Indholdsfortegnelse for Understanding Human Anatomy i Understanding Human Anatomy

Despite experiencing them first hand, human bodies never ceases to fascinate us. In this brand-new special edition, we’ve compiled a selection of articles from the How It Works archives as well as brand new features to explain the complexities of our own anatomy. Find out what we are made of, from the smallest cells to the largest organs, and how each and every element works together to create complex systems and what makes us, well… us. Discover what can go wrong, how modern science can fix it and how much more we still have to learn about our incredible bodies.…

Cells are life and cells are alive. You are here because every cell inside your body has a specific function and a very specialised job to do. There are many different types of cell, each one working to keep the body’s various systems operating. A single cell is the smallest unit of living material in the body capable of life. When grouped together in layers or clusters, however, cells with similar jobs to do form tissue, such as skin or muscle. To keep these cells working, there are thousands of chemical reactions going on all the time. All animal cells contain a nucleus, which acts like a control hub telling the cell what to do and contains the cell’s genetic information (DNA). Most of the material within a cell is…

Stem cells are incredibly special because they have the potential to become any kind of cell in the body, from red blood cells to brain cells. They are essential to life and growth, as they repair tissues and replace dead cells. Skin, for example, is constantly replenished by skin stem cells. Stem cells begin their life cycle as generic, featureless cells that don’t contain tissue-specific structures, such as the ability to carry oxygen. Stem cells become specialised through a process called differentiation. This is triggered by signals inside and outside the cell. Internal signals come from strands of DNA that carry information for all cellular structures, while external signals include chemicals from nearby cells. Stem cells can replicate many times – known as proliferation – while others such as nerve…

There are two major parts to the human skeleton: the axial bones and the appendicular bones. The axial bones form the central core of the skeletal system, including the skull, spine, rib cage and pelvis. These bones have a protective role, supporting the central nervous system and protecting the vital organs from damage. The appendicular bones are attached to this central support, and include the bones of the arms and legs. Their major function is movement, providing rigid jointed structures onto which the muscles are attached. The skull The skull is constructed out of 22 plates of flat bone, 21 of which are permanently fused together. The other is the mandible, or jawbone. They are made from a thick layer of organised cortical bone, sandwiched around a centre of spongy…

Muscles are often taken for granted. Responsible for every move you make, the primary goal of a muscle is to turn energy into motion. Muscles are broken down into three categories. Skeletal muscles are the type that people in the gym train and what individuals are most commonly aware of. Smooth are the involuntary muscles such as blood vessels, airways and your bladder. The final category is cardiac, the muscles of the heart. It is skeletal muscle, however, that allows humans to both shape their bodies and increase their strength. Skeletal muscles are incredibly complex, designed to contract when asked to perform any action. If you perform a bicep curl, for example, your brain will send a signal to the nerve cells indicating that it’s time for the biceps to…

A team at the Salk Institute in California estimate that the brain can store around 1 petabyte of information, stuffed into the connections between nerve cells. That’s around 2,000 years worth of MP3 music or 223,000 DVDs. And, incredibly, it’s possible to watch memories being made. The Weizmann Institute in Israel and UCLA in the US captured memory formation in action. Patients watched clips of videos and were then asked to recall what they’d seen. The neurons that lit up when they watched the first time lit up again as they relived the experience inside of their heads – a bit like an echo in the brain. Recent research from the US and Japan suggests that these echoes are actually stored twice – once in the hippocampus and again in…

Scientists have known for the past 100 years or so that the spinal cord is actually part of the brain. According to Melillo, while the brain has grey matter on the outside (protected by the skull) and protected white matter on the inside, the spinal cord is the reverse: the grey matter is inside the spinal cord and the white matter is outside.…

Sensory memory The body is constantly bombarded by sensory signals and most incoming sensory information is retained for less than a second before it is forgotten. Transfer The hippocampus integrates incoming sensory information, collecting it together as a single experience. It works together with the cortex to prioritise which information to store and which to forget. Short-term memory Without concentrating too hard, short-term memory can hold around seven items for 20 to 30 seconds. Collecting information into discrete chunks, like splitting a phone number up into sections, can help the brain retain more. Acoustic encoding Short-term memory tends to be based on sound, also known as echoic memory. When remembering a phone number, it often helps to rehearse it vocally in your head. Long-term memory The hippocampus is essential for the…

