An ankle sprain occurs when the joint’s ligament is stretched or torn. Ligaments are bands or sheets of regular, tough fibrous tissue that connect bones together. Symptoms of an ankle sprain include swelling and discoloration near the affected area. Ankle sprains may be classified as follows:

• Type I sprain – ligaments stretched

• Type II sprain – ligaments slightly torn

• Type III sprain – ligaments completely torn

Treatment for a Type I sprain should include rest, ice, compression and immobilization, and elevation of the affected area. This is easy to remember if you think of the acronym RICE. If you suspect a ligament is torn or completely severed, see your medical care professional for treatment.

The nervous system enables a person to blink to prevent harmful substances from getting in the eyes. During the normal course of a day, a person blinks an average of 15 times a minute to keep the eyes healthy. The lacrimal gland provides lubricating fluid for the eyes. The eyelid moves fluid from the lacrimal gland and across the eye. Blinking also provides the eyes with protection from foreign objects.

When the eye becomes irritated, the lacrimal gland produces extra tears to wash out impurities. Excess fluid drains through the tear ducts and into the nasal cavity. An abundance of tears draining through the nasal cavity may cause the nose to run and a person to sniffle.

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The brain is composed of more than a thousand million neurons. Specific groups of them, working in concert, provide us with the capacity to reason, to experience feelings, and to understand the world. They also give us the capacity to remember numerous pieces of information.

The 3 major components of the brain are the cerebrum, cerebellum, and brain stem.

The cerebrum is divided into is left and right hemispheres, each composed of a frontal, temporal, parietal, and occipital lobes. The cerebral cortex (gray matter) is the outside portion of the cerebrum and provides us with functions associated with conscious thought. The grooves and folds increase the cerebrum’s surface area, allowing us to have a tremendous amount of gray matter inside of the skull. Deep to the gray matter is the cerebral "white matter". The white matter provides for the communication between the cortex and lower central nervous system centers.

The cerebellum is located near the base of the head. It creates automatic programs so we can make complex movements without thinking.

The brain stem connects the brain with the spinal cord and is composed of 3 structures: the midbrain, pons, and medulla oblongata. The brain stem provides us with automatic functions that are necessary for survival.

The two lungs are the primary organs of the respiratory system. Other components of the respiratory system conduct air to the lungs, such as the trachea (windpipe) which branches into smaller structures called bronchi.

The process of breathing (respiration) is divided into two distinct phases, inspiration (inhalation) and expiration (exhalation). During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward.  This increases the size of the thoracic cavity and decreases the pressure inside.  As a result, air rushes in and fills the lungs.

During expiration, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases.  As a result, the lungs contract and air is forced out.

During intercourse, sperm are released into the vagina near the cervix, swim through the uterus and travel up the fallopian tubes. Sperm are composed of 3 parts: a head, a middle section, and a tail. The tail propels the sperm, which is powered by energy cells stored in the middle section. The head of the sperm contains the man’s genetic material and an enzyme-filled acrosomal cap needed to help the sperm penetrate through the outer membrane of the egg.

As an egg released by an ovary travels through a fallopian tube, it may encounter hundreds of sperm that have survived to reach this point in their journey. Eventually, one sperm may succeed in breaking through the egg’s outer membrane.

After penetrating the egg’s outer membrane, the sperm releases its nucleus, which unites with the nucleus from the egg. Fertilization or conception occurs when the sperm fuses with the egg to form a fertilized egg (zygote).

Coronary artery bypass graft surgery (CABG) is an invasive procedure that involves taking a section of vein from the leg and grafting it onto a location on the heart, which allows blood to bypass the blocked portion of the coronary artery.

The procedure begins with the surgeon making a cut in the leg and removing a section of vein. Both ends of the vein are tied-off in the leg and cut is closed. The chest is opened and the blood is rerouted through a heart-lung machine. The heart is then stopped.

The surgeon locates the blocked coronary artery and attaches the section of vein taken from the leg to the aorta and to the coronary artery below the blocked segment of the artery. The surgeon may do as many bypasses on as many blocked coronary arteries as the patient needs.

