The Digestive System

The digestive system is made up of the gastrointestinal tract—also called the GI tract or digestive tract—and the liver, pancreas, and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. The hollow organs that make up the GI tract are the mouth, esophagus, stomach, small intestine, large intestine, and anus. The liver, pancreas, and gallbladder are the solid organs of the digestive system.

The small intestine has three parts. The first part is called the duodenum. The jejunum is in the middle and the ileum is at the end. The large intestine includes the appendix, cecum, colon, and rectum. The appendix is a finger-shaped pouch attached to the cecum. The cecum is the first part of the large intestine. The colon is next. The rectum is the end of the large intestine.

Bacteria in your GI tract, also called gut flora or microbiome, help with digestion. Parts of your nervous and circulatory systems also help. Working together, nerves, hormones, bacteria, blood, and the organs of your digestive system digest the foods and liquids you eat or drink each day.

The Intestinal System

The intestinal system consists of hollow organs, including the oesophagus, stomach, small bowel, and large bowel, that hold and move nutrients through your body.

The intestinal system is part of the overall digestive system.

The stomach, which receives food from the oesophagus, is located in the upper left quadrant of the abdomen. The stomach is divided into the fundic, cardiac, body, and pyloric regions. The lesser and greater curvatures are on the right and left sides, respectively, of the stomach.

The small intestine extends from the pyloric sphincter to the ileocecal valve, where it empties into the large intestine. The small intestine finishes the process of digestion, absorbs the nutrients, and passes the residue on to the large intestine.

The small intestine is divided into the duodenum, jejunum, and ileum. The small intestine follows the general structure of the digestive tract in that the wall has a mucosa with simple columnar epithelium, submucosa, smooth muscle with inner circular and outer longitudinal layers, and serosa. The absorptive surface area of the small intestine is increased by plicae circulares, villi, and microvilli.

The large intestine is larger in diameter than the small intestine. It begins at the ileocecal junction, where the ileum enters the large intestine, and ends at the anus. The large intestine consists of the colon, rectum, and anal canal.

The wall of the large intestine has the same types of tissue that are found in other parts of the digestive tract but there are some distinguishing characteristics. The mucosa has a large number of goblet cells but does not have any villi. The longitudinal muscle layer, although present, is incomplete. The longitudinal muscle is limited to three distinct bands, called taeniae coli, that run the entire length of the colon.

Unlike the small intestine, the large intestine produces no digestive enzymes. Chemical digestion is completed in the small intestine before it reaches the large intestine. Functions of the large intestine include the absorption of water and electrolytes and the elimination of faeces.

The Circulatory System

Your heart and blood vessels make up the circulatory system. The main function of the circulatory system is to provide oxygen, nutrients and hormones to muscles, tissues, and organs throughout your body. Another function of the circulatory system is to remove waste from cells and organs so your body can dispose of it.

Your heart pumps blood to the body through a network of arteries and veins (blood vessels). Your circulatory system can also be defined as your cardiovascular system. Cardio means heart, and vascular refers to blood vessels.

The circulatory system’s function is to move blood throughout the body. This blood circulation keeps organs, muscles, and tissues healthy and working to keep you alive.

The circulatory system also helps your body get rid of waste products. This waste includes:

• Carbon dioxide from respiration (breathing)
• Other chemical by-products from your organs
• Waste from things you eat and drink

Your circulatory system functions with the help of blood vessels that include arteries, veins, and capillaries. These blood vessels work with your heart and lungs to continuously circulate blood through your body.

Here’s how:

1. The heart’s bottom right pumping chamber (right ventricle) sends blood that’s low in oxygen (oxygen-poor blood) to the lungs. Blood travels through the pulmonary trunk (the main pulmonary artery).
2. Blood cells pick up oxygen in the lungs.
3. Pulmonary veins carry the oxygenated blood from the lungs to the heart’s left atrium (upper heart chamber).
4. The left atrium sends the oxygenated blood into the left ventricle (lower chamber). This muscular part of the heart pumps blood out to the body through the arteries.
5. As it moves through your body and organs, blood collects and drops off nutrients, hormones and waste products.
6. The veins carry deoxygenated blood and carbon dioxide back to the heart, which sends the blood to the lungs.
7. Your lungs get rid of the carbon dioxide when you exhale.