The back of the eye is covered in a layer of light-sensitive cells. When photons of light hit the pigments inside the cells, it triggers a cascade of signals that are transmitted to the brain. First, they move through interneurones and then to neurones known as ganglion cells. These cells are cross-linked, able to compare adjacent signals, filtering out some of the information. This helps to improve contrast and definition. The neurones travel across the back surface toward the optic nerve, which then relays the information into the brain. As the two optic nerves enter the brain, they cross over, coming together at the optic chiasm. Here, signals from the left side of both eyes are diverted to the left side of the brain, and vice versa, allowing the images…

Vocal cords, also known as vocal folds, are situated in the larynx, which is placed at the top of the trachea. They are layers of mucous membranes that stretch across the larynx and control how air is expelled from the lungs in order to make certain sounds. The primary usage of vocal cords within humans is in order to be abl to communicate with eachother and it is hypothesised that human vocal cords actually developed to the extent we see now to facilitate advanced levels of communication in response to the formation of social groupings during phases of primate, and specifically human, evolution. As air is expelled from the lungs, the vocal folds vibrate and collide to produce a range of sounds. The type of sound emitted is effected by…

The ductless glands in the endocrine system use chemicals called hormones to communicate with and control the cells in our bodies. They secrete different types of hormones directly into the bloodstream which then target specific organs. The target organs contain hormone receptors that respond to the chemical instructions supplied by the hormone. There are 50 different types of hormone in the body and they all consist of three basic types: peptides, amines and steroids. Steroids include the testosterone hormone. This is not only secreted by the cortex of the adrenal gland, but also from the male and female reproductive organs and by the placenta in pregnant women. The majority of hormones are called peptides that consist of short chains of amino acids. They are secreted by the pituitary and parathyroid…

Blood is a delicate balancing act, with the body constantly regulating oxygen flow, iron content and clotting ability. Unfortunately, there are several genetic conditions and chronic illnesses that can disturb the balance, sometimes with deadly consequences Haemophilia This rare genetic blood disorder severely inhibits the clotting mechanism of blood, causing excessive bleeding, internal bruising and joint problems. Platelets are essential to the clotting and healing process, producing threads of fibrin with help from proteins in the bloodstream called clotting factors. People who suffer from haemophilia – almost exclusively males – are missing one of those clotting factors, making it difficult to seal off blood vessels after even minor injuries. Sickle cell anaemia Anaemia is the name for any blood disorder that results in a dangerously low red blood cell count.…

Cholesterol is a fatty molecule in the blood and, generally speaking, having too much is bad: it can stick to the walls of your arteries and increase the risk of heart disease. Most foods, with the exception of offal, eggs and shellfish, don’t contain any cholesterol, but many contain saturated fats, which are turned into cholesterol by the liver. However, cholesterol itself isn’t unhealthy – in fact, you wouldn’t be able to live without it. The body uses cholesterol as a kind of padding, coating the outer membranes of all our cells and insulating nerve fibres, helping signals travel properly to and from the brain. It’s also a fundamental part of the endocrine system, as all steroid hormones (eg oestrogen) are synthesised from cholesterol. A good diet and regular exercise…

Checking your pulse is a good way to track your heart rate. To get an accurate reading, monitor it while you’re resting, as rhythm can change throughout the day, especially during exercise. You can check your radial pulse on your wrist or neck, because arteries pass close to the skin in these areas. For your wrist pulse, turn one hand over with the palm facing up. Place the index and middle finger from your other hand below the wrist crease, next to the tendon. Use gentle pressure and feel for a beating pulse. For the pulse in your neck, place the same two fingers below the jawline next to your windpipe. Find your pulse and use the second hand on a watch to count the number of beats per minute.…

