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. One fluid compartment is
cellular fluid.
– 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
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
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
Table 31.1, p.747
Composition of Blood
• 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
Go to GOB ch 27-2 here
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
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
Go to GOB 27-3 here
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
– 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
– when muscle contracts, both H+ and CO2 are
produced
Fig. 31.UN, p.749
Blood as a Carrier of O2
– an oxygen dissociation curve
Chem Connect 31C, p.750
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
O
HbO2 + CO2
Hb-C-O- + H+ + O2
Carbaminohemoglobin
– each heme can carry four CO2
– CO2 is also carried in red blood cells as H2CO3
CO2 + H2 O
carbonic
anhydrase
H2 CO3
Fig. 31.UN, p.754
Fig. 31.UN, p.751
Fig. 31.UN, p.751
Urine
• Urine
– normal urine contains about 4% dissolved
waste products
– the pH of urine varies from 5.5 to 7.5
O
– the main solute is urea
H2 N-C-NH2
– other organic solutes present include
O
O
H2 N-C-N-CH2 -C-O
CH3
Creatine
CH3
N
NH
NH
O
Creatinine
O
N
H
-
COO
Hippuric acid
– urine also contains inorganic ions such as
Na+, Ca2+, Mg2+, Cl-, PO43-, SO42-, and HCO3-
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 cell-lining of the
walls of the distal
tubules reabsorb the
CO2 that was lost in the
glomeruli
the lost HCO3- ions are
replaced by the kidneys
CO2 + H2 O
H2 CO3
HCO3
-
+
+ H
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 HCO3are 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
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 antidiuretic
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
more H2O is also retained to
solvate these ions
thus, increased aldosterone
production allows the body to
retain more H2O
as the concentration of Na+ in the
blood increases, the
concentration of Cl- (necessary to
maintain electrical neutrality) also
increases
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
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
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
Fig. 31.UN, p.754
Fig. 31.UN, p.754