Once each bypass graft is placed, it is checked for leaks. Following this, the heart is restarted. Once the heart is beating again, the surgeon will remove its attachments to the heart-lung machine and sew the openings closed. Following this the chest is closed. A pacemaker may be inserted during the procedure to help control any heart rhythm problems the patient may have.

Coughing is a sudden expulsion of air from the lungs through the epiglottis at an amazingly fast speed (estimated at 100 miles per hour). With such a strong force of air, coughing is the body’s mechanism for clearing the breathing passageways of unwanted irritants.

In order for a cough to occur, several events need to take place in sequence. First, the vocal cords open widely, allowing additional air to pass through into the lungs. Then the epiglottis closes off the windpipe (larynx), and simultaneously, the abdominal and rib muscles contract, increasing the pressure behind the epiglottis. With the increased pressure, the air is forcefully expelled, and creates a rushing sound as it moves very quickly past the vocal cords. The rushing air dislodges the irritant, making it possible to breathe comfortably again.

Digestion is the process in which food is broken down into nutrients used by the body. Food passes from the mouth through the esophagus to the stomach. The stomach churns the food and breaks it down further with its contents of hydrochloric acid and an enzyme called pepsin.

The process of breaking food down in the stomach takes a few hours. From there, it goes to the duodenum where it is broken down further by digestive bile produced by the liver and stored in the gallbladder along with enzymes from the pancreas. Enzymes are chemicals that speed up the digestion of specific types of food. For example, the enzyme trypsin breaks down the protein in steak, lipase helps to break down fat, and lactase breaks down the sugar in milk.

Once everything is broken down, the small intestine absorbs the nutrients the body needs. From there the nutrients go into the bloodstream and to the liver, where poisons are removed. Undigested food and water continue through the small intestine and go into the large intestine, where water is reabsorbed. Finally, feces are eliminated through the rectum and anus.

Directional Coronary Atherectomy (DCA) is a minimally invasive procedure to remove the blockage from the coronary arteries and allow more blood to flow to the heart muscle and ease the pain caused by blockages.

The procedure begins with the doctor injecting some local anesthesia into the groin area and putting a needle into the femoral artery, the blood vessel that runs down the leg. A guide wire is placed through the needle and the needle is removed. An introducer is then placed over the guide wire, after which the wire is removed. A different sized guide wire is put in its place.
 
Next, a long narrow tube called a diagnostic catheter is advanced through the introducer over the guide wire, into the blood vessel. This catheter is then guided to the aorta and the guide wire is removed. Once the catheter is placed in the opening or ostium of one the coronary arteries, the doctor injects dye and takes an x-ray.

If a treatable blockage is noted, the first catheter is exchanged for a guiding catheter. Once the guiding catheter is in place, a guide wire is advanced across the blockage, then a catheter designed for lesion cutting is advanced across the blockage site. A low-pressure balloon, which is attached to the catheter adjacent to the cutter, is inflated such that the lesion material is exposed to the cutter.

The cutter spins, cutting away pieces of the blockage. These lesion pieces are stored in a section of the catheter called a nosecone, and removed after the intervention is complete. Together with rotation of the catheter, the balloon can be deflated and re-inflated to cut the blockage in any direction, allowing for uniform debulking. 

A device called a stent may be placed within the coronary artery to keep the vessel open. After the intervention is completed the doctor injects contrast media and takes an x-ray to check for any change in the arteries.  Following this, the catheter is removed and the procedure is completed.

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A woman is born with all of the egg cells she will release throughout her lifetime. Starting at about age 12 through menopause, a woman’s reproductive cycle releases an egg about once a month.

Hormonal messages from the brain instruct the ovaries to develop several follicles in which a single dominant follicle in one of the ovaries will release an egg for fertilization. During this time, other hormones instruct the uterine lining to thicken in preparation for nourishing a fertilized egg.

There are several hormones that regulate the reproductive cycle. Follicle stimulating hormone (FSH) stimulates preparation of the egg for fertilization by instructing a follicle to begin dividing it’s genetic material (chromosomes).

