Circulation and the Heart (Pumps and Fluid flow)
The heart and circulatory system, called thecardiovascular system, make up the network that delivers blood to the body's tissues. With each heartbeat, blood is sent throughout our bodies, carrying oxygen and nutrients to all of our cells.
Every day the approximately 10 pints (5 liters) of blood in your body travel many times through about 60,000 miles (96,560 kilometers) of blood vessels that branch and cross, linking the cells of our organs and body parts. From the hard-working heart, to our thickest arteries, to capillaries so thin that they can only be seen through a microscope, the cardiovascular system is our body's lifeline.
The circulatory system is composed of the heart and blood vessels, including arteries, veins, and capillaries. Our bodies actually have two circulatory systems: the pulmonary circulation is a short loop from the heart to the lungs and back again, and the systemic circulation (the system we usually think of as our circulatory system) sends blood from the heart to all the other parts of our bodies and back again.
The Heart
The heart is the key organ in the circulatory system. As a hollow, muscular pump, its main function is to push blood throughout the body. It usually beats from 60 to 100 times per minute, but can go much faster when necessary. It beats about 100,000 times a day, more than 30 million times per year, and about 2.5 billion times in a 70-year lifetime.
The heart gets messages from the body that tells it when to pump more or less blood depending on a person's needs. When we're sleeping, it pumps just enough to provide for the lower amounts of oxygen needed by our bodies at rest. When we're exercising or frightened, the heart pumps faster to get more oxygen to our bodies. Oxygen is carried by proteins called hemoglobin in the blood and give the blood its red color. These transport the oxygen to all parts of the body. When something goes wrong with these or when we lose blood we are unable to distribute the oxygen effectively.
The heart has four chambers that are enclosed by thick, muscular walls. It is positioned between the lungs and just to the left of the middle of the chest cavity. The bottom part of the heart is divided into two chambers called the right and left ventricles, which pump blood out of the heart. The upper part of the heart is made up of the other two chambers of the heart, the right and left atria, receive the blood entering the heart.
The Role of Blood Vessels
Blood vessels carrying blood away from the heart are calledarteries. They are the thickest blood vessels, with muscular walls that contract to keep the blood moving away from the heart and through the body. In the systemic circulation, oxygen-rich blood is pumped from the heart into the aorta. This large artery curves up and back from the left ventricle, then heads down in front of the spinal column into the abdomen.Two coronary arteries branch off at the beginning of the aorta and divide into a network of smaller arteries that provide oxygen and nourishment to the muscles of the heart.
Unlike the aorta, the body's other main artery, the pulmonary artery, carries oxygen-poor blood. From the right ventricle, the pulmonary artery divides into right and left branches, on the way to the lungs where blood picks up oxygen.
Arterial walls are composed of three layers: the endothelium is on the inside and provides a smooth lining for blood to flow over as it moves through the artery; the media is the middle part of the artery, made up of a layer of muscle and elastic tissue; the adventitia is the tough covering that protects the outside of the artery. As they get farther from the heart, the arteries branch out into arterioles, which are smaller and less flexible.
Blood vessels that carry blood back to the heart are called veins. They are not as muscular as arteries, but they contain valves that prevent blood from flowing backward. Veins have the same three layers that arteries do, but they are thinner and less flexible. The two largest veins are the superior and inferior vena cavae. The terms superior and inferior do not mean that one vein is better than the other, but that they are located above (superior) and below (inferior) the heart.
A network of tiny capillaries connects the arteries and veins. Even though they are tiny, the capillaries are one of the most important parts of the circulatory system because it is through them that nutrients and oxygen are delivered to the cells. In addition, waste products such as carbon dioxide are also removed by the capillaries.
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Musculoskeletal System (bridges and levers)
Every time you walk, sit into a chair, or pick up your cat, you are using your bones, muscles, and joints. All of these important body parts in collaboration allow us to stand, walk, run, exercise or just sit down.
