Human Digestive System

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Your digestive system turns the food you eat into nutrients, which the body uses for energy, growth and cell repair. The digestion process starts in your mouth. Biting and chewing breaks the food into pieces that are more easily digested, while saliva mixes with food to begin the process of breaking it down into a form your body can absorb and use.

The throat (pharynx) is a muscular valve that controls airflow into the lungs and directs solids and liquids towards the stomach through the oesophagus.

The oesophagus is a muscular tube extending from the throat to the stomach. The oesophagus delivers food to the stomach through a series of contractions, called peristalsis. Just before the connection to the stomach, there is a "zone of high pressure", called the lower oesophageal sphincter which prevents food from passing back into the oesophagus.

The stomach is a sac-like organ with strong muscular walls. In addition to holding the food, it's also a mixer and grinder. The stomach secretes acid and powerful enzymes that continue the process of breaking down the food. When it leaves the stomach, food is the consistency of a liquid or paste. From there, the food moves to the small intestine.

The liver filters the blood coming from the digestive tract before it passes to the rest of the body. The liver also detoxifies chemicals and metabolises drugs. The liver secretes bile via the gallbladder back to the intestines.

The pancreas plays an essential role in converting the food we eat into fuel for the body's cells. It has two main functions—the exocrine function that helps digestion, and a vital endocrine function that regulates blood sugar.

The small intestine is a 6m long tube, loosely coiled in the abdomen, made up of three segments, the duodenum, jejunum, and ileum. The small intestine continues the process of breaking down food using segmentation (bi-directional peristaltic contractions) and enzymes released by the pancreas and bile from the liver. Bile aids in the digestion of fat and eliminates waste products from the blood. The duodenum is mainly responsible for breaking down food, with the jejunum and ileum being mainly responsible for the absorption of nutrients into the bloodstream.

The final stage of the journey for solids is through the large intestine to the rectum for final discharge through the anus.

Human Nervous System

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The nervous system is a sophisticated collection of nerves and specialised cells called neurons that transmit signals between different parts of the body. It is essentially the body's electrical wiring.

The brain and spinal cord together form the central nervous system; all the other nerves in the body are the peripheral nervous system. The nerves themselves are cylindrical bundles of fibres.

The brain is the central computer of the nervous system. It receives messages from the body, processes the information, and sends signals back to tell the body how to respond. The spinal cord is the main information highway of the nervous system and connects the brain to all the nerves that help you sense the world.

The peripheral nervous system includes sensory nerves that receive information from your sensory organs (your eyes, ears, nose, tongue and skin) and motor nerves that carry information to muscles throughout your body. There are many other kinds of nerves in the peripheral nervous system that have special functions for communicating with your internal organs (e.g. heart, lungs and liver.)

Neurons send signals to other cells through thin fibres called axons, which cause chemicals known as neurotransmitters to be released at junctions called synapses. There are 86 billion neural connections in the average human brain.

Human Muscles

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Muscles are a type of soft tissue found in humans and most animals. Muscles allow us to consciously move our limbs so we can jump in the air, chew our food and perform other conscious actions. They are also responsible for many sub-conscious processes, such as keeping our hearts pumping, moving food through our guts, and even making us blush.

Our muscles need signals from our brains and energy from our food to contract and move. There are two types of muscles. Striated muscles consist of multiple fibres arranged in parallel, whereas smooth muscles have interconnected cells that form layers.

The skeletal muscles that move our body parts are striated muscles, and we actively control these with our brain. We are not able to control all the striated muscles, like those that keep our hearts pumping. These muscles work on their own without us even thinking about it.

Specific molecules within the muscle fibres, particularly actin and myosin, allow striated muscles to contract rapidly, allowing us to move. The energy for muscles comes from the food that we eat.

The contractions that smooth muscles produce tend to be more gradual than those produced by striated muscle. An example is the slow and controlled movement of food through the digestive system.

