Download Physiology Lecture 9

Blood, and Circulation
Blood serves numerous functions, including the transport of respiratory gases,
nutritive molecules, metabolic wastes, and hormones.
Blood travels through the body in a system of vessels leading from and returning to
the heart.
Functions of the Circulatory System
The functions of the circulatory system can be divided into three broad areas:
transportation, regulation, and protection.
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1. Transportation.
All of the substances essential for cellular metabolism are transported by the
circulatory system. These substances can be categorized as follows:
a. Respiratory. Red blood cells, or erythrocytes, transport oxygen from the heart
to the cells, and carbon dioxide from tissue to the heart.
b. Nutritive. The blood carries absorbed products of digestion through the liver
to the cells of the body
c. Excretory. Metabolic wastes (such as urea), excess water and ions, and other
molecules not needed by the body are carried by the blood to the kidneys and
excreted in the urine.
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2. Regulation.
a. Hormonal. The blood carries hormones from their site of origin to distant target
tissues where they perform a variety of regulatory functions.
b. Temperature. When the ambient temperature is high, diversion of blood from
deep to superficial vessels helps cool the body.
when the ambient temperature is low, the diversion of blood from superficial to
deeper vessels helps keep the body warm.
3. Protection.
a. Clotting. The clotting mechanism protects against blood loss when vessels are
damaged.
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b. Immune. The immune function of the blood is performed by the leukocytes
(WBCs) that protect against many disease-causing agents (pathogens).
Major Components of the Circulatory System
The circulatory system consists of two subdivisions: the cardiovascular system and
the lymphatic system.
The cardiovascular system consists of the heart and blood vessels.
The lymphatic system, which includes lymphatic vessels and lymphoid tissues
within the spleen, thymus, tonsils, and lymph nodes.
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The heart
A four-chambered double pump. Its pumping action creates the pressure head
needed to push blood through the vessels to the lungs and body cells.
At rest, the heart of an adult pumps about 5 liters of blood per minute.
At this rate, it takes about 1 minute for blood to be circulated to the most distal
extremity and back to the heart.
Blood vessels
A tubular network that permits blood to flow from the heart to all the living cells of
the body and then back to the heart.
Arteries carry blood away from the heart, whereas veins return blood to the heart.
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Arteries and veins are continuous with each other through smaller blood vessels.
Arteries branch extensively to form a “tree” of smaller vessels called arterioles.
Blood passes from the arterial to the venous system in microscopic capillaries,
which are the most numerous of the blood vessels.
All exchanges of fluid, nutrients, and wastes between the blood and tissues occur
across the walls of capillaries.
Blood flows through capillaries into microscopic veins called venules, which deliver
blood into larger veins that eventually return the blood to the heart.
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Composition of the Blood
Blood consists of formed elements (blood cells) that are suspended in a fluid called
plasma.
The total blood volume in the average-size adult is about 5 liters, constituting about
8% of the total body weight.
When a blood sample is centrifuged, the heavier formed elements are packed into
the bottom of the tube, leaving plasma at the top.
The formed elements constitute approximately 45% of the total blood volume, and
the plasma accounts for the remaining 55%.
Red blood cells compose most of the formed elements; the percentage of red blood
cell volume to total blood volume in a centrifuged blood sample (the hematocrit ) is
36% to 46% in women and 41% to 53% in men.
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The constituents of blood.
Blood cells become packed at the bottom of the test tube when whole blood is centrifuged,
leaving the fluid plasma at the top of the tube. Red blood cells are the most abundant of the
blood cells—white blood cells and platelets form only a thin, light-colored “buffy coat” at the
interface between the packed red blood cells and the plasma.
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Representative Normal Plasma Values
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Plasma
Plasma is a straw-colored liquid consisting of water and dissolved solutes.
Plasma contains Na+ and other ions, as well as organic molecules such as
metabolites, hormones, enzymes, antibodies, and other proteins.
Plasma Proteins
The major three types of proteins are albumins, globulins, and fibrinogen.
