Anatomy & Physiology For Dummies, 3rd Ed.

Chapter 16

Ten (Or So) Chemistry Concepts Related to Anatomy and Physiology


check Understanding the nature of energy

check Getting a handle on fluid properties, osmosis, and polarity

check Transferring electrons with redox reactions

Biology is a very special application of the laws of chemistry and physics. Biology follows and never violates the laws of the physical sciences, but this fact can sometimes be obscured in the complexity and other special characteristics of biological chemistry and physics.

This chapter contains a review of some of the principles of chemistry and physics that have special application in anatomy and physiology. Some of these principles overlap — for example, probability is one factor that drives the process of diffusion. Although what follows are oversimplified explanations of very profound and complex matters, we hope they help you better understand anatomy and physiology.

Energy Can Neither Be Created Nor Destroyed

The first law of thermodynamics is that energy can be neither created nor destroyed — it can only change form. Throughout any process, the total energy in the system remains the same. This law is one of the fundamental concepts in physics, chemistry, and biology.

Energy is the ability to bring about change or to do work. It exists in many forms, such as heat, light, chemical energy, and electrical energy. Light energy can be captured in chemical bonds, such as in the process of photosynthesis. In physiological processes, the energy in the bonds of ATP is transformed into work when the chemical bonds are broken — to move things, for example, and to generate heat. (And where did the energy in ATP come from in the first place? Ultimately, the sun via photosynthesis.)

Although the total energy in a system always remains the same, the energy available for biological processes does not. Cells can use energy only in certain specific forms. A physiological process that uses ATP doesn’t use all the energy stored in those chemical bonds, but the leftover energy isn’t in a form that can be used in another physiological process. It is “lost” to physiology, mostly as heat flowing out into the surrounding environment.

Everything Falls Apart

In our universe, energy is required to create “order” — for example, to build the atomic and molecular aggregations we call “matter” or “stuff.” Without continuous input of energy (maintenance), stuff falls apart. No news here for dwellers in the real world. As a physicist might put it, all systems tend toward increasing entropy (disorder). This is the second law of thermodynamics.

Energy always moves from a point of higher concentration to a point of lower concentration, never the reverse. For example, where two adjacent objects are of different temperatures, heat flows only from the warmer object (higher energy) to the cooler object (lower energy). A state of order contains more energy than a state of disorder because of the energy that went into building the state of order. Energy flows outward into the relative chaos of disorder.

remember Because living systems are highly ordered, the implications of the second law of thermodynamics are profound for physiology. The law means that physiological homeostasis (the maintenance of order) is an active process that requires energy. The energy that must be applied to drive any physiological process comes from releasing the chemical bonds in ATP. It means that physiological reactions proceed in only one direction — they aren’t reversible (unlike, say, sodium and chlorine ions that go into solution in water and then reorganize back into salt crystals spontaneously when the water is removed).

The ultimate physiological implication of the second law is the inevitability of death.

Everything’s in Motion

Particles in a solution fly around constantly and collide with one another all the time. This kind of motion is called Brownian motion. The higher the temperature, the more frequent and harder the collisions. It’s the reason why any reaction that can happen will happen, because (most of) the particles required for the reaction will collide sooner or later. (But see the “Probability Rules” section that follows.) This is especially important when considering all the molecules (such as glucose and ions) that move through membranes by simple or facilitated diffusion.

Brownian motion is also a mechanism of entropy. Each of the molecular collisions converts energy in the molecules to heat, in which form the energy is transferred to the surroundings.

Probability Rules

Everything that can happen will happen — some of the time. Other times, it won’t. The proportion of times it does happen depends on a lot of factors. If a solution contains large numbers of each of two molecules required for a reaction, the different types will collide frequently. So, concentration affects the chances that a reaction will actually occur. The higher the solution’s temperature, the more frequently molecules will collide and facilitate the reaction. But almost never will every possible reaction actually happen. Just by chance, some of these molecules won’t meet up with their counterpart molecule. That’s life. The chance, or randomness, can be quantified as probability. As with this hypothetical reaction, so with everything else related to biology and physiology: Probability, not certainty, rules.

By the way, the existence of life itself is highly improbable. And the probability of the existence of the uniqueness that is you is more improbable still.

Polarity Charges Life

A molecule is said to be polar when the positive and negative electrical charges are separated between one side of the molecule and the other because of unequal electron sharing. For example, a molecule of water is polar because the oxygen hogs the electrons concentrating the negative charge on the oxygen atom. So the water molecule has a positive charge at one end and a negative charge at the other, similar to a magnet. It attracts and holds other polar molecules. Methane is nonpolar because the carbon shares the electrons with the four hydrogen atoms uniformly.