Although the pathways that regulate contraction in striated and smooth muscles are very different, calcium is the common key molecular messenger in the process.

Striated muscles receive their triggers from the brain via motor neurons, whereas smooth muscle cells are activated by neuronal signalling or by hormones. This results in calcium rushing into the muscle, activating the actin and myosin and causing the muscle to contract.

Some smooth muscles are in a permanent state of contraction, and the muscles that line our blood vessels are in this category. A constant supply of calcium allows these muscles to regulate blood flow. For example, when the muscles that line the blood vessels in our face relax, we blush.

Human Skeleton

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The skeletal system includes all the bones and joints in the body. Each bone is a complex living organ made up of many cells, protein fibres, and minerals. The skeleton provides support and protection (skull and ribs) for the soft organs that make up the rest of the body. The bones have attachment points for muscles and tendons to allow movements at the joints. The red bone marrow produces new blood cells inside of our bones. Bones act as the body’s warehouse.

The skeletal system in an adult body is made up of 206 individual bones arranged into two major divisions:
• The axial skeleton (80 bones) runs along the body’s vertical axis and includes the skull, ribs, sternum, and vertebral column.
• The appendicular skeleton (126 bones) makes up the limbs, the shoulder, and pelvic girdles.

The skeleton makes up about 30-40% of an adult’s body mass and consists of non-living bone matrix and many tiny bone cells. Roughly half of the bone mass is water, while the other half is collagen protein and solid crystals of calcium carbonate and calcium phosphate.

Living bone cells are found on the surfaces of bones and in small cavities inside the bone matrix. Although these cells make up very little of the total bone mass, they are essential for bone growth and repair during daily wear and injury.

The bone matrix is our calcium bank, storing and releasing calcium ions into the blood as needed for the proper function of the nervous and muscular systems. Red bone marrow is found in the hollow space inside of bones known as the medullary cavity and produces red and white blood cells in a process known as haematopoiesis. Red bone marrow also stores iron used to form haemoglobin in red blood cells.

Five Basic Senses

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Humans have five basic senses: touch, sight, hearing, smell and taste. The sensory organs (skin, eyes, ears, nose and tongue) send information to the brain to help us perceive the world around us.

For touch, several distinct sensations in the skin are communicated to the brain through specialized neurons.

For sight, light passes through the lens of an eye. It is focussed onto the retina where photoreceptors called rods (for black and white) and cones (for colour) convert the information into electrical impulses sent to the brain. The iris (coloured part of the eye) controls how much light enters the eye.

For hearing, sound waves pass from the outer ear, through the auditory canal and onto the eardrum. In the middle ear, delicate bones transfer the vibrations to fine hairs in the inner ear, which produce electrical signals that the brain decodes as sound.

For smell, there are millions of nerve endings in the olfactory cleft in the roof of the nasal cavity, that transmit complex smell information directly to the brain. The number of unique smell combinations is almost infinite.

For taste, receptors around our tongue, the roof of the mouth and throat detect the four primary tastes: salty, sweet, sour and bitter. Recently a fifth taste, umami (or savoury) has been added.

The Eye

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The eye is a fused two-piece unit, The anterior (front) segment is made up of the cornea, iris and lens. The cornea is transparent and more curved, and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 12 mm in diameter, and 0.5 mm thick near its centre. The posterior (rear) chamber constitutes the remaining five-sixths; its diameter is typically about 24 mm. The iris is the circular pigmented area surrounding the centre of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles.

Light energy enters the eye through the cornea, then the pupil and then the lens, before falling on the light-sensitive cells of the retina (photoreceptor cones and rods) that generate electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle.

When the eye views an object far away, the rays of light entering the eye are parallel. The ciliary muscles and suspensory ligaments cause the lens to flatten. Conversely, when the object is close to the eye, the light enters at an angle and the muscles and ligaments allow the lens to fatten (convex) so the eye can focus on the close object. When the eye is unable to adjust the lens shape sufficiently, then glasses can be worn to correct the focus.

