CONCEPT MAP: THE GENERATION OF BIOCHEMICAL ENERGY
29.1 Body Water and Its Solutes
All body f luids have water as the solvent; in fact, the water content of the human body averages about 60% (by weight). Physiologists describe body water as occupying two different “compartments”—the intracellular and the extracellular compartments. We have looked primarily at the chemical reactions occurring in the intracellular f luid (the f luid inside cells), which includes about two-thirds of all body water (Figure 29.1). We now turn our attention to the remaining one- third of body water, the extracellular fluid, which includes mainly blood plasma (the f luid portion of blood) and interstitial f luid (the f luid that fills the spaces between cells).
To be soluble in water, a substance must be an ion, a gas, a small polar molecule, or a large molecule having many polar, hydrophilic (water-loving) or ionic groups on its surface. All four types of solutes are present in body f luids. The majority are inorganic ions and ionized biomolecules (mainly proteins), as shown in the com- parison of blood plasma, interstitial f luid, and intracellular f luid in Figure 29.2.
Although these f luids have different compositions, their osmolarities are the same; that is, they have the same number of moles of dissolved solute particles (ions or molecules) per liter. The osmolarity is kept in balance by the passage of water across cell membranes by osmosis, which occurs in response to osmolarity differences.
Inorganic ions, known collectively as electrolytes (Section 9.9), are major contribu- tors to the osmolarity of body fluids and they move about as necessary to maintain charge balance. Water-soluble proteins make up a large proportion of the solutes in blood plasma and intracellular fluid; 100 mL of blood contains about 7 g of protein.
Intracellular fluid Fluid inside cells.
Extracellular fluid Fluid outside cells.
Blood plasma Liquid portion of the blood: an extracellular fluid.
Interstitial fluid Fluid surrounding cells: an extracellular fluid.
Osmolarity Amount of dissolved sol- ute per volume of solution.
CONCEPTS TO REVIEW
A. Solutions (Sections 9.1, 9.2, 9.10) B. Osmosis and Osmotic Pressure
(Section 9.12) C. Dialysis (Section 9.13) D. pH
(Sections 10.7, 10.8) that participate in inflammation, the
immune response, and blood clotting.
4. How do red blood cells participate in the transport of blood gases?
THE GOAL: Be able to explain the rela- tionships among O2 and CO2 transport, and acid–base balance. ( D.) 5. How is the composition of urine
controlled?
THE GOAL: Be able to describe the transfer of water and solutes during urine formation and give an overview of the composition of urine. ( B, C.) 1. How are body fluids classified?
THE GOAL: Be able to describe the major categories of body fluids, their general composition, and the exchange of solutes between them. ( A, B.)
2. What are the roles of blood in main- taining homeostasis?
THE GOAL: Be able to explain the composition and functions of blood.
( B, C.)
3. How do blood components par- ticipate in the body’s defense mechanisms?
THE GOAL: Be able to identify and describe the roles of blood components CHAPTER GOALS
Plasma 8%
Interstitial fluid 25%
Minor components
3%
Intracellular fluids 64%
▲Figure 29.1
Distribution of body water.
About two-thirds of body water is intracellular—within cells. The extra- cellular fluids include blood plasma, fluids surrounding cells (interstitial), and such minor components as lymph, cerebrospinal fluid, and the fluid that lubricates joints (synovial fluid).
In osmosis, water moves across a semipermeable membrane from the more dilute solution to the more con- centrated solution (see Section 9.12).
Blood proteins are used to transport lipids and other molecules, and they play essential roles in blood clotting (Section 29.5) and the immune response (Section 29.4).
The blood gases (oxygen and carbon dioxide), along with glucose, amino acids, and the nitrogen-containing by-products of protein catabolism, are the major small molecules in body fluids.
Blood travels through peripheral tissue in a network of tiny, hair-like capillaries that connect the arterial and venous parts of the circulatory system (Figure 29.3). Capillaries are where nutrients and end products of metabolism are exchanged between blood and interstitial fluid. Capillary walls consist of a single layer of loosely spaced cells. Water and many small solutes move freely across the capillary walls in response to differences in fluid pressure and concentration (see Figure 29.3).
