Excretion and Nervous System

As living things carry out their life processes, they produce waste substances that can be harmful if they pile up. The process of removing harmful metabolic wastes from the body is called excretion. In animals this is done by special organs, but plants do not have such organs. Instead, plants get rid of their wastes in simpler, slower ways. This is partly because many plant wastes are not very harmful and can even be stored safely or reused, so plants do not need a complex excretory system like animals do.

Plants produce some wastes in the form of gases. During the day, plants give off oxygen as a waste product of photosynthesis, and at all times they give off carbon dioxide as a waste of respiration. These waste gases leave the plant mainly through tiny pores called stomata on the surface of the leaves (and through similar pores called lenticels on stems). Plants also lose excess water as water vapour through the stomata, a process called transpiration, which helps remove unwanted water.

For their solid and liquid wastes, plants use clever methods of storage and removal. Some wastes are stored in leaves, bark, or fruits that the plant will eventually shed. When a plant sheds its old leaves (leaf fall) or its bark, the wastes stored in them are removed from the plant's body. So leaf fall and the peeling of bark are natural ways in which a plant throws out accumulated wastes along with the parts being shed.

Many plant wastes are stored as useful or harmless substances in different parts of the plant. These include resins (sticky substances stored in stems, like the resin from pine trees), gums (such as the gum from the babool or acacia tree), latex (a milky fluid, from which rubber is made, as in the rubber tree), and various oils and tannins. Although these begin as waste products, humans make great use of them. So, unlike animals, plants excrete through stomata (gases and water vapour), by shedding leaves and bark, and by storing wastes as resins, gums, latex (rubber), and similar substances — a simple but effective way of handling their wastes.

As the body's cells work, they produce harmful nitrogen-containing wastes, chiefly urea, which forms when the body breaks down extra proteins. These wastes, along with excess water and salts, must be removed from the blood, and this is the job of the human excretory system. The main organs of this system are a pair of kidneys, two tubes called ureters, a urinary bladder, and a tube called the urethra. Together they filter the blood and remove wastes from the body in the form of a liquid called urine.

The kidneys are the chief excretory organs. They are two bean-shaped organs, reddish-brown in colour, located in the back of the abdomen, one on each side of the backbone. The kidneys' main job is to filter the blood, removing urea, excess water, and salts while keeping the useful substances. Blood is brought to each kidney by a large blood vessel, is cleaned inside, and the filtered blood returns to circulation. The waste removed becomes urine, which then leaves the kidney.

From each kidney, the urine flows down a narrow tube called the ureter. The two ureters carry urine from the kidneys to the urinary bladder, a muscular, stretchable bag that stores the urine until it is convenient to release it. When the bladder is full, the urine is passed out of the body through a final tube called the urethra. The release of urine from the body is called urination (or micturition). This pathway — kidneys → ureters → bladder → urethra — is the route by which liquid waste leaves the body.

The real filtering work of the kidney is done by about a million tiny units in each kidney called nephrons. The nephron is the functional unit of the kidney — the basic structure that actually filters the blood. Each nephron has a cup-shaped part (where blood is filtered) connected to a long, coiled tubule (where useful substances are taken back and wastes are concentrated into urine). Blood is first filtered in the cup-shaped part, and then, as the filtered fluid travels along the tubule, the body reabsorbs what it needs, leaving urine behind. So the human excretory system, working through millions of nephrons in the kidneys, keeps the blood clean by removing urea and other wastes as urine.

The kidneys turn blood into urine through the work of millions of nephrons. The making of urine is not a single step but happens in three stages: filtration, reabsorption, and secretion. Through these stages, the nephron removes wastes from the blood while carefully keeping back everything the body still needs. The result is urine — a liquid that contains the body's harmful wastes and surplus water but almost none of the useful substances.

The first stage is filtration. Blood enters a tiny knot of capillaries inside the cup-shaped part of the nephron, where it is under pressure. This pressure forces water and small dissolved substances — including water, glucose, salts, amino acids, and urea — out of the blood and into the nephron. Large things, such as blood cells and proteins, are too big to pass through and stay in the blood. So filtration produces a fluid (the filtrate) that is like blood plasma but without the big cells and proteins.

