Lecture 15
The Circulatory System – Blood and the Heart
Introduction
 Functions of the Circulatory System
o Transport
 Blood carries oxygen from the lungs to all of the body’s tissues.
 Blood carries carbon dioxide from the body’s tissues and carries it
to the lungs to be removed from the body
 Blood picks up nutrients from the digestive system and delivers
them to all of the body’s tissues
 Blood carries metabolic wastes to the kidneys for removal
 Blood helps to regulate body temperature by carrying heat to the
body surface for removal
o Protection
 Blood plays several roles in inflammation, a mechanism for
limiting the spread of infection
 White blood cells destroy microorganisms and cancer cells
 Antibodies and other blood proteins neutralize toxins and help to
destroy pathogens
 Platelets secrete factors that initiate blood clotting and other
processes for minimizing blood loss
o Regulation
 By absorbing or giving off fluid under different conditions, the
blood capillaries help to stabilize fluid distribution in the body
 By buffering acids and bases, blood proteins help to stabilize the
pH of the extracellular fluids
o Formed elements:
 Erythrocytes
 Red blood cells
o Have two functions
 Pick up oxygen from the lungs and deliver it
to tissues in other parts of the body
 Pick up carbon dioxide from tissues and
unload it in the lungs
o Most abundant formed elements in the blood
 Leukocytes
 White blood cells
o Least abundant, but provide protection against
infectious microorganisms and other pathogens
 Platelets
 Cell fragments
o Used primarily to form clots
Components and General Properties of Blood
 Blood Plasma – clear extracellular marix
o Has no anatomy that we can study visually
o It’s a complex mixture of water, proteins, nutrients, electrolytes,
nitrogenous wastes, hormones, and gases
 Proteins are the most abundant plasma solutes by weight and play
a variety of roles
 Albumin
o Smallest and most abundant plasma protein
o Transports various plasma solutes and buffer the
pH of the blood plasma
o Contributes to viscosity and osmolarity of blood to
affect blood volume, blood pressure, and blood flow
 Viscosity – thickness, or resistance to flow
 Osmolarity – concentration of particles that
cannot pass through the walls of blood
vessels
 Globulins
o Alpha Globulins
 Transport hemoglobin released by dead
erythrocytes
 Transport copper, lipids, fat soluble
vitamins, and hormones
o Beta Globulins
 Transport iron and lipids
 Aid in destruction of toxins and
microorganisms
o Gamma Globulins
 Consists of antibodies that combat
pathogens
 Fibrinogen
o Soluble precursor of fibrin, which forms the
framework of a blood clot

Erythrocytes
o Form and Function
 Discoid cells with thick rim and thin sunken center
 Lose nearly all of their organelles during their development
 They lack mitochondria, and rely on anaerobic fermentation to
produce ATP
 They have no nucleus, so they cannot produce proteins or undergo
mitosis
 They have glycoproteins on their outer surface that determine
blood type
 The cytoplasm of erythrocytes consists of a 33% solution of
hemoglobin
 Hemoglobin is best known for oxygen transport
 It also transports carbon dioxide and buffers blood pH
 The bioconcave shape increases surface area to volume ratio which
enables oxygen and carbon dioxide to diffuse quickly to and from
the hemoglobin
o Quantity of Erythrocytes
 Quantity determines the amount of oxygen the blood can carry
 The RBC (red blood count) is normally 4.6 to 6.2 million
RBCs/microliter in men and 4.2 to 5.4 million in women
 Hematocrit is the percentage of blood volume composed of
RBCs
o It normally ranges from 42% and 52% in men
o It’s usually between 37% and 48% in women
o Hemoglobin
 This is an iron-containing gas-transport protein normally found
only in RBCs
 It consists of four protein chains called globins and non-protein
heme groups bound to each protein chain
 Attached to each heme is a ferrous iron, which is the binding site
for oxygen
 Carbon dioxide that is transported by hemoglobin attaches to the
globin, rather than the heme
o The Erythrocyte Life Cycle
 Erythropoiesis is the formation of new red blood cells.
 Hemopoiesis is the formation of all formed elements in the blood
 In the embryo, the yolk sac produces blood cells
 In the fetus, blood cells are produced by the bone marrow,
liver, spleen, and thymus
 In an adult, the bone marrow and thymus continue to
produce blood cells
o The spleen continues to produce lymphocytes
 Red blood cells, originate as bone marrow stem cells called
pluripotent stem cells (which have the potential to develop into
multiple mature cell types)
 The cells pass through several stages of transformation over 3-5
days
 Reduction in size
 Increase in cell number
 Synthesis of hemoglobin
 Loss of nucleus and other organelles
 Erythrocytes live for about 120 days and then become trapped in
(and destroyed by) the spleen
o Blood Types
 Blood types differ because different glycoproteins and glycolipids
may be found on the surface of the erythrocyte