The Nervous System

Your nervous system guides almost everything you do, think, say or feel. It controls complicated processes like movement, thought and memory. It also plays an essential role in the things your body does without thinking, such as breathing, blushing and blinking.

Your nervous system affects every aspect of your health, including your:

• Thoughts, memory, learning, and feelings
• Movements, such as balance and coordination
• Senses, including how your brain interprets what you see, hear, taste, touch and feel
• Sleep, healing and aging
• Heartbeat and breathing patterns
• Response to stressful situations
• Digestion, as well as how hungry and thirsty you feel
• Body processes, such as puberty

This complex system is the command center for your body. It regulates your body’s systems and allows you to experience your environment.

A vast network of nerves sends electrical signals to and from other cells, glands, and muscles all over your body. These nerves receive information from the world around you. Then the nerves interpret the information and control your response. It’s almost like an enormous information highway running throughout your body.

The nervous system has two main parts. Each part contains billions of cells called neurons, or nerve cells. These special cells send and receive electrical signals through your body to tell it what to do.

The main parts of the nervous system are:

Central nervous system (CNS): Your brain and spinal cord make up your CNS. Your brain uses your nerves to send messages to the rest of your body. Each nerve has a protective outer layer called myelin. Myelin insulates the nerve and helps the messages get through.

Peripheral nervous system: Your peripheral nervous system consists of many nerves that branch out from your CNS all over your body. This system relays information from your brain and spinal cord to your organs, arms, legs, fingers and toes. Your peripheral nervous system contains your:

Somatic nervous system, which guides your voluntary movements.
Autonomic nervous system, which controls the activities you do without thinking about them.

The Immune System

Your immune system is a large network of organs, white blood cells, proteins (antibodies) and chemicals. This system works together to protect you from foreign invaders (bacteria, viruses, parasites, and fungi) that cause infection, illness and disease.

Your immune system works hard to keep you healthy. Its job is to keep germs out of your body, destroy them or limit the extent of their harm if they get in.

When your immune system is working properly: When your immune system is working properly, it can tell which cells are yours and which substances are foreign to your body. It activates, mobilizes, attacks and kills foreign invader germs that can cause you harm. Your immune system learns about germs after you’ve been exposed to them too. Your body develops antibodies to protect you from those specific germs.

An example of this concept occurs when you get a vaccine. Your immune system builds up antibodies to foreign cells in the vaccine and will quickly remember these foreign cells and destroy them if you are exposed to them in the future. Sometimes doctors can prescribe antibiotics to help your immune system if you get sick. But antibiotics only kill certain bacteria. They don’t kill viruses.

When your immune system is not working properly: When your immune system can’t mount a winning attack against an invader, a problem, such as an infection, develops. Also, sometimes your immune system mounts an attack when there is no invader or doesn’t stop an attack after the invader has been killed. These activities result in such problems as autoimmune diseases and allergic reactions.

Your immune system is made of up a complex collection of cells and organs. They all work together to protect you from germs and help you get better when you’re sick. The main parts of the immune system are:

• White blood cells
• Lymph nodes
• Spleen
• Tonsils and adenoids
• Thymus
• Bone marrow
• Skin, mucous membranes and other first-line defences
• Stomach and bowel

The Respiratory System

The respiratory system is the network of organs and tissues that help you breathe. It includes your airways, lungs and blood vessels. The muscles that power your lungs are also part of the respiratory system. These parts work together to move oxygen throughout the body and clean out waste gases like carbon dioxide.

The respiratory system has many functions. Besides helping you inhale (breathe in) and exhale (breathe out), it:

• Allows you to talk and to smell
• Warms air to match your body temperature and moisturizes it to the humidity level your body needs
• Delivers oxygen to the cells in your body
• Removes waste gases, including carbon dioxide, from the body when you exhale
• Protects your airways from harmful substances and irritants

The respiratory system has many different parts that work together to help you breathe. Each group of parts has many separate components.