Polarity underlies a number of physical properties of a substance, including surface tension, solubility, and melting and boiling points. In physiology, polarity strongly determines which molecules form bonds and which don’t — like how oil and water don’t mix. More specifically for the study of physiology, lipids and water don’t mix. Living cells use this principle to control the flow of substances into and out of the cell.

Lipids are a large and varied group of organic compounds, including fats and oils. All lipids have hydrophobic portions to them — that is, they don’t mix with water. Why not? Because a lipid is nonpolar, so it can’t form bonds with water. Water molecules push nonpolar molecules aside to get closer to other polar molecules.

Visualize a party where some people gather around the TV to watch a game and others congregate in the kitchen. The game watchers are the polar entities (supporters of one team or the other), and the other folks are the nonpolar entities (sharing an interest in nonpolar subjects like biological development and the effect of heat on complex organic substances). To belabor the analogy: The polar entities, after they’ve taken their positions close to other polar entities (on the couch), tend to maintain their state and position relative to those they’ve bonded with, while vibrating in place. The nonpolar entities move around relative to one another (milling about the kitchen), and they hold and release each other (children) easily and often. A different set of polar entities (the teenagers) conduct different physiological processes in seclusion from both the nonpolar and the other polar entities.

Water Is Special

Water is arguably the most important molecule in physiology. It accounts for around 60 percent of an adult’s body weight. Water’s strong polarity gives it characteristics that make it uniquely suited to providing its numerous functions.

Water has a high specific heat. A substance’s specific heat is the amount of heat required to raise the temperature of 1 gram of the substance 1 degree Celsius. Because water has a high specific heat, it can absorb heat from our active physiological process without increasing the body temperature.

The polarity of water also separates molecules from each other; dissolving them. This makes it useful as a method of transport (like in blood). This also makes it an ideal environment for chemical reactions to occur. As such, nearly all our metabolic reactions take place in water.

Fluids and Solids

Physiological processes, generally speaking, take place in fluids, and the properties of fluids are very important in these processes.

In everyday conversation, “fluid” means “liquid,” something that’s usually water-based, like juice, broth, or tea. In physics and chemistry, though, a watery solution is one kind of fluid, whether it’s one you’d care to drink or not. Air is another kind of fluid. Fats are fluids, even when they’re solid: Butter is exactly the same substance whether cold or warm, and so is every other form of fat. Technically speaking, glass and pure metals are fluids!

Salt, in contrast, is a solid. Salt (NaCl) crystals flow out of their containers in every kitchen and dining room, yes, but that doesn’t make salt a fluid. It’s got to do with the molecular structure. In solids, atoms are tightly packed together in a geometrically precise formation called a crystalline lattice. Sodium chloride is the model for this: Equal numbers of sodium and chlorine ions, each linked to six other ions, all pull each other in as tightly as the forces of polarity (electrical charge) require and allow. Solids are rigid at the molecular level; once bound together in a crystalline lattice, every atom in the molecule remains in place relative to its surrounding molecules.

In fluids, things move around more. Components come together in various ways — carbon dioxide and molecular oxygen (O2) dissolve from air into water and back into air (in the lungs). Fluids take the shape of their container. Air flows into and fills your alveoli. A watery mass in your stomach changes shape with every churning contraction. Gaseous fluids can be easily compressed because the molecules are already so far apart. However, the compressibility of liquids is very limited because the water molecules are already held together just about as tightly as they can be made to go.

Under Pressure

Boyle’s law describes the inverse relationship between the volume and pressure of a gas. If nothing else changes, such as temperature, an increase in volume brings about a decrease in pressure. When the pressure drops in a fixed space, it creates a vacuum.

The mechanisms of breathing utilize Boyle’s law. When the diaphragm contracts, it increases the volume of the lungs, which decreases the pressure. The vacuum pulls air in through the upper respiratory tract. It’s also a driving force for the cardiac cycle — opening and closing valves to move blood through the chambers of the heart.

Redox Reactions Transfer Electrons

The concept of reduction-oxidation (or redox) reactions is basically this: An electron is transferred from one chemical entity (atom or molecule) to another. The entity that receives the electron is said to be REDuced. The entity that releases the electron is said to be OXidized. In a redox reaction, the reduction of one entity is always balanced by the oxidation of another. The entities are called a redox pair. The redox reaction changes the oxidation state of both entities. In some cases, the oxidized entity undergoes another reaction to acquire another electron. Note that this isn’t a simple reversal of a redox reaction but a new reaction that involves another electron “donor” and frequently requires an enzyme catalyst.

tip Here’s a clever mnemonic to help with the terminology: OIL RIG — Oxidation Is Losing, Reduction Is Gaining (electrons, that is).