The follicle then releases estrogen, the hormone that prepares the lining of the uterus to receive a fertilized egg. Increased levels of estrogen in the bloodstream cause a small structure in the brain, the pituitary gland, to stop releasing the hormone FSH, and to start releasing luteinizing hormone (LH).

LH causes the follicle to enlarge rapidly and to release its egg in a process known as ovulation. Once the egg is out of the follicle, the follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for the fertilized egg. The remaining cells of the follicle shrink into a hormone producing mass of cells called a corpus luteum.

The egg is swept into the fallopian tube by its waving structures called fimbriae. Fertilization of the egg usually occurs in the fallopian tube. From there, it is transported to the uterus and implants itself in the uterine wall, where it is nourished by the uterine lining. In the ovary, the corpus luteum produces progesterone so that the egg can develop into a fetus.

If the egg is not fertilized within 24 hours after its release from the ovary, it stops developing and dissolves before reaching the uterus. The absence of a fertilized egg causes the body to stop releasing the hormones that prepare the uterus for implantation. In response, the uterus sheds its lining over a period of four to five days in a process known as menstruation.

The prostate gland is located underneath the bladder and is about the size of a chestnut. Part of the urethra is encased within the prostate gland. As a man ages, the prostate typically enlarges in size in a process called benign hypertrophy (non-cancerous enlargement).

The enlarged prostate crowds its surrounding structures and may cause the urethra to narrow. The narrowed urethra results in several of the symptoms of benign prostatic hypertrophy (BPH). Symptoms may include a slowed or delayed start in urination, the need to urinate frequently during the night, difficulty in emptying the bladder, a strong, sudden urge to urinate, and incontinence. Less than half of all men with BPH have symptoms of the disease, or their symptoms are minor and do not restrict their life style.

BPH is a normal physiological process of aging and treatment options are available. The choice of the appropriate treatment is based on the severity of the symptoms, the extent to which they effect lifestyle, and the presence of other medical conditions. Men with BPH should consult with their physician yearly to monitor the progression of the symptoms and decide the best course of treatment as needed.

Geriatrics,UrologyReproductive System,Urinary SystemShockwave

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The ears begin their development during the fifth week of pregnancy. Ear formation starts from a few small bulges called branchial arches. Portions of the branchial arches form into structures called auricular hillocks. The auricular hillocks grow and join together to form the outer ears.

During the fifth month, the inner and middle parts of the ear develop, but won’t be completely finished until birth.

Twins occur in about 1% of all pregnancies in which 30% are identical (maternal, monozygotic) twins and 70% are non-identical (fraternal, dizygotic) twins.

A single baby is formed when an egg cell is fertilized by a single sperm cell to form a zygote. The zygote divides to form a structure composed of hundreds of cells called a blastocyst. The blastocyst implants into the uterine lining and will grow into a single baby.

Identical twins start out from a single fertilized egg cell (zygote). Unlike a single baby, the fertilized egg cell will split into two separate embryos during the two-cell stage (day 2), early blastocyst stage (day 4), or late blastocyst stage (day 6).

The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. The earlier the splitting occurs, the more independently the twins will develop in the uterus. Twins that split during the late blastocyst stage will share an amnion, chorion, and amniotic sac.

Non-identical twins develop from two fertilized egg cells (zygotes). During ovulation, two egg cells are released and fertilized by two different sperm cells. Non-identical twin embryos develop separately each having their own chorion, amnion, and placenta.

Air first enters the body through the mouth or nose, quickly moves to the pharynx (throat), passes through the larynx (voice box), enters the trachea, which branches into a left and right bronchus within the lungs and further divides into smaller and smaller branches called bronchioles.  The smallest bronchioles end in tiny air sacs, called alveoli, which inflate during inhalation, and deflate during exhalation.

Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs. It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli.

The walls of the alveoli actually share a membrane with the capillaries in which oxygen and carbon dioxide to move freely between the respiratory system and the bloodstream. Oxygen molecules attach to red blood cells, which travel back to the heart.  At the same time, the carbon dioxide molecules in the alveoli are blown out of the body with the next exhalation. 