Your airways deliver air to your lungs. Your airways are a complicated system that includes your:

Mouth and nose: Openings that pull air from outside your body into your respiratory system.

Sinuses: Hollow areas between the bones in your head that help regulate the temperature and humidity of the air you inhale.

Pharynx (throat): Tube that delivers air from your mouth and nose to the trachea (windpipe).

Trachea: Passage connecting your throat and lungs.

Bronchial tubes: Tubes at the bottom of your windpipe that connect into each lung.

Lungs: Two organs that remove oxygen from the air and pass it into your blood. From your lungs, your bloodstream delivers oxygen to all your organs and other tissues.

Muscles and bones help move the air you inhale into and out of your lungs. Some of the bones and muscles in the respiratory system include your:

Diaphragm: Muscle that helps your lungs pull in air and push it out.

Ribs: Bones that surround and protect your lungs and heart.

When you breathe out, your blood carries carbon dioxide and other waste out of the body.

Other components that work with the lungs and blood vessels include:

Alveoli: Tiny air sacs in the lungs where the exchange of oxygen and carbon dioxide takes place.

Bronchioles: Small branches of the bronchial tubes that lead to the alveoli.

Capillaries: Blood vessels in the alveoli walls that move oxygen and carbon dioxide.

Lung lobes: Sections of the lungs — three lobes in the right lung and two in the left lung.

Pleura: Thin sacs that surround each lung lobe and separate your lungs from the chest wall.

Some of the other components of your respiratory system include:

Cilia: Tiny hairs that move in a wave-like motion to filter dust and other irritants out of your airways.

Epiglottis: Tissue flap at the entrance to the trachea that closes when you swallow to keep food and liquids out of your airway.

Larynx (voice box): Hollow organ that allows you to talk and make sounds when air moves in and out.

The Urinary System

The urinary system works as a filter, removing toxins and wastes from your body through urine. It uses a series of tubes and ducts to pass this waste. These tubes are connected to your blood vessels and digestive system. Your urinary system helps the rest of your body work properly.

Your urinary system filters your blood to get rid of what your body doesn’t need. It eliminates extra water and salt, toxins, and other waste products. Different parts of the urinary system perform tasks including:

• Filtering blood
• Separating the toxins you don’t need from the nutrients you do need
• Storing and carrying urine out of your body

Your kidneys are an essential part of filtering your blood. Here’s how the urinary system works:

1. Your blood enters each kidney through lots of little arteries
2. Your kidneys filter your blood, separating toxins from nutrients
3. Vitamins, minerals, nutrients and proteins return to your bloodstream
4. Waste products and urine move through your ureters to your bladder. Your bladder stores urine until you use the toilet
5. Urine leaves your body through your urethra

The kidneys, ureters, bladder and urethra make up the urinary system. They all work together to filter, store and remove liquid waste from your body.

Here’s what each organ does:

Kidneys: These organs work constantly. They filter your blood and make urine, which your body eliminates. You have two kidneys, one on either side of the back of your abdomen, just below your rib cage. Each kidney is about as big as your fist.

Ureters: These two thin tubes inside your pelvis carry urine from your kidneys to your bladder.

Bladder: Your bladder holds urine until you’re ready to empty it (pee). It’s hollow, made of muscle, and shaped like a balloon. Your bladder expands as it fills up. Most bladders can hold up to 2 cups of urine.

Urethra: This tube carries urine from your bladder out of your body. It ends in an opening to the outside of your body in the penis (in men) or in front of the vagina (in women).

The Glandular System

Your glandular, or endocrine system, is made up of several organs called glands. These glands, located all over your body, create and secrete (release) hormones.

Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, skin, muscles and other tissues. These signals tell your body what to do and when to do it.

Your endocrine system continuously monitors the amount of hormones in your blood. Hormones deliver their messages by locking into the cells they target so they can relay the message.

