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pacemaker, artificial: A small electronic device generally placed in the chest to correct an irregular heartbeat. It generates small electrical pulses that prompt the heart to beat at a normal pace.

pacemaker, natural: A specialized cluster of cells called the sinoatrial node in the top of the right atrium. The pacemaker produces a steady flow of beat now signals that flash across the atria and then pass through the atrioventricular node to the ventricles.

PAD: Abbreviation for peripheral artery disease, a condition caused by atherosclerosis in the arteries in the legs or leading to them.

painkillers: Drugs that relieve pain.

palate: The tissues that make up the roof of the mouth.

palliative care: Treatment that relieves the symptoms of a serious illness, but does not cure the disease itself.

palpate: To examine a part of the body by touching it carefully.

palpitation: Sensation that the heart is beating rapidly or irregularly.

palsy: Paralysis in part of the body, often with loss of sensation and uncontrolled body movements.

pancreas: A gland in the abdomen that produces digestive enzymes and hormones.

pandemic: A disease outbreak affecting large populations or a whole region, country, or continent.

pannus: An abnormal layer of tissue that forms over joints or the cornea of the eye.

papillary muscles: Threads of muscle that pull the heart valves between the upper and lower chambers of the heart closed during heart contractions.

paraplegia: Paralysis of the legs and lower part of the body.

parasomnias: Sleep disorders, such as night terrors or sleep walking, that periodically interfere with sleep.

parasympathetic nervous system: Part of the nervous system that calms body systems excited by the stress hormones.

parathyroid glands: Glands responsible for releasing a hormone that controls calcium levels and influences bone loss and growth.

parathyroid hormone: A hormone that controls levels of calcium and phosphorous in the blood and influences bone loss and growth.

parietal lobe: Part of the brain. It plays a role in sensory processes, like pain and touch, and language.

Parkinson's disease: A brain disorder that causes movement problems, including shaking, difficulty walking, and rigidity in muscles.

paroxysm: A sudden, violent attack or convulsion; or the worsening of symptoms or recurrence of disease.

paroxysmal hemicrania: A rare form of headache. Sufferers experience a severe throbbing, drilling pain on one side of the face or behind the eye.

passive immunity: Immunity that is conferred by another, such as a mother's antibodies protecting her baby during gestation and shortly after birth.

patch test: A test used to diagnose whether a rash was caused by a reaction to certain allergens, such as poison ivy or a cosmetic ingredient, or an irritant such as soap.

patella: The thick bone that protects the knee joint; also known as the kneecap.

pathogen: A tiny organism such as a virus, bacterium, or parasite that can invade the body and produce disease.

pathology: The underlying abnormalities that contribute to or are characteristic of a disease.

patient-controlled analgesia: A method that allows a person to control, within limits, the amount and timing of pain medication he or she receives. It is usually done by pressing a button to release the medication from a computerized pump into an IV.

pattern-recognition receptors: Proteins that recognize classes of pathogens and stimulate the innate immune system to signal the adaptive immune system.

PCOS: Abbreviation for polycystic ovary syndrome, an inherited disorder characterized by the formation of abnormal cysts in enlarged ovaries; a leading cause of female infertility and a common cause of excess facial or body hair (hirsutism).

PDE5 inhibitors: Abbreviation for phosphodiesterase type 5 inhibitors, drugs that can help a man achieve and maintain an erection.

peak bone mass: The greatest amount of bone tissue that a person has during his or her life.

peak flow: A measure of how fast a person can blow air out of the lungs.

peak flow meter: A device to assess lung function, often used to diagnose and monitor asthma.

Pediculus humanus capitis: A blood-sucking parasite commonly known as the louse (plural, lice) that can cause an itchy scalp; infestations are highly contagious and especially common in school-age children.

pellagra: A rare disease brought on by severe niacin deficiency that causes diarrhea, dermatitis, and dementia.

pelvic floor: The sling of muscles that support the intestines and bladder, as well as the uterus in women. Weak or damaged pelvic floor muscles can cause incontinence or diminished sexual pleasure, among other problems.

penile prosthesis: An inflatable or bendable device that is implanted in the penis to allow a man with erectile dysfunction to have erections when he wishes.

pepsin: Enzymes secreted by the stomach to break down protein.

peptic ulcer: A raw, crater-like sore in the lining of the stomach or duodenum that causes burning stomach pain.

percutaneous diskectomy: Surgical removal of part of a spinal disk that is bulging out abnormally and pressing on a nerve root or the spinal column.

percutaneous electrical nerve stimulation: A pain relief therapy that uses needles to deliver low-voltage electrical current under the skin to stop pain signals from reaching the brain.

