The Human Body: An Orientation

An Overview of Anatomy and Physiology

• two complementary branches of science provide concepts that help us understand the human body
  • Anatomy: the study of the structure of body parts and their relationships to one another
  • has a certain appeal because it is concrete
  • body structures can be seen, felt, and examined closely so there isn’t a need to imagine what they look like
  • Physiology: the study of the function of the body (how all the body parts work and carry out their life-sustaining activities
  • only explainable in terms of underlying anatomy

Topics of Anatomy

• Anatomy is a broad field with many subdivisions, each providing information to be a course in itself
• Gross/Macroscopic Anatomy: the study of large body structures visible to the naked eye
  • ex. heart, lungs, and kidneys
• the term anatomy (derived from Greek words meaning to cut apart) relates closely to gross anatomy because studies with preserved animals or their organs are dissected to be examined
• regional anatomy: all of the structures in a particular region of the body are examined at the same time
• systemic anatomy: the anatomy of the body is studied system by system
  • ex. when studying the cardiovascular system, you would examine the heart and all of the blood vessels in the body
• surface anatomy: the study of internal structures as they relate to the overlying skin surface
  • used during the identification of bulging muscles underneath a bodybuilder’s skin
• microscopic anatomy: concerns structures too small to be seen with the naked eye
  • for most of these studies, extremely thin slices of body tissues are stained and mounted on microscope slides to be examined under the microscope
• subdivisions under microscopic anatomy include
  • cytology: the study of cells in the body
  • histology: the study of body tissues
• developmental anatomy: the study of structural changes occurring throughout the life span
  • embryology: a subdivision of developmental anatomy that focuses on on the developmental changes that occur before birth
• some highly specialized branches are used for diagnosis and scientific research
  • pathological anatomy: the study of structural change by diseases
  • radiographic anatomy: the study of internal structures as visualized by x-ray imaging or specialized scanning procedures
  • molecular biology: the study of the structure of biological molecules
  • molecular biology is actually a separate branch of biology, but falls under the same umbrella as anatomy
• one of the most important ways to study anatomy is to master anatomical terminology
• other ways include observation, manipulation, and in a living person, palpitations, and auscultations
• although most observations you will make are made with the naked eye or the help of a microscope, there are many tools that we can use that won’t disrupt the body

Topics of Physiology

• physiology, just like anatomy, has many subdivisions which help consider the operation of certain structures or systems
  • renal physiology: the study of kidney function and urine production
  • neurophysiology: the study of the workings of the nervous system
  • cardiovascular physiology: the study of the operation of the heart and blood vessels
• anatomy provides us with an image of the body’s architecture while physiology reveals the body’s dynamic nature
• physiology mainly focuses on on events at the cellular or molecular level
  • due to the fact that the body’s abilities to function depends on those cells
• understanding of physiology rests on principle of physics
  • helps explain electrical currents blood pressure, and the way muscles use bones to cause body movements among other things

Complementarity of Structure and Function

• although you can study anatomy and physiology separately, they are actually inseparable because function always represents structure
• Principle of Complementarity of Structure and Function: what a structure can do depends on its specific form
  • Ex. Bones can protect and support organs because they contain hard mineral deposits

Levels of Structural Organization

• the human body has many levels of structural organization
• Chemical level: the simplest level of organization
  • at this level, atoms combine to form molecules such as water and proteins
  • molecules then combine to form organelles, the basic component of microscopic cells
• Cellular level: the cell is the smallest unit of living things
  • All cells have common function, but may vary in size and shape, which reflects their unique functions in the body
• Simpler organisms are composed of a single cell, but for more complex organisms like humans, the hierarchy goes on
• Tissue level: tissues are groups of similar cells that have a common function
  • four basic types of tissue include
  • epithelium: lines body cavities and covers the body surfaces
  • muscle: provides movement
  • connective: supports and protects body organs
  • nervous: provides a means of rapid internal communication by transmitting electrical impulses
  • each tissue plays a characteristic role in the body
• Organ level: extremely complex functions become possible
  • An organ is a discrete structure composed of at least two tissue types that performs a specific function for the body
  • Ex. Function of the Stomach
  • the lining of the stomach is an epithelium that produces digestive juices
  • the majority of the wall is muscle which helps churn and mix stomach contents
  • the connective tissue reinforces the soft muscular walls
  • the nerve fibers increase digestive activity by stimulating the muscle to contract more vigorously and the glands to secrete more digestive juices
• Organ System: Organs that work together to establish a common purpose
  • Ex. The heart and blood vessels of the cardiovascular system make sure that your blood is flowing to every part of the body to deliver oxygen and nutrients to all cells
• Organism level: the highest level of organization
  • represents the sum total of all structural levels working together to promote life