Albumins
Account for most (60% to 80%) of the plasma proteins and are the smallest in size.
They are produced by the liver and provide the osmotic pressure needed to draw
water from the surrounding tissue fluid into the capillaries.
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This action is needed to maintain blood volume and pressure.
Globulins
They are grouped into three subtypes: alpha globulins, beta globulins, and gamma
globulins.
The alpha and beta globulins are produced by the liver and function in transporting
lipids and fat-soluble vitamins.
Gamma globulins are antibodies produced by lymphocytes and function in
immunity.
Fibrinogen
Accounts for only about 4% of the total plasma proteins, is an important clotting
factor produced by the liver.
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During the process of clot formation, fibrinogen is converted into insoluble threads
of fibrin.
Thus, the fluid from clotted blood, called serum, does not contain fibrinogen but is
otherwise identical to plasma.
Plasma Volume
If the body should lose water, the remaining plasma becomes excessively
concentrated.
This is detected by receptors in the hypothalamus, resulting in a sensation of thirst
and the release of antidiuretic hormone (ADH) from the posterior pituitary.
ADH promotes water retention by the kidneys, which helps compensate for the
dehydration and lowered blood volume.
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This regulatory mechanism, together with others that influence plasma volume, are
very important in maintaining blood pressure.
The Formed Elements of Blood
The formed elements of blood include two types of blood cells: erythrocytes, or
red blood cells, and leukocytes, or white blood cells.
1- Erythrocytes
They are flattened, biconcave discs about 7 μm in diameter and 2.2 μm thick.
Their shape provides an increased surface area through which gas can diffuse.
Erythrocytes lack nuclei and mitochondria. Their life span is about 120 days.
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Older erythrocytes are removed from the circulation by phagocytic cells in the liver,
spleen, and bone marrow.
Erythrocyte contains hemoglobin molecules, which give blood its red color.
Each hemoglobin molecule consists of four protein chains called globins, each of
which is bound to one heme, a red- pigmented molecule that contains iron.
The iron group of heme is able to combine with oxygen in the lungs and release
oxygen in the tissues.
The heme iron is recycled from old RBCs in the liver and spleen. This iron travels in
the blood to the bone marrow attached to a protein carrier called transferrin.
This recycled heme iron supplies most of the body’s need for iron.
A dietary deficiency in iron reduces the ability of the bone marrow to produce
hemoglobin, and can result in iron-deficiency anemia.
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2- Leukocytes
Leukocytes differ from erythrocytes in that, they contain nuclei and mitochondria
and can move in an amoeboid fashion.
WBCs are almost invisible under the microscope unless they are stained; therefore,
they are classified according to their staining properties.
- Granular leukocytes; have granules in their cytoplasm. They inclde, eosinophils,
basophils, and neutrophils.
- Agranular leukocytes, those without clearly visible granules, and include
lymphocytes and monocytes.
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Neutrophils
are the most abundant type of leukocyte, accounting for 50% to 70%
of the leukocytes in the blood.
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At maturation, the nuclei of neutrophils become lobulated, with 2-5 lobes, so that
called polymorphonuclear leukocytes (PMNs).
Lymphocytes are usually the second most numerous type of leukocyte; they are
small cells with round nuclei and little cytoplasm.
Monocytes are the largest of the leukocytes and generally have kidney- or
horseshoe-shaped nuclei.
There are smaller numbers of plasma cells, which are derived from lymphocytes.
Plasma cells produce and secrete large amounts of antibodies.
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Platelets
Platelets, or thrombocytes, are the smallest of the formed elements and are
fragments of large cells called megakaryocytes, which are found in bone marrow.
Platelets lack nuclei but, like leukocytes, are capable of amoeboid movement.
Platelets survive for five 5-9 days before being destroyed by the spleen and liver.
Platelets play an important role in blood clotting, and they constitute most of the
mass of the clot.
Platelets that attach together in a blood clot release serotonin, which stimulates
constriction of blood vessels, thus reducing the flow of blood to the injured area.
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Formed Elements of the Blood
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