The ear is divided into three regions: the outer ear, middle ear and inner ear.

When sound waves enter the ear canal, they cause the eardrum to vibrate. The vibration moves the three bones in the middle ear, called the ossicles. The ossicles are also known as the hammer (malleus), anvil (incus), and stirrup (stapes). These tiny bones transfer and amplify sound waves to the oval window, which is located behind the stirrup.

When the oval window vibrates, it moves fluid across a membrane inside the cochlea. The fluid causes the membrane to move. Specialized hair cells translate this movement into nerve impulses, which are sent to the brain through the vestibulocochlear nerve. The brain interprets the impulses as sound.

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The urinary tract includes the kidneys, ureters, bladder and urethra. Within each kidney, urine flows from the outer cortex to the inner medulla. The renal pelvis is the funnel through which urine exits the kidney and enters the ureter.

As urine can become very concentrated as it passes through the kidneys. When the urine becomes too concentrated, calcium, uric acid salts and other chemicals dissolved in the urine can crystallize, forming a kidney stone (renal calculus).

Usually the calculus is the size of a small pebble. But ureters are very sensitive to being stretched, and when stones form and distend it, the stretching can be very painful. Often, people may not know they have kidney stones until they feel the painful symptoms resulting from a stone being stuck anywhere along the urinary tract. Fortunately, small stones typically passed out of the kidneys and through the ureters on their own without causing any problems.

However, stones can become more problematic when they block the flow of urine. A staghorn kidney stone may obstruct the entire kidney. Fortunately, these stones are the exception rather than the rule.

At 1 month, the baby growing inside the mother’s uterus is very small. The baby is so small she could fit in the palm of your hand and is about the size of your thumbnail.

Over the next 9 months, the baby will grow more inside the uterus until she is ready to be born.

To make a baby, a man’s sperm meets and joins with a woman’s egg cell inside her body. Inside the man’s sperm are a set of instructions that tell the baby to be a boy or a girl.

The instructions in the man’s sperm cell can either carry the letter "X" or the letter "Y". If the letter is an "X", it means the baby will be a girl. If the letter is a "Y", the baby will be a boy.

When the baby is in the mother’s uterus, it can’t eat or breathe on its own, so it needs some help. The baby has a little tube that goes to its middle called the umbilical cord. The umbilical cord goes to the placenta, which connects to the mother’s uterus.

Here’s how it works. First, the food that the mother eats and air that she breathes get into her bloodstream as very tiny pieces called molecules.

These molecules, or tiny pieces of food and air, travel through the mother’s bloodstream to her placenta. From there, they go to the umbilical cord and into the baby’s body. That’s how the baby eats and breathes inside the uterus.

After a baby is born, the umbilical cord goes away. Guess what’s left? You’re belly button.

Two things are needed to make a baby: a sperm cell and an egg cell. A man makes the sperm cell inside his body and a woman makes the egg cell inside her body.

Both the sperm cell and egg cell are very small. You would need a microscope to see them in real life. A microscope is like a magnifying glass, only much stronger.

When the sperm cell and the egg cell meet each other, they make a tiny baby that’s smaller than a grain of salt. The baby will grow inside a special place in woman’s body called the uterus. After about nine months, the baby will come out as a little boy or girl.

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Muscles perform four important body functions: maintain body posture, stabilize the joints, provide mobility, and generate heat that the body requires.

The body contains three types of muscle to perform these functions:
• Smooth muscle - involuntary muscle found in the walls of body organs; functions without conscious control
• Cardiac muscle - involuntary muscle found only in the walls of the heart; functions without conscious control
• Skeletal muscle - attaches to and covers the bony skeleton to provide movement of the body; the only type of muscle under voluntary or conscious control

The nervous system is composed of two divisions, the central nervous system (CNS) and peripheral nervous system (PNS). The CNS contains the brain and the spinal cord and the PNS consists of thousands of nerves that connect the spinal cord to muscles and sensory receptors.

