The most abundant formed elements in blood, erythrocytes are red, biconcave disks packed with an oxygen-carrying compound called hemoglobin. The hemoglobin molecule contains four globin proteins bound to a pigment molecule called heme, which contains an ion of iron. In the bloodstream, iron picks up oxygen in the lungs and drops it off in the tissues; the amino acids in hemoglobin then transport carbon dioxide from the tissues back to the lungs. Erythrocytes live only 120 days on average, and thus must be continually replaced. Worn-out erythrocytes are phagocytized by macrophages and their hemoglobin is broken down. The breakdown products are recycled or removed as wastes: Globin is broken down into amino acids for synthesis of new proteins; iron is stored in the liver or spleen or used by the bone marrow for production of new erythrocytes; and the remnants of heme are converted into bilirubin, or other waste products that are taken up by the liver and excreted in the bile or removed by the kidneys. Anemia is a deficiency of RBCs or hemoglobin, whereas polycythemia is an excess of RBCs.
anemia
deficiency of red blood cells or hemoglobin
bilirubin
yellowish bile pigment produced when iron is removed from heme and is further broken down into waste products
biliverdin
green bile pigment produced when the non-iron portion of heme is degraded into a waste product; converted to bilirubin in the liver
carbaminohemoglobin
compound of carbon dioxide and hemoglobin, and one of the ways in which carbon dioxide is carried in the blood
deoxyhemoglobin
molecule of hemoglobin without an oxygen molecule bound to it
erythrocyte
(also, red blood cell) mature myeloid blood cell that is composed mostly of hemoglobin and functions primarily in the transportation of oxygen and carbon dioxide
ferritin
protein-containing storage form of iron found in the bone marrow, liver, and spleen
globin
heme-containing globular protein that is a constituent of hemoglobin
heme
red, iron-containing pigment to which oxygen binds in hemoglobin
hemoglobin
oxygen-carrying compound in erythrocytes
hemosiderin
protein-containing storage form of iron found in the bone marrow, liver, and spleen
hypoxemia
below-normal level of oxygen saturation of blood (typically <95 percent)
macrophage
phagocytic cell of the myeloid lineage; a matured monocyte
oxyhemoglobin
molecule of hemoglobin to which oxygen is bound
polycythemia
elevated level of hemoglobin, whether adaptive or pathological
reticulocyte
immature erythrocyte that may still contain fragments of organelles
sickle cell disease
(also, sickle cell anemia) inherited blood disorder in which hemoglobin molecules are malformed, leading to the breakdown of RBCs that take on a characteristic sickle shape
thalassemia
inherited blood disorder in which maturation of RBCs does not proceed normally, leading to abnormal formation of hemoglobin and the destruction of RBCs
transferrin
plasma protein that binds reversibly to iron and distributes it throughout the body
1. Which of the following statements about mature, circulating erythrocytes is true?
A) They have no nucleus.
B) They are packed with mitochondria.
C) They survive for an average of 4 days.
D) All of the above
A
2. A molecule of hemoglobin ________.
A) is shaped like a biconcave disk packed almost entirely with iron
B) contains four glycoprotein units studded with oxygen
C) consists of four globin proteins, each bound to a molecule of heme
D) can carry up to 120 molecules of oxygen
C
3. The production of healthy erythrocytes depends upon the availability of ________.
A) copper
B) zinc
C) vitamin B12
D) copper, zinc, and vitamin B12
D
4. Aging and damaged erythrocytes are removed from the circulation by ________.
A) myeoblasts
B) monocytes
C) macrophages
D) mast cells
C
5. A patient has been suffering for 2 months with a chronic, watery diarrhea. A blood test is likely to reveal ________.
A) a hematocrit below 30 percent
B) hypoxemia
C) anemia
D) polycythemia
D
1. A young woman has been experiencing unusually heavy menstrual bleeding for several years. She follows a strict vegan diet (no animal foods). She is at risk for what disorder, and why?
She is at risk for anemia, because her unusually heavy menstrual bleeding results in excessive loss of erythrocytes each month. At the same time, her vegan diet means that she does not have dietary sources of heme iron. The non-heme iron she consumes in plant foods is not as well absorbed as heme iron.
2. A patient has thalassemia, a genetic disorder characterized by abnormal synthesis of globin proteins and excessive destruction of erythrocytes. This patient is jaundiced and is found to have an excessive level of bilirubin in his blood. Explain the connection.
Bilirubin is a breakdown product of the non-iron component of heme, which is cleaved from globin when erythrocytes are degraded. Excessive erythrocyte destruction would deposit excessive bilirubin in the blood. Bilirubin is a yellowish pigment, and high blood levels can manifest as yellowed skin.