The problem is that this filtrate still contains many useful substances that the body must not lose. The second stage, reabsorption, solves this. As the filtrate travels along the long, coiled tubule of the nephron, the body takes back (reabsorbs) into the blood the substances it needs — all the glucose, most of the water, and the required salts and amino acids. What is left behind is mainly the wastes and surplus water that the body does not need. Reabsorption makes sure that filtering the blood does not waste valuable materials.

The third stage is secretion, in which a few extra wastes and substances (such as certain ions and drugs) are actively added from the blood into the tubule, to fine-tune the urine and keep the blood balanced. After these three stages, the fluid remaining in the nephron is urine. Urine is mostly water (about 95%), with urea as the main waste, plus salts and small amounts of other substances. It does not normally contain glucose or proteins — if it does, it can be a sign of illness. So urine is formed by filtration (removing materials from the blood), reabsorption (taking back the useful ones), and secretion (adding a few extra wastes).

The kidneys are the main excretory organs, but the body has helper organs that also remove some wastes. One of the most important is the skin, which excretes wastes in the form of sweat (perspiration). The skin contains millions of tiny sweat glands that produce sweat and release it onto the skin's surface through tiny openings called pores. While its main job is to cool the body, sweating also removes some wastes, so the skin acts as a minor excretory organ alongside the kidneys.

Sweat is a watery fluid, very similar in some ways to dilute urine. It is mostly water, but it also contains small amounts of salts (such as common salt, which makes sweat taste salty) and traces of urea and other wastes. When sweat is released and removed from the body, these dissolved wastes are removed too. In this way, the skin shares a little of the kidneys' work, helping to get rid of excess water, salts, and a small amount of urea.

The skin's most important role connected with sweating, however, is thermoregulation — keeping the body's temperature steady. Humans must keep their body temperature close to 37 °C, whatever the weather. When the body becomes too hot — for example during exercise or in hot weather — the sweat glands produce more sweat, which spreads over the skin. As this sweat evaporates from the skin, it takes heat away from the body, cooling it down. This is why we sweat more when we are hot and feel cooler as the sweat dries.

The skin helps control temperature in other ways too. When the body is hot, the blood vessels near the skin widen, bringing more warm blood close to the surface so that heat is lost to the air. When the body is cold, these vessels narrow to keep the warm blood deeper and reduce heat loss, and sweating decreases. Tiny muscles can also make body hairs stand up (giving "goose bumps") to trap warm air. So the skin is both a helper in excretion (removing water, salts, and a little urea in sweat) and the body's main organ for thermoregulation, keeping our temperature steady.

The body must constantly respond to what is happening around and inside it — to light, sound, touch, heat, and much more. The system that controls and coordinates these responses is the nervous system, and its basic working unit is a special cell called the neuron (nerve cell). A neuron is the structural and functional unit of the nervous system — the cell that carries messages, in the form of tiny electrical signals called nerve impulses, from one part of the body to another. Neurons are among the longest cells in the body, some stretching over a metre.

A neuron has three main parts. At one end is the cell body (cyton), which contains the nucleus and most of the cell's organelles; it controls the cell's activities. Branching out from the cell body are many short, thread-like extensions called dendrites. The dendrites receive messages (impulses) from other neurons or from sense organs and carry them toward the cell body. So information enters the neuron through the dendrites and cell body.

From the cell body extends a single, long fibre called the axon. The axon carries the impulse away from the cell body toward the next neuron, a muscle, or a gland. The axon may be very long, allowing a single neuron to carry a message a great distance. Many axons are wrapped in a fatty covering called the myelin sheath. This sheath acts like the insulation around an electrical wire: it protects the axon and speeds up the movement of the impulse along it, so messages travel faster.

At the far end of the axon, the impulse must pass to the next cell, but neurons do not quite touch each other. There is a tiny gap between the end of one neuron and the beginning of the next, called a synapse. The impulse crosses this gap with the help of chemicals, then continues along the next neuron. So a neuron is built to carry messages in one direction: dendrites receive the impulse → the cell body processes it → the axon (often covered by a myelin sheath) carries it away → and the synapse passes it to the next cell. This design lets the nervous system send signals quickly all over the body.