There are six extraocular muscles that control eye movements. By having two eyes, stereo vision, animals are able to judge distance to objects.

The Ear

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The ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts — the outer ear, the middle ear and the inner ear. The outer ear consists of the pinna and the ear canal. Since the outer ear is the only visible portion of the ear in most animals, the word "ear" often refers to the external part alone. The middle ear includes the tympanic cavity and the three ossicles. The inner ear sits in the bony labyrinth, and contains structures which are key to several senses: the semicircular canals, which enable balance and eye tracking when moving; the utricle and saccule, which enable balance when stationary; and the cochlea, which enables hearing. The ears of vertebrates are placed somewhat symmetrically on either side of the head, an arrangement that aids sound localisation.

The ear develops from the first pharyngeal pouch and six small swellings that develop in the early embryo called otic placodes, which are derived from ectoderm.

The ear has been adorned by earrings and other jewelry in numerous cultures for thousands of years, and has been subjected to surgical and cosmetic alterations.

The ear may be affected by disease, including infection and traumatic damage. Diseases of the ear may lead to hearing loss, tinnitus and balance disorders such as vertigo, although many of these conditions may also be affected by damage to the brain or neural pathways leading from the ear.

The Heart

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The heart is an organ that pumps blood throughout the body via the circulatory system, supplying oxygen and nutrients to muscles and tissues and removing carbon dioxide and other waste products.

The heart is a muscle, about the size of your fist, in the middle of your chest tilted slightly to the left. Each day your heart beats around 100,000 times, circulating about five litres (eight pints) of blood. Replenished blood is distributed through a network of arteries, returning through a complementary network of veins. In between are tiny capillaries with very thin walls, which allow them to exchange compounds (such as oxygen and carbon dioxide, water, nutrients and waste) with surrounding tissues. Waste products are removed from the blood in the kidneys.

The right side of the heart receives deoxygenated blood and sends it to the lungs, which remove the carbon dioxide and replenish with oxygen. The left side receives blood from the lungs and pumps it to all parts of the body. Arteries are strong and their elastic walls help keep blood pressure consistent. Veins have thinner walls than arteries.

When exercising, the heart beats faster to get oxygen and nutrients to the muscles, increasing from an idling rate of 60 beats per minute to almost 200 bpm. The heart can take strain without you being aware of it. Over a long period of time your diet can influence the condition of your blood vessels and valves of the heart. Monitoring blood pressure is essential to know the condition of your circulatory system.

Lungs

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Human lungs are remarkable organs responsible for the crucial task of breathing, which provides oxygen to the body's cells and removes carbon dioxide, a waste product of metabolism. Each person has two lungs, situated in the chest cavity on either side of the heart. The lungs are spongy in texture and pinkish in color, resembling two large air-filled balloons.

The lungs are divided into sections called lobes – three on the right lung and two on the left. Inside the lungs are a vast network of airways, including the trachea (windpipe), bronchi, and bronchioles, which branch out like tree limbs and end in small air sacs called alveoli. It is within these alveoli that the exchange of gases occurs: oxygen from the air enters the bloodstream, while carbon dioxide from the bloodstream exits the body when we exhale.

The process of breathing, also known as respiration, is controlled by the respiratory system, which includes the lungs, airways, and muscles involved in breathing. When we inhale, the diaphragm – a dome-shaped muscle beneath the lungs – contracts and moves downward, creating space for the lungs to expand. This allows air to rush into the lungs. When we exhale, the diaphragm relaxes, and the lungs recoil, pushing air out of the body.

Human lungs are essential for survival, as they enable the body to receive the oxygen it needs to function properly. Maintaining lung health is crucial for overall well-being, and practices such as regular exercise, avoiding smoking and pollution, and maintaining a healthy diet can help support lung function.

