15 THE CARDIOVASCULAR SYSTEM: BLOOD
Plasma: liquid portion of blood; it transports proteins, hormones, electrolytes, organic nutrients, and waste
Erythrocytes: the cellular components of blood, which transport oxygen and carbon dioxide.
Leukocytes: white blood cells; defend the body against pathogens.
Platelets: cell fragments; critical for the formation of blood clots to prevent the loss of blood.
Overview of the Composition of Blood: The Hematocrit
The fractional contribution of erythrocytes to the blood is called hematocrit, which is determined by centrifuging
a sample of blood. Separated by density.
Hematocrit is calculated by determining the percentage of whole blood that consists of erythrocytes:
o Hematocrit = (height of erythrocyte column/height of whole blood column) x 100.
It indicates whether a person has a normal complement of erythrocytes.
Men: 42-55% of blood volume. Women: 37-47%.
A low hematocrit indicates a lower than normal concentration of erythrocytes in the blood.
A high hematocrit indicates a higher than normal concentration of erythrocytes in the blood, called polycythemia
(normal adaptive response in low-oxygen environments)
Plasma is an aqueous solution in which a great variety of solutes are dissolved (i.e. proteins, small nutrients,
metabolic waste products, gases, and electrolytes).
The composition of plasma is very similar to that of interstitial fluid.
The albumins, which are synthesized by the liver, are the most abundant plasma proteins and make a large
contribution to the osmotic pressure of plasma, which affects the movement of fluid across capillaries.
The globulins encompass a wide variety of proteins that transport lipids, steroid hormones, and other substance
in the blood (important for the blood’s ability to form clots and defending the body against foreign substances).
Fibrinogen is synthesized by the liver and is a key substance in the formation of blood clots.
Serum is plasma from which fibrinogen and other clotting proteins have been removed.
Erythrocytes (red blood cells) are the most abundant cells in the blood, numbering about 5 million per cubic
millimeter of blood.
Erythrocytes lack nuclei, mitochondria, and other organelles, such as ribosomes, that are necessary for
manufacturing proteins (transcription).
They are shaped like disks and are about 7 m in diameter and a little more than 2 m thick. Often biconcave
disks to give more surface area, suitable for exchange.
The shape of erythrocytes is due to the presence of cytosolic protein called spectrin. It is a fibrous protein that
forms a network linked to the plasma membrane. It is flexible giving erythrocytes the ability to bend and flex
OXYGEN AND CARBON DIOXIDE TRANSPORT
The major function of erythrocytes is to transport oxygen and carbon dioxide in the blood.
They have a high capacity for carrying hemoglobin and carbonic anhydrase.
Hemoglobin binds, and thus transports, oxygen and carbon dioxide, whereas carbonic anhydrase is essential for
the transport of carbon dioxide only.
HEMOGLOBIN’S REVERSIBLE BINDING OF OXYGEN AND CARBON DIOXIDE
o 4 polypeptide chains of 2 types (2 alpha and 2 beta), each of which has an iron-containing ring structure
o The iron in hemoglobin is present in the ferrous form (Fe ), which imparts a red color to the
erythrocytes and thus blood.
o This iron is the site to which oxygen binds. Because of the 4 heme groups, each hemoglobin can bind for
4 oxygens. 15 THE CARDIOVASCULAR SYSTEM: BLOOD
CARBONIC ANHYDRASE AND THE CARBON DIOXIDE-BICARBONATE REACION
o Carbonic anhydrase is another protein found in erythrocytes that is critical in the transport of gases.
Carbonic anhydrase is an enzyme that catalyzes the reversible conversion of carbon dioxide and water
to carbonic acid. C2 + H2O (carbonic anhydrase) H2CO 3
o The pathway continues with the reversible dissociated of carbonic acid to yield a hydrogen ion and a
bicarbonate ion. H2CO3 H + HCO . 3
LIFE CYCLE OF ERYTHROCYTES
New erythrocytes are produced on a regular basis, at a rate of approx. 2-3 million per second or 200 billion per
The bone marrow has the enormous task of producing these erythrocytes by a process called erythropoiesis,
while the spleen removes the old erythrocytes from the blood.
o All blood cells develop from precursor cells called hematopoietic (blood-forming) stem cells, located in
the bone marrow.
o The development of a particular type of blood cell depends on cytokines called hematopoietic growth
o The HGF that stimulates erythrocyte production is erythropoietin.
o The HGFs involved in leukocyte production include colony-stimulating factors and interleukins.
o Erythropoietin is released form certain cells in the kidney in response to low oxygen levels in blood.
o The last cell stage prior to developing into the mature erythrocyte is the reticulocyte, a red blood cell
with some ribosomes still present in the cytoplasm (web-like structure).
o Dietary requirements for erythrocyte production are iron, folic acid, and vitam12 B . The iron is needed
to synthesize hemoglobin. Folic acid and vitamin 12are necessary for the synthesis of DNA.
o The effects of folic acid and vitamin on erythropoiesis are most notable because of rapid production
o A shortage of dietary elements for erythpoiesis can produce either a reduction in the amount of
hemoglobin per cell or a reduction in the number of erythrocytes in the blood either of which reduces
the oxygen carrying capacity of the blood (anemia).
o Iron-deficiency anemia: lack of iron in diet; erythrocytes are smaller than normal due to a decrease in
the amount of hemoglobin per erythrocyte.
o Pernicious anemia: lack of vitamin 12; larger than normal erythrocyte size but a decrease in the
number of erythrocytes.
FILTERING AND DESTRUCTION OF ERYTHROCYTES BY THE SPLEEN
o The spleen is a lymphoid organ that stores blood cells and removes old erythrocytes from the
o The iron is removed; the resulting heme is converted to bilirubin, a yellow compound.
o Bilirubin is then released into the bloodstream, where it gives a yellowish tinge.
o The bilirubin travels to the liver, where it is catabolized further.
o Most products of bilirubin catabolism are secreted in the bile to the small intestine and ultimately
excreted in the feces, imparting a brownish tinge to the feces.
o Jaundice: which causes the skin and the whites of the eyes to take on a yellowish color; plasma bilirubin
levels are elevated due to liver diseases, blockage of bile duct, or excessive hemolysis of erythrocytes in
o The iron that was released by hemoglobin catabolism is recycled to form new hemoglobin. It is
transported in blood bound to a protein called transferrin.
o Transferrin picks up iron from the gastrointestinal tract or from the spleen and transports the iron to
the red bone marrow for erythrocyte synthesis, or to the liver where some iron can be stored bound to
the protein ferritin.
o Some iron is also stored bound to ferritin in the spleen and in cells lining the small intestine.
4000-10 000 per cubic millimeter. Less numerous than erythrocytes.
Leukocytes are nucleated and posses all the normal cellular machinery, and they are thus the only fully
functional cells in the blood 15 THE CARDIOVASCULAR SYSTEM: BLOOD
The presence of leukocytes outside blood vessels results from their mobility, which allows them to squeeze
through pores in capillaries and migrate through tissues.
This ability to migrate is important to defend the body against invading microorganisms and other foreign
Neutrophils, eosinophils, and basophils are called granulocytes because they have prominent protein-containing
vesicles in their cytoplasm known as cytoplasmic granules.
Monocytes and lymphocytes do not have prominent granules and thus are sometimes called agranulocytes.
Neutrophils constitute about 50-80% of all leukocytes and are capable of one of the most important defense
activities in the body: phagocytosis.