The primary organs used for respiration in humans are the lungs. Humans have two lungs, with the left lung being divided into two lobes and the right into three. Lungs have between 300–500 million alveoli, which is actually where gas exchange occurs. Respiration of oxygen breaks into four main stages: ventilation, pulmonary gas exchange, gas transportation and peripheral gas exchange. Each stage is crucial in getting oxygen to the body’s tissue, and removing carbon dioxide. Ventilation and gas transportation need energy to occur, as the diaphragm and the heart are used to facilitate these actions, whereas gas exchanging is passive. Air is drawn into the lungs at a rate of between 10-20 breaths per minute while resting, through either your mouth or nose by diaphragm contraction, and travels through the…

The small intestine is actually one of the most important elements of our digestive system, which enables us to process food and absorb nutrients. On average, it sits at a little over six metres, that is 19.7 feet, long with a diameter of 2.5-3 centimetres, 1-1.2 inches. The small intestine is made up of three different distinctive parts: the duodenum, jejunum and the ileum. The duodenum actually connects the small intestine to the stomach and is the key place for further enzyme breakdown, following already passing through the stomach, turning food into an amino acid state. While the duodenum is very important in breaking food down, using bile and enzymes from the gallbladder, liver and pancreas, it is actually the shortest element of the small bowel, only averaging about 30…

A person’s reproductive system is made up of a collection of organs and a network of hormone releasing glands that function for the purpose of sexual reproduction. Both male and female reproductive systems are divided into three sections: the gonads, internal and external genitalia. The gonads of each system produce reproductive cells, called gametes. For people assigned male at birth (AMAB) the gonads are the testes and amongst those assigned female at birth (AFAB) the gonads are the ovaries. The rest of the system’s glands and organs are collectively known as genitalia. Each system is governed by chemical messengers called hormones, which dictate how and when reproductive cells are made and when the body is ready to make a baby.…

Physical defences Human anatomy subscribes to the notion that good fences make good neighbours. Your skin, made up of tightly packed cells and an antibacterial oil coating, keeps most pathogens from ever setting foot in body. Your body’s openings are well-fortified too. Pathogens that you inhale face a wall of mucus-covered membranes in your respiratory tract, optimised to trap germs. Pathogens that you digest end up soaking in a bath of potent stomach acid. Tears flush pathogens out of your eyes, dousing bacteria with a harsh enzyme for good measure. It’s true: while you’re simply sitting around watching TV, trillions and trillions of foreign invaders are launching a full scale assault on your cells. Collectively known as pathogens, these attackers include bacteria, single-celled creatures that live to eat and reproduce;…

The human body is composed of an estimated 7 octillion atoms, making up over 75 trillion cells. At the atomic level, the human body comprises about 60 elements, but the function of many of them is unknown. In fact, 99 per cent of the human body is made from just six elements. Like all other life discovered to date, we are carbon-based; the biomolecules that make up our bodies are constructed using frameworks of carbon atoms. Carbon is almost unique among the elements; it is small in size and can make four covalent bonds to other atoms, allowing it to form the backbone of key molecules that make up the human body. The bonds are strong enough to hold the molecules in a stable structure, but not so strong that…

The human skeleton is crucial for us to live. Our bones make up a strong, flexible framework that protects our vital organs, keeps our shape and allows our bodies to move, as well as being a stem-cell reserve. Bones also produce blood cells within bone marrow and store minerals we need released on a daily basis. As an adult you will have around 206 bones, but you are born with over 270, which continue to grow, strengthen and fuse after birth until around 18 in females and 20 in males. Skeletons actually do vary between sexes in structure also. One of the most obvious areas is the pelvis as a female must be able to give birth, and therefore hips are comparatively shallower and wider. The cranium also becomes more…

The brain makes up just two per cent of our total body weight, but crammed inside are approximately 86 billion neurons, surrounded by 180,000 kilometres of insulated fibres connected at 100 trillion synapses. It’s a vast biological supercomputer. The cells in the brain communicate with each other using electrical signals. When a message is sent, thousands of microscopic channels open, allowing positively charged ions to flood across the membrane. Afterwards, more than 1 million miniature pumps in each cell will move the ions back again, ready for the next impulse. The cell bodies of the neurons, and their connections, are contained within the grey matter, which consumes 94 per cent of the oxygen delivered to the brain. Different areas are responsible for different functions, and wiring them together is a…

