Body Fluids Body fluids serve as a medium

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 CO 2

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 O 2 • 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 Fe 2+ ion – because each Hb contains four hemes, it can carry four O 2 – the ability of Hb to carry O 2 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 O 2 – the oxygen-carrying capacity of Hb is also affected by its environment – a slight change in p. H 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 CO 2 – as p. H decreases, more oxygen is released for an active muscle than for a muscle at rest – similarly, active muscle produces CO 2 which accumulates and further enhances the release of O 2 – when muscle contracts, both H+ and CO 2 are produced

Fig. 31. UN, p. 749

Blood as a Carrier of O 2 – an oxygen dissociation curve

Chem Connect 31 C, p. 750

Transport of CO 2 in Blood • CO 2 also binds to Hb – as O 2 is released from Hb. O 2, CO 2 becomes bound to the terminal NH 2 group of each polypeptide chain of Hb – the product formed is called carbaminohemoglobin – each heme can carry four CO 2 – CO 2 is also carried in red blood cells as H 2 CO 3

Fig. 31. UN, p. 754

Fig. 31. UN, p. 751

Urine • Urine – normal urine contains about 4% dissolved waste products – the p. H of urine varies from 5. 5 to 7. 5 – the main solute is urea – other organic solutes present include – urine also contains inorganic ions such as Na+, Ca 2+, Mg 2+, Cl-, PO 43 -, SO 42 -, and HCO 3 -

Buffer Production -Kidney • Among the waste products in the blood are H+ – H+ is neutralized by the HCO 3 - ions that are part of the blood’s buffer system – when the blood reaches the lungs, H 2 CO 3 is decomposed by carbonic anhydrase and CO 2 is exhaled

the cell-lining of the walls of the distal tubules reabsorb the CO 2 that was lost in the glomeruli the lost HCO 3 - ions are replaced by the kidneys

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 HCO 3 - ions move from the cells into the capillaries – thus, H+ ions picked up at the tissues and temporarily neutralized in the blood by HCO 3 are finally pumped out into the urine – at the same time, the HCO 3 - 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 H 2 O 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 H 2 O is also retained to solvate these ions thus, increased aldosterone production allows the body to retain more H 2 O 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 O 2 delivery or CO 2 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