Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
1
Lecture Name: The Blood and its Cellular Elements
Lecturer Name: Dr. Basim A. Al-Ka'abi
Department: Medical Physiology
Stage: First Stage Medical Students
Lecture Objectives:
By the end of this lecture the students should be able to know the following:
Definition and meaning of blood as an organ.
The cellular elements (red and white cells) of the blood.
The subdivisions of these cellular elements with their various functions.
References:
Barrett, K et al. (2018). Ganong's Review of Medical Physiology. Twenty-
sixth edition. USA.
Guyton, A and Hall, J (2015). Text Book of Medical Physiology.
Thirteenth edition. Philadelphia, USA.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
2
Blood physiology
The blood
-Blood is a specialized connective tissue consisting of cellular elements
suspended in a pale yellow fluid called the plasma.
-The cells make up approximately 45% of the total blood volume.
-The blood is one of the largest organs of the body, with a volume of about
5 liters and a weight of about 5.5 kg in an average 70 kg man.
-Blood circulates throughout the body, supporting the function of all other
body tissues.
-Normal peripheral blood is composed of three types of cell, red blood
cells, white blood cells and platelets.
The cellular elements
A- Red blood cells (erythrocytes).
B- White blood cells (leucocytes).
C- Platelets (thrombocytes).
(A) Red blood cells (RBCs)
-Mature RBCs, or erythrocytes, are the most numerous of the blood cells;
about 5*10
12
red cells are normally present in each liter of blood.
-Red cells are biconcave discs of approximately 7.5 microns in diameter
and 2 microns thick, but their extreme flexibility allows them to squeeze
through capillaries of less than 5 microns in diameter.
-They survive in the circulation for about 120 days before being
sequestered in the spleen and consumed by the phagocytic cells of the
reticulo-endothelial system.
-Less than 1% of RBCs are the newly formed reticulocytes, which take
about 1-2 days to develop into mature red cells.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
3
-The red cell membrane is freely permeable to water and anions that
transverse the membrane in less than 1 second, and is relatively
impermeable to cations.
-The major function of red cells is to transport hemoglobin, which in turn
carries oxygen from the lungs to the tissues; and transport CO
2
from tissues
back to the lungs. In addition, hemoglobin is an excellent acid-base buffer.
-Red blood cells contain a large quantity of carbonic anhydrase enzyme,
which catalyzes the reaction between CO
2
and water.
-The rapidity of this reaction makes it possible for the water in the blood
to react with large quantities of CO
2
and thereby transport it from the
tissues to the lungs in the form of the bicarbonate ion (HCO
3
-
).
-The percentage of the total blood volume comprised of red blood cells is
called the hematocrit, and this is normally about 40% in women and about
45% in men.
(B) White blood cells (leucocytes)
-The leucocytes are the mobile units of the body’s protective system.
-They are formed partially in the bone marrow (granulocytes, monocytes
and few lymphocytes) and partially in the lymph tissue (lymphocytes and
plasma cells); but after formation, they are transported in the blood to the
different parts of the body where they are to be used.
-The number of white blood cells in the blood is normally only 1/600 the
number of red blood cells.
Leucocytes are of two main types
(1) Granular leucocytes.
(2) Agranular leucocyte.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
4
(1) Granular leucocytes
-Are the most numerous. Always contain specific granules, and they are
characterized by the presence of many lobed nuclei to which they are
known as polymorph nuclear leucocytes.
There are three types of granular leucocytes:
(a) Neutrophils
-They are the most numerous of the leukocytes in human blood, which
constitute about 50-70% of the total white blood cells.
-The neutrophil nucleus is highly polymorphous which usually consist of
from 3-5 irregular ovoid lobes connected by a thin chromatin strand.
-Their cytoplasm contains numerous fine granules, which are special types
of lysosomes that contains principally hydrolytic enzymes.
-Neutrophils constitute the first line of defense against invading organism
so the main function of them is bacterial killing by phagocytosis.
-They are highly mobile, highly phagocytic, and are attracted out of the
blood into tissue areas where tissue destruction is occurring by a process
called chemotaxis, which means attraction by the destruction products
from the damaged tissues.
(b) Eosinophils
-They normally constitute about 1-4 % of the total white blood cells.
-Their nucleus is usually bilobed.
-This name is derived from the staining characteristic of their large
cytoplasmic granules which stain strongly with the acidic dye eosin.