Messages are passed from one nerve cell to the next by chemical messengers called neurotransmitters. Each has a slightly different effect and by looking at what happens when neurotransmitter levels change, we are discovering that different combinations play a role in a range of complex emotions. Acetylcholine excites the nerve cells that it touches, triggering more electrical activity. It plays a role in wakefulness, attention, learning and memory, and abnormally low levels are found in the brains of people with dementia caused by Alzheimer’s disease. Dopamine is a chemical that also excites nerve cells. It plays a vital role in the control of movement and posture, and low levels of dopamine underlie the muscle rigidity that exists in Parkinson’s disease. Dopamine is also used in the brain’s reward circuitry and…

Baby In order to fit through the birth canal, human babies must be born well before their brains have finished developing, so their brains grow rapidly in their first years. Experiences prompt the development of new connections between nerves, and by the time a baby is two years old, it has 1.5 times as many synapses as an adult. Infant Support cells, known as glia, provide protection, insulation and nutrition for the brain’s nerve cells. Throughout childhood, they continue to migrate and grow. During the first two to three years of a child’s life, the insulating white matter of the still-developing brain begins to form. Child By the age of ten or 11, the rapid development of new connections in the brain has ended and a period of trimming and…

CT Computed tomography (CT) scans use X-rays to build a three-dimensional image of the brain. The radiation travels at different speeds through different tissues, allowing a density map to be produced. It provides purely structural information and is useful for identifying tumours. PET Positron emission tomography uses safe radioactive isotopes to measure brain activity. By labelling oxygen or sugar with radioactive tags, blood flow in the brain can be monitored. The tags emit low-energy radiation and as blood is diverted to active regions of the brain, the emissions pinpoint the locations. fMRI Functional magnetic resonance imaging detects the amount of oxygen present in the blood, allowing brain activity to be mapped. When regions of the brain become more active, their demand for blood rises and they light up on the…

The structure of the human eye is so complex that it’s difficult to believe that it’s not the product of intelligent design but by looking at the eyes of other animals, scientists have shown that it evolved very gradually from a simple light-dark sensor over the course of around 100 million years. The eye functions in a very similar way to a camera, with an opening through which the light enters, a lens for focusing and a light-sensitive membrane at the back. The amount of light that enters the eye is controlled by the circular and radial muscles in the iris, which contract and relax to alter the size of the pupil. The light first passes through a tough protective sheet called the cornea, and then moves into the lens.…

In optics, a lens is a transparent substance (usually glass) that is used to form an image of an object by focusing rays of light to a designated point. In eyeglasses, lenses are used to correct a wide variety of eye defects including myopia, hyperopia and astigmatism. This is achieved by shaping the lenses into various optical profiles, the most common of which is ophthalmic convex-concave. Here, both the front and back of the lens surface has a positive radius, resulting in a convergent front surface and divergent back surface. This difference creates the corrective power of the lens, so in order to treat hyperopia a convergent lens is used, where the front surface is stronger than the rear (narrowing the incoming light’s focus); for myopia, the reverse is used,…

The network of blood vessels in the human body must cope with different volumes of blood travelling at different pressures. These blood vessels come in a multitude of different sizes and shapes, from the large, elastic aorta down to very tiny, one-cell-thick capillaries. Blood is the ultimate multitasker. It carries oxygen for various tissues to use, nutrients to provide energy, removes waste products and even helps you warm up or cool down. It also carries vital clotting factors which stop us bleeding. Blood comes in just two varieties; oxygen-rich (oxygenated) blood is what the body uses for energy, and is bright red. After it has been used, this oxygen-depleted (deoxygenated) blood is returned for recycling and is actually dark red (not blue, as is often thought). Blood is carried in…

Think of blood as the body’s emergency response team to an injury. Platelets emit signals that encourage blood vessels to contract, stemming blood loss. The platelets then collect around the wound, reacting with a protein in plasma to form fibrin, a tissue that weaves into a mesh. Blood flow returns and white blood cells begin their hunt for bacteria. Fibroblasts create beds of fresh collagen and capillaries to fuel skin cell growth. The scab begins to contract, pulling the growing skin cells closer together until damaged tissue is replaced. Injury When the skin surface is cut, torn or scraped deeply enough, blood seeps from broken blood vessels to fill the wound. To stem the flow of bleeding, the blood vessels around the wound constrict. Haemostasis Activated platelets aggregate around the…

