Dorsal CavityThe dorsal cavity contains the central nervous system,and consists of the cranial cavity and the vertebral orspinal cavity. The dorsal cavity is a continuous one;that is, no wall or boundary separates its subdivisions.The cranial cavity is formed by the skull and containsthe brain. The spinal cavity is formed by the backbone(spine) and contains the spinal cord. The membranesthat line these cavities and cover the brain and spinalcord are called the meninges.Ventral CavityThe ventral cavity consists of two compartments, thethoracic cavity and the abdominal cavity, which areseparated by the diaphragm. The diaphragm is a large,dome-shaped respiratory muscle. It has openings forthe esophagus and for large blood vessels, but otherwiseis a wall between the thoracic and abdominal cavities.The pelvic cavity may be considered asubdivision of the abdominal cavity (there is no wallbetween them) or as a separate cavity.Organization and General Plan of the Body 13Body Parts and AreasAnatomic positionCranialOrbitalNasalBuccalAxillaryUmbilicalVolarPatellarPlantarPoplitealFemoralInguinalIliacBrachialMammaryPectoralDeltoidCervicalParietalOccipitalLumbarSacralGlutealPerinealA BFrontalTemporalSternalAntecubitalAntebrachialPedalScapularFigure 14. Body parts and areas. The body is shown in anatomic position. (A) Anteriorview. (B) Posterior view. (Compare with Table 12.)QUESTION: Name a body area that contains a bone with a similar name. Can you nametwo more?

As you will see, the terminology presented in thischapter is used throughout the text to describe theanatomy of organs and the names of their parts. Allorgans of the body contribute to homeostasis, thehealthy state of the body that is maintained by constantand appropriate responses to internal and externalchanges. In the chapters that follow, you will finddetailed descriptions of the physiology of each organand organ system, and how the metabolism of each isnecessary to homeostasis. We will now return to aconsideration of the structural organization of thebody and to more extensive descriptions of its levels oforganization. The first of these, the chemical level, isthe subject of the next chapter.

When you hear or see the word chemistry, you maythink of test tubes and Bunsen burners in a laboratoryexperiment. However, literally everything in our physicalworld is made of chemicals. The paper used forthis book, which was once the wood of a tree, is madeof chemicals. The air we breathe is a mixture of chemicalsin the form of gases. Water, gasoline, and dietsoda are chemicals in liquid form. Our foods arechemicals, and our bodies are complex arrangementsof thousands of chemicals. Recall from Chapter 1 thatthe simplest level of organization of the body is thechemical level.This chapter covers some very basic aspects ofchemistry as they are related to living organisms, andmost especially as they are related to our understandingof the human body. So try to think of chemistrynot as a complicated science, but as the air, water, andfood we need, and every substance that is part of us.ELEMENTSAll matter, both living and not living, is made of elements,the simplest chemicals. An element is a substancemade of only one type of atom (therefore, anatom is the smallest part of an element). There are 92naturally occurring elements in the world around us.Examples are hydrogen (H), iron (Fe), oxygen (O),calcium (Ca), nitrogen (N), and carbon (C). In nature,an element does not usually exist by itself but rathercombines with the atoms of other elements to formcompounds. Examples of some compounds importantto our study of the human body are water (H2O), inwhich two atoms of hydrogen combine with one atomof oxygen; carbon dioxide (CO2), in which an atom ofcarbon combines with two atoms of oxygen; and glucose(C6H12O6), in which six carbon atoms and sixoxygen atoms combine with 12 hydrogen atoms.The elements carbon, hydrogen, oxygen, nitrogen,phosphorus, and sulfur are found in all living things. Ifcalcium is included, these seven elements make upapproximately 99% of the human body (weight).More than 20 different elements are found, in varyingamounts, in the human body. Some of these arelisted in Table 21. As you can see, each element has astandard chemical symbol. This is simply the first (andsometimes the second) letter of the elements Englishor Latin name. You should know the symbols of theelements in this table, because they are used in text-24 Some Basic ChemistryTable 21 ELEMENTS IN THEHUMAN BODYPercent ofAtomic the BodyElements Symbol Number* by WeightHydrogen H 1 9.5Carbon C 6 18.5Nitrogen N 7 3.3Oxygen O 8 65.0Fluorine F 9 TraceSodium Na 11 0.2Magnesium Mg 12 0.1Phosphorus P 15 1.0Sulfur S 16 0.3Chlorine Cl 17 0.2Potassium K 19 0.4Calcium Ca 20 1.5Manganese Mn 25 TraceIron Fe 26 TraceCobalt Co 27 TraceCopper Cu 29 TraceZinc Zn 30 TraceIodine I 53 