how the human body works essay

Essay on Human Body

Students are often asked to write an essay on Human Body in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Human Body

The marvel of the human body.

The human body is a complex, wonderful machine. It consists of many parts, all working together.

Body Systems

There are many systems in our body, like the skeletal system that provides structure, and the muscular system that allows movement.

Organs and Cells

Organs like the heart and lungs perform vital functions. Our body is also made up of trillions of tiny cells.

Body’s Defenses

The immune system protects us from disease, while the skin serves as a barrier against the outside world.

250 Words Essay on Human Body

Introduction.

The human body, a complex biological system, is a marvel of evolution. It comprises numerous organs, tissues, and cells, all working in harmony to maintain life. This essay delves into the intricacies of the human body, highlighting its major components and their functions.

Structural Organization

At the most basic level, cells form the building blocks of the human body. These cells group together to form tissues, which further combine to create organs. Each organ has a specific function, contributing to the body’s overall health and survival.

Systems of the Human Body

The body is divided into several systems, including the nervous, circulatory, respiratory, digestive, endocrine, and musculoskeletal systems. Each of these systems plays a crucial role. For instance, the nervous system, which includes the brain and spinal cord, controls body functions and enables cognition and consciousness.

Maintenance and Regulation

The human body’s homeostasis is fundamental to its functioning. This involves maintaining a constant internal environment, such as body temperature and pH balance. The endocrine system, with its hormones, and the nervous system play significant roles in this regulation.

In conclusion, the human body is an intricate and efficient system, a testament to the wonders of nature and evolution. Its complexity and functionality are a constant subject of study, offering endless possibilities for research and advancement in the field of medicine and biology.

500 Words Essay on Human Body

Structural complexity.

The human body is composed of several levels of structural organization. At the most basic level, we find cells – the building blocks of life. These cells group together to form tissues, which in turn combine to form organs. The organs then work together in organ systems to perform specific functions. The human body comprises eleven organ systems, each with its own unique role, yet they all work in unison to maintain homeostasis.

Functional Dynamics

The functionality of the human body is a testament to the intricate design and coordination of its systems. The circulatory system, for instance, is responsible for the transportation of nutrients, oxygen, and waste products around the body. The nervous system, on the other hand, acts as the body’s control center, sending and receiving signals to and from different parts of the body. The respiratory system facilitates the exchange of gases, while the digestive system breaks down food into nutrients that the body can use. These systems, among others, work in a coordinated fashion to ensure the smooth functioning of the body.

The Symbiotic Relationship

Adaptability and resilience.

One of the most remarkable characteristics of the human body is its adaptability and resilience. It can adapt to various environmental conditions, from the freezing temperatures of the Arctic to the scorching heat of the desert. The immune system, a complex network of cells, tissues, and organs, defends the body against harmful microorganisms, demonstrating the body’s resilience. Furthermore, the body has remarkable healing capabilities, with systems in place to repair damage and restore function.

In conclusion, the human body is an intricately designed system that showcases the marvels of evolution. Its structural complexity, functional dynamics, symbiotic relationship between systems, and adaptability and resilience are awe-inspiring. Understanding the human body not only allows us to appreciate the marvel that it is but also equips us with the knowledge to take better care of it. Indeed, the human body is a testament to the sophistication and beauty of biological engineering.

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What are the systems of the body? Fast facts about the human body and how it works

Learn all about the human body's many systems and some of its individual organs, both vital and vestigial.

The human body is a complex network of systems that work together to keep life-sustaining processes running smoothly. These systems break down food for fuel, clear away waste, repair damaged tissues and DNA, fight infectious germs and monitor the outside world so we can move through it safely. 

Many scientists spend their days working to understand how each bodily system performs its jobs, how the systems interact, and what can happen when one or more of them falter. Such malfunctions can stem from aging or disease, for instance, and through medical care, doctors aim to get derailed systems back on track. 

