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Illustration showing different internal systems (Photo by De Agostini via Getty Images/De Agostini via Getty Images)

CHAPTER ONE: THE CHEMICAL BASIS OF HUMAN PHYSIOLOGY

Human physiology is the study of how the human body works. Understanding the physiological basis of life requires a thorough grasp of its chemistry – the interplay and interconnection between the different states of matter and the hierarchical organization of the human body.

Such organization forms the fundamental blocks of the human body – from atoms, molecules, organelles, cells, and tissues, to organs, systems, and the entire organism. It’s noteworthy that life materializes at the level of a cell. The cell thus becomes the basic unit of life.

Atoms are neutral: they become ions when they either lose or gain electrons. Upon combination through chemical bonds, atoms form molecules. Through an array of atomic and molecular interactions, organelles emerge. They are then specially packaged and enclosed in a membrane to form cells. 

Cells are the fundamental units of life: they are capable of sustaining life, like protozoa. The human body has trillions of cells specially organized at various hierarchical levels to perform distinct functions. 

ATOMS AND MOLECULES

Atoms are key to the hierrachical organization of human physiology.
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Atoms like carbon (C), hydrogen (H), and oxygen (O) are composed of particles – protons, neutrons, and electrons. Protons and neutrons are part of the atomic nucleus: electrons are within the orbits. Protons carry a positive charge; electrons have a negative charge, and neutrons carry no charge. 

Some orbit the outer shells of the electrons habited within the orbits: they are called outer-shell electrons or valence electrons. They determine the chemical properties of atoms, including the nature of the bonds formed. 

Atoms having less than four outer-shell electrons tend to lose them in chemical reactions to become cations (positively charged ions). Those having more than four tend to gain more electrons to become anions (negatively charged ions).

The aim of losing, gaining, or sharing outer-shell electrons is to stabilize the atoms/ions. By convention, atoms are stable if their outer-shell orbits eight electrons. We term it the octet rule. 

CHEMICAL BONDS

Chemical bonds are integral to formation of compounds that constitute the fundamentals of the chemical basis of physiology
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Three classical chemical bonds exist between atoms – covalent, ionic, and hydrogen bonds.

Covalent bonds involve sharing of electrons among atoms. They are strong bonds. Electron sharing can be equal to forming non-polar bonds or unequal to forming polar bonds.

Ionic bonds involve the complete transfer of electrons. The atoms involved in the process become ions – either cations or anions. These bonds are relatively weak. The compounds formed dissociate in water to form electrolytes in solution. 

Hydrogen bonds exist between hydrogen atoms and either oxygen or nitrogen atoms. They seem to be the weakest among the three bonds: however, we cannot underestimate their importance. 

When atoms or a group of atoms combine, they form molecules. These molecules can either be organic if they contain carbon or inorganic if they do not contain carbon. 

INORGANIC MOLECULES: WATER

Water is the most versatile molecule in the field of physiology
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Among the inorganic molecules, the most abundant and crucial is water. It accounts for about 50 to 70% of the human body. An average 70-kg man has about 42 liters of water.

Water exhibits exquisite characteristics that render it extremely important in the physiology of life – solvency, cohesion, thermostability, and reactivity. 

Water dissolves polar molecules. It organizes non-polar molecules – the universal solvent. 

Hydrogen bonds among water molecules are responsible for the cohesiveness of water. This cohesiveness creates a force on the surface of the water molecules at the fluid-gas interface that gives water that elasticity. We term it surface tension. 

Surface tension helps insects float but it collapses the respiratory air sacs. Its reduction is crucial in respiratory physiology
Credit: Getty images

It is so strong that insects can crawl on the water without sinking. It has fascinating physiological importance in the lungs, as we shall highlight in subsequent chapters. Water interacts with other substances to exhibit adhesive properties. Along the walls of test tubes or syringes, this interaction is mediated by oxygen or hydrogen atoms. The interaction is responsible for the inward meniscus in either test tubes or syringes that the fluid a horizontal half-moon shape. The adhesive forces formed are always greater than the corresponding cohesive forces.

