibers. Osteocytes are located within cavities of this matrix.


3. Muscle: Elongated cells that have the specialized function of contraction. Muscle tissue consists of cells, the muscle fibers, which have the ability to contract, allowing movement. There are three types of muscle tissue:

3a. Skeletal: Skeletal muscle tissue is found in skeletal muscles and consists of relatively long cylindrical muscle fibers, which bear stripes. Their contraction is under our control.


3b. Cardiac Muscle: The muscular tissue of the heart (myocardium) is located only in the walls of the heart. Its muscle fibers are cylindrical, have striations, but do not obey our will.


3c. Smooth: Smooth muscle tissue mainly lines the walls of structures such as blood vessels and the gastrointestinal tract. It consists of spindle-shaped and non-striated muscle fibers that do not obey our will.


4. Nervous: Cells with branches that extend far from the cell body. The nervous tissue consists of nerve cells or neurons and neuroglial cells. Neurons are specialized cells with branches for the production and transmission of nervous impulses. Neuroglial cells support, insulate and nourish the neurons. We observe that different types of cells may coexist in a tissue, but they participate in the same function.


Some of these systems are of particular interest to sports (skeletal, muscular, nervous, etc.) and are analyzed subsequently.