Here's a quick rundown of the systems of the human body, its vital organs and its "vestigial" organs, as well as a few fascinating facts about how the body works.

What are the different systems of the human body? 

Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living. Some organs and tissues play roles in multiple systems at once.

Related: Strange, two-faced brain cells confirmed to exist, and they may play a role in schizophrenia  

Circulatory : The job of the circulatory system is to move blood, nutrients, oxygen, carbon dioxide and hormones around the body. It consists of the heart, blood, blood vessels, arteries and veins. According to the Cleveland Clinic , the adult human body's network of blood vessels is more than 60,000 miles (around 100,000 kilometers) long. 

Digestive: The digestive system consists of a series of connected organs that together allow the body to break down and absorb nutrients from food and remove waste. It includes the mouth, esophagus, stomach, small intestine , large intestine, rectum and anus. The large intestine is home to microorganisms that are collectively called the gut microbiome and influence our health in various ways . The liver and pancreas also have roles in the digestive system because they produce digestive juices filled with enzymes to break down the components of food, such as carbohydrates , fats and proteins , according to the National Institute of Diabetes and Digestive and Kidney Diseases .

Endocrine: The endocrine system consists of a network of glands that secrete hormones — long-range chemical messengers that regulate how cells and tissue function — into the blood. These hormones, in turn, travel to different tissues and regulate many bodily functions, such as metabolism , growth and sexual function, according to Johns Hopkins Medicine . For example, the pancreas releases the hormones insulin and glucagon to regulate blood sugar . Conditions like diabetes and insulin resistance arise from the body having too little insulin or not responding to it adequately. 

Related: Meet the 'exclusome': A mini-organ just discovered in cells that defends the genome from attack

Immune: The immune system is the body's defense against bacteria , viruses and other pathogens that may be harmful. Components of the system include the lymph nodes , which contain infection-fighting cells called lymphocytes. These lymphocytes are one of many types of leukocyte , or white blood cell. The immune system also includes the spleen , the bone marrow and a gland called the thymus . The immune system can learn to recognize antigens — proteins on the surface of bacteria, fungi and viruses — and alert the body to their presence. Some immune cells make proteins called antibodies that attach to these antigens and mark invaders for destruction. 

Lymphatic: The lymphatic system includes the lymph nodes, lymph ducts and lymph vessels and is considered part of the immune system. Its main job is to make and move lymph , a clear fluid that contains white blood cells. The lymphatic system also removes excess lymph fluid from the body's tissues and returns it to the blood.

Nervous: The nervous system controls both voluntary actions, such as conscious movements, and involuntary actions,like breathing, and it sends signals to and detects signals from different parts of the body. Conscious actions are controlled by the somatic nervous system, while involuntary actions are controlled by the autonomic nervous system. The autonomic nervous system dictates whether we're in " rest and digest " or " fight or flight " mode. The nervous system can further be split up into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system, or the nerves connecting the CNS to every other part of the body.

Muscular: The body's muscular system consists of hundreds of muscles that aid movement, blood flow and other bodily functions, according to the Library of Congress . There are three types of muscle: skeletal, which is connected to bone and helps with voluntary movement; smooth, which is found inside organs and helps to move substances through them; and cardiac, which is found in the heart. The body's largest muscle by mass is the gluteus maximus, but the two latissimus dorsi are the largest in terms of surface area.

Related: Why is it harder for some people to build muscle than others?

Reproductive: The reproductive system allows humans to produce offspring. The male reproductive system includes the penis and the testes , which produce sperm. The female reproductive system includes the vagina, uterus and ovaries, which produce eggs. During fertilization, a sperm cell will fuse with an egg cell that, in a successful pregnancy, will then implant in the uterus. The fertilized egg will then mature into what's called a blastocyst, then an embryo and, finally, a fetus. A placenta forms to support this process. 

