Download Physiology Cardiovascular System Blood • Overview o Components

Physiology
Cardiovascular System
Blood
• Overview
o Components
Blood is the only fluid tissue in the body. It appears to be a thick, homogeneous
liquid, but the microscope reveals that blood has both cellular and liquid
components
Blood is a specialized type of connective tissue
• Collagen and elastic fibers typical of other connective tissue are absent
from blood, but dissolved fibrous proteins become visible as fibrin
strands during blood clotting.
Major components of whole blood include:
• Plasma
o 55% of whole blood
o Least dense component (floats to the top in a centrifuge)
• Buffy Coat = a thin, whitish layer at the erythrocyte-plasma junction
o Leukocytes and platelets
o <1% of whole blood
• Erythrocytes
o 45% of whole blood
Hematocrit = “blood fraction”; total blood volume
occupied by erythrocytes
Normal hematocrit values vary
• Healthy males = 47% +/- 5%
• Healthy females = 42% +/- 5%
o Most dense component (sinks to the bottom in a centrifuge)
o Physical Characteristics
Depending on amount of oxygen it is carrying, blood color varies from scarlet
(oxygen rich) to dark red (oxygen poor).
Blood is more dense than water
Blood is slightly alkaline (pH between 7.35 and 7.45)
Blood temperature (38-degrees C or 100.4-degrees F) is higher than body
temperature
Blood accounts for about 8% of body weight
Blood volume is about 1.5 gallons (approx. 5 L)
o Functions
Distribution
• Delivering oxygen from lungs and nutrients from digestive tract to all
body cells
• Transporting metabolic waste products from cells to elimination sites
• Transporting hormones from the endocrine organs to their target
organs
Regulation
• Maintaining appropriate body temperature by absorbing and
distributing heat throughout the body and to the skin surface to
encourage heat loss
Maintaining normal pH in body tissues. Many blood proteins and other
bloodborne solutes act as buffers to prevent excessive or abrupt
changes in blood pH that could jeopardize normal cell activities.
• Maintaining adequate fluid volume in the circulatory system. Salts and
blood proteins act to prevent excessive fluid loss from the bloodstream
into the tissue spaces.
Protection
• Preventing blood loss. When a blood vessel is damaged, platelets and
plasma proteins initiate clot formation, halting blood loss.
• Preventing infection. Drifting along in blood are antibodies,
complement proteins, and white blood cells, all of which help defend
the body against foreign invaders such as bacteria and viruses.
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Blood Plasma
o A straw-colored, sticky fluid
o Mostly water (about 90%), plasma contains over 100 different dissolved solutes
o When blood starts to become too acidic (acidosis), both the respiratory system and the
kidneys are called into action to restore plasma’s normal, slightly alkaline pH.
o Steroids and hormones are carried by plasma proteins
Formed Elements
o Erythrocytes
Red blood cells that transport oxygen
Cells with no nuclei or organelles
Shaped like biconcave discs—flattened discs with depressed centers
They are little more than “bags” of hemoglobin (Hb)
• Hemoglobin = RBC protein that makes the RBC red and functions in gas
transport by binding easily to oxygen
Disorders of Erythrocytes:
• Anemias = “lacking blood”
o A condition where blood has abnormally low oxygen-carrying
capacity
o Anemic individuals are fatigued, often pale, short of breath, and
chilly
o Common Causes
An insufficient number of red blood cells
• i.e., hemorrhagic anemia resulting from blood
loss
Low hemoglobin content
• i.e., iron-deficiency anemia
Abnormal hemoglobin
• i.e., sickle-cell anemia
o Leukocytes
White blood cells that protect the body in various ways
Only complete cells with nuclei and all the usual organelles in the blood
Diapedesis = “leaping across”; ability of WBCs to slip out of the capillary blood
vessels and go to areas where they are most needed to mount inflammation or
immune responses.
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Whenever WBCs are mobilized for action, the body speeds up their production
and twice the normal number may appear in the blood within a few hours.
• Leukocytosis = a WBC count of over 11,000 cells/ul; a normal
homeostatic response to an infection in the body.
