The Digestive System This may sound strange, but your digestive system is basically one big tube that starts at your mouth and ends at your anus. The shape of the tube changes along its length, getting bigger in some places and smaller in others. The process of digestion breaks down your food into small pieces so that the nutrients can be absorbed into your body and used for maintenance, growth, and repair. Any materials that cannot be absorbed are eliminated in the feces. Digestion takes place in four stages: ingesting absorbing digesting eliminating Stage 1: Ingesting Ingesting, or ingestion, is the starting point of the digestion process. This process begins when you bite off a piece of apple or take a sip of milk. Stage 2: Digesting You start digesting your food the moment you start chewing. This process involves several body structures along the digestive tube. Mouth When food enters your mouth, it undergoes both mechanical and chemical digestion. Mechanical digestion occurs when you use your teeth and tongue to chew food into pieces small enough for you to allow. Each small piece of food is called a bolus. Before you swallow the bolus, it is covered in saliva, which helps it continue its journey down the digestive tube. Saliva is produced from salivary glands in your mouth. Saliva also contains an enzyme called amylase. An enzyme is a protein found in your body that helps speeds up chemical reactions. Chemical digestion occurs when amylase begins to break down the food bolus by breaking down complex carbohydrates into simple carbohydrates.
Stomach Your stomach is like a stretchable muscular bag. In fact, your stomach can stretch to hold about 2 L of food or liquid. Inside the stomach is gastric juice, which is made from hydrochloric acid, mucus, and enzymes and is secreted by your stomach's lining. Gastric juice is very acidic and the stomach walls are lined with mucus that protects the tissue from being damaged by the acid. One of the reasons the gastric juice must be acidic is that the enzyme pepsin, which breaks down protein, needs an acidic environment in which to function. As each bolus drops into the stomach, it is covered by gastric juice and is slowly mixed around by the contractions of the stomach muscles. The bolus breaks down into a liquid called chyme. This process can take from two to six hours. Esophagus As the food moves from your mouth, it passes through the pharynx. The pharynx is where your airway passage and the rest of your digestive system meet. A small flap of flesh, called the epiglottis, covers the airway tube. When you swallow, the flap covers the airway and your food carries on to the esophagus. The esophagus is the part of the digestive tube that connects the pharynx and stomach. If you have ever choked on some food or a drink, your epiglottis has opened up, and the food or liquid has gone down your airway passage instead of your esophagus. The esophagus is long and muscular. The bolus is pushed through the esophagus in a process called peristalsis. The process of peristalsis is like using your hands to squeeze a marble down through a small rubber tube. The action of your hands squeezing the tube is like the muscles of the esophagus pushing each bolus to the stomach. Small intestine Once the liquid chyme leaves the stomach, it empties into the small intestine. This organ is a tube about 6 m long and 2.5 cm in diameter. The first metre of the small intestine is called the duodenum. The second stage of the digestion process is complete once the food particles leave the duodenum. Within the first 30 cm of the small intestine are ducts or tubes that connect to other organs, such as the pancreas, liver, and gallbladder. The pancreas is a small organ that produces digestive enzymes that pass into the small intestine. These enzymes help in the further breakdown of the carbohydrates, protein, and fat in the chyme. The liver produces a substance called bile. Bile is stored in the gallbladder. The function of bile is to break the globs of fat into smaller droplets, similar to how dish detergent breaks up grease. This process helps the pancreatic enzymes break down the fat into even smaller particles.
Stage 3: Absorbing Small intestine Now the nutrients are ready to be absorbed by the remaining 5 m of the small intestine. This process is called absorption.to help increase the rate of absorption of nutrients into your body, the small intestine is covered in villi (singular: villus). Villi are structures that look like folds in the wall of the small intestine. Each of these folds increases the surface area available to take in nutrients. In fact, if you were to calculate the total surface area of the small intestine, you would find that it takes up an area about the size of a tennis court, or about 250 m 2. If the small intestine did not have villi, the total surface area would be only about 0.5 m 2, which would make absorption difficult. Food, typically, takes five to six hours to pass through the small intestine by peristalsis. Large intestine Your large intestine is 5 cm wide and 1.5 m long. Its main function is to take undigested material from the small intestine and reabsorb the water and some minerals. Peristalsis continues to move the undigested food along the digestive tube. By the time the undigested material leaves your large intestine, 90 percent of the water that entered your small intestine has been reabsorbed. This process takes about 12 to 24 hours. The role of bacteria in digestion Many different types of bacteria live in your digestive system. Beneficial bacteria assist in the breakdown and absorption of food. For example, bacteria in the large intestine use undigested material to make vitamins, such as vitamin K. Vitamin K helps your blood to clot. At birth, babies do not yet have bacteria in their digestive system. Doctors will give them a shot of vitamin K to last until they have enough bacteria to begin producing it. Sometimes we ingest bacteria that are not beneficial, such as salmonella, which cause food poisoning. Stage 4: Eliminating Any undigested materials left at this point in the digestion process are called feces. Feces are the solid waste products of the digestion process and are stored in the rectum until they are eliminated through the anus. This stage is called elimination, which is the end of the digestion process. The whole process, from ingesting food to eliminating waste, takes approximately 20 to 30 hours.