Skeletal: Our bodies are supported by the skeletal system , which contains between 206 and 213 bones in an adult human body, due to slight variations in people's anatomy, according to the medical resource StatPearls . These bones are connected by tissues called tendons, ligaments and cartilage. As infants, humans have about 300 bones , but some fuse together as the child grows. The skeleton not only helps us move but is  also involved in the production of blood cells and the storage of calcium. The teeth are also part of the skeletal system, but they aren't considered bones . The smallest bones in the body are found in the ear, and the largest is the femur, or thigh bone, which is also one of the heaviest body parts .

Respiratory: The respiratory system allows us to take in oxygen and expel carbon dioxide through breathing. It includes the lungs ; trachea, or windpipe; and the diaphragm, a muscle that pulls air into and pushes air out of the lungs.

Urinary: The urinary system helps eliminate a waste product called urea, which is produced when certain foods are broken down. The system includes the two kidneys; two ureters, or tubes leaving the kidneys; the bladder; two sphincter muscles; and the urethra. The kidneys filter blood in the body to make urine that then travels down the ureters to the bladder and exits the body through the urethra.

Integumentary: The skin, hair and nails make up the integumentary system. Skin is the body's largest organ . It protects our innards from the outside world, serving as our first defense against bacteria, viruses and other pathogens, for instance. Our skin also helps regulate body temperature and eliminate waste through perspiration, or sweat. 

Related: Scientists discover new way humans feel touch  

What are the body's vital organs?

Click the purple circles to learn about the body's vital organs, including the brain, lungs, heart, liver and kidneys. They're considered vital because you need a functioning brain, heart, liver, at least one kidney and at least one lung to survive. That said, there are medical devices and treatments that can make up for a loss of function in these organs, at least temporarily — for example, ECMO machines can do the work of the heart and lungs, and dialysis can filter the blood of people with kidney failure.

• The average adult male body contains about 36 trillion cells , the average adult female body contains 28 trillion cells and a 10-year-old has about 17 trillion. 
• It's often said that there are 78 organs in the human body , but the number actually differs depending on whom you ask. 
• There's a popular idea that the body replaces itself every seven years . But that's not really true, because tissues renew themselves at different rates. 
• Oxygen is the most common element in the human body , followed by carbon. 
• The average adult body contains about 1.2 to 1.5 gallons (4.5 to 5.5 liters) of blood . 
• Humans' average body temperature has fallen slightly over time, so it's no longer 98.6 degrees Fahrenheit (37 degrees Celsius). 
• The most detailed map of the human brain to date contains more than 3,300 types of brain cells . 

What are vestigial organs?

There are arguably some parts of the human body that don't serve any useful purpose, such as the male nipple. That said, the usefulness of some organs is still up for debate , as scientists have often judged the worth of body parts before discovering their purposes. 

Broadly speaking, vestigial body parts are defined as those that have lost their original physiological significance to humans over the course of evolutionary history. The idea is that, while we inherited them from an ancient ancestor, we could really do without them in the modern day. 

— Scientists just discovered a new way cells control their genes — it's called 'backtracking'

— New part of the body found hiding in the lungs

— Scientists stumble upon a new part of a cell in one of the most studied animals on Earth  

Wisdom teeth are held up as one example of a vestigial body part, as the modern human jaw is often too small to accommodate a third set of molars. Some people also carry remnants of a vomeronasal organ that is largely thought to be nonfunctional in humans; animals use equivalent organs to detect each other's pheromones. 

Some scientists consider the human tailbone, or coccyx, vestigial because it's no longer a full-blown tail. But it's far from useless, as it still anchors many muscles, ligaments and tendons. And the appendix has gotten a bad rap for supposedly being both vestigial and useless, but more recently, scientists have uncovered possible functions for the long-maligned body part. 

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun ? Send us your questions about how the human body works to [email protected] with the subject line "Health Desk Q," and you may see your question answered on the website!

Editor's note: This page was last updated on April 5, 2024.

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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.