Kidneys

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Human kidneys are vital organs responsible for filtering waste products and excess fluids from the bloodstream to produce urine. Each person has two kidneys, located on either side of the spine, just below the rib cage. The kidneys are bean-shaped organs, reddish-brown in color, and about the size of a fist.

The main function of the kidneys is to remove waste products, such as urea and creatinine, from the blood and excrete them in the form of urine. Additionally, the kidneys regulate the body's fluid balance, electrolyte levels, and blood pressure by adjusting the concentration and volume of urine produced.

Inside each kidney are tiny structures called nephrons, which are responsible for the filtration and purification of blood. Each nephron consists of a filtering unit called the glomerulus, surrounded by a tubule. As blood passes through the glomerulus, waste products and excess fluids are filtered out and collected in the tubule, while essential substances such as water, electrolytes, and glucose are reabsorbed back into the bloodstream.

The kidneys also play a crucial role in the production of hormones that regulate various bodily functions. For example, they produce erythropoietin, which stimulates the production of red blood cells, and renin, which helps regulate blood pressure.

Maintaining kidney health is essential for overall well-being, as kidney dysfunction can lead to serious health problems, including kidney disease, kidney stones, and electrolyte imbalances. Practices such as staying hydrated, maintaining a healthy diet, and avoiding excessive consumption of alcohol and certain medications can help support kidney function..

Liver

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The human liver is a vital organ with multiple functions essential for maintaining overall health and well-being. It is the largest internal organ in the body and is located in the upper right side of the abdomen, just below the diaphragm. The liver is reddish-brown in color and has a soft, spongy texture.

One of the liver's primary functions is to filter and detoxify blood coming from the digestive tract before it circulates throughout the rest of the body. It removes harmful substances such as toxins, drugs, and alcohol from the bloodstream, converting them into less harmful compounds that can be excreted from the body.

The liver also plays a crucial role in metabolism, the process by which the body converts food into energy. It helps regulate blood sugar levels by storing glucose as glycogen and releasing it into the bloodstream when needed. Additionally, the liver synthesizes proteins necessary for blood clotting, immune function, and transport of nutrients throughout the body.

Another important function of the liver is the production of bile, a greenish-yellow fluid that helps digest fats in the small intestine. Bile is produced in the liver and stored in the gallbladder until it is needed for digestion. The liver continuously produces bile to aid in the digestion and absorption of fats from the diet.

Maintaining liver health is crucial for overall well-being, as liver dysfunction can lead to serious health problems, including liver disease, hepatitis, and liver cancer. Practices such as maintaining a healthy diet, limiting alcohol consumption, and avoiding exposure to toxins can help support liver function and prevent liver-related complications.

Lungs

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Human lungs are remarkable organs responsible for the crucial task of breathing, which provides oxygen to the body's cells and removes carbon dioxide, a waste product of metabolism. Each person has two lungs, situated in the chest cavity on either side of the heart. The lungs are spongy in texture and pinkish in color, resembling two large air-filled balloons.

The lungs are divided into sections called lobes – three on the right lung and two on the left. Inside the lungs are a vast network of airways, including the trachea (windpipe), bronchi, and bronchioles, which branch out like tree limbs and end in small air sacs called alveoli. It is within these alveoli that the exchange of gases occurs: oxygen from the air enters the bloodstream, while carbon dioxide from the bloodstream exits the body when we exhale.

The process of breathing, also known as respiration, is controlled by the respiratory system, which includes the lungs, airways, and muscles involved in breathing. When we inhale, the diaphragm – a dome-shaped muscle beneath the lungs – contracts and moves downward, creating space for the lungs to expand. This allows air to rush into the lungs. When we exhale, the diaphragm relaxes, and the lungs recoil, pushing air out of the body.

Human lungs are essential for survival, as they enable the body to receive the oxygen it needs to function properly. Maintaining lung health is crucial for overall well-being, and practices such as regular exercise, avoiding smoking and pollution, and maintaining a healthy diet can help support lung function.