Join us as we unravel all nine metres of your digestive system The digestive tract is a long, muscular tube that runs the entire length of your body. It’s separated into five distinct sections, each with its own function. Digestion begins in the mouth. As you chew your food, saliva is released, providing a lubricant and kick-starting the breakdown of carbohydrates with the enzyme amylase. Touch receptors in your mouth tell you when to swallow, and as your tongue comes up, the food is pushed to the back of your throat. As you swallow, you pass control of digestion over to your automatic motor functions. A flap of skin called the epiglottis folds down to cover the voice box, and the entrance to the lungs, and then a wave pushes…

The stomach’s major role is as a reservoir for food; it allows large meals to be consumed in one go before being gradually emptied into the small intestine. A combination of acid, protein-digesting enzymes and vigorous churning action breaks the stomach contents down into an easier-to-process liquid form, preparing food for absorption in the bowels. In resting state, the stomach contracts and the internal surface of the organ folds into ridges, or rugae. When we start eating, however, the stomach begins to distend; the rugae flatten, allowing the stomach to expand, and the outer muscles relax. It can accommodate about a litre of food without discomfort. The expansion of the stomach activates stretch receptors, which trigger nerve signalling that results in increased acid production and powerful muscle contractions to churn…

Enzymes are often called ‘biological catalysts’, and their job is to speed up chemical reactions. You are full of dissolved chemicals with the potential to come together or break apart to form the biological building blocks that you need to stay alive, but the reactions happen too slowly on their own. Enzymes are molecules with ‘active sites’ that lock on to other molecules, bringing them close together so that they can react, or bending their structures so that they can combine or break apart more easily. The enzymes themselves do not actually get involved in the reactions; they just help them to happen faster. Some of the most well-known enzymes are those in your digestive system. These are important for breaking down the molecules in your food. However, these aren’t…

Kidneys are two bean-shaped organs situated halfway down the back just under the ribcage, on each side of the body, and weigh 115-170 grams (4-6 ounces) each, dependent on the individual’s sex and size. The left kidney is often a little larger than the right and due to the effectiveness of these organs, individuals born with one kidney can survive with little or no health problems. Indeed, the body can operate normally with a 30-40 percent decline in kidney function. This decline would rarely even be noticeable and shows just how effective the kidneys are at filtering out waste products as well as maintaining mineral levels and blood pressure throughout the body. The kidneys manage to control all of this by working with other organs and glands across the body…

The pancreas is a pivotal organ within the digestive system. It sits inside the abdomen, behind the stomach and the large bowel, adjacent to the spleen. In humans, it has a head, neck, body and tail. It is connected to the first section of the small intestine, the duodenum, by the pancreatic duct, and to the bloodstream via a rich network of vessels. When it comes to the function of the pancreas, it is best to think about the two types of cell it contains: endocrine and exocrine. The endocrine pancreas is made up of clusters of cells called islets of Langerhans, which in total contain approximately 1 million cells and are responsible for producing hormones. These cells include alpha cells, which secrete glucagon, and beta cells which generate insulin.…

From the external appendage of the penis and its testicular companions, to the internal bladder-hugging prostate gland, both the internal and external portions of the male reproductive system are designed to carry out three main functions; the creation and storage of productive cells, the release of vital hormones and for sexual pleasure. At the start of the male reproductive system are the oval-shaped glands called the testes, also referred to as the testicles. The predominant function of the testes is to act as a factory for the production of reproductive cells called sperm. These sperm factories take their orders from a hormone, which is also produced in the testes, called testosterone. When a testosterone production order is received by a cluster of tubes within the testes called seminiferous tubules, they…

Humans reproduce by sexual reproduction, whereby the genetic information of two people is mixed. The female set of genetic instructions comes from eggs, while sperm provide the males genetic contribution. Both the egg and sperm hold instructions encoded by deoxyribonucleic acid (DNA), which contains all the information needed to form a functional human body. The cells’ DNA is wrapped up tightly into large structures called chromosomes. Both the egg cell and the sperm cell carry 23 chromosomes, which once aligned in an embryo will form 23 pairs that will be carried by every somatic cell in a mature human body. Two of these chromosomes form a pair known as sex chromosomes, as they define the sex of the baby during development. Every egg cell carries an X chromosome, and a…