Trace*Atomic number is the number of protons in the nucleus ofthe atom. It also represents the number of electrons thatorbit the nucleus.books, articles, hospital lab reports, and so on. Noticethat if a two-letter symbol is used for an element, thesecond letter is always lowercase, not a capital. Forexample, the symbol for calcium is Ca, not CA. CA isan abbreviation often used for cancer.ATOMSAtoms are the smallest parts of an element that havethe characteristics of that element. An atom consists ofthree major subunits or particles: protons, neutrons,and electrons (Fig. 21). A proton has a positive electricalcharge and is found in the nucleus (or center) ofthe atom. A neutron is electrically neutral (has nocharge) and is also found in the nucleus. An electronhas a negative electrical charge and is found outsidethe nucleus orbiting in what may be called an electroncloud or shell around the nucleus.The number of protons in an atom gives it itsatomic number. Protons and neutrons have mass andweight; they give an atom its atomic weight. In anelectrons (_); therefore, an atom is electrically neutral.The electrons, however, are important in thatthey may enable an atom to connect, or bond, to otheratoms to form molecules. A molecule is a combinationof atoms (usually of more than one element) thatare so tightly bound together that the moleculebehaves as a single unit.Each atom is capable of bonding in only very specificways. This capability depends on the number andthe arrangement of the electrons of the atom.Electrons orbit the nucleus of an atom in shells orenergy levels. The first, or innermost, energy levelcan contain a maximum of two electrons and is thenconsidered stable. The second energy level is stablewhen it contains its maximum of eight electrons. Theremaining energy levels, more distant from thenucleus, are also most stable when they contain eightelectrons, or a multiple of eight.A few atoms (elements) are naturally stable, oruninterested in reacting, because their outermostenergy level already contains the maximum number ofelectrons. The gases helium and neon are examples ofthese stable atoms, which do not usually react withother atoms. Most atoms are not stable, however, andtend to gain, lose, or share electrons in order to filltheir outermost shell. By doing so, an atom is capableof forming one or more chemical bonds with otheratoms. In this way, the atom becomes stable, becauseits outermost shell of electrons has been filled. It isthese reactive atoms that are of interest in our study ofanatomy and physiology.CHEMICAL BONDSA chemical bond is not a structure, but rather a forceor attraction between positive and negative electricalcharges that keeps two or more atoms closely associatedwith each other to form a molecule. By way ofcomparison, think of gravity. We know that gravity isnot a thing, but rather the force that keeps our feeton the floor and allows us to pour coffee with consistentsuccess. Molecules formed by chemical bondingoften have physical characteristics different from thoseof the atoms of the original elements. For example,the elements hydrogen and oxygen are gases, butatoms of each may chemically bond to form moleculesof water, which is a liquid.The type of chemical bonding depends upon thetendencies of the electrons of atoms involved, as youwill see. Four kinds of bonds are very important to thechemistry of the body: ionic bonds, covalent bonds,disulfide bonds, and hydrogen bonds.IONIC BONDSAn ionic bond involves the loss of one or more electronsby one atom and the gain of the electron(s) byanother atom or atoms. Refer to Fig. 22 as you readthe following.An atom of sodium (Na) has one electron in its outermostshell, and in order to become stable, it tends tolose that electron. When it does so, the sodium atomhas one more proton than it has electrons. Therefore,it now has an electrical charge (or valence) of _1 andis called a sodium ion (Na_). An atom of chlorine hasseven electrons in its outermost shell, and in order tobecome stable tends to gain one electron. When itdoes so, the chlorine atom has one more electron thanit has protons, and now has a charge (valence) of _1.It is called a chloride ion (Cl_).When an atom of sodium loses an electron to anatom of chlorine, their ions have unlike charges (positiveand negative) and are thus attracted to oneanother. The result is the formation of a molecule ofsodium chloride: NaCl, or common table salt. Thebond that holds these ions together is called an ionicbond.Some Basic Chemistry 25Second energy levelFirst energy levelProton [+]NeutronNucleusElectrons [--]Figure 21. An atom of carbon. The nucleus containssix protons and six neutrons (not all are visible here). Sixelectrons orbit the nucleus, two in the first energy leveland four in the second energy level.QUESTION: What is the electrical charge of this atom asa whole?