-They phagocytose antibody-antigen complex.
-Eosinophils are produced in large numbers in persons with parasitic
infestation.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
5
-The parasites are usually too large to be phagocytized, but the eosinophils
attach themselves to the surface and release lethal substances that can kill
many of the parasites.
-Large numbers of eosinophils also appear in the blood in allergic
conditions and may help detoxify toxins that are released by allergic
reactions.
(c) Basophils
-These cells are difficult to find in human blood, since they constitute only
about 0.5-1 % of the total number of leucocytes.
-The nucleus often is irregular in outline and partially constricted into two
lobes (like an S shape).
-The cytoplasmic granules are round and variable in size, which stain with
basic dyes.
-The basophils are very similar to mast cells located immediately outside
many of the capillaries in the body.
-Basophils and mast cells are important for allergic reaction.
-Since the IgE type of antibody that causes allergic reaction binds to mast
cells and basophils causing them to release various inflammatory products
that in turn cause many of the manifestations of allergic reactions.
-Also, basophils and mast cells liberate heparin into the blood, a substance
that can prevent blood coagulation; as well as histamine and small
quantities of bradykinin
-Eosinophils can reduce the inflammation, thus preventing spread of local
inflammatory process, so the number of eosinophils is greatly increased in
allergic conditions and parasitic infestation.
-Basophils differ from neutrophils in that they are not phagocytic.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
6
(2) Agranular leucocytes
-These cells have cytoplasm that appears homogenous and nuclei that are
almost spherical in shape.
There are two types of agranular leucocytes:
(a) Lymphocytes
-Lymphocytes are the second most common white cell in the peripheral
blood, with a range of 20-40 % of circulating white blood cells.
-The majority of the lymphocytes are small in size (smaller than
monocytes), spherical cells, with small amount of cytoplasm surrounding
dense, round nucleus.
-Most of lymphocytes are formed in lymph nodes, thymus and spleen.
-Lymphocytes are divided into two major populations, which play distinct
roles in specific immunity.
-One of these populations, the T-lymphocytes, is responsible for forming
the activated lymphocytes that provide the cell-mediated immunity.
-The other type, the B-lymphocytes, is responsible for forming the
antibodies that provide humoral immunity.
-In the blood, 70-80% of small lymphocytes are T cells and 15-20% are B
cells.
(b) Monocytes
-Monocytes are phagocytic leucocytes that play a major role in defending
the body against pathogenic organism and foreign cells.
-The monocytes are larger than neutrophils, and have abundant cytoplasm
in relation to the nucleus.
-The nuclei of monocytes are of kidney-shaped.
-Monocytes enter the circulation from the bone marrow but after about 24
hours, they enter the tissues to become tissue macrophages.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
7
-The tissue macrophage system has generally been called the reticulo-
endothelial system.
-The macrophages migrate in response to chemotactic stimuli and engulf
and kill bacteria by phagocytosis.
Reference values of WBCs (in cubic milliliters)
Total WBCs 4000-11000
Neutrophils 2100-7200 (55-65%)
Lymphocytes 1500-4000 (20-40%)
Monocytes 200-800 (4 -10%)
Eosinophils 0-450 (1-3%)
Basophils 0-200 (0-1%)
Life span of the WBCs
-The life of the granulocytes after being released from the bone marrow is
normally 4-8 hours circulating in the blood and another 4-5 days in tissues
where they are needed.
-In times of serious tissue infection, this total life span is often shortened
to only a few hours because the granulocytes proceed more rapidly to the
infected area, perform their functions, and, in the process, are themselves
destroyed.
-The monocytes also have a short transit time, 10-20 hours in the blood,
before travelling through the capillary membranes into the tissues.
-Once in the tissues, they swell too much larger sizes to become tissue
macrophages, and, in this form, can live for months unless destroyed by
phagocytic functions.
-Lymphocytes enter the circulatory system continually, along with
drainage of lymph from the lymph nodes and other lymphoid tissue.
Dr. Basim Al-Ka'abi
Blood Physiology
First Lecture
8
-After a few hours, they pass out of the blood back into the tissues by
diapedesis.
-Then they re-enter the lymph and return to the blood repeatedly; thus,
there is continual circulation of lymphocytes through the body.
-Lymphocytes have life spans of weeks or months depending on the body's
need for these cells.