one of the formed elements in the peripheral blood
, constituting the great majority of the cells in the blood. (For immature forms see erythrocytic series
.) In humans the normal mature erythrocyte is a biconcave disk without a nucleus, about 7.7 micrometers in diameter, consisting mainly of hemoglobin
and a supporting framework called the stroma
. Erythrocyte formation (erythropoiesis
) takes place in the red bone marrow in the adult, and in the liver, spleen, and bone marrow of the fetus. It requires an ample supply of dietary elements such as iron, cobalt, copper, amino acids, and certain vitamins. Called also red cell or corpuscle
and red blood cell or corpuscle
The functions of erythrocytes include transportation of oxygen and carbon dioxide. They owe their oxygen-carrying ability to hemoglobin
, a combination of an iron-containing prosthetic group (heme)
with a protein (globin).
Hemoglobin attracts and forms a loose connection with free oxygen, and its presence enables blood to absorb some 60 times the amount of oxygen that the plasma by itself absorbs. Oxyhemoglobin is red, which gives oxygenated blood its red color. Erythrocytes are stored in the spleen, which acts as a reservoir for the blood system and discharges the cells into the blood as required. The spleen may discharge extra erythrocytes into the blood during emergencies such as hemorrhage or shock.
Erythrocytes also are important in the maintenance of a normal acid-base balance
, and, since they help determine the viscosity of the blood, they also influence its specific gravity. Their average life span is 120 days. They are subjected to much wear and tear in circulation and eventually are removed by cells of the reticuloendothelial system
, particularly in the liver, bone marrow, and spleen. In spite of this constant destruction and production of erythrocytes, the body maintains a fairly constant number, between 4 and 5 million per mm3
of blood in women and 5 to 6 million per mm3
in men. A decreased number constitutes one form of anemia
Erythrocytes are destroyed whenever they are exposed to solutions that are not isotonic to blood plasma. If they are placed in a solution that is more dilute than plasma (distilled water for example) the cells will swell until osmotic pressure bursts the cell membrane. If they are placed in a solution more concentrated than plasma, the cells will lose water and shrivel or crenate. It is for this reason that solutions to be given intravenously must be isotonic to plasma.
Aged red cells are ingested by macrophages in the spleen and liver. The iron is transported by the plasma protein transferrin
to the bone marrow, where it is incorporated into new red cells. The heme group is converted to bilirubin, a bile pigment secreted by the liver. About 180 million red blood cells are destroyed every minute. Since the number of cells in the blood remains more or less constant, this means that about 180 million red blood cells are manufactured every minute.
Determination of the red blood cell volume is usually done as a preliminary step in determination of the total blood volume
. A radioactive substance, usually chromium, is used to “tag” cells of a sample of blood drawn from the patient. The sample is then reintroduced into the circulating blood and subsequent samples are taken to be evaluated for degree of radioactivity. The degree of dilution is used to calculate total blood volume.
The events in the life of erythrocytes. Nucleated red blood cell (RBC) precursors stimulated by erythropoietin form erythrocytes in the bone marrow. Normal synthesis of hemoglobin occurs only in the presence of nutrients, iron, vitamin B12, and folic acid. Mature RBCs are released into circulation. The old or defective RBCs are degraded in the spleen. Iron and globin are reutilized immediately. Bilirubin is released in bile into the intestine. From Damjanov, 1996.
erythrocyte protoporphyrin test EP test
; a screening test for lead toxicity; erythrocyte protoporphyrin levels are determined by direct fluorometry of whole blood or fluorescence analysis of whole blood extracts. Levels will be increased in either lead poisoning or iron deficiency.
erythrocyte sedimentation rate the rate at which erythrocytes settle out of unclotted blood in one hour. The test is based on the fact that inflammatory processes cause an alteration in blood proteins, resulting in aggregation of the red cells, which makes them heavier and more likely to fall rapidly when placed in a special vertical test tube. Normal ranges vary according to the type of tube used, each type being of a different size. The most common methods and the normal range for each are: Wintrobe method, 0 to 6.5 mm per hour for men, 0 to 15 mm per hour for women; and Westergren method, 0 to 15 mm per hour for men, 0 to 20 mm per hour for women.
The erythrocyte sedimentation rate is often inconclusive and is not considered specific for any particular disorder. It is most often used as a gauge for determining the progress of an inflammatory disease such as rheumatic fever, rheumatoid arthritis, or a respiratory infection. The information provided by this test must be used in conjunction with results from other tests and clinical evaluations.
the speed or frequency with which an event or circumstance occurs per unit of time, population, or other standard of comparison.
a fictitious summary rate statistically adjusted to remove the effect of a variable, such as age or sex, to permit unbiased comparison between groups having different compositions with respect to these variables. See also crude rate
and specific rate
attack rate in the analysis of acute outbreaks of disease, the proportion of persons who are exposed to the disease during the outbreak who do become ill.
basal metabolic rate an expression of the rate at which oxygen is utilized in a fasting subject at complete rest as a percentage of a value established as normal for such a subject. Abbreviated BMR.