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Human body systems

Author: Jana Vasković, MD • Reviewer: Nicola McLaren, MSc Last reviewed: November 03, 2023 Reading time: 24 minutes

The human body is a biological machine made of body systems; groups of organs that work together to produce and sustain life. Sometimes we get lost while studying about cells and molecules and can’t see the forest for the trees. It can be helpful to step back and look at the bigger anatomical picture.

This topic page will provide you with a quick introduction to the systems of the human body, so that every organ you learn later on will add a superstructure to the basic concept you adopt here.

Skeletal system

Muscular system, cardiovascular system.

• Respiratory system

Central nervous system

Peripheral nervous system, somatic and autonomic nervous systems.

• Digestive system

Urinary system

Endocrine system.

• Lymphatic system

Reproductive system

• Integumentary system

Related articles

The skeletal system is composed of bones and cartilages . There are two parts of the skeleton; axial and appendicular. The axial skeleton consists of the bones of the head and trunk . The appendicular skeleton consists of the bones within the limbs, as well as supporting pectoral and pelvic girdles .

There are 206 bones in an adult human body. The place at which two bones are fitted together is called the joint or articulation. Joints are supported by cartilages and reinforced with ligaments . Functions of the skeletal system are mechanical support, movement , protection, blood cell production, calcium storage and endocrine regulation.

Elements of the skeletal system are adjusted to the function of the body part they support. Thus, the anatomy of bones, joints and ligaments is studied topographically, as the bones of the; head and neck , thorax , abdomen , upper and lower limbs .

Get started with skeletal system anatomy by checking out the study unit and custom quiz below.

The muscular system consists of all the body muscles. There are three muscle types ; smooth , cardiac and skeletal muscles. Smooth muscle is found within walls of blood vessels and hollow organs such as the stomach or intestines. Cardiac muscle cells form the heart muscle, also called the false . Skeletal muscles attach to the bones of the body.Among these three, only skeletal muscles can be controlled consciously and enable us to produce body movement, while the function of other two muscle types is regulated by the autonomic nervous system and is absolutely unconscious.

Histologically, skeletal and cardiac muscle fibers are arranged in a repetitive fashion giving a striped appearance, hence are called striated muscle .

Smooth muscle does not contain repetitive sarcomeres , thus is non-striated muscle.

Learn all about the muscular system in the study unit below, or consolidate what you already learned with our fully customizable quiz.

The cardiovascular system is comprised of the heart and the circulatory system of blood vessels. The heart is composed of four chambers; two atria and two ventricles . Blood enters the heart through the upper chambers of the left and right atria and exits via the left and right ventricles. Heart valves prevent the backflow of blood.

The heart acts as a two-way pump. The right side of the heart pumps deoxygenated blood into the pulmonary circulation of the lungs , where the blood is reoxygenated again. While the left side of the heart simultaneously pumps oxygenated blood into the systemic circulation, distributing it to the peripheral tissues . The regular pumping, or heartbeat , is controlled by the conduction system of the heart .

The circulatory system, also called the vascular system, consists of arteries, veins and capillaries . They all comprise a continuous network of vessels which act to carry blood around the body. Blood leaves the heart via arteries , these progressively reduce in size to continue as smaller arterial vessels called arterioles . Arterioles end in a web of even smaller vessels called capillaries . The exchange of gases and nutrients occurs through the capillary walls.

Small veins, called venules , leave from capillaries and gradually increase their lumen on the way to the heart to end as veins . There is a certain histological difference between arteries and veins , but their main functional difference reflects the direction in which they conduct blood: the arteries convey blood from the heart to the periphery, whereas the veins convey blood from the periphery to the heart. 

There are three separate circuits to the circulatory system.

• The pulmonary circulation which carries blood between the heart and the lungs;
• The coronary circulation which supplies blood to the muscle of the heart;
• And the systemic circulation which carries blood to the rest of the body.

Major arteries within the systemic circulatory system are the aorta and its branches, while the main representatives of the veins are the superior vena cava and inferior vena cava .