percutaneous transluminal coronary angioplasty: A procedure used to open blocked coronary arteries. A small, thin tube with a tiny balloon at its tip is inserted into a narrowed coronary artery. The balloon is then inflated to widen the narrowed area. A stent may be put in place to hold the artery open. Percutaneous transluminal coronary angioplasty is also known as balloon angioplasty.

perforation: A hole, such as an ulcer, in an organ or tissue.

performance anxiety: Concern about sexual performance that is so severe that it leads to sexual dysfunction.

perfusion: Passage of a fluid through a specific organ or an area of the body.

perfusion defect: A test result that indicates abnormal blood flow or areas of damaged or dead heart muscle.

pericarditis: Inflammation of the pericardium, the heart's sac-like covering.

pericardium: The fibrous sac that surrounds the heart and the roots of the major blood vessels.

perimenopause: The transition time in a woman's life that begins when ovaries produce less estrogen and menstruation becomes less frequent, and ends when the ovaries no longer produce eggs and menstruation stops.

perineum: The area of skin between the vagina and anus in women, and between the scrotum and anus in men.

periodic limb movement disorder: A sleep disorder in which the legs jerk or cramp repeatedly during the sleep.

periodization: An exercise strategy that varies reps, sets, and resistance to alternate heavier and lighter workouts over a period of time.

periodontal disease: Diseases, including gingivitis and periodontitis, that attack the gum tissue and the structures supporting the teeth. Also called gum disease.

periodontitis: An advanced stage of gum disease that attacks the teeth's supporting structures.

peripheral artery disease: A condition caused by atherosclerosis in the arteries in the legs or leading to them.

peripheral nervous system: The parts of the nervous system outside of the brain, brain stem, and spinal cord.

peripheral neuropathy: Damage to the long nerves radiating from the spine to the arms and legs.

peripheral vascular disease: Narrowing of blood vessels that supply blood to the legs, arms, stomach, or kidneys.

peripheral vision: Side vision, or what the eye perceives outside the direct line of vision.

peristalsis: Wavelike movement of intestinal muscles that propels food through the digestive tract.

peritoneal dialysis: Treatment for kidney failure that uses a machine to remove toxins from the bloodstream.

peritonitis: Inflammation of the membrane that line the abdominal cavity and surrounds most abdominal organs.

pernicious anemia: A form of anemia in which red blood cells enlarge and decrease in size due to an inability to properly absorb vitamin B12.

personal emergency response system: A device worn around the neck or wrist that allows a person to call for help by pressing a button.

pessary: A device placed in the vagina to support or correct the position of the uterus, rectum, or bladder.

pessimism: A characteristic frame of mind that leads a person to expect negative outcomes and to view the world as a negative or fearful place.

petechiae: Pinpoint-sized red or purple spots that appear in clusters on the skin, caused by bleeding under the skin.

PET scan: Abbreviation for positron emission tomography, a medical imaging test that uses a radioactive substance to assess organ and tissue function in the body and to look for disease.

Peyronie's disease: Scarring of some of the tissue inside the penis, causing the penis to bend at an angle during an erection.

pH monitoring: A test to determine whether stomach acid is backing up into the esophagus. For this test, a sensing probe is inserted through the nose and positioned above the lower esophageal sphincter.

phacoemulsification: A method of cataract removal. It uses ultrasound waves to break up the clouded lens of the eye so it can be suctioned out with a needle.

phagocytes: Cells that can ingest other cells, bacteria, and foreign particles.

phase shift disorder: Sleep problem that results when a person's internal clock becomes out of sync with external time. This can be a problem for people who work the night shift.

phase-2 enzyme: A helpful enzyme that seems to clear toxins and help prevent cancer-causing substances from binding to DNA.

phenothiazines: Powerful tranquilizing drugs used to treat schizophrenia and, sometimes, severe nausea.

phenylpropanolamine: A decongestant drug used to treat nasal congestion and sometimes mild incontinence.

pheochromocytoma: A rare adrenal gland tumor that secretes hormones that narrow blood vessels and increase blood pressure.

phlegm: Thick, sticky mucus secreted by mucous membranes, such as the sinuses.

phonophobia: Sensitivity to noise, often experienced during a migraine attack.

phosphodiesterase type 5: An enzyme that breaks down substances that help the penis become erect and maintain an erection.

photocoagulation: Use of a laser to seal off blood vessels.

photodynamic therapy: A medical treatment that uses a light source to activate a photosensitizing drug (one that becomes activated by light exposure). Often used in oncology, dermatology, and cosmetic surgery.