Platelets
Cell fragments that help stop bleeding
Essential for the clotting process
They stick to the damaged site forming a temporary plug that helps seal the
break.
The Heart
• Anatomy
o About the size of a fist, the heart is less than a pound.
o Heart rests on the superior surface of the diaphragm lying anterior to vertebral column
and posterior to sternum.
o Situated between the 2nd and 5th intercostal space
• Pathway of Blood through the Heart
o Heart has four chambers
Left Atrium
• Receive blood from the lungs back to the heart
Left Ventricle
• Pumps oxygen-rich blood to the rest of the body via the systemic circuit
o Systemic Circuit = blood vessels that functionally carry blood to
and from all body tissues.
Right Atrium
• Receives blood from the body
Right Ventricle
• Pumps oxygen-poor blood into the lungs via the pulmonary circuit
o Pulmonary Circuit = serves gas exchange; blood vessels that
carry blood to and from the lungs
• Physiology
o The Cardiac Cycle
Includes all events associated with the blood flow through the heart during one
complete heartbeat.
Systole = contraction period
Diastole = relaxation period
Blood Vessels
• Arterial System
o Arteries can be divided into three groups
Elastic Arteries
• Thick-walled arteries near the heart
• Largest in diameter; approx. 2.5cm to 1 cm and most elastic
• They are pressure reservoirs—expanding and recoiling as blood is
ejected from the heart. This provides a continuous blood flow without
stopping or becoming intermittent.
Muscular Arteries
Aka, distributing arteries
Deliver blood to specific body organs and account for most of the
named arteries studied in the lab.
• Internal diameter ranges from that of a little finger (1 cm) to a pencil
lead (0.3 mm).
Arterioles
• Smallest of all the arteries
• Minute-to-minute blood flow into the capillary beds is determined by
arteriole diameter which varies in response to changing neural,
hormonal and local chemical influences.
• When arterioles constrict, the tissues served are largely bypassed.
• When arterioles dilate, blood flow into the local capillaries increases
dramatically.
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Capillaries
o The smallest blood vessels
o They have a very thin wall
o Given their location and thinness of their walls, capillaries are ideally suited for their
role—exchange of materials (gases, nutrients, hormones, and so on) between blood and
the interstitial fluid
o Capillary Beds
Capillaries do not function independently
They form interweaving networks
Microcirculation = flow of blood from an arteriole to a venule through a capillary
bed.
Amount of blood entering a capillary bed is regulated by local chemical
conditions and arteriolar vasomotor nerve fibers.
A bed may be flooded with blood or almost completely bypassed, depending on
conditions in the body or in that specific organ.
Venous System
o Blood is carried from the capillary beds toward the heart by veins.
o Venules = formed when capillaries unite
o Veins = venules join to form veins
Physiology of Circulation
o Any fluid driven by a pump through a circuit of closed channels operates under
pressure, and the nearer the fluid is to the pump, the greater the pressure exerted on
the fluid.
o Blood flows through the blood vessels along a pressure gradient, always moving from
higher- to lower-pressure areas.
o The pumping action of the heart generates blood flow; pressure results when flow is
opposed by resistance.
o Arterial Blood Pressure
Reflects two factors
• How much the elastic arteries close to the heart can be stretched
• The volume of blood forced into them at any time
o If the amount of blood entering and leaving the elastic arteries
in a given period were equal, arterial pressure would be
constant
Systolic pressure = as the left ventricle contracts and expels blood into the
aorta, it imparts kinetic energy to the blood which stretches the elastic aorta as
aortic pressure reaches its peak.
• In healthy adults, this pressure peak averages 120 mm Hg
• Blood moves forward into the arterial bed because the pressure in the
aorta is higher than the pressure in the more distal vessels.
Diastolic pressure = as the aortic valve closes, it prevents blood from flowing
back into the heart and the walls of the aorta recoil, maintaining sufficient
pressure to keep the blood flowing forward into the smaller vessels.
• Aortic pressure drops to its lowest level—approx. 70 to 80 mm Hg in
health adults.
o Essentially, the volume and energy of blood stored in the elastic
arteries during systole are given back during diastole.
Pulse pressure (pulse) = the difference between the systolic and diastolic
pressures.