The Excretory System You ingest food, liquids and air through your mouth, but only excess gases (such as carbon dioxide) are expelled from your mouth. Solid waste, such as indigestible material and dead cells, is expelled at the end of the digestive system. The Excretory System removes liquid waste from your body through the urinary tract. The excretory system is made up of two main organs, the kidneys and the bladder, and three tubes, two ureters and the urethra. Kidneys The human body has two kidneys. Blood is delivered to the kidneys by the arteries of the circulatory system. The kidneys filter waste material, like the nitrogen compound urea, and excess water from the blood. The liver also plays an important role in excretion as it breaks down harmful compounds, such as ammonia, and turns them into compounds that the body can safely remove, such as urea. As the blood is filtered in the kidneys, urine (coming from urea ) is formed. The kidneys control how much water is reabsorbed into the body, and are therefore also responsible for maintaining the balance of water in the body. Once formed, urine passes down the ureters to the bladder. Bladder Urine is stored in the bladder. When the bladder is full, the urine is flushed from the body through the urethra. This process is called urination.
The Circulatory System The circulatory system consists of the heart, arteries, capillaries, and veins. Arteries carry blood away from the heart, and veins carry blood to the heart. In your capillaries, oxygen is released into your body and carbon dioxide is collected. In your lungs, oxygen is collected and carbon dioxide is released into the alveoli. Your heart is a pump that pushes approximately 4 L of blood per minute through your body. If an average heart beats about 70 times per minute, and you live to be 80 years old, that's almost three billion beats. The Heart Your heart is a muscular organ that pumps blood throughout your body. It is made of four chambers: two atria (singular: atrium) and two ventricles. The atria allow the blood to move from the body into the heart. The ventricles pump the blood out of the heart. It may seem backward that the atria are labelled "right" and "left" on the diagram. But imagine the heart as it is positioned in your body. The right atrium refers to the right side of your body, not the right side of the diagram. The muscle around the left ventricle is larger than that around the right ventricle. The left ventricle pumps your blood to your body. The right ventricle pumps blood to your lungs, a much shorter distance. The walls of the atria are much thinner because they only have to pump the blood a short distance into the ventricles. Between the chambers are valves. Each valve allows blood to flow in only one direction. If you have ever listened to your heart through a stethoscope, the "lub dub" sound you hear is the opening and closing of the valves. The heart is part of the circulatory system, which is like an enormous highway system that moves blood throughout your body. The "roads" that make up the circulatory system are your blood vessels. Blood moves along these "roads" from your heart to all parts of your body and back again. All your blood vessels lined up end to end would be about 100 000 km in length. That is about 2.5 times around Earth at the equator! Blood Vessels Blood is carried from your heart throughout your body in three types of blood vessels: arteries, capillaries, and veins. Arteries are blood vessels that carry blood away from your heart. Most arteries carry bright red, oxygenated (oxygen-containing) blood. Your blood is under pressure after it is pumped from your heart.
The pressure keeps it flowing in one direction. Your arteries are thick-walled and elastic and can withstand this pressurized flow. The largest artery in your body is the aorta. It has a width of about 25 mm or the width of a penny. As blood moves farther away from your heart, your arteries branch into smaller and smaller arteries. Eventually, the smallest arteries, the arterioles, which have a diameter of about 0.5 mm, reach the capillaries. Capillaries are a network of tiny blood vessels that act like a highway interchange. Here oxygen, nutrients, and glucose diffuse through the very thin walls of the capillaries into the fluid that surrounds tissue cells. In exchange, carbon dioxide and other waste materials that have diffused out of these cells diffuse into the capillaries. After leaving the capillaries, your blood no longer contains oxygen. This deoxygenated blood is carried back to your heart through tiny venules and into your veins. The superior vena cava and inferior vena cava empty blood into the heart.