Wound healing happens in four key stages: haemostasis, inflammation, proliferation and remodelling. Haemostasis means ‘blood halting’ in Greek, and is the first crucial part in closing a wound. The body’s first line of defence when wounded is to constrict the blood vessels in the affected area to minimise blood loss. Platelets then start to stick to the exposed tissue, becoming activated and encouraging more and more platelets to clump together to plug the gap. Once this plug is in place, a mesh of fibrin fibres starts to form around it, trapping passing blood cells and forming a sturdy clot that holds the wound closed until it can be repaired. This process only takes a matter of minutes, and once the bleeding stops, the local blood vessels dilate again, allowing immune…

Bacteria are vital to our survival. They help us digest food and help develop our advanced immunity while we’re young. However, they can also cause diseases. Antibiotics were discovered in 1928 and almost overnight they changed what were once fatal diseases into trivial infections. But despite effective medication, bacteria are smart and know how to fight back. They can change their shape, hide in plain sight and alter their surface appearance. After an attack from antibiotics, the few bacteria left over are the strongest of the bunch and are resistant to the current crop of antibiotics; this is true survival of the fittest. The lasting bacteria then multiply, become stronger and are able to survive attacks from other antibiotics. This is known as ‘multidrug resistance’ and has led to certain…

The human body changes as it ages, and the peak time for organ functionality is thought to be around the age of 30. The body has amazing capacity for regeneration, but cells can only divide a finite number of times, and as we get older our ability to repair damaged tissue decreases. Dramatic changes, such as the menopause, produce obvious effects on the body. Female sex hormones are not just involved in reproduction, but also play a role in other processes, such as the maintenance of bone density. In the absence of oestrogen, bone mineral density decreases, which can lead to osteoporosis. A similar, but less dramatic, effect can be seen in men as testosterone levels begin to drop. Similar decline in functionality can be observed throughout the human body;…

Surrounded by cytoplasm, the nucleus contains a cell’s DNA and controls all of its functions and processes such as movement and reproduction. There are two main types of cell: eukaryotic and prokaryotic. Eukaryotic cells contain a nucleus while prokaryotic do not. Some eukaryotic cells have more than one nucleus – called multinucleate cells – occurring when fusion or division creates two or more nuclei. At the heart of a nucleus you’ll find the nucleolus; this particular area is essential in the formation of ribosomes. Ribosomes are responsible for making proteins out of amino acids which take care of growth and repair. The nucleus is the most protected part of the cell. In animal cells it is located near the centre and away from the membrane for maximum cushioning. As well…

Breaking it down How the various bones fit together to form the skull The human skull is made up of 22 bones that fall into two primary groups: the cranium, which consists of eight ‘cranial’ bones, and the facial skeleton, which consists of 14 facial bones. These bones are joined together by fibrous joints known as sutures. Unique to the skull, once these joints have fused together by the age of around 30 to 40, they are immovable. The cranium consists of a roof part – known as the calvarium – and a complex base part. The calvarium helps to cover the cranial cavity, which the brain occupies, along with flat bones at the top and sides. The base of the brain case is divided into large spaces, and has…

Within weeks of fertilisation, neural progenitors start to form; these stem cells will go on to become all of the cells of the central nervous system. They organise into a neural tube when the embryo is barely the size of a pen tip, and then patterning begins, laying out the structural organisation of the brain and spinal cord. At its peak growth rate, the developing brain can generate 250,000 new neurons every minute. By the time a baby is born, the process still isn’t complete. But, by the age of two, the brain is 80 per cent of its adult size. 20 watts Your brain is incredibly efficient, using less energy than a standard lightbulb Brain formation This astonishing structure is formed and refined as pregnancy progresses 4 weeks Brain…

Nerves are the transmission cables that carry brain waves in the human body, says Sol Diamond, an assistant professor at the Thayer School of Engineering at Dartmouth. According to Diamond, nerves communicate these signals from one point to another, whether from your toenail up to your brain or from the side of your head.…