photophobia: Sensitivity to light, often experienced during a migraine attack.

photopsia: A sensation of sparks or flashes of light across the visual field.

photorefractive keratectomy: Laser surgery used to reshape the cornea in order to correct vision problems.

photorejuvenation: A cosmetic procedure that uses intense pulsed light to remove wrinkles and improve skin tone and texture.

physiatrist: A physician who specializes in physical medicine, pain, and rehabilitation. These doctors diagnose and treat sports injuries and degenerative conditions like arthritis or low back pain, and oversee rehab for patients with severe impairments resulting from trauma, stroke, and other conditions.

physical activity: Any voluntary body movements that burn calories, including walking up stairs, vacuuming a floor, going for a brisk stroll, or engaging in a structured program of exercise.

physical dependence: The process through which the body becomes accustomed to a psychoactive drug or rewarding behavior and misses it if it's taken away. People with physical dependence who stop or cut down on their substance or activity of choice might develop uncomfortable withdrawal symptoms.

physical fitness: A state of being physically sound and healthy; having the ability to perform physical activity well.

phytochemicals: Substances made by plants that have biological effects in the human body. Examples include isoflavones, lycopene, lutein, and zeaxanthin.

phytoestrogen: A plant component that mimics the effects of estrogen hormones in the body.

Pick's disease: A brain disorder that causes dementia, neurotic behavior, and gradual changes in personality and emotional control.

pineal gland: A gland located in the middle of the brain, between the brain's two hemispheres, that produces melatonin in response to declining light.

Pittsburgh Compound B: A substance that binds to amyloid proteins in the brain (markers for Alzheimer's disease), making them visible under positron emission tomography (PET) scanning.

pituitary gland: The so-called master gland, located at the base of the brain. It controls and regulates the thyroid and other glands throughout the endocrine system.

placebo: A false or inactive medication or treatment that may still offer relief despite being ineffective. In clinical trials, the effectiveness of a new drug is often tested against a placebo.

placebo effect: A change or improvement in symptoms that is due to a dummy medication or treatment (placebo) rather than a real drug or treatment.

plantar fascia: Connective tissue in the foot which joins the heel bone to the ball of the foot.

plantar fasciitis: An inflammation of the plantar fascia; the leading cause of heel pain.

plantar wart: A wart on the sole of the foot, caused by a virus.

plaque: 1) A layer of bacteria that forms on the surface of a tooth and can cause dental disease. 2) A fatty deposit in or on the walls of an artery, as part of atherosclerosis.

plasma: The fluid in which blood cells are suspended.

plasma cells: White blood cells that produce large quantities of antibodies as part of an immune system response.

plasmin: An enzyme that breaks down the protein involved in clotting blood (fibrin), dissolving the clot.

platelet: A colorless, disk-shaped cell in the blood that is necessary for clotting.

podiatrist: A physician who specializes in the medical, surgical, and orthopedic management of foot and ankle disorders.

poliosis: A localized patch of gray or white hair on the scalp, eyebrows, or eyelashes.

polycystic ovary syndrome: An inherited disorder characterized by the formation of abnormal cysts in enlarged ovaries; a leading cause of female infertility and a common cause of hirsutism.

polydipsia: Excessive thirst.

polymyositis: A rare disease in which the muscles become inflamed and weak.

polyp: An abnormal, noncancerous growth that protrudes from mucous membranes, like those found in the sinuses and colon lining.

polysomnography: A sleep study that examines brain waves and other measures of physiological functioning.

polyunsaturated fat: A type of fat that is abundant in soybean, corn, cottonseed, safflower, and sunflower oils, as well as in fatty fish. One type, omega-3 fats, are especially important for cardiovascular health.

polyuria: Excessive urination.

positive psychology: A branch of psychology that studies mental health rather than illness, focusing on how life can be more happy and fulfilling.

positron emission tomography: A medical imaging test that uses a radioactive substance to assess organ and tissue function in the body and to look for disease. Commonly called a PET scan.

posterior chamber: The part of the eye behind the iris and in front of the lens that is filled with aqueous humor.

posterior heel bursitis: An inflammation of the bursa sac surrounding the joint in the heel of the foot, causing swelling and pain.

posterior keyhole foraminotomy: A minimally invasive surgical procedure that is sometimes an option for repairing a herniated disk.

posterior myocardial infarction: Heart attack involving the rear wall of the heart muscle.

postherpetic neuralgia: Nerve pain caused by the herpes zoster virus, also known as shingles.

postmenopausal osteoporosis: Bone loss caused by lower estrogen levels associated with menopause. Sometimes called type I osteoporosis.