In biological systems, redox reactions are highly controlled and very important. Chemical energy is stored in electron bonds and released (made available for work) by redox reactions. Redox reactions are commonly part of signaling pathways. A change in the oxidative state of some molecules carries information. A change in the oxidation state of an entity can affect its polarity, which, in turn, affects its solubility in water and thus its ability to enter or leave a cell through the cell membrane. An entity that becomes more soluble can also become more available metabolically, which can be very important for some metal ions like iron and calcium.

remember Redox reactions play a crucial role in both of the most important reactions in biology: photosynthesis and cellular respiration. Photosynthesis is the reduction of carbohydrate to glucose and the oxidation of water molecules to molecular oxygen, using light energy. (Molecular oxygen is O2 — the oxygen atoms from two water molecules joined together.) In cellular respiration, glucose is oxidized to CO2 and O2 is reduced to water.

Body System



Cardiovascular system (see Chapter 9)

Heart increases in size.

There is an increased risk of thrombosis (clotting) and heart attack.


Fat is deposited in and around the heart muscle.

Varicose veins develop.


Heart valves thicken and stiffen.

There is a rise in blood pressure.


Resting and maximum heart rates decrease.


Pumping capacity declines.


Arteries decrease in diameter and lose elasticity.


Digestive system (see Chapter 11)

Teeth may be lost.

There is an increased risk of hiatal hernia, heartburn, peptic ulcers, constipation, hemorrhoids, and gallstones.


Peristalsis slows.

Rates of colon cancer and pancreatic cancer increase in the elderly.


Pouches form in the intestines (in a condition known as diverticulosis).


Liver requires more time to metabolize alcohol and drugs.


Endocrine system (see Chapter 8)

Glands shrink with age, decreasing hormone release.

Numerous homeostatic mechanisms are disrupted.


The metabolic rate decreases.

Lymphatic system (see Chapter 13)

Thymus gland shrinks with age.

Cancer risk increases.


Number and effectiveness of T lymphocytes decrease with age.

Infections are more common in elderly.


Autoimmune diseases (such as arthritis) increase.

Integumentary system (see Chapter 4)

Epidermal cells are replaced less frequently.

The skin loosens and wrinkles.


Adipose tissue in face and hands decreases.

Sensitivity to cold increases.


There is a loss and degeneration of fibers in dermis (collagen and elastin).

The body is less able to adjust to increased temperature.


Fewer blood vessels and sweat glands are present.

Hair grays and skin becomes paler.


Melanocytes decrease.

Hair thins.


Number of hair follicles decreases.


Muscular system (see Chapter 6)

Muscle tissue deteriorates and is replaced by connective tissue or fat.

The muscles lose strength.


Fewer mitochondria are in muscle cells.

Endurance decreases due to fewer mitochondria.


Neuromuscular junction degenerates.

There is a decrease in response and overall function.

Nervous system (see Chapter 7)

Brain cells die and are not replaced.

Learning, memory, and reasoning decrease.


Cerebral cortex of the brain shrinks.

Reflexes slow.


There is decreased production of neurotransmitters.

Alzheimer’s disease occurs in elderly people.


There is a loss of sensory input (smell, vision, hearing, and so on).

Reproductive system (see Chapter 14)

Females: Menopause occurs between 45 and 55 years of age and causes cessation of ovarian and uterine cycles, so eggs are no longer released, and hormones such as estrogen and progesterone are no longer produced.

Osteoporosis and wrinkling of skin occur, and there is an increased risk of heart attack.


Males: Possible decline in testosterone level after age 50; enlarged prostate gland; decreased sperm production.

Impotence and decreased sex drive occur.

Respiratory system (see Chapter 10)

Breathing capacity declines.

There is decreased efficiency of gas exchange.


Thickened capillaries, loss of elasticity in muscles of rib cage.

Risk of infections such as pneumonia increases.

Skeletal system (see Chapter 5)

Cartilage calcifies, becoming hard and brittle.

Bones become thinner and weaker.


Bone resorption occurs faster than creation of new bone (loss of bone matrix).

More time is required for bones to heal if they break.


Osteoporosis risk increases.

Urinary system (see Chapter 12)

Kidney size and function decrease.

Wastes build up in the blood.


There is decreased bladder capacity.

Incontinence occurs.


The prostate gland in men is enlarged.

The risk of kidney stones increases.


The urge to urinate is more frequent.


Urinary tract infections are more likely.