Dictionary

Body systems

Anatomy and the conditions of each body system, from the heart and lungs to the skin.

meconiumA newborn's first stools: thick, sticky and dark green to black.

medialToward the middle line of the body.

medullaThe inner part of an organ; most often the medulla oblongata, the lowest part of the brainstem.

meiosisCell division that makes eggs or sperm, each with half the usual number of chromosomes.

melaninThe dark pigment that gives color to the skin, hair and eyes.

melanocyteA cell in the bottom layer of the skin's outer layer that makes melanin, the pigment that gives skin and hair their color.

melanomaA cancer that starts in melanocytes, the cells that give skin its color.

melatoninA hormone made by the pineal gland in the brain, mostly at night, that helps control the sleep-wake cycle.

melenaBlack, sticky stool that looks like tar.

membraneA thin layer of tissue that covers, lines or separates parts of the body, or the outer layer of a cell.

menarcheA person's first menstrual period.

meningesThe three thin layers of tissue that cover and protect the brain and spinal cord.

meningitisInflammation of the meninges, the membranes that cover the brain and spinal cord.

menopauseThe time in a woman's life when menstrual periods stop for good, counted once there has been no period for 12 months in a row.

menorrhagiaMenstrual periods that are very heavy or last longer than usual.

menstrualHaving to do with menstruation, the monthly shedding of the lining of the uterus as bleeding from the vagina.

menstrual cycleThe monthly series of hormone-driven changes in the ovaries and uterus that prepare the body for possible pregnancy.

menstrual periodThe days each month when the lining of the uterus is shed and passes out of the vagina as bleeding.

menstruationThe monthly shedding of the lining of the uterus, which leaves the body as blood through the vagina.

mental illnessA health condition that changes a person's thinking, mood or behavior and causes distress or problems in daily life.

mental statusA person's state of mind at a given time: how alert they are, what they know and how they think, speak and behave.

metabolicRelating to metabolism, the chemical processes that turn food into energy and build and break down substances in the body.

metabolic acidosisA state in which the blood is too acidic because the body has too much acid or has lost too much bicarbonate.

metabolic alkalosisAn acid-base imbalance in which the blood holds too much bicarbonate, so its pH rises above 7.45.

metabolic syndromeA group of risk factors, such as a large waist, high blood pressure, high blood sugar and abnormal blood fats, occurring together.

metabolismAll the chemical processes in the body's cells that make energy, build tissue and break substances down.

metastasisThe spread of cancer cells from where the cancer started to other parts of the body.

MHAn abbreviation with more than one meaning, most often mental health or malignant hyperthermia; the setting shows which.

MHCMajor histocompatibility complex: a group of genes that make the markers cells use to show the immune system what is self and what is not.

MIAn abbreviation most often meaning myocardial infarction (heart attack); it can also mean mitral insufficiency or motivational interviewing.

micronutrientA vitamin or mineral that the body needs in small amounts to work and grow normally.

midlineAn imaginary line down the center of the body; also, a type of IV catheter placed in a vein of the upper arm.

migraineA long-term neurological disorder that causes repeated attacks of moderate to severe headache.

miliaTiny, white, raised bumps on the skin, most often on a newborn's nose, chin and cheeks.

mineralA natural element, such as calcium, iron or potassium, that the body needs in small amounts.

mineralocorticoidA steroid hormone, or a drug that acts like one, that makes the kidneys keep sodium and water and remove potassium.

mitosisCell division that makes two new cells, each with the same full set of chromosomes as the original.

mitral valveThe heart valve between the left atrium and the left ventricle.

mobilityThe ability to move freely and easily, such as turning in bed, getting up and walking.

moleA small, usually brown spot or bump on the skin made of a cluster of pigment cells.

Mongolian spotsFlat, blue-gray patches of skin present at birth, now called congenital dermal melanocytosis.

Montgomery tuberclesSmall raised bumps on the areola, the darker skin around the nipple.

moodA person's inner emotional state that lasts over time, as the person describes it.

mood disorderA mental health condition in which a person's mood is far lower or higher than usual for a long time.

morbidityThe state of having a disease or health problem, or how often a disease occurs in a group of people.

Moro reflexA newborn reflex in which a sudden noise or loss of support makes the baby fling the arms out and then bring them back in.

motilityThe ability to move on its own, such as the movement of food through the gut or the swimming of sperm.

motor neuronA nerve cell that carries signals from the brain or spinal cord out to a muscle to make it move.

mucosaThe moist lining of body passages that open to the outside, such as the mouth, nose, gut and bladder.

mucousRelating to mucus or making mucus, as in mucous membrane.