The pituitary gland senses when your hormone levels rise, and tells other glands to stop producing and releasing hormones. When hormone levels dip below a certain point, the pituitary gland can instruct other glands to produce and release more. This process, called homeostasis, works similarly to the thermostat in your house. Hormones affect nearly every process in your body, including:

• Metabolism (the way you break down food and get energy from nutrients)
• Growth and development
• Emotions and mood
• Fertility and sexual function
• Sleep
• Blood pressure

Sometimes glands produce too much or not enough of a hormone. This imbalance can cause health problems, such as weight gain, high blood pressure and changes in sleep, mood and behaviour. Many things can affect how your body creates and releases hormones. Illness, stress and certain medications can cause a hormone imbalance.

The endocrine system is made up of organs called glands. Glands produce and release different hormones that target specific things in the body. You have glands all over your body, including in your neck, brain and reproductive organs. Some glands are tiny, about the size of a grain of rice or a pea. The largest gland is the pancreas, which is about 6 inches long.

The main glands that produce hormones include:

Hypothalamus: This gland is located in your brain and controls your endocrine system. It uses information from your nervous system to determine when to tell other glands, including the pituitary gland, to produce hormones. The hypothalamus controls many processes in your body, including your mood, hunger and thirst, sleep patterns and sexual function.

Pituitary: This little gland is only about the size of a pea, but it has a big job. It makes hormones that control several other glands such as the thyroid gland, adrenal glands, ovaries and testicles. The pituitary gland is in charge of many different functions, including how your body grows. It’s located at the base of your brain.

Thyroid: Your thyroid is a butterfly-shaped gland in the front of your neck. It’s responsible for your metabolism (how your body uses energy).

Parathyroid: These four tiny glands are no larger than a grain of rice. They control the level of calcium in your body. For your heart, kidneys, bones and nervous system to work, you need the right amount of calcium.

Adrenal: You have two adrenal glands, one on top of each kidney. They control your metabolism, blood pressure, sexual development and response to stress.

Pineal: This gland manages your sleep cycle by releasing melatonin, a hormone that causes you to feel sleepy.

Pancreas: Your pancreas is part of your endocrine system, and it plays a significant role in your digestive system too. It makes a hormone called insulin that controls the level of sugar in your blood.

Ovaries: In women, the ovaries release sex hormones called estrogen, progesterone and testosterone. Women have two ovaries in their lower abdomen, one on either side.

Testes: In men, the testes (testicles) make sperm and release the hormone testosterone. This hormone affects sperm production, muscle strength and sex drive.

The Structural System

The structural, or skeletal system, is your body’s central framework. It consists of bones and connective tissue, including cartilage, tendons, and ligaments. It’s also called the musculoskeletal system.

The skeletal system has many functions. Besides giving us our human shape and features, it:

Allows movement: Your skeleton supports your body weight to help you stand and move. Joints, connective tissue and muscles work together to make your body parts mobile.

Produces blood cells: Bones contain bone marrow. Red and white blood cells are produced in the bone marrow.

Protects and supports organs: Your skull shields your brain, your ribs protect your heart and lungs, and your backbone protects your spine.

Stores minerals: Bones hold your body’s supply of minerals like calcium and vitamin D.

The skeletal system is a network of many different parts that work together to help you move. The main part of your skeletal system consists of your bones, hard structures that create your body’s framework — the skeleton. There are 206 bones in an adult human skeleton. Each bone has three main layers:

Periosteum: The periosteum is a tough membrane that covers and protects the outside of the bone.

Compact bone: Below the periosteum, compact bone is white, hard, and smooth. It provides structural support and protection.

Spongy bone: The core, inner layer of the bone is softer than compact bone. It has small holes called pores to store marrow.

The other components of your skeletal system include:

Cartilage: This smooth and flexible substance covers the tips of your bones where they meet. It enables bones to move without friction (rubbing against each other). When cartilage wears away, as in arthritis, it can be painful and cause movement problems.

Joints: A joint is where two or more bones in the body come together. There are three different joint types. The types of joints are:

Immovable joints: Immovable joints don’t let the bones move at all, like the joints between your skull bones.

Partly movable joints: These joints allow limited movement. The joints in your rib cage are partly movable joints.

Movable joints: Movable joints allow a wide range of motion. Your elbow, shoulder, and knee are movable joints.

Ligaments: Bands of strong connective tissue called ligaments hold bones together.

Tendons: Tendons are bands of tissue that connect the ends of a muscle to your bone.