Transcript Document

31
General, Organic, and
Biochemistry, 7e
Bettelheim,
Brown, and March
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31-1
31 Chapter 31
Body Fluids
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31 Body Fluids
• Body fluids serve as a medium for carrying
nutrients to and waste products from cells, and
for carrying the chemical communicators that
coordinate activities among cells
• extacellular fluids: all body fluids not inside cells;
collectively, they make up about 25% of a person’s
body weight
• interstitial fluid: the extracellular fluid that surrounds
most cells and fills the space between them; makes up
about 17% of body weight
• blood plasma: the fluid that flows through arteries and
veins; makes up about 5% of body weight
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31 Body Fluids
• other body fluids occurring in lesser amounts are
urine, lymph, cerebrospinal fluid, aqueous humor, and
synovial fluid
• Blood can exchange substances with other body
fluids
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31 Body Fluids
• there is only limited exchange, however, between blood
and cerebrospinal fluid because of the blood-brain
barrier
• the blood-brain barrier is permeable to water, oxygen,
carbon dioxide, glucose, alcohols, and most
anesthetics
• it is impermeable or only slightly permeable to
electrolytes such as Na+, K+, and Cl-; also to many
higher-molecular weight compounds
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31-5
31 Functions of Blood
• Blood
• caries O2 from the lungs to tissues
• carries CO2 from tissues to the lungs
• carries nutrients from the digestive system to tissues
• carries waste products from tissues to the excretory
organs
• by means of its buffers, maintains the pH of the body
(with the help of the kidneys)
• maintains a constant temperature
• carries hormones from the endocrine glands to their
target locations
• carries infection-fighting leukocytes and antibodies
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31-6
31 Composition of Blood
• The three main cellular elements in blood are:
1. Erythrocytes
• are very specialized cells whose main function is to
carry O2 to cells and CO2 away from them
• are formed in bone marrow
• have a half-life of about 32 days
• are removed by the liver and spleen and destroyed
2. Leukocytes (white blood cells)
• are formed in the bone marrow
• most of the different leukocytes destroy invading
bacteria or other foreign substances by phagocytosis
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31 Composition of Blood
3. Platelets or thrombocytes
• are formed in the bone marrow and spleen
• control bleeding when there is a cut or injury
• Plasma
• the fluid remaining after all cellular elements have been
removed from whole blood by centrifugation
• is 92% water
• the dissolved solids are mainly proteins (7%)
• the remaining 1% contains glucose, lipids, enzymes,
vitamins, hormones, and waste products such as urea
and CO2
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31-8
31 Composition of Blood
• if plasma is allowed to stand, it forms a clot, a gel-like
substance
• serum: the clear liquid that can be extracted from blood
plasma
• serum contains all the components of plasma but lacks
fibrinogen that makes blood clot
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31-9
31 Blood as a Carrier of O2
• The oxygen carriers in blood are hemoglobin (Hb)
molecules, which are located in erythrocytes
• the active sites are the hemes; at the center of each
heme is an Fe2+ ion
• because each Hb contains four hemes, it can carry four
O2
• the ability of Hb to carry O2 depends on how much
oxygen is in the environment
• as shown by an oxygen dissociation curve, each heme
has a cooperative effect on the other hemes
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31 Blood as a Carrier of O2
• an oxygen dissociation curve
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31 Blood as a Carrier of O2
• the oxygen-carrying capacity of Hb is also affected by
its environment
• a slight change in pH of the environment, for example,
changes Hb’s oxygen-binding capacity
• Bohr effect: the relationship between the oxygencarrying capacity of Hb and the levels of H+ and CO2
• when muscle contracts, both H+ and CO2 are produced
• as pH decreases, more oxygen is released for an active
muscle than for a muscle at rest
• similarly, active muscle produces CO2 which
accumulates and further enhances the release of O2
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31 Transport of CO2 in Blood
•
CO2 also binds to Hb
• as O2 is released from HbO2, CO2 becomes bound to the
terminal NH2 group of each polypeptide chain of Hb
• the product formed is called carbaminohemoglobin
HbO2 + CO2
O
Hb-C-O- + H+ + O2
Carb amin oh emoglob in