Your brain is an incredible thing. It is one of the most complex structures in the known universe, and for decades, scientists have been teasing it apart to find out what it’s made of and how it works. The brain is an electrical and chemical circuit, and nerve cells, or neurons, are the components. They each have a cell body, which contains their genetic code, an axon to transmit electrical impulses, and dendrites to receive them. They are connected together at junctions known as synapses. When an impulse arrives, packets of molecules are released, passing the message on. Each neuron makes hundreds, or even thousands, of connections, producing the complicated patterns that drive human thought. There are hundreds of different types of neuron in the brain, categorised according to their…

Stage 1 The first stage of sleep is the transition period. It is very light and lasts just a few minutes. As the brain shuts down, there can be some twitching as the muscles relax. Stage 2 As people enter the second stage of sleep, their breathing and heart rate slow down and their body temperature drops. Around half of sleep time is spent in stage-2 sleep. Stage 3 The third stage of sleep is described as ‘deep sleep’ and is characterised by the presence of a slow delta-wave pattern, representing the underlying activity of the brain stem. Stage 4 We spend about ten per cent of the night in this deep sleep stage. Breathing is rhythmic and there is little muscle movement. Blood pressure drops and growth and repair…

Severe If the injury is severe, the patient is no longer able to respond to sensory stimulation. Their eyes remain closed and there is no response at all to verbal cues. Moderate When brain injury is more severe, verbal communication starts to break down and patients no longer respond normally to pain. Mild With mild brain injury, patients may be confused, but they remain aware, conscious and conversational.…

Open your eyes, and you are met with an array of different colours, but amazingly you can only detect three different wavelengths of light, corresponding to green, blue, and red. Combining these three signals in the brain creates millions of different shades. Each eye has between 6 and 7 million cone cells, containing one of three colour-sensitive proteins known as opsins. When photons of light hit the opsins, they change shape, triggering a cascade that produces electrical signals, which in turn transmit the messages to the brain. Well over half of our cone cells respond to red light, around a third to green light, and just two per cent to blue light, giving us vision focused around the yellow-green region of the spectrum. The vast majority of the cone cells…

Cameras and human eyes both focus light using a lens. This structure bends the incoming wavelengths so that they hit the right spot on a photographic plate, or on the back of the eye. A camera lens is made from solid glass, and focuses on near and distant objects by physically moving closer or further away. A biological lens is actually squishy, and it focuses by physically changing shape. In the eye, this process is known as ‘accommodation’, and is controlled by a ring of smooth muscle called the ciliary muscle. This is attached to the lens by fibres known as suspensory ligaments. When the muscle is relaxed, the ligaments pull tight, stretching the lens until it is flat and thin. This is perfect for looking at objects in the…

The human spine is made up of 33 vertebrae, 24 of which are articulated (flexible) and nine of which normally become fused. They are situated between the base of the skull to the pelvis, where the spine trails off into the coccyx (the tail bone). The primary functions of the spine is to support the torso and head, which protect vital nerves and the spinal cord and allow the individual to move. By sitting closely together, separated only by thin intervertebral discs which work as ligaments and effectively form joints between the bones, the vertebrae form a strong pillar structure which holds the head up and allows for the body to remain upright. It also produces a base for ribs to attach to and to protect vital internal organs. Vertebrae…

“Red blood cells are so numerous because they perform the most essential function of blood” Blood is the river of life. It feeds oxygen and essential nutrients to living cells and carries away waste. It transports the foot soldiers of the immune system, white blood cells, which seek out and destroy invading bacteria and parasites. Then it speeds platelets to the site of injury or tissue damage, triggering the body’s process of self-repair. Blood looks like a thick, homogenous fluid, but it’s actually more like a watery current of plasma – a straw-coloured, protein-rich fluid – carrying billions of microscopic solids consisting of red blood cells, white blood cells and cell fragments that are called platelets. The distribution is far from equal. Over half of our blood is plasma, 45…

