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What Is the Human Body?

The human body is the physical structure of a human being. This includes a person's head, hair, neck, torso (with thorax and abdomen), arms, hands, legs, and feet. A person's body is made up of many different types of cells, which in turn create tissues and organs. These tissues and organs form systems that work together to keep things balanced, like sugar and oxygen (O2) levels in a person's blood. Scientists study the human body through the fields of anatomy, physiology, histology, and embryology.

The Systems of the Human Body

To make it easier to understand, scientists typically study specific parts of the human body, separated into these groups, or systems:

  • Circulatory system: Responsible for transporting blood, oxygen, and nutrients throughout the body using the heart, blood vessels (veins and arteries), and blood.
  • Digestive system: Responsible for breaking down food, absorbing nutrients, and eliminating waste.
  • Endocrine system: Glands that produce hormones and regulate various bodily functions such as growth, metabolism, and reproduction.
  • Immune system: The body's defense system that protects against harmful pathogens (bacteria and viruses) and helps fight infections and diseases.
  • Integumentary system: Includes the skin, hair, and nails, providing protection, regulating temperature, and sensing the environment.
  • Lymphatic system: The network of vessels, tissues, and organs that help remove waste, toxins, and excess fluid from the body and supports the immune system.
  • Musculoskeletal system: The muscles, bones, joints, and connective tissues that provide structure, support, and enable movement.
  • Nervous system: The complex network of nerves and cells that transmit signals throughout the body, allowing for coordination, sensory perception, and control of bodily functions.
  • Reproductive system: The system involved in reproduction and the production of offspring.
  • Respiratory system: The system responsible for the exchange of oxygen (O2) and carbon dioxide (CO2) in the body through organs like the lungs, trachea, and diaphragm.
  • Urinary system: The system involved in the production, storage, and elimination of urine.

Top Facts About The Human Body for Kids

  • 1. Uncoiled, a person's DNA would be long enough to stretch to the Moon and back—over 150,000 times.

    Unraveled from its coils, the DNA molecule from a single human cell would stretch about 2 m (6.6 ft) long. In our bodies, we have approximately 30 to 37 trillion cells. This means the total length of DNA would be approximately 60 to 74 trillion meters, or 60 to 74 billion kilometers (37.3 to 46 billion miles)!

  • 2. Babies and kids have 64 more bones than adults.

    An adult human typically has 206 bones in their body, while a newborn baby has around 270 bones. During development, some bones in a baby's body are not fully fused with one another and consist of separate parts connected by cartilage. Over time, as babies grow, some of these bones fuse together, resulting in a decrease in the total number of bones. By adulthood, many of the separate bones have fused into larger structures, such as the skull, pelvis, and spine, reducing the overall count to 206.

  • 3. If our intestines were unfolded, we'd be taller than our houses.

    Our intestines are folded within our abdomens to fit into a pretty small space. These folds increase the surface area available for nutrient absorption during the digestion.

    While the length of a human's intestines can vary depending on factors such as age, height, and individual differences, on average, the combined length of the small and large intestines in an adult human is estimated to be around 5 meters (16.4 feet)! The small intestine is the longer of the two, measuring about 6 to 7 m (20 to 23 ft) in length, while the large intestine is shorter, typically around 1.5 to 1.8 m (5 to 6 ft) long.

  • 4. The smallest bone in the human body is about half the size of a grain of rice.

    Located deep inside the ear, the stirrup, or stapes, measures only around 3 mm in length. Its purpose is to transfer vibrations from the outer ear—our earlobes, ear canal, and eardrums—to the inner ear—the cochlea, semicircular canals, and auditory nerves—where sounds are processed into signals our brains can understand.

  • 5. A person's red blood cells can circulate through their entire body in less than a minute.

    While circulation can vary slightly depending on heart rate, blood vessel size, and a person's health, an average estimate is that it takes approximately 20 to 30 seconds for a red blood cell to complete one full trip through the body. This estimation is based on the average speed of blood flow, which is approximately 5 L (1.3 gal) every minute in a resting adult.

  • 6. Your nose can remember and recognize tens of thousands of different scents.

    Biologists estimate this number based on the number of scent receptors in the human nose and the brain's ability to process and store this information. Individual capabilities may vary, and genetics, age, and exposure to specific smells can also influence scent memory. Often, strong memory associations also play a role in whether or not we remember certain scents.

    However, researchers have also found that trained professionals in scent-related fields, such as perfumers and winemakers, can identify and differentiate many more smells than the average person. This suggests that with training and experience, our scent memory capacity can be improved.

  • 7. The average human body carries nearly ten times as many bacterial cells as human cells.

    The human body is host to a vast ecosystem of microorganisms, including bacteria, viruses, fungi, and other microbes, collectively known as the "human microbiota." Biologists estimate that the number of bacterial cells inhabiting the human body is approximately 10 times greater than the number of human cells! Luckily, these bacteria cells are much, much smaller than our own.

    This estimation is based on studies that have analyzed the microbial populations in different areas, such as the skin, digestive system, and mouth. The human microbiota plays crucial roles in digestion, immune system development, vitamin synthesis, and protection against harmful pathogens.

  • 8. In just a couple days, you could probably fill up a 2 L bottle with your own saliva.

    On average, a healthy adult human produces about 0.5 to 1.5 L (.13 - .40 gal) of saliva per day. That being said, saliva production can depend greatly on age, hydration level, oral health, and overall health conditions.

  • 9. The heaviest human brain on record had a mass of just over that of a typical red building brick.

    The average mass of a human brain is around 1.3 to 1.4 kg (2.9 - 3.1 lbs). However, there have been recorded cases of individuals with larger-than-average brain sizes. One such case involved the brain of an individual weighing approximately 2.3 kg (5.1 lbs)!