postmenopause: The period in a woman's life lasting from the end of perimenopause until the end of life.

postpartum: Pertaining to the period after giving birth.

post-traumatic headache: A persistent headache resulting from a head or neck injury, sometimes lasting for a year or more.

post-traumatic stress disorder: A prolonged reaction to a traumatic event. Post-traumatic stress disorder can cause crippling anxiety and leading to other problems, such as sleep disturbances, depression, and substance abuse. Often referred to as PTSD.

post-void residual: The amount of urine left in the bladder after urinating.

power: Force times speed of movement. It reflects how quickly a given force is exerted.

power training: An emerging field of physical medicine aimed at boosting the ability to exert strength quickly, especially in relation to practical, day-to-day tasks.

prazosin: A member of a class of drugs called alpha blockers. Prazosin eases the symptoms of benign prostatic hyperplasia by relaxing smooth muscle tissue in the capsule that surrounds the prostate.

precursor: A substance that the body can convert into the active form of a vitamin. One example is beta carotene, which the body can convert into vitamin A as needed.

prediabetes: A fasting blood sugar level above a healthy level, but still below the level used to diagnose diabetes. An individual with prediabetes is at increased risk for developing diabetes.

preeclampsia: High blood pressure during pregnancy accompanied by such signs as protein in the urine and swelling of the hands and feet; can progress to eclampsia, characterized by seizures.

prehypertension: Blood pressure that is above normal but not high enough to qualify as hypertension. An individual with prehypertension is at increased risk for developing hypertension.

preload reduction: A method of reducing cardiac workload by decreasing the pressure of blood entering the heart.

premature atrial contraction: An early beat in an atria that feels like the heart skipped a beat.

premature ejaculation: Ejaculation that occurs before or immediately after penetration; this can interfere with a couple having a mutually satisfying sexual experience.

premature ventricular contraction: An early beat in a ventricle that feels like the heart skipped a beat.

presbycusis: Age-related hearing loss caused by the death of hair cells in the inner ear.

presbyopia: Age-related difficulty focusing the eyes at close range, as the flexible lens of the eye becomes less elastic.

priapism: An erection that lasts longer than three hours. Emergency medical treatment is required to prevent permanent damage to the penis.

prick test: A commonly used skin test to confirm hypersensitivity to a broad range of allergens.

primary angioplasty: Use of angioplasty as the first treatment to open a blocked artery that is causing a heart attack, rather than using clot-busting (thrombolytic) drugs.

primary hypertension: High blood pressure with no known cause. Also known as essential hypertension.

primary osteoporosis: Bone loss that results from a normal physiological process, such as menopause or aging.

prion: The smallest known infectious agent; unlike a virus or bacterium it is made entirely of protein and contains no nucleic acid or chromosomes.

probate: A public, legal process supervised by the courts after a person dies that helps ensure debts are paid and assets are properly owned and correctly distributed.

probiotic: Live microorganisms used to benefit health, such as the L. acidophilus bacteria found in yogurt.

procedural memory: The long-term memory of skills and procedures, or how-to knowledge. Also called implicit memory.

processes: Bony projections that extend in several directions from each vertebra bone in the spine.

prodrome: A group of early symptoms preceding a given disease or condition. For example, a migraine prodrome (fatigue, hunger, nervousness) may occur hours or days before the headache strikes.

progesterone: A female steroid hormone produced by the ovaries that prepares the uterine lining for pregnancy.

progestin: A synthetic compound that produces effects similar to those of the hormone progesterone.

progestogen: Any hormone having the same effect as progesterone in the body; refers to both natural progesterone and synthetic progestin.

prognosis: A prediction on how a person's disease will progress in the future.

progressive muscle relaxation: A mind/body technique for inducing the relaxation response that involves isolating, tensing, and relaxing specific sets of muscles in sequence.

prolapse: A condition in which an organ or other body part drops from its normal position.

prolapsed disk: When part of a spinal disk bulges out of a tear or weak spot in the disk's tough outer shell. Also known as herniated disk.

proliferative retinopathy: An advanced stage of diabetic eye disease characterized by the development of new blood vessels that grow into the vitreous cavity; they are fragile and may bleed and cause loss of vision.

prophylaxis: Steps taken to prevent a particular disease or condition, such as taking nitroglycerin to prevent angina.

proprioception: The ability to sense the position of one's body in space, in relation to other objects.

proptosis: Forward bulging or displacement of an organ, especially of an eye. See exophthalmos.

prospective study: A type of research method that collects data on a group of people at the start of the study and then follows them into the future, gathering data over time.