Bones and What They Do
From our head to our toes, bones provide support for our bodies and help form our shape. The skull protects the brain and forms the shape of our face. The spinal cord is a passageway for messages between the brain and the body and protected by the backbone, or spinal column.
The ribs form a cage that shelters the heart, lungs, liver, and spleen, and the pelvis helps protect the bladder, intestines, and in women, the reproductive organs. Although they are very light, bones are strong enough to support our entire weight. The human skeleton has 206 bones, which begin to develop before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification. Ossification is when the cartilage is replaced by hard deposits of calcium phosphate and stretchy collagen, the two main components of bone. It takes about 20 years for this process to be completed.
The bones of children are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of columns of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. Because girls mature at an earlier age than boys, this process is completed at an earlier age.
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Growing Bones
Bone-building continues throughout life, as the body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells: osteoblasts, which make new bone and help repair damage; osteocytes, mature bone cells which help continue new born formation; and osteoclasts, which break down bone and help to sculpt and shape it. Osteoclasts are very active in kids and teens, working on bone as it is remodeled during growth. They also play an important role in the repair of fractures.
Bones are made up of calcium, phosphorus, sodium, and other minerals, as well as the protein collagen. Calcium is needed to make bones hard, which allows them to support body weight. Bones also store calcium and release some into the bloodstream when it's needed by other parts of the body. The amount of certain vitamins and minerals that you eat, especially vitamin D and calcium, directly affects how much calcium is stored in the bones.
The soft bone marrow inside many of the bones is where most of the blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound. White blood cells help the body fight infection.
Bones are made up of two types of bone tissues: compact bone is the solid, hard, outside part of the bone. This type of bone makes up the most of the human skeleton. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves from the periosteum, the bone's outer membrane covering; cancellous bone, which looks like a sponge, is inside the compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae. This is where red and white blood cells are formed in the marrow.
Bones are fastened to other bones by long, fibrous straps called ligaments. Cartilage, a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.
Muscles and What They Do
Bones don't work alone — they need help from the muscles and joints. Muscles pull on the joints, allowing us to move. They also help your body perform other functions so we can grow and remain strong, such as chewing food and then moving it through the digestive system.
The human body has more than 650 muscles, which make up half of a person's body weight. They are connected to bones by tough, cord-like tissues called tendons, which allow the muscles to pull on bones. If you wiggle your fingers, you can see the tendons on the back of your hand move as they do their work.
Humans have three different kinds of muscle:
Skeletal muscle is attached to bone, mostly in the legs, arms, abdomen, chest, neck, and face. Skeletal muscles are called striated because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control their movement). They can contract (shorten or tighten) quickly and powerfully, but they tire easily and have to rest between workouts.
Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. Generally, we can't consciously control our smooth muscles; rather, they are controlled by the nervous system automatically, the reason they are called involuntary. Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system. Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily.
Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.
Our Nonstop Muscles
Even as we sit perfectly still, muscles throughout our bodies are constantly moving. Muscles enable the heart to beat, the chest to rise and fall during breathing, and blood vessels to help regulate the pressure and flow of blood through the body. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy.
The movements our muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.
When we decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.
The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it is in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain will send a message to the muscles in your arm and you then move it. When you run, the messages to the brain are more involved, because many muscles have to work together in rhythm.
Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position so they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. When the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the bicep contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.
Joints and What They Do
Joints occur where two bones meet. They make the skeleton flexible — without them, movement would be impossible. Joints allow our bodies to move in many ways. Some joints open and close like a hinge (such as knees and elbows), whereas others allow for more complicated movement — a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement.
Joints are classified by their range of movement. Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which must be immovable to protect the brain. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.
Partially movable, or cartilaginous, joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility.
Freely movable, or synovial, joints move in many directions. The main joints of the body — found at the hip, shoulders, elbows, knees, wrists, and ankles — are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily. Three kinds of freely movable joints play a big part in voluntary movement:
hinge joints allow movement in one direction, as seen in the knees and elbows; pivot joints allow a rotating or twisting motion, like that of the head moving from side to side; ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone.
