Erythrocytes
The structure of the erythrocytes is well suited to their primary function of oxygen transport in
the blood. Each millilitre of blood contains about 5 billion erythrocytes, commonly reported
clinically in a red blood cell count as 5 million cells per cubic millimetre (mm3). They are flat,
disc-shaped cells intended in the middle on both sides (they biconcave discs 8 μm in diameter,
2 μm thick at the outer edges, and 1 μm thick in the centre).
The unique shape of the erythrocytes makes their primary function of oxygen transport in the
blood more efficient in two ways:
1. The biconcave shape provides a larger surface area for diffusion of oxygen across the
membrane.
2. The thinness of the cell enables oxygen to diffuse rapidly between the exterior and the
interior regions of the cell.
The flexibility of their membrane, which makes it possible for them to travel through the
narrow, tortuous capillaries, enables the delivery of oxygen without rupturing in the process.
Red blood cells are able to deform as narrow as 3 μm in diameter. But the most important
feature that enables erythrocytes to transport oxygen is the haemoglobin. Haemoglobin is a
pigment and because of its iron content, it appears reddish when combined with oxygen and
bluish when deoxygenated.
Haemoglobin can also combine with:
1.
2.
3.
Carbon dioxide
The acidic hydrogen-ion portion (H+) of ionized carbonic acid
Carbon monoxide
Haemoglobin (erythrocytes) plays the key role in oxygen transport while contributing
significantly to carbon dioxide transport and the buffering capacity of blood.
A single erythrocyte is stuffed with several hundred million haemoglobin molecules. Because
of that erythrocytes contain no nucleus, organelles, or ribosomes. They are extruded during
the cell development.
Only a few crucial, non-renewable enzymes remain within mature erythrocytes: these are
glycolytic enzymes and carbonic anhydrase.
Glycolytic enzymes: necessary for generating the energy needed to fuel the active transport
mechanisms involved in maintaining proper ionic concentrations within
the cell.
Erythrocytes cannot use the oxygen they are carrying for energy production. Erythrocytes
lacking the mitochondria must rely entirely on glycolysis for ATP formation.
Carbonic anhydrase: catalyses a key reaction that ultimately leads to the conversion of
metabolically produced carbon dioxide into bicarbonate ion (HCO3-),
which is the primary form in which carbon dioxide is transported in the
blood.
Erythrocytes – Replacement and production
Erythrocytes have a short life span and must be replaced at the average rate of 2 to 3 million
cells per second. This is the price erythrocytes pay for their generous content of haemoglobin.
Without DNA and RNA, erythrocytes cannot synthesize proteins for cellular repair, growth
and division or for renewal of enzyme supplies.
Erythrocytes are able to survive an average of only 120 days. In this time each erythrocyte
travels about 700 miles through the entire circulatory system. In time, their irreparable plasma
membrane becomes fragile and shows the tendency to rupture as the cell squeezes through
tight spots in the vascular system. Most of the old erythrocytes have their final destination in
the spleen, because this organ has a narrow, winding capillary network.
Erythropoiesis (erythrocyte formation):
Erythrocytes are produced in the bone marrow (soft, highly cellular tissue that fills the
internal cavities of bones). The erythrocyte production takes place in a process known as
erythropoiesis.
In children most of the bones are filled with red bone marrow, which is capable of erythrocyte
production. As a person becomes older, fatty yellow bone marrow, which is incapable of
erythrocyte production gradually replaces red bone marrow. Red bone marrow in adults only
remains in the sternum, the vertebrae, ribs, base of the skull and in the epiphysis of long limb
bones. The red bone marrow synthesizes also leukocytes and platelets as well as erythrocytes,
because the bone marrow includes undifferentiated pluripotent stem cells. Regulatory factors
act on the hemopoietic marrow to govern the type and number of cells generated and
discharged in to the blood.
Usually the number of circulating erythrocytes remains constant. But when the erythrocyte
concentration in the blood is decreased, the primary stimulus that increases erythrocyte
production is the reduced oxygen delivery to the tissues (not the number of erythrocytes in the
blood). But the reduced oxygen delivery to the tissues acts not directly on the bone marrow to
increase erythropoiesis. In fact the reduced oxygen delivery to the kidneys stimulates them to
secrete the hormone erythropoietin into the blood. This hormone in turn stimulates
erythropoiesis.
Erythropoietin acts on derivatives of undifferentiated stem cells that are already cells
committed to become erythrocytes. It stimulates their proliferation and maturation into mature
erythrocytes. Once normal oxygen delivery to the kidneys is achieved, erythropoietin
secretion stops until it is needed once again.
In response to severe loss of erythrocytes (haemorrhage or abnormal destruction of young
circulating erythrocytes) the rate of erythropoiesis can be increased to more than six times.
But the preparation of an erythrocyte involves several steps, such as synthesis of haemoglobin
and extrusion of the nucleus and the organelles. Cells closest to maturity need a few days to
be released from the bone marrow. For less developed and newly proliferated cells it will take
several weeks before reaching maturity. When demands for erythrocytes production are high
(haemorrhage) the bone marrow may release large numbers of immature erythrocytes, known
as reticulocytes (contain residual ribosomes and organelle remnants). Their presence above
the normal value of 0,5 % to 1,5 % of the total number of circulating erythrocytes indicates a
high rate of erythropoietic activity. At very high rates, there can be 30 % of the circulating
erythrocytes at the immature reticulocyte stage.
In addition to erythropoietin, testosterone (major male sex hormone) increases the basal rate
of erythropoiesis. This hormone is responsible for the normally higher hematocrit in male
compared to female. Additional erythrocytes in male provide extra oxygen capacity to meet
the needs of the larger muscles in male.