Although all neurons share the same basic structure, they do different jobs depending on where they are and what kind of message they carry. Based on their function, neurons are of three main types: sensory neurons, motor neurons, and interneurons (also called relay or association neurons). Together, these three types form the pathways along which information flows: from the sense organs, to the brain or spinal cord, and out to the muscles and glands. Understanding them shows how the body senses a change and then responds to it.

Sensory neurons carry messages from the sense organs (receptors) toward the central nervous system (the brain and spinal cord). The sense organs — such as the eyes, ears, skin, nose, and tongue — detect a change in the surroundings, called a stimulus (for example, touching something hot). The sensory neurons pick up this information and carry the impulse inward to the brain or spinal cord. Because they carry messages toward the centre, sensory neurons are also called afferent neurons.

Motor neurons carry messages in the opposite direction — from the central nervous system out to the effectors, which are the muscles and glands that carry out the response. After the brain or spinal cord has decided what to do, motor neurons carry the command outward to make a muscle contract or a gland release a substance. For example, a motor neuron carries the order to the arm muscles to pull the hand away from a hot object. Because they carry messages away from the centre, motor neurons are also called efferent neurons.

Interneurons (relay neurons) are found within the central nervous system — in the brain and spinal cord. Their job is to connect neurons to one another: they receive impulses from sensory neurons and pass them on to motor neurons (or to other interneurons), linking the incoming message to the outgoing response. They act as the "middle-men" that process and relay information. So the three types work as a team: sensory neurons bring information in, interneurons connect and process it in the brain or spinal cord, and motor neurons carry the response out to the muscles and glands.

The message that a neuron carries is called a nerve impulse. A nerve impulse is a tiny electrical signal that travels rapidly along a neuron, from the dendrites, through the cell body, and down the axon. This electrical signal is created by the movement of charged particles (ions) across the neuron's membrane. To understand how it works, we look at two states of the neuron: its resting state and its active state, described by the resting potential and the action potential.

When a neuron is not carrying a message, it is in its resting state, described by the resting potential. In this state, the inside of the neuron is slightly negatively charged compared with the outside, because of an uneven balance of ions across the membrane. The neuron is like a battery that is charged up and ready but not yet "firing." It stays this way until a strong enough stimulus comes along to set off an impulse.

When the neuron is stimulated strongly enough, it suddenly changes for a brief moment — this change is the action potential, the nerve impulse itself. At the stimulated point, ions rush across the membrane, briefly making the inside positively charged instead of negative. This sudden flip is the electrical signal. The action potential does not stay in one place: it travels along the axon like a wave, each point setting off the next, so the impulse moves quickly from one end of the neuron to the other. After it passes, the neuron quickly returns to its resting state, ready to fire again.

When the impulse reaches the end of the axon, it must cross the synapse — the tiny gap to the next neuron. Since the gap is not bridged by the cell, the electrical impulse cannot simply jump across. Instead, the arrival of the impulse causes the neuron to release special chemicals called neurotransmitters into the synapse. These chemicals diffuse across the gap and stimulate the next neuron, starting a fresh impulse in it. In this way, the message is passed on chemically across each synapse and then continues as an electrical impulse along the next neuron. So a nerve impulse is an electrical signal (a travelling action potential) within a neuron, handed on by chemical neurotransmitters across the synapse — allowing messages to race through the nervous system.

All the neurons of the body are organised into the human nervous system, the body's master control and coordination system. It receives information about changes inside and outside the body, decides how to respond, and sends out commands to the muscles and glands — all in a fraction of a second. For study, the nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). Together they reach every part of the body.

The central nervous system (CNS) is the control centre. It is made of two organs: the brain and the spinal cord. The brain, protected inside the skull, is the main coordinating centre — it receives information, thinks, decides, remembers, and controls most of the body's activities. The spinal cord, protected inside the backbone (vertebral column), is a thick bundle of nerves running down the back; it connects the brain to the rest of the body and also controls many quick automatic responses (reflexes). Because both organs are so important, they are well protected by bone.

The peripheral nervous system (PNS) is made of all the nerves that branch out from the brain and spinal cord and spread throughout the body. These nerves act like the body's wiring, connecting the CNS to the sense organs, muscles, and glands. The PNS carries information into the CNS (through sensory neurons) and carries commands out from the CNS (through motor neurons). So while the CNS does the deciding, the PNS does the communicating, linking the control centre to every part of the body.