The molecules act as antigens
 Antigens are substances capable of evoking an immune
reaction
 The blood plasma contains antibodies that react against
incompatible antigens on foreign RBCs, so some blood
types are incompatible for blood transfusions
Leukocytes
 Form and Function –
o Protect against pathogens
o Retain their organelles throughout life
 Types of Leukocytes
o Granulocytes – have cytoplasm that contains lysosomes and other
organelles that appear as conspicuous colored granules in stained
blood films
 Neutrophils –
 have granules that stain a very light lilac color at a
neutral pH (don’t stain a dark pink or a dark blue)
 Most abundant WBCs
 Primary job is to destroy bacteria
o Phagocytize bacteria and digest them
o Release toxic mix of chemicals that forms a
killing zone to kill bacteria (but themselves
too in the process)
 Eosinophils –
 have rosy to orange-colored granules (“stain pink”)
 Abundant in mucous membranes
 Secrete chemicals that weaken or destroy relatively
large parasites such as hookworms and tapeworms
 Phagocytize and dispose of inflammatory
chemicals, antigen-antibody complexes, and
allergens (foreign antigens that trigger allergies)
 Basophils –
 Have dark violet staining granules (“stain blue”)
 Rarest of the WBCs
 Secrete chemicals which aid in the body’s defense
processes
o Histamine is a vasodilator that widens the
blood vessels
 This speeds the flow of blood to
injured tissue
 This makes the blood vessels more
permeable, so that components such
as neutrophils and clotting proteins
can get into the connective tissues
more quickly
o Heparin is an anticoagulant

In inhibits blood clotting, so it
promotes the mobility of WBCs in
the area
o Agranulocytes – white blood cells that lack visible granules in the
cytoplasm
 Monocytes
 Largest of the WBCs
 Become macrophages after leaving the blood stream
o Macrophages are highly phagocytic cells
that consume up to 25% of their own
volume per hour
o They destroy dead or dying host and foreign
cells, pathogenic chemicals and
microorganisms, and other foreign matter
o They can chop up or “process” foreign
antigens and display fragments of them on
the cell surface to alert the immune system
to the presence of a pathogen
o Types of macrophages
 Dendritic cells are in the epidermis
and mucous membranes
 Microglia are in the CNS
 Alveolar macrophages are in the
pulmonary alveoli
 Hepatic macrophages are in the liver
sinusoids
 Lymphocytes
 Smallest WBCs
 Nucleus fills most of the cell
 Most are involved in specific immunity
o Immunity is a defense in which the body
recognizes a certain antigen it has
encountered before and mouse such a quick
response that a person notices little or no
symptoms of illness
o Certain T lymphocytes attack foreign cells
directly
o B lymphocytes fight pathogens by
differentiating into plasma cells and
secreting antibodies
o Life Cycle of Leukocytes
 Granulocytes and monocytes stay in red bone marrow until
they are needed
 Lymphocytes begin developing in bone marrow, but
migrate elsewhere



B lymphocytes mature in bone marrow and disperse
and colonize the lymph nodes, spleen, tonsils, and
mucous membranes
 T lymphocytes begin development in bone marrow
but then migrate to the thymus and mature there.
o Mature T lymphocytes disperse from the
thymus and colonize the same organs as B
lymphocytes
 Natural killer (NK) cells develop in bone marrow
Circulating WBCs don’t stay in the blood for long
 4 to 20 hours
 Macrophages may live in tissue for a few years,
however
Platelets
o Platelets are not cells, but small fragments of marrow cells
o Platelets have a complex internal structure that includes lysosomes,
mitochondria, microtubules, and granules
o Platelets have multiple functions:
 They secrete vasoconstrictors, which cause blood vessels to
constrict
 They stick together to form temporary platelet plugs to seal
small breaks in injured blood vessels
 They secrete procoagulants, or clotting factors, which
promote blood clotting
 They secrete chemicals that attract neutrophils and
monocytes to sites of inflammation
 They secrete growth factors that stimulate mitosis in
fibroblasts and smooth muscle and thus help to maintain
and repair blood vessels