How one of your hardest-working muscles keeps your blood pumping Your heart began to beat when you were a four-week-old foetus in the womb. Over the course of the average lifetime, it will beat over 2 billion times. The heart is made of four chambers separated into two sides. The right side receives deoxygenated blood from the body, and pumps it towards the lungs, where it picks up oxygen from the air you breathe. The oxygenated blood returns to the left side of the heart, where it is sent through the circulatory system, delivering oxygen and nutrients around the body. The heart’s pumping action is coordinated by muscular contractions generated by electrical currents that trigger regular cardiac contractions called systole. The upper chambers – the atria – receive the blood…

Ribs are not merely armour for the organs inside our torsos, as we reveal here… The ribcage – also known as the thoracic cage or thoracic basket – is easily thought of as just a framework protecting your lungs, heart and other major organs. Although that is one key function, the ribcage does so much more. It provides vital support as part of the skeleton and, simply put, breathing wouldn’t actually be possible without it. All this means that the ribcage has to be flexible. The conical structure isn’t just a rigid system of bone – it’s actually both bone and cartilage. The ribcage comprises 24 ribs, joining in the back to the 12 vertebrae making up the middle of the spinal column. The cartilage portions of the ribs meet…

The intestine splits into two distinct parts, the small intestine and the large intestine. The small intestine is where the food goes through final stages of digestion and nutrients are absorbed into the blood stream, the large intestine is where waste is stored until it’s ultimately expelled through the anus. Both the small and large intestines can be further divided into sections, the duodenum, jejunum and ileum are the three distinct sections of the small intestine and the cecum, colon and rectum are the sections of the large intestine. As well as storing waste, the large intestine removes water and salt from the waste before it is expelled. Muscle contractions and mucosa are essential for the intestine to work properly, and we see a variation of mucosa, called villi, present…

There is an increasing amount of evidence to suggest that the activity of bacteria in your gut can significantly affect your brain. This relationship is called the gut-brain axis, and while its exact mechanisms and significance haven’t been deciphered, it’s thought that the microbes in your digestive tract are responsible for complex interactions between your digestive system and nervous, endocrine and immune systems. Your intestines are filled with bacteria that help break down food, produce vital nutrients and defend us against harmful bacteria. Scientists speculate that gut microbes can send signals to the brain via three different methods. The first involves bacteria releasing neurotransmitters (chemicals that help to transmit nerve impulses) to trigger the neurons in your digestive tract, which in turn send signals to your brain via the vagus…

The pelvis is an essential part of the skeleton that acts as a junction between the abdomen and the legs. It consists of eight key structures: the sacrum, ilium, ischium, pubic bone, pubic symphysis, obturator foramen, acetabulum and coccyx, which are laid out symmetrically. Together these make up the bony pelvis. The role of the pelvis is threefold. Firstly, it serves as a connecting mechanism between the torso and the legs. Secondly, it is a vital support and balance structure for the upper body. And thirdly it provides a protective, containing cradle for the intestines, bladder and internal sex organs. The pelvis – technically referred to as the pelvic girdle in this context – consists of a pair of hipbones connected to the base of the vertebral column. Each hipbone…

Inside the uterus Discover the anatomy of the female reproduce system Like its male counterpart, the female reproductive system also consists of both external and internal components. The external female genitalia, commonly known as the vulva, consists of a sensory organ called the clitoris, lubricating glands, and folds of skin known as the labia majora and labia minora around the vaginal opening. Internally, a complex system of glands and organs work together to not only produce female sexual cells (commonly called eggs), but also provide an environment for a developing baby to grow. At the start of the reproductive system are a pair of almond-shaped organs called the ovaries. In the same way that the male testes spend their time manufacturing sperm cells, their female counterparts produce premature egg cells,…

When you’ve got a sore throat, the cause doesn’t always seem important. Some microscopic nasty is waging war with your immune system, it hurts, and you just want to feel better. But whether it’s bacteria or a virus on the rampage is actually very important. Bacteria are each made from just a single, primitive cell. Their insides are separated from the outside by a fatty membrane and a flexible coat of armour called the cell wall. Their genetic information is carried on loops of DNA, and these contain tiny factories called ribosomes, which use the genetic code to produce the molecules that the bacteria need to grow, divide and survive. Viruses, on the other hand, are not technically alive. They carry genetic information containing the instructions to build more virus…