The cohesive properties of water enable its molecules to cling together
Credit: Getty images

Water has a high specific heat capacity. It implies that water absorbs tremendous amounts of heat before its temperature changes by a substantial difference. The temperature of the water is thus stable across a wide array of temperature variations within the environment. This characteristic is vital to the physiology of life for two reasons: (1) metabolic processes occur within the body and generate vast quantities of heat with modest changes in the core body temperature. We measure the energy in these processes in terms of calories. We can define a calorie as the amount of heat energy required to raise the temperature of 1g of water by 1 degree Celsius. (2) When water (sweat) vaporizes, it loses heat (specific heat of vaporization): this confers body cooling. 

Water is either a reactant or product in many metabolic reactions. In the process, different compounds are synthesized or broken down. Hydrolysis occurs if water is used to break a chemical bond. Dehydrogenation reactions involve removing a water molecule to form compounds. In the former, energy is liberated; in the latter, energy is consumed.

A variety of nuts and seeds arranged on a wooden table
Credit: Getty Images

4% of the human body weight is made up of minerals. Minerals are inorganic elements from animals, plants, and soil. Calcium and phosphorous account for 75% of the minerals in the body. 

Minerals are either macro or micro (trace) elements. Macro-minerals include sodium, potassium, chloride, magnesium, iron, calcium, and phosphorous. 

Micro-elements include zinc, boron, selenium, copper, cobalt, et cetera. Trace elements are needed in small quantities. Different foods provide various minerals as illustrated below.

MINERALSDIETARY SOURCES
Ironliver, meat, beans, nuts, dried fruits, whole grains, brown rice, dark-green leafy vegetables
CalciumMilk, cabbage, okra, broccoli, nuts, bony fish, green leafy vegetables, dairy, cheese
IodineWhole grains, cereals, shellfish, sea fish
PotassiumTurkey, chicken, beef, shellfish, fish, milk, nuts and seeds, vegetables, fruits like bananas
SodiumSeaweed, fruits, vegetables, cheese, meat
MagnesiumSpinach, whole grain bread, fish, nuts, meat, dairy foods
ZincPumpkin, Sunflower, poppy, cashew nuts, pecan, wheat germ, dairy, meat, shellfish
PhosphorousRice, oats, whole grain bread, poultry, fish, red meat, dairy foods
SeleniumEggs, meat, fish, whole grain bread, nuts
ManganeseGreen vegetables like peas, nuts, tea
CopperNuts, Offal, shellfish
CobaltBroccoli, spinach, oats, nuts, fish
ChromiumSpices, lentils, oats, whole grains, meat
BoronNuts, fruits, green vegetables

ACID-BASE BALANCE

Blood pH is one the most tightly controlled parameters in human physiology
pH acidic basic alkaline scale gauge measuring acidity or alkalinity.
Credit: Getty images

A tight balance between body acids and bases must occur to ensure optimal physiological function. We measure such balance using the pH scale. The pH of any solution is the negative log of the concentration of hydrogen ions in that solution. Acids donate protons (H+), whereas bases accept protons. Water is key to the normal integrity of acid-base balance by providing either hydrogen or hydroxyl ions.

The relatively constant pH of body fluids is upon the normal integrity of the buffering systems. A buffer is a chemical substance that resists any drastic change in the pH of the solution. It accomplishes this by affecting the concentrations of the hydrogen ions within that solution. It can be a weak acid that absorbs hydroxyl ions or a weak base that absorbs hydrogen ions. Buffer systems in the human body include hemoglobin, phosphate, and bicarbonate ions among others. 

Blood pH is maintained between 7.35 and 7.45. The respiratory and renal systems are the primary systems involved in maintaining an optimal acid-base balance.

ORGANIC MOLECULES

Foods are crucial to the optimal physiology of life
A large group of food. The assortment includes fruits, vegetables, meat, fish and dairy products. The food is mostly sorted by type. The dairy products are in the center, while the rest of the food is circled around the dairy products.
Credit: Getty images

Among the organic molecules, carbohydrates, proteins, lipids, and nucleic acids are of paramount importance to the physiology of life. The ability of carbon to share all four outer-shell electrons gives it versatility in how it interacts with the body to be part of almost every molecule that contributes to the functioning of the human body.

Carbohydrates are the principal sources and storage of energy for the body. They supply the carbon skeleton for cellular components including the structures that make up the cell membrane. They exist as monosaccharides, disaccharides, and polysaccharides.