Learn everything about the heart, arteries and veins faster with our cardiovascular system diagrams, quizzes and free worksheets .

Major functions of the cardiovascular system include transportation of oxygen, nutrients and hormones throughout the body within the blood, and as well as eliminating carbon dioxide and other metabolic waste.

Learn more about the major arteries, veins and nerves of the body with Kenhub resources!

The respiratory system consists of a series of organs; the nasal cavity , pharynx , larynx , trachea , bronchi , bronchioles and lungs ( alveoli ). The nasal cavity and pharynx are together called the upper respiratory system , while the remainder of the organs comprise the lower respiratory system .

Respiratory system organs, with the exception of the alveoli, function to conduct air into the lungs aided by the muscles of respiration (mainly the diaphragm and intercostal muscles ).

Once air is in the lungs it enters alveoli (the site of gas exchange) and interacts with blood transported by the pulmonary circulation. Here carbon dioxide is removed from, and oxygen returned to, the blood. Thus the major respiratory system function is to bring oxygen into the body and expel carbon dioxide. 

Fortify your knowledge about the respiratory system with this content we have prepared for you.

Nervous system

Nervous system controls how we interact with and respond to our environment, by controlling the function of the organs in our other body systems. The nervous system organs are the brain , spinal cord and sensory organs. These are connected by neurons , which act to transmit neural signals around the body. 

Morphologically and topographically, the nervous system is divided into the central (CNS) and peripheral (PNS) nervous systems. Whilst functionally, the nervous system is considered as two parts; the somatic (SNS) or voluntary nervous system, and the autonomic (ANS) or involuntary nervous system.

The  central nervous system definition is that it receives information from the body’s environment and generates instructions, thereby controlling all the activities of the human body. This two-way information flow into, and out of, the CNS is conveyed by the peripheral nervous system. 

The CNS consists of the brain and spinal cord. The brain is placed within the  neurocranium , and is formed from the cerebrum , cerebellum and brainstem ( pons and  medulla oblongata ). The central parts of the CNS are occupied by spaces called ventricles filled with cerebrospinal fluid (CSF) . The spinal cord is placed within the vertebral column . The spinal canal extends through the central part of the spinal cord. It is also filled with CSF and it communicates with the ventricles of the brain.

The CNS is made of neurons and their processes ( axons ). Gray matter is made of neuron cell bodies, it is found in the cerebral cortex and the central portion of the spinal cord. White matter is made of axons, which combine and build neural pathways . The gray matter is where the instructions generate, while the white matter is the path through which the instructions travel toward the organs.

The peripheral nervous system definition is that it conducts information from the CNS to the target tissues, and from the target tissues to the CNS. It consists of nerves and their ganglia . Nerves that carry information from peripheral sense organs (for example eye , tongue , nasal mucosa, ear , skin ) to the CNS are called the ascending, afferent or sensory nerve fibers. Fibers that carry information from the CNS to the periphery (muscles and glands) are the descending, efferent , motor or secretory nerve fibers.

A ganglion is a cluster of neural tissue outside of the CNS, made of neuronal cell bodies. Ganglia can be both sensory and autonomic. Sensory ganglia are associated with spinal nerves and some cranial nerves ( V , VII , IX , X ). 

Peripheral nerves emerge from the CNS. There are 12 pairs of cranial nerves which arise from the brain, and 31 pairs of spinal nerves which extend from the spinal cord. Cranial nerves are named I to XII, determined by their skull exit location (anterior to posterior). Spinal nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal nerve , depending on vertebral level from which they arise. In certain areas of the body peripheral nerves interconnect, creating neural networks called plexuses . Notable plexuses are the:

• Cervical plexus (C1-C4) – innervates the back of the head , some  neck muscles , pericardium and diaphragm via great auricular, transverse cervical nerve , lesser occipital, supraclavicular, and phrenic nerves .
• Brachial plexus (C5-T1) – innervates the upper limb with nerves such as median , ulnar , radial , musculocutaneous  and  axillary nerve .
• Lumbar plexus (L1-L4) – innervates the muscles and the skin of the abdomen and pelvis , as well as thigh muscles via iliohypogastric, ilioinguinal, genitofemoral , lateral femoral cutaneous, obturator, femoral nerves .
• Sacral plexus (S1-S4, with branches from L4, L5) – innervates the muscles and skin of parts of the pelvis, posterior thigh , lower leg and foot via the following nerves; gluteal, sciatic , posterior femoral cutaneous, pudendal, nerve to piriformis, nerve to obturator internus , and nerve to quadratus femoris . 

The somatic nervous system (SNS) and autonomic nervous system (ANS) are divisions of the peripheral nervous system, with information conveyed through the cranial and spinal nerves. 

The somatic nervous system definition is that it allows voluntary control over our movements and responses. It conveys sensory and motor information between the skin, sensory organs, skeletal muscles and the CNS; establishing communication of the human body with its environment and response to outside stimuli. Major somatic peripheral nerves include the median nerve, sciatic nerve and femoral nerve. 

The autonomic nervous system definition is that it controls all the internal organs unconsciously, through the associated smooth muscle and glands . Functionally, the ANS is divided into sympathetic   (SANS) and parasympathetic   (PANS) autonomic nervous systems. The sympathetic nervous system definition is informally known as producing the „flight or fight“ state as it is the part of the ANS which is mostly active during stress.PANS dominates during rest, and is more active in „rest and digest“ or „feed and breed“ activities. The centers of SANS and PANS are within the brainstem and spinal cord, and they communicate with SANS and PANS ganglia located throughout the body. Note that there isn’t any pure SANS or pure PANS nerve, instead their fibers are added to the specific somatic nerves, making them mixed.

The digestive system function is to degrade food into smaller and smaller compounds, until they can be absorbed into the body and used as energy. It consists of a series of gastrointestinal tract organs and accessory digestive organs.

The digestive system organs spread from the mouth to the anal canal. So it’s actually a tube consisting of the mouth , pharynx , esophagus , stomach , small intestine , large intestine , and anal canal . Accessory digestive organs assist with the mechanical and chemical food breakdown, these are the tongue, salivary glands , pancreas , liver and gallbladder .

Master the digestive system anatomy starting with this study unit and custom quiz:

Urinary system is a body drainage system comprised of the group of organs that produce and excrete urine. It consists of the kidneys, ureters , urinary bladder and urethra .

Kidneys  are paired bean-shaped organs placed retroperitoneally. The kidneys have a rich blood supply provided by the renal artery . Nephrons within the kidneys filter the blood that passes through their web of capillaries ( glomerulus ). The blood filtrate then passes through a series of tubules and collecting ducts, eventually forming the final ultrafiltrate, urine . Urine passes into the ureters , tubes of smooth muscle that convey urine from the kidneys to the urinary bladder . The bladder is a hollow muscular organ that collects and stores urine before disposal by urination (micturition). Functions of the urinary system include; elimination of body waste, regulation of blood volume and blood pressure, regulation of electrolyte levels and blood pH.

Get started with the urinary system with these resources:

The endocrine system is a collection of specialised organs (endocrine glands) scattered throughout the body that act to produce hormones. The main organs of the endocrine system can be seen in the diagram below.

With regards to the endocrine system function; hormones produced by the endocrine system act to regulate a wide variety of bodily functions, such as triiodothyronine which regulates metabolism, or estrogen and progesterone which regulate the menstrual cycle. Endocrine glands secrete hormones directly into the circulatory system to regulate the function of distant target organs. 

We have you covered with everything you need to know about the endocrine system here.