• each heme can carry four CO2
• CO2 is also carried in red blood cells as H2CO3
CO2 + H2 O
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carbonic
anhydrase
H2 CO3
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31 Blood Cleansing - Kidneys
• The kidney is a superfiltration machine
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31
Blood Cleansing Kidneys
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31 Urine
• Urine
• normal urine contains about 4% dissolved waste
products
• the pH of urine varies from 5.5 to 7.5
O
H 2 N-C-NH2
• the main solute is urea
• other organic solutes present include
O
O
H2 N-C-N-CH2 -C-OCH3
Creatine
CH3
N
NH
NH
O
Creatin ine
O
N
H
COO-
Hip puric acid
• urine also contains inorganic ions such as Na+, Ca2+,
Mg2+, Cl-, PO43-, SO42-, and HCO3-
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31 Buffer Production -Kidney
• Among the waste products in the blood are H+
• H+ is neutralized by the HCO3- ions that are part of the
blood’s buffer system
H+ + HCO3
H2 CO3
• when the blood reaches the lungs, H2CO3 is
decomposed by carbonic anhydrase and CO2 is
exhaled
• the lost HCO3- ions are replaced by the kidneys
• the cell-lining of the walls of the distal tubules reabsorb
the CO2 that was lost in the glomeruli
CO2 + H2 O
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H2 CO3
HCO3 - + H+
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31 Buffer Production -Kidney
• the H+ ions move into the urine where they are partially
neutralized by a phosphate buffer
• to compensate for the loss of positive ions, Na+ ions
from the tubules enter the cells
• as this happens, Na+ and HCO3- ions move from the
cells into the capillaries
• thus, H+ ions picked up at the tissues and temporarily
neutralized in the blood by HCO3- are finally pumped
out into the urine
• at the same time, the HCO3- ions lost in the lungs are
regained by the blood in the distal tubules
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31 Water and Salt Balance
• The balance in the kidneys between filtration and
reabsorption is under hormonal control
• the production of urine is called diuresis
• vasopressin promotes reabsorption of water
• in the absence of vasopressin, only the proximal
tubules reabsorb water, and too much water passes
into the urine
• in the presence of vasopressin, water is also
reabsorbed in the distal tubules
• thus, vasopressin causes blood to retain more water
and produces a more concentrated urine
• any agent that reduces the volume of urine is called an
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antidiuretic
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31 Water and Salt Balance
• usually the vasopressin level is sufficient to maintain
the proper level of H2O in tissues
• in severe dehydration, the hormone aldosterone helps
to maintain proper fluid levels
• aldosterone controls the Na+ concentration in blood
• in the presence of aldosterone, the reabsorption of Na+
increases
• as the concentration of Na+ in the blood increases, the
concentration of Cl- (necessary to maintain electrical
neutrality) also increases
• more H2O is also retained to solvate these ions
• thus, increased aldosterone production allows the
body to retain more H2O
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31 Blood Pressure
• Blood pressure is maintained by
• the total volume of blood
• the pumping of the heart
• the muscles that surround the blood vessels and
provide the proper resistance to blood flow
• Blood pressure is controlled by several very
complex systems, some of them operating within
seconds and some that take days to react to a
change in blood pressure
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31 Blood Pressure
• In the event of a hemorrhage, three different
control systems begin to operate within seconds
• baroreceptors in the neck detect the drop in pressure
and send signals to the heart to pump harder and to
the muscles surrounding the blood vessels to contract
and thus restore pressure
• chemical receptors on the cells detect less O2 delivery
or CO2 accumulation and also send nerve signals
• the central nervous system reacts to an O2 deficiency
by a feedback mechanism
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31 Blood Pressure
• Hormonal control
• hormonal controls act somewhat more slowly and may
take minutes or even days
• the kidneys secrete an enzyme called renin
• renin acts on an inactive blood protein called
angiotensinogen, converting it to angiotensin
• antiotensin is a potent vasoconstrictor
• Long-term renal control
• when blood pressure falls, the kidneys retain more
water and salt, thus increasing blood volume and
pressure
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31 Body Fluids
End
Chapter 31
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