The Respiratory System The respiratory system is made up of structures and organs that move oxygen into your body and carbon dioxide out of your body. Your lungs are important organs in this system because they deliver oxygen for cellular respiration and excrete carbon dioxide. Breathing is the process that brings oxygen into your body. Breathing begins when you inhale air through your nose or mouth. The air is warmed and filtered by tiny hairs, called cilia, and sticky mucus in your nose. The cilia and mucus trap dirt and other particles and either push these particles back out the nostril or down into the digestive system. The air continues down into the pharynx, past the epiglottis and through your larynx, or voice box. The larynx contains your vocal cords, which vibrate as air passes through them, producing the sounds of your voice. Then the air goes into the airway passage called the trachea. You can feel your trachea by gently rubbing your hand up and down on your neck. The bumps you feel are C-shaped bands of cartilage that maintain the tubular shape of the trachea and are similar to the ridges of a vacuum hose. At the base of the trachea branch two tubes called the bronchi (singular: bronchus). Air passes through both of these tubes and goes directly to your lungs. Air continues down the bronchi, which branch into smaller air tubes called bronchioles. Finally, the air reaches tiny, thin-walled sacs called alveoli (singular: alveolus). There are millions of alveoli at the ends of the bronchioles, and this is where gas exchange, or respiration takes place. Gas Exchange in the Alveoli Each alveolus is surrounded by capillaries. This is where the respiratory structures and circulatory structures interact. Deoxygenated blood coming from the capillaries surrounds the alveolus. Carbon dioxide diffuses into the alveolus, and oxygen diffuses the other way into the capillaries and binds with the red blood cells. The oxygenated blood returns to the heart to be pumped to the rest of the body.
The Immune System Infectious diseases are caused by pathogens. The immune system attacks and destroys invaders such as pathogens that enter the body. There are two lines of defence. The first line of defence is the skin and the linings of the body's internal systems. The second line of defence has two parts: innate and acquired. An innate, or built-in, response is quick and general; an acquired response is slower and more specific. In the acquired response, the immune system makes specific antibodies that attach to antigens and destroy them. The acquired response involves B cells, helper T cells, killer T cells, and memory B cells. Table 1 shows the four general categories of transmission for infectious diseases. Table 1 Transmission Method Direct contact Indirect contact Water and food Animal bites Example Shaking hands or sharing drinking containers or bodily fluids with an infected person. Being near an infected person who sneezes without covering his or her mouth. Some pathogens can travel up to 5 m and infect people within that range. Eating foods, such as eggs and some meats, that are infected with Salmonella bacteria. Drinking water infected with E. coli bacteria can also result in serious illness. Being bitten by an animal carrying the rabies virus. First Line of Defence Pathogens are constantly attacking your body. But your immune system has powerful defences to fight them. The first line of defence against infectious diseases is the skin and the linings of all internal body systems. The skin is a physical barrier that stops most pathogens from entering the body. The sweat and oil on your skin, which are slightly acidic, also prevent some pathogens from growing on the surface of your body. Inside the body, other defences include the acidic gastric juice secreted by the lining of the stomach. Gastric juice can destroy pathogens, such as bacteria, that enter the stomach. The mucus and cilia that line your nose prevent pathogens from entering your respiratory system Second Line of Defence If a pathogen makes it past the first line of defence, your body can mount an immune response to the attack and destroy the invader. This is the immune system's second line of defence. Whether or not the response succeeds depends very much on your body's ability to distinguish between cells that belong to it and cells that do not. Imagine that each of the cells of your body wears a uniform. Your immune system recognizes the uniforms of cells that belong to your body. It also recognizes the uniforms of unknown cells that do not belong. These unknown cells are invaders, and the immune system usually attacks them. The second line of defence includes two types
of immune response. All living things can mount an innate immune response, a response they were born with. Animals with backbones, called vertebrates, can also mount an acquired immune response. Innate Immune Response The innate immune response to invading pathogens is quick and general, or non -specific. This means the response is the same for anything the body recognizes as an invader. Innate responses are usually mounted against invading bacteria and some viruses, such as the common cold virus. The first action in the innate immune response is a flow of fluid, cells, and dissolved substances from the blood to the site of infection. This action causes a fever, swelling, and redness in the area. The swelling and redness is called inflammation. Along with inflammation, there is an increase in types of white blood cells called phagocytes. Their role is to fight infection. Phagocytes roam the body searching for invader cells and swallowing them. Increasing the number of phagocytes in an area of infection is one way the immune system destroys pathogens. Acquired Immune Response The acquired immune response is a highly specific attack on a particular pathogen. Your immune system can mount an acquired immune response in two ways. Both ways involve types of white blood cells called B cells and T cells. This response can take up to a week to develop, because your body needs time to develop the exact response needed to defeat the particular invader. When a pathogen is detected by your white blood cells they signal for special cells called Helper T Cells that recognize the invader. The Helper T Cells then activate B Cells which make antibodies to bind to the invader and destroy it. Antibodies can attach to and destroy pathogens. The antibodies either prevent the pathogen from infecting body cells or mark it for destruction by other white blood cells. Another form of T Cell called Killer T Cells can go to the site of the infection and attack the pathogen directly. The Helper T Cells continue to stimulate the B Cells to produce antibodies until there are no more pathogens. Then the Helper T Cells and the B Cells die to conserve the body s energy. Special cells called Memory T and Memory B Cells remain alive so that if the same pathogen attacks the body again the body can fight back faster. This is called acquired immunity, and is why you can only have some diseases, like chickenpox, once