Monosaccharides are also known as simple sugars. They include glucose, fructose, and galactose. Glucose is the most common source of energy in the body. Fructose is found in fruits. And galactose is part of milk sugar. 

When two simple sugars combine, they form disaccharides. Examples include sucrose, lactose, and maltose. Sucrose is also known as table sugar – a combination of glucose and fructose. Glucose and galactose from the milk sugar, lactose. And two glucose molecules form the grain sugar, maltose. 

Three or more simple sugars form polysaccharides. Most of them are polymers of glucose – glycogen, cellulose, and starch. 

Like carbohydrates, lipids also contain carbon, hydrogen, and oxygen; however, they don’t have the 2:1 H:O ratio of carbohydrates. They are non-polar. Lipids of physiological importance include fatty acids, triglycerides, phospholipids, and steroids.

Fatty acids can have between 4 and 24 carbon atoms. The fatty acid has both a methyl (CH3) group and a carboxyl (-COOH) group at either end. They are rich sources of energy. They exist in two forms – saturated and unsaturated fatty acids. 

The storage form of lipids is triglyceride. It forms from one glycerol and three fatty acids. It is the most abundant lipid in our bodies and diet. Its energy generated is twice that of carbohydrates.

Instead of one glycerol and three fatty acids, phospholipids replace the three fatty acids with a phosphate group and a nitrogen compound. Phospholipids have both a polar head and a non-polar tail. They are thus called amphiphilic molecules. They are the most abundant lipids in the cell membranes. 

Another form of lipids is steroids. They have 17 of their carbon atoms arranged in four rings. Cholesterol is the most abundant steroid in the human body. It is an integral component of cell membranes and a precursor of several vital steroidal molecules – hormones like cortisol, estrogen, and testosterone among others.

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Proteins are the most versatile and complex organic molecules in the body. They contain carbon, hydrogen, oxygen, nitrogen, and sulfur atoms. Their building blocks are called amino acids. Proteins exist as primary, secondary, tertiary, or quaternary structures depending on the degree of folding. Primary proteins assume a linear shape that amino acid residues that are joined together by covalent peptide bonds. When these linear sequences join to form alpha or beta sheets by hydrogen bonds, they constitute the secondary structure proteins. Further interaction between the functional groups to form three-dimensional structures is the hallmark of tertiary proteins. When several polypeptides are joined by hydrogen and other bond linkages, they form quaternary proteins. 

Proteins can take on structural, catalytic, transport, contractile, immunological, or regulatory functions within the body. 

ENERGY AND THERMODYNAMICS

Metabolism
Metabolism. Anabolic and catabolic reactions, metabolic (biochemical) reactions, growth, energy production. (Photo by: Encyclopedia Britannica/Universal Images Group via Getty Images)

Metabolic processes consume and generate energy. Simply put, energy is the capacity to do work. Such work may involve movement and locomotion. 

Energy exists in two forms – kinetic and potential energy. The two forms of energy can be converted from one form to another. 

Kinetic energy is the energy of motion whereas potential energy is the energy stored. The conversion of energy to work underlies the principles of thermodynamics.

The first law of thermodynamics states that energy can be converted from one form to another but cannot be created or destroyed. Within the universe, energy is constant. The principle underlies the transfer and storage of energy in various forms.

However, during metabolism, energy can be lost as energy. The second law of thermodynamics asserts that in every transfer of energy: some become heat and cease to do useful work. It also notes that natural spontaneous processes move from a state of order to a state of disorder. We refer to this state of disorder as entropy (randomness).

A person who consumes about 100 Kcal from food may lose 60% of it as heat and 40% may be used for body movements. The energy from food constitutes potential energy which then generates 40% kinetic energy: in other words, the body is only 40% efficient.

It is noteworthy the efficiency of the body further plummets to about 30%, which is used for muscle contraction. The 70% is lost as heat.

There are various chemical reactions in the body that primarily store, release, or transfer energy. Their sum constitutes metabolism. Metabolism may comprise the breakdown of complex molecules into simple structures (catabolism) or the building of complex molecules from simple ones (anabolism). All these processes may require energy (endergonic) or release energy (exergonic).


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IAmDrSsekandi

MBChB (MUK), Graduate Fellow, Department of Physiology, Makerere University Founder and Content Creator Peer reviewer, Associate Editor

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