Part of the nervous system controls activities that happen automatically, without our conscious thought — such as the heartbeat, breathing, and digestion. This is the autonomic nervous system. It works on its own to keep these vital processes running smoothly, whether we are awake, asleep, or not paying attention. For example, it keeps the heart beating and the gut moving food along without us having to think about it. So the human nervous system is made of the CNS (brain and spinal cord) as the control centre and the PNS as the connecting network, with the autonomic nervous system quietly managing the body's involuntary, automatic activities.

The brain is the main control centre of the body — the organ that thinks, feels, remembers, and directs almost everything we do. It sits protected inside the skull, cushioned by fluid, and is connected to the rest of the body through the spinal cord and nerves. The brain has several parts, but three are especially important to know: the cerebrum, the cerebellum, and the medulla oblongata (brain stem). Each part has its own special duties, and together they keep the body working and let us interact with the world.

The cerebrum is the largest part of the brain, forming most of its bulk, with a deeply folded surface. It is the centre of thinking, intelligence, memory, reasoning, and the will. The cerebrum lets us think and learn, make decisions, remember things, imagine, and feel emotions. It also receives information from the sense organs — so that we can see, hear, smell, taste, and feel — and it controls all our voluntary actions, the movements we decide to make, such as writing or walking. In short, the cerebrum is responsible for our conscious thoughts and deliberate actions.

The cerebellum lies below and behind the cerebrum. Its main job is to look after balance, posture, and the coordination of muscle movements. While the cerebrum may decide to make a movement, the cerebellum makes sure the movement is smooth, accurate, and well-balanced. Thanks to the cerebellum, we can stand upright without falling, walk steadily, ride a bicycle, and pick up objects precisely. If the cerebellum did not work well, our movements would become clumsy and unbalanced.

The medulla oblongata is the lowest part of the brain, where it joins the spinal cord (it is part of the brain stem). Its great importance is that it controls many involuntary (automatic) actions that keep us alive — including the heartbeat, breathing, blood pressure, swallowing, coughing, sneezing, and vomiting. These happen by themselves, without our thinking about them, under the medulla's control. So the three parts share the work of the brain: the cerebrum thinks and controls voluntary actions, the cerebellum maintains balance and coordinates movement, and the medulla oblongata controls the vital involuntary actions that keep the body alive.

Some of our responses happen instantly and automatically, before we even have time to think. If you accidentally touch a hot pan, your hand jerks away at once; if something flies toward your eye, you blink immediately. Such a sudden, automatic, and quick response to a stimulus is called a reflex action. Reflex actions are protective — they help the body react fast to avoid harm. They are different from ordinary actions because they are not consciously decided; they happen on their own.

It is useful to compare voluntary and involuntary actions here. A voluntary action is one we decide to do consciously, using our will — such as raising a hand, writing, or kicking a ball; these are controlled by the cerebrum. An involuntary action happens without our conscious control — such as the heartbeat or digestion. A reflex action is a special kind of quick involuntary action that responds to a stimulus, and it is controlled mainly by the spinal cord, not the brain — which is exactly why it is so fast.

The pathway that a reflex action follows is called the reflex arc. In a typical reflex, such as pulling the hand away from a hot object, the steps are: a receptor (in the skin) detects the stimulus (heat); a sensory neuron carries the impulse to the spinal cord; in the spinal cord, an interneuron passes the message straight to a motor neuron; the motor neuron carries the command to the effector (the arm muscle), which contracts and pulls the hand away. The message does not need to travel all the way to the brain first, which is why the response is so quick.

Why is the reflex arc designed to go through the spinal cord rather than the brain? Because going to the brain and back would take more time, and in a dangerous situation even a moment's delay could cause harm. By letting the spinal cord handle the response directly, the body reacts in the shortest possible time to protect itself. The brain is usually informed a moment later, which is why we feel the pain just after the hand has already moved. Common examples of reflex actions include the knee-jerk (the leg kicks when the knee is tapped), withdrawal of the hand from heat or a pin-prick, blinking, sneezing, and coughing. So a reflex action is a fast, automatic, protective response, carried out through the reflex arc via the spinal cord.