prostaglandins: A group of chemicals that have hormone-like actions; prostaglandins help regulate blood pressure and contraction of smooth-muscle cells (for example, those in the lining of the blood vessels).

prostate cancer: Cancer of the prostate gland.

prostate gland: A walnut-shaped gland at the base of the male bladder. It produces a fluid that forms part of semen.

prostate-specific antigen: A protein produced by the prostate. Elevated levels may indicate the presence of cancer, benign prostatic hyperplasia, or prostatitis. Often referred to as PSA.

prostatic carcinoma: Another name for prostate cancer.

prostatic urethral stent: A small, springlike cylinder, designed to relieve pressure from an enlarged prostate and improve urine flow by widening a narrowed urethra.

prostatism: A blockage at the base of the bladder that reduces or prevents the flow of urine into the urethra, the tube that carries urine out of the body (also known as bladder outlet obstruction). Typical symptoms include feeling the need to urinate right away yet having to strain to do so, having a weak urinary stream, dribbling after urinating, feeling as though the bladder has not been emptied completely, needing to urinate frequently, or experiencing urinary incontinence.

prostatitis: An inflammation of the prostate gland, sometimes caused by a bacterial infection, which may result in painful or difficult urination.

prosthesis: An artificial device such as a hearing aid, artificial joint, or dentures that substitutes for a missing body part.

protease inhibitor: A class of drugs that help fight retrovirus infections; commonly prescribed to treat human immunodeficiency virus (HIV).

protein: One of the three major nutrients (along with carbohydrates and fats). It is used by the body for building and repairing tissues. Protein is derived primarily from animal sources but can be obtained from nuts and seed, some grains, and other plant sources.

protocol: A plan that lays out the procedures that will be followed in conducting a physical examination, a research study, or the treatment of a disease.

pruritus: Itching.

PSA: Abbreviation for prostate-specific antigen, a protein produced by the prostate. Elevated levels may indicate the presence of cancer, benign prostatic hyperplasia, or prostatitis.

PSA velocity: The rate at which a man's PSA level increases over time.

pseudoephedrine: A decongestant drug that may also relieve mild incontinence.

pseudogout: Arthritis caused by crystals of calcium pyrophosphate dihydrate in the joints. Also known as calcium pyrophosphate dihydrate disease.

psoriasis: A common skin disease characterized by thickened patches of inflamed red skin; sometimes accompanied by painful joint swelling and stiffness.

psychodynamic therapy: A form of therapy that focuses on how life events, desires, and close relationships lead to conflict, symptoms such as anxiety or depression, and difficulty in managing life's tasks.

psychogenic: Symptoms and illnesses that have a psychological cause, rather than a physical one.

psychogenic erectile dysfunction: Difficulty in getting or maintaining erections because of a psychological cause, such as stress or depression.

psychosomatic: Symptoms and illnesses that involve both the mind and the body, in which psychological stress may result in physical illness.

PTCA: Abbreviation for percutaneous transluminal coronary angioplasty, a procedure used to open blocked coronary arteries.

ptosis: A drooping of the eyelid attributed to weakened muscles.

PTSD: Abbreviation for post-traumatic stress disorder, a prolonged reaction to a traumatic event. PTSD can cause crippling anxiety and leading to other problems, such as sleep disturbances, depression, and substance abuse.

puborectalis muscle: A pelvic floor muscle that forms a sling around the rectum and helps maintain fecal continence.

pudendal arteries: Arteries supplying blood to the genital area.

pudendal nerve: The nerve that carries sensation from the genital area to the central nervous system.

pulmonary: Pertaining to the lungs.

pulmonary edema: A condition caused by excess fluid accumulating in the lungs, making breathing difficult.

pulmonary embolism: Blockage of one or more arteries in the lungs by a blood clot that formed elsewhere, often in the legs. Typically accompanied by sudden shortness of breath, chest pain, and cough.

pulmonary veins: The veins that carry blood from the lungs to the left atrium.

pulp: Tissue containing nerves and blood vessels that fills the chamber at the center of the tooth.

pulse pressure: The difference between systolic blood pressure and diastolic blood pressure (systolic pressure minus diastolic pressure = pulse pressure). Pulse pressure may help predict heart disease risk.

pump failure: When the heart muscle becomes so weak that it can no longer pump enough blood to the rest of the body.

pupil: The dark, circular opening in the middle of the iris of the eye.

purulent: Formed of or containing pus.

pus: A thick, yellow or green liquid that is composed of dead cells and bacteria, most often found at the site of a bacterial infection.

pyloric sphincter: A muscular valve at the lower end of the stomach that opens to the duodenum.

pyridoxine: A form of vitamin B6.