The  lymphatic system  is a network of lymphatic vessels that drains excess tissue fluid (lymph) from the intercellular fluid compartment, filters it through lymph nodes, exposes it to lymphocytes (white blood cells) of the immune system and returns the fluid to the circulatory system. The lymphatic system consists of lymph, lymphatic plexuses, lymphatic vessels, lymph nodes and lymphoid organs. The lymphatic system function is to; convey and eliminate toxins and waste from the body; recirculate proteins; and defend the body from microorganisms.

Lymph is a watery tissue fluid with a similar consistency to blood plasma. It starts as interstitial fluid which occupies the spaces between cells. Excess fluid is picked up by lymphatic capillaries and transported through lymphatic plexuses into lymphatic vessels , filtering through lymph nodes along its journey. Superficial lymphatic vessels are found in the subcutaneous tissue alongside veins. They drain into deep lymphatic vessels that follow the arteries. Lymphatic vessels empty into larger lymphatic trunks, which unite to form one of the two main collecting ducts; the thoracic duct and the right lymphatic duct .

The thoracic duct begins at the cisterna chyli , collecting lymph from the left side of head, neck and thorax, left upper limb, abdomen and both lower limbs and draining it into the left venous angle (junction of the left internal jugular and left subclavian veins). The right lymphatic duct drains the rest of the body and empties into the right venous angle. From the venous angles, cleaned lymph is returned to the circulatory system, rejoining with the fluid of the blood. Note that the central nervous system was previously thought to have no lymphatic vessels. However, recent research has shown its lymph is drained by lymph vessel-like structures found in the meninges.

Lymphatic system organs are divided into primary and secondary organs. Primary lymphatic organs produce lymphocytes and release them into lymphatic vessels. The two primary lymphoid organs are the thymus and red bone marrow . Secondary lymphatic organs include lymph nodes, tonsils , appendix and spleen . Lymph nodes are masses of lymphocyte containing lymphoid tissues, attached to lymphoid vessels. Lymph nodes function to filter cellular debris, foreign pathogens, excess tissue fluid, and leaked plasma proteins. There are aggregations of lymph nodes at key points around the body (cervical, axillary , tracheal, inguinal, femoral, and deep nodes related to the aorta).

The reproductive system, or genital system, is a system of internal and external sex organs which work together to contribute towards the reproduction process. Unlike other systems of organs, the genital system has significant differences among sexes.

The external female sex organs , also known as the genitals, are the organs of the vulva (the labia, clitoris, and vaginal opening). The internal sex organs are the ovaries , fallopian tubes , uterus and vagina . The vulva provides an entry to, and protection, for the vagina and uterus, as well as the proper warmth and moisture that aids in its sexual and reproductive functions. In addition, it is important for the sexual arousal and orgasm in females.

The vagina is the canal leading from the outside of the body to the cervix (neck) of the uterus. Ovaries secrete hormones and produce egg cells, which are transported to the uterus fallopian tubes . The uterus provides protection, nutrition, and waste removal for the developing embryo and fetus. In addition, contractions in the muscular wall of the uterus contribute to pushing out the fetus at the time of birth.

The external male sex organs are the testes and penis , while the internal are the epididymis, ductus deferens and accessory glands. Functionally, they can be grouped into three categories.The first category is for sperm production (the testes ), and storage ( epididymis ). The second category organs produce ejaculatory fluid; the ductus deferens and the accessory glands ( seminal vesicles and prostate ). The final category is those used for copulation and deposition of the sperm, these include the penis , urethra and ductus deferens.

The integumentary system is the set of organs that forms the external covering of the body. It includes the skin, skin appendages , sweat glands and sensory receptors.

The skin is the largest organ of the body. It has three layers; epidermis, dermis and hypodermis. The epidermis is a thick keratinized epithelium made of multiple cell layers. Underneath the epidermis is the dermis , a layer of connective tissue that contains blood vessels and nerves that supply the skin. The underlying fascia, also called the hypodermis , consists of fat , connective tissue and skin appendages (hair, nails, sebaceous and sweat glands).The integumentary system functions are various. It forms a continuous layer that protects the body from various damaging events, such as external injuries, loss of water and heat, and the carcinogenic effects of UV rays. It also excretes waste, contains sensory receptors to detect pain, sensation, pressure, and temperature, and provides for vitamin D synthesis.