birth rate the number of live births in a geographic area in a defined period, usually one year, relative to some specified population. For the crude birth rate, it is the average total population or the midyear population in the area during the period. Specific birth rates for subsets of the population may also be calculated, for example, an age-specific birth rate is limited to the population of females of a defined age range.
case fatality rate the number of deaths due to a specific disease as compared to the total number of cases of the disease.
one giving the total number of events occurring in an entire population over a period of time, without reference to any of the individuals or subgroups within the population. See also adjusted rate
and specific rate
the number of deaths in a certain period of time divided by the total of a given population. The crude death rate
is the ratio of the number of deaths in a geographic area in one year divided by the average population in the area during the year. The age-specific death rate
is the ratio of the number of deaths occurring in a specified age group to the average population of that group. The cause-specific death rate
is the ratio of the number of deaths due to a specified cause to the average total population. Called also mortality rate
Historic example of death rates (per 100,000) for leading causes of death for men aged 25–44 years. From Centers for Disease Control and Prevention: Morbidity and Mortality Weekly Report 42:483, 1993.
DEF rate an expression of dental caries experienced in primary teeth, calculated by adding number of those requiring filling (D), decayed teeth requiring extraction (E), and those that have already been successfully filled (F); missing primary teeth are not included in the calculation.
DMF rate an expression of the condition of the permanent teeth based on the number of teeth decayed, missing (or indicated for removal), and filled or bearing restorations. It is calculated by adding the number of carious permanent teeth requiring filling (D), carious ones requiring extraction (Mr), ones previously extracted because of caries (Mp), and permanent teeth (F).
dose rate the amount of any therapeutic agent administered per unit of time.
the death rate
in a specific group of persons simultaneously affected by some event or circumstances, such as a natural disaster.
a measure of fertility in a defined population over a specified period of time, usually one year; particularly the general fertility rate
, but also including more specific rates such as those for females of a given parity or a particular age range or that describing the completed rate for females who have finished childbearing.
fetal death rate the ratio of the number of fetal deaths in one year to the total number of both live births and fetal deaths in that year.
five-year survival rate an expression of the number of survivors with no trace of a given disease five years after each has been diagnosed or treated for the disease.
general fertility rate the most widely used measure of fertility; the number of live births in a geographic area in a year per 1000 women of childbearing age, which is usually defined as age 15 to 44 years.
glomerular filtration rate an expression of the quantity of glomerular filtrate formed each minute in the nephrons of both kidneys, calculated by measuring the clearance of specific substances, e.g., inulin or creatinine.
growth rate an expression of the increase in size of an organic object per unit of time.
heart rate the number of contractions of the cardiac ventricles per unit of time (usually per minute).
the risk of developing a particular disease during a given period of time; the numerator of the rate is the number of new cases during the specified time period and the denominator is the population at risk during the period. Compare prevalence r.
infant mortality rate the ratio of the number of deaths in one year of children less than one year of age to the number of live births in that year.
maternal mortality rate
a rate in which the numerator is the number of maternal deaths ascribed to puerperal causes in one year; the number of live births in that year is often used as the denominator, although to make a true rate the denominator should be the number of pregnancies (live births plus fetal deaths). Called also puerperal mortality rate
mendelian rate an expression of the numerical relations of the occurrence of distinctly contrasted mendelian characteristics in succeeding generations of hybrid offspring.
metabolic rate an expression of the amount of oxygen consumed by the body cells.
neonatal mortality rate the ratio of the number of deaths in one year of children less than 28 days of age to the number of live births in that year.
perinatal mortality rate the ratio of the number of the sum of fetal deaths after 28 or more weeks of gestation (stillbirths) and deaths of infants less than 7 days of age in one time period and population to the sum of the number of live births and fetal deaths after 28 or more weeks of gestation (stillbirths) in that same time period and population.
postneonatal mortality rate the ratio of the number of deaths in a given year of children between the 28th day of life and the first birthday relative to the difference between the number of the live births and neonatal deaths in that year; the denominator is sometimes simplified, less correctly, to the number of live births. The ratio is sometimes approximated as the difference between the infant mortality rate and the neonatal mortality rate.
the number of people in a population who have a disease at a given time; the numerator is the number of existing cases of disease at a specified time and the denominator is the total population. Time may be a point or a defined interval, and is traditionally the former if unspecified. Compare incidence r.
the rate of the pulse
, measured as number of pulsations in an artery per unit of time; normally between 60 and 80 per minute in an adult.
respiration rate the number of inhalations and exhalations per unit of time, usually measured by observation of chest movements and averaging 16 to 20 per minute in an adult.
the rate at which a sediment is deposited in a given volume of solution, especially when subjected to the action of a centrifuge; see also erythrocyte sedimentation rate.
in cardiac pacing
, the rate, expressed in units of mV/msec, at which an R wave reaches peak amplitude; it represents the maximum rate of change of amplifier output voltage.
a rate that applies to a specific demographic subgroup, e.g., individuals of a specific age, sex, or race, giving the total number of events in relation only to that subgroup. See also adjusted rate
and crude rate