Solutes that can cross membranes freely (passive diffusion) move from regions of high solute concentration to regions of low solute concentration. On the arterial ends of capillaries, blood pressure is higher than interstitial fluid pressure and solutes and water are pushed into interstitial fluid. On the venous ends of the capillaries, blood pressure is lower, and water and solutes from the surrounding tissues are able to reen- ter the blood plasma. The combined result of water and solute exchange at capillaries is that blood plasma and interstitial fluid are similar in composition (except for protein content; see Figure 29.2).
Plasma Proteins 200
150
100
50
0
Org. acid SO42− HPO42−
Cl− HCO3−
ANIONS
Plasma Mg2+ Ca2+ K+ Na+ 200
150
100
50
0 Interstitial
fluid CATIONS
Milliequivalents per liter (mEq/L)
Intracellular fluid
Interstitial fluid
Intracellular fluid
▲ Figure 29.2
The distribution of cations and anions in body fluids.
Outside cells, Na+ is the major cation and Cl- is the major anion. Inside cells, K+ is the major cation and HPO4 2- is the major anion. Note that at physiological pH, proteins are negatively charged.
S E C T I O N 2 9 . 1 Body Water and Its Solutes 873
In addition to blood capillaries, peripheral tissue is networked with lymph capil- laries (Figure 29.4). The lymphatic system collects excess interstitial fluid, debris from cellular breakdown, and proteins and lipid droplets too large to pass through capillary walls. Interstitial fluid and the substances that accompany it into the lymphatic sys- tem are referred to as lymph, and the walls of lymph capillaries are constructed so that lymph cannot return to the surrounding tissue. Ultimately, lymph enters the blood- stream at the thoracic duct.
Arteries
Capillaries Vein
• • •
•
•
From heart To heart ◀ Figure 29.3
The capillary network.
Solute exchange between blood and interstitial fluid occurs across capillary walls.
Loose connective
tissue Endothelial
cells Lymphatic
capillary
Blood capillaries
Artery
Vein
Interstitial fluid
◀ Figure 29.4
Blood and lymph capillaries.
The arrows show the flow of fluids in and out of the various components of peripheral tissue.
Exchange of solutes between the interstitial fluid and the intracellular fluid occurs by crossing cell membranes. Here, major differences in concentration are maintained by active transport (transport requiring energy) against concentration gradients (from regions of low concentration to regions of high concentration) and by the impermeabil- ity of cell membranes to certain solutes, notably the sodium ion (Figure 29.5). Sodium ion concentration is high in extracellular fluids and low in intracellular fluids, whereas potassium ion concentrations are just the reverse: high inside cells and low outside cells (see Figure 29.2).
Interstitial fluid
Intracellular fluid
Tissue cell
Capillary
Blood plasma
Blood cells Protein
Protein
H2O H2O H2O
Na+
Na+
Na+
K+
Active transport
▶ Figure 29.5
Exchange among body fluids.
Water exchanges freely in most tissues, with the result that the osmolarities of blood plasma, interstitial fluid, and intracellular fluid are the same. Large proteins cross neither capillary walls nor cell membranes, leaving the inter- stitial fluid protein concentration low.
Concentration differences between interstitial fluid and intracellular fluid are maintained by active transport of Na+ and K+.
KEY CONCEPT PROBLEM 29.1
The drug cisplatin is used to treat various forms of cancer in humans. As with many other drugs, the difficult part in designing the cisplatin molecule was to have a structure that ensures transport into the cell. The equilibrium reaction that takes place in the body when cisplatin is administered is
Cl Cl
Pt
Cisplatin Monoaquacisplatin
(aq)+H2O(l) NH3
NH3
Cl H2O
Pt (aq)+Cl−(aq) NH3
NH3
+
(This is an example of ligand exchange.) Which form of cisplatin would you expect to exist inside the cell (where chloride concentrations are small)? Which form of cisplatin would you expect to exist outside the cell (where chloride concentrations are high)? Which form—cisplatin or monoaquacisplatin—enters the cell most readily? Why?