Go through these resources to reinforce your knowledge of the skin:

References: 

• Haines, D. E., Mihailoff, G. A. (2018). Fundamental neuroscience for basic and clinical applications. Philadelphia, PA: Elsevier.
• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
• Netter, F. (2019). Atlas of Human Anatomy (7th ed.). Philadelphia, PA: Saunders.
• Standring, S. (2016). Gray's Anatomy (41st ed.). Edinburgh: Elsevier Churchill Livingstone.
• Tamura, R., Yoshida, K., & Toda, M. (2019). Current understanding of lymphatic vessels in the central nervous system. Neurosurgical Review, 43(4), 1055–1064. https://doi.org/10.1007/s10143...

Article, review and layout:

• Jana Vaskovic
• Nicola McLaren

Illustrations:

• Digestive system (anterior view) - Begoña Rodriguez
• Skeletal system (an overview) - Irina Münstermann
• Cardiovascular system (a diagram) - Begoña Rodriguez
• Respiratory system (a diagram) - Begoña Rodriguez
• Nervous system (an overview) - Begoña Rodriguez
• Cranial nerves (a diagram) - Paul Kim
• Digestive system (a diagram) - Begoña Rodriguez
• Organs of the endocrine system (a diagram) - Begoña Rodriguez
• Lymphatic system (a diagram) - Begoña Rodriguez
• Integumentary system (a diagram) - Paul Kim

Articles within this topic:

• Anatomy of breathing
• Autonomic nervous system
• Central nervous system and brain lymphatics
• Circulatory (cardiovascular) system
• Clinical case: Schwannoma of the nasal cavity
• Development of the central nervous system
• Development of the digestive system
• Digestive system quizzes and free learning tools
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human body systems

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The human body is an incredibly complex structure, with cells , tissues , and organs assembled into highly organized systems that work together to perform an astonishing array of functions—from seeing and hearing to breathing and digesting food to running, playing a musical instrument, and problem-solving. Each of the major systems of the body is essential to the survival of the human organism, and each serves as a critical companion to the others. The muscle and skeletal systems, for example, act together to move the body. The skeletal system also protectively houses internal organs, such as the brain , heart , and lungs , while the muscle system aids in breathing and interacts with the nervous system to coordinate sensory and muscle responses. The respiratory , circulatory , renal , and digestive systems likewise are interconnected, working together to supply oxygen and blood to tissues, remove wastes, and break down and utilize nutrients . Learn more about these amazing systems through our human body interactive.

Introduction to the Human Body

The human body is a complex, highly organized structure made up of unique cells that work together to accomplish the specific functions necessary for sustaining life.

The biology of the human body includes

Physiology (how the body functions)

Anatomy (how the body is structured)

Anatomy is organized by levels, from the smallest components of cells to tissues and organs and to organ systems .

Gross anatomy is the study of the body's organs as seen with the naked eye during visual inspection and when the body is cut open for examination (dissection).

Cellular anatomy is the study of cells and their components, which can be observed only with the use of special techniques and special instruments such as microscopes.

Molecular anatomy (often called molecular biology) is the study of the smallest components of cells at the biochemical level.

Anatomy and physiology change remarkably between fertilization and birth. After birth, the rate of anatomic and physiologic changes slows, but childhood is still a time of remarkable growth and development ( see Physical Growth of Infants and Children ). Some anatomic changes occur past adulthood, but the physiologic changes in the body's cells and organs are what contribute most to what we experience as aging ( see Changes in the Body With Aging ).

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INFOGRAPHIC

The organization and structure of the human body.

The human body is made up of a complex structure of systems that all work together. There are several levels of organization to this structure, with each level more complex than the last.

Biology, Health

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