A peripheral nerve is composed of nerve bundles (fascicles) that contain hundreds of individual nerve fibers (neurons). Neurons consist of dendrites, axon, and cell body. The dendrites are the tree-like structures that receive signals from other neurons and from special sensory cells that sense the body’s surrounding environment. The cell body is the headquarters of the neuron and contains its genetic information in the form of DNA. The axon transmits signals away from the cell body to other neurons.

Many neurons are insulated like pieces of electrical wire. This insulation protects them and also allows their signals to move faster along the axon. Without this insulation, signals from the brain might never reach the outlying muscle groups in the limbs.

The operation of the nervous system depends on the flow of communication between neurons. For an electrical signal to travel between two neurons, it must first be converted to a chemical signal, which then crosses a space of about a millionth of an inch wide. The space is called a synapse, and the chemical signal is called a neurotransmitter.

Neurotransmitters allow the billions of neurons in the nervous system to communicate with one another, making the nervous system the master communication system of the body.

Osteoarthritis is the most common form of arthritis and is associated with the aging process. Osteoarthritis is a chronic disease causing the deterioration of the cartilage within a joint.

For most people, the cause of osteoarthritis is unknown, but metabolic, genetic, chemical, and mechanical factors play a role in its development. Symptoms of osteoarthritis include loss of flexibility, limited movement, and pain and swelling within the joint. The condition results from injury to the cartilage, which normally absorbs stress and covers the bones, so they can move smoothly.

The cartilage of the affected joint is roughened and becomes worn down. As the disease progresses, the cartilage becomes completely worn down and the bone rubs on bone. Bony spurs usually develop around the margins of the joint.
Part of the pain results from these bone spurs, which can restrict the joint’s movement as well.

Percutaneous Transluminal Coronary Angioplasty (PTCA) is a minimally invasive procedure to open up blocked coronary arteries, allowing blood to circulate unobstructed to the heart muscle.

The procedure begins with the doctor injecting some local anesthesia into the groin area and putting a needle into the femoral artery, the blood vessel that runs down the leg. A guide wire is placed through the needle and the needle is removed. An introducer is then placed over the guide wire, after which the wire is removed. A different sized guide wire is put in its place.
 
Next, a long narrow tube called a diagnostic catheter is advanced through the introducer over the guide wire, into the blood vessel. This catheter is then guided to the aorta and the guide wire is removed. Once the catheter is placed in the opening or ostium of one the coronary arteries, the doctor injects dye and takes an x-ray.

If a treatable blockage is noted, the first catheter is exchanged for a guiding catheter. Once the guiding catheter is in place, a guide wire is advanced across the blockage, then a balloon catheter is advanced to the blockage site. The balloon is inflated for a few seconds to compress the blockage against the artery wall.  Then the balloon is deflated.

The doctor may repeat this a few times, each time pumping up the balloon a little more to widen the passage for the blood to flow through. This treatment may be repeated at each blocked site in the coronary arteries. A device called a stent may be placed within the coronary artery to keep the vessel open. Once the compression has been performed, contrast media is injected and an x-ray is taken to check for any change in the arteries. Following this, the catheter is removed and the procedure is completed.

The growing embryo requires nutrition and oxygen, and a disposal system for the waste products of its own metabolism. All of this is accomplished by the placenta, which allows the growing embryo to eat and breathe while in the mother’s uterus.

Following implantation of the fertilized egg into the uterine lining, the outer layer of the embryo develops spaces called lacunae. The lacunae filled up with blood from the mother’s uterine lining. Small projections from the embryo’s chorionic layer reached out into the uterine lining. The chorionic layer is one of the membranes that surround the embryo and help it implant. Blood vessels begin to form beneath this chorionic layer.

Around day 21, the embryo’s bloodstream and the mother’s bloodstream are in such close contact that nutrients and oxygen can cross from mother to embryo. The two bloodstreams are separated by a thin collection of tissues in the placenta called the blood barrier. This barrier permits small particles like nutrients and oxygen to pass from the mother to the embryo and allows waste products to pass from the embryo back to the mother.

The blood barrier also prevents many large or potentially harmful particles from entering the embryo’s bloodstream. The red blood cells do not cross from the mother’s bloodstream to the embryo’s bloodstream.