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Immune System (The defense system: army navy marines)
The immune system, which is made up of special cells, proteins, tissues, and organs, defends people against germs and microorganisms every day. Generally, the immune system does a great job of keeping people healthy and preventing infections, but sometimes there are problems with the immune system that can lead to illness and infection.
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The immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade body systems and cause disease.
The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. The cells involved are white blood cells, or leukocytes, which come in two basic types that come together to seek out and destroy disease-causing organisms or substances.
Leukocytes are produced or stored in many locations in the body, including the thymus, spleen, and bone marrow. For this reason, they're called the lymphoid organs. There are also clumps of lymphoid tissue throughout the body, primarily as lymph nodes, that house the leukocytes. The leukocytes circulate through the body between the organs and nodes via lymphatic vessels and blood vessels. In this way, the immune system works in a coordinated manner to monitor the body for germs or substances that might cause problems.
The two basic types of leukocytes are: phagocytes, cells that chew up invading organisms; and lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them. A number of different cells are considered phagocytes. The most common type is the neutrophil, which primarily fights bacteria. If doctors are worried about a bacterial infection, they might order a blood test to see if a patient has an increased number of neutrophils triggered by the infection. Other types of phagocytes have their own jobs to make sure that the body responds appropriately to a specific type of invader.
The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and either stay there and mature into B cells, or they leave for the thymus gland, where they mature into T cells. B lymphocytes and T lymphocytes have separate functions: B lymphocytes are like the body's military intelligence system, seeking out their targets and sending defenses to lock onto them. T cells are like the soldiers, destroying the invaders that the intelligence system has identified.
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When antigens, foreign substances that invade the body, are detected, several types of cells work together to recognize them and respond to them. These cells trigger the B lymphocytes to produce antibodies, specialized proteins that lock onto specific antigens.
Once produced, these antibodies continue to exist in a person's body, so that if the same antigen is presented to the immune system again, the antibodies are already there to do their job. So if someone gets sick with a certain disease, like chickenpox, that person typically doesn't get sick from it again.
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This is how immunizations prevent certain diseases. An immunization introduces the body to an antigen in a way that doesn't make someone sick, but does allow the body to produce antibodies that will then protect the person from future attack by the germ or substance that produces that particular disease.
Although antibodies can recognize an antigen and lock onto it, they are not capable of destroying it without help. That's the job of the T cells, which are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. Some T cells are actually called "killer cells." T cells also are involved in helping signal other cells, like phagocytes, to do their jobs.
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Antibodies also can neutralize toxins (poisonous or damaging substances) produced by different organisms. Lastly, antibodies can activate a group of proteins, called complement, that are also part of the immune system. Complement assists in killing bacteria, viruses, or infected cells. All of these specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.
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Proteins and Enzymes (cars and engines)
At any given moment, all of the work being done inside any cell is being done by enzymes or proteins. Enzymes have extremely interesting properties that make them little chemical-reaction machines. The purpose of an enzyme in a cell is to allow the cell to carry out chemical reactions very quickly. They do this by bringing two molecules that need to react in very close proximity so they shorten the time it takes for two or more reactants to find each other and form a product. Enzymes are therefore catalysts of change in the body which is why they are extremely important. These reactions allow the cell to build things or take things apart as needed. This is how a cell grows and reproduces. At the most basic level, a cell is really a little container full of chemical reactions that are made possible by enzymes.
Enzymes are made from amino acids, and they are proteins. When an enzyme is formed, it is made by stringing together between 100 and 1,000 amino acids in a very specific and unique order. The chain of amino acids then folds into a unique shape. That shape allows the enzyme to carry out specific chemical reactions -- an enzyme acts as a very efficient catalyst for a specific chemical reaction. The enzyme speeds that reaction up tremendously.
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