It’s important to keep the two bloodstreams separate since the blood type of the mother and embryo could be different. If the mother’s blood type is positive, and her embryo’s blood type is negative, then the mother’s blood cells would treat the embryo as an invading foreign organism, and try to destroy it.

The placenta and its blood barrier are important for supplying the growing embryo with nutrition and oxygen, removing its waste products, and preventing harmful substances from getting into the embryo’s bloodstream.

A woman is born with all of the egg cells she will release throughout her lifetime. Starting at about age 12 through menopause, a woman’s reproductive cycle releases an egg about once a month.

Hormonal messages from the brain instruct the ovaries to develop several follicles in which a single dominant follicle in one of the ovaries will release an egg for fertilization. During this time, other hormones instruct the uterine lining to thicken in preparation for nourishing a fertilized egg.

There are several hormones that regulate the reproductive cycle. Follicle stimulating hormone (FSH) stimulates preparation of the egg for fertilization by instructing a follicle to begin dividing it’s genetic material (chromosomes).

The follicle then releases estrogen, the hormone that prepares the lining of the uterus to receive a fertilized egg. Increased levels of estrogen in the bloodstream cause a small structure in the brain, the pituitary gland, to stop releasing the hormone FSH, and to start releasing luteinizing hormone (LH).

LH causes the follicle to enlarge rapidly and to release its egg in a process known as ovulation. Once the egg is out of the follicle, the follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for the fertilized egg. The remaining cells of the follicle shrink into a hormone producing mass of cells called a corpus luteum.

The egg is swept into the fallopian tube by its waving structures called fimbriae. Fertilization of the egg usually occurs in the fallopian tube. From there, it is transported to the uterus and implants itself in the uterine wall, where it is nourished by the uterine lining. In the ovary, the corpus luteum produces progesterone so that the egg can develop into a fetus.

If the egg is not fertilized within 24 hours after its release from the ovary, it stops developing and dissolves before reaching the uterus. The absence of a fertilized egg causes the body to stop releasing the hormones that prepare the uterus for implantation. In response, the uterus sheds its lining over a period of four to five days in a process known as menstruation.

This animation shows the process of red blood cell formation and the components that comprise blood.

Blood carries various substances that must be brought to one part of the body or another. Red blood cells are an important element of blood. Their job is to transport oxygen to the body’s tissues in exchange for carbon dioxide, which is carried to and eliminated by the lungs.

Red blood cells are formed in the red bone marrow of bones. Stem cells in the red bone marrow called hemocytoblasts give rise to all of the formed elements in blood. If a hemocytoblast commits to becoming a cell called a proerythroblast, it will develop into a new red blood cell.

The formation of a red blood cell from hemocytoblast takes about 2 days. The body makes about two million red blood cells every second.

Blood is made up of both cellular and liquid components. If a sample of blood is spun in a centrifuge, the formed elements and fluid matrix of blood can be separated from each other. Blood consists of 45% red blood cells, less than 1% white blood cells and platelets, and 55% plasma.

The skeletal muscles are under voluntary (conscious) control most of the time. However, skeletal muscle movement can also by induced by involuntary reflexes.

Reflexes are involuntary reactions to a stimulus such as the burning of the hand. As soon as a hot substance contacts the hand, pain receptors in the skin send a signal to the spinal cord. In turn, the spinal cord sends a signal back to the arm muscles that instruct the hand to pull away. The arm flexed as it withdrew, which is known as a flexor (withdrawal) reflex. There are many other reflexes that protect the body as well.

If the body did not have the reflexes to withdraw quickly from a painful stimulus, we would be at risk for serious injury.

The eye is the organ of sight and is shaped as a slightly irregular hollow sphere. Various structures in the eye enable it to translate light into recognizable images. Among these are the cornea, the lens, and the retina.

Light first passes through the cornea, a clear dome-like structure covering the iris, or colored part, of the eye. The cornea bends, or refracts, the light onto the lens. The light is then refracted a second time while passing through the lens, finally focusing on the retina. The retina is the light sensitive part of the eye. Impulses travel down the optic nerve to the occipital lobe of the brain, which then interprets the image in the correct perspective.

The shape of the eye is very important in keeping the things we see in focus. If the shape of the eye changes, it affects a person’s vision.

Normally, light is precisely focused onto the retina at a location called the focal point. A nearsighted eye is longer from front to back than a normal eye causing light to be focused in front of the retina instead of directly onto it. This makes it difficult to see objects that are far away. Glasses with concave lenses are used to correct nearsightedness. The concave lens focuses light back onto the focal point of the retina.

Farsightedness occurs when the length of the eye is too short. Light is focused at a point behind the retina, making it difficult to see objects that are up close. A convex lens is used to correct farsightedness because it directs the focal point back onto the retina.

A baby’s sex is determined at the time of conception. When a baby is conceived, the X or Y chromosome carried by the sperm cell fuses with the X chromosome in the egg cell. The chromosome combination determines whether the baby will be female or male. An XX combination means the baby will be a girl and XY means it will be a boy.

Even though gender is determined at conception, the fetus doesn’t develop its external sexual organs until the fourth month of pregnancy. At seven weeks after conception, the front of the fetus appears to be sexually indifferent, looking neither like a female or a male.

Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either female or male organs. This process is called sexual differentiation. If the fetus is female, it will produce hormones called estrogens. If the fetus is a male, it will produce hormones called androgens.

Hormones will instruct a common structure called the genital tubercle to either form the clitoris in the female or the penis in the male. The clitoris and penis are called sexual analogs because they originate from the same structure.

A baby’s skeleton begins as fragile membranes and cartilage. As the fetus develops, the membranes and cartilage turn into bone in a process called ossification.

During the third month of development, the membranes on the side and back of the fetus’ skull start to ossify. Bone tissue slowly grows over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

Close to birth, the bones of the skull still have gaps between them called fontanelles. The fontanelles allow room for the baby’s brain to grow and enable the head to be compressed during delivery.

Most of the bones of the skeleton start off as cartilage, such as the arms, legs, ribs, fingers, and backbone. From the second month until the end of the third month, the cartilage in the middle of the bones begins to ossify outward. Bones continue to grow in this manner until adulthood, allowing them to increase in their length and width.

Skeletal muscle is well-organized body tissue, composed in a complex array of smaller and smaller structures. Each skeletal muscle is composed of many units called muscle fascicles. The fascicles are bound together by a type of connective tissue called fascia.

Fascicles are composed of smaller organizational units called muscle fibers.

Smaller strands called myofibrils organize muscle fibers. The myofibrils move as skeletal muscle contracts. It is the interaction of the myofibrils as they slide and pull along side each other that gives skeletal muscle its functional ability to do work and move things.

Putting it all back together, myofibrils compose muscle fibers, muscle fibers make-up muscle fascicles, and muscle fascicles are bound together by fascia to compose skeletal muscle.

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Normal, healthy lungs are compared to the lungs of a long-term smoker.

The lungs are the primary respiratory organs. They act as filters for the air the body breathes in and normally are a healthy pink color.

Filtering smoke from the air breathed in can do damage to the lung tissue as seen in a smoker’s lung. Over time, carbon molecules from inhaled smoke deposit in the lung tissue, giving it a blackened appearance.

Smoking can eventually lead to the formation of tumors and other serious lung diseases. Smoking has also been linked to diseases that affect the cardiovascular system, such as atherosclerosis, which can lead to a heart attack or stroke.

From a side view of the head and neck, this animation shows the structures involved in snoring. The common causes for snoring are also discussed

Snoring affects many of people during their sleep when the airway become partially blocked, forcing the lungs to inhale harder to compensate for the lack of air entering the body. The snoring sound results from the vibration the soft palate and the uvula.

Several factors are thought to cause snoring, including poor muscle tone, too much alcohol, heavy smoking, colds or allergies, obesity, and obstruction by enlarged adenoids and tonsils.

Usually, snoring is not an indication of an underlying disorder. However, people who snore and have quiet periods lasting more than 10 seconds may have some degree of sleep apnea.