Maintaining Life

Necessary Life Functions

• What does this highly organized body do?
• Like all other complex organisms, human beings
  • Maintain their boundaries
  • Move
  • Respond to environmental changes
  • Take in and digest nutrients
  • Carry out metabolism
  • Dispose of wastes
  • Reproduce themselves
  • Grow
• The multicellular state and parceling out of vital body functions to several different organ systems causes an interdependence of all body cells
• Organ systems do not work in isolation
  • They work cooperatively to promote the good health of the body

Maintaining Boundaries

• Every organism must maintain their boundaries so that its internal environment remains separate from the external environment surrounding it
• In single-celled organisms, the boundary is a limiting membrane that encloses its contents and admits necessary substances while denying entry of harmful substances
• Similarly, our cells also have such membranes
• To protect our entire body, our body is enclosed by the integumentary system
  • The integumentary system protects our organs from drying out as well

Movement

• Responsiveness/Irritability: the ability to sense changes (stimuli) in the environment and then respond to them
  • Ex. If you cut your hand on broken glass, withdrawal reflex occurs
  • you move your hand away from the painful stimulus
  • It is not necessary to think about it, it will just happen
  • When CO2 in your blood raises to dangerous levels, chemical sensors respond by sending messages to brain centers controlling respiration to speed up your breathing rate
• Because of how irritable nerve cells are, they are most associated with responsiveness
  • However, all cells are irritable to some extent

Digestion

• Digestion: the breaking down of ingested foodstuffs to simple molecules that can be absorbed into the blood
• This blood is distributed throughout all body cells by the cardiovascular system
• In single-celled organisms, the cell itself is the “digestion factory” while the complex organism, the digestive system performs this for the whole body

Metabolism

• Metabolism: a broad term that includes all reactions that occur within body cells
• This includes
  • Catabolism: breaking down substances into their simpler building blocks
  • Anabolism: synthesizing more complex cellular structures from simpler substances
  • Using nutrients and oxygen to produce ATP
• Metabolism depends on the digestive system as well as the respiratory system to make nutrients and oxygen available to the blood
• It also depends on the cardiovascular system to circulate the nutrient-rich blood
• It is regulated by hormones secreted by the endocrine system

Excretion

• Excretion: the process of removing excreta (wastes) from the body
• For the proper functioning of the body, we must get rid of the nonuseful products created by digestion and metabolism
• Many organs play a part in excretion
  • The digestive system rids the body of indigestible food residues through feces
  • The urinary system disposes of nitrogen-containing metabolic wastes (urea) in urine
  • CO2 is disposed through the body by the respiratory system

Reproduction

• Reproduction: can occur at the cellular or organismic level
• In the cellular level, an individual cell goes through mitosis, which is the splitting of one cells into two
• In a human being, there is a reproductive system, which is tasked in making a whole new person
  • When sperm unites with an egg, it creates a fertilized egg, which will then develop into a baby in the mother’s body
• Because males produce sperm and females produce ova (eggs), the reproductive systems for each respective sex are different
  • The female reproductive system also is the site for the fertilization of eggs by sperm, and the place where the fertilized egg will become a fetus

Growth

• Growth: increase in size of a body part or the organism
• Usually accomplished by increasing the number of cells
  • However, individual cells also increase in size while they aren’t dividing
• For true growth to occur, constructive activities must be occurring at a faster rate than destructive activities

Survival Needs

• Ultimate goal of all body systems: maintaining life
• Life is extremely fragile and requires several factors to be present
• Survival needs include
  • Nutrients (food)
  • Oxygen
  • Water
  • Appropriate temperature and atmospheric pressure

Nutrients

• Taken in via the diet
• They contain chemical substances used for energy and cell building
• Majority of plant-derived foods contain carbohydrates, vitamins, and minerals while animal-derived foods contain proteins and fats
• Carbohydrates are the major energy source for body cells
• Proteins, and to a certain extent, fats, are essential for building cell structures
• Fats also cushion organs, which provides insulating layers and provides a source of energy-rich fuel
• Certain vitamins and minerals are required for chemical reactions to occur
  • Calcium is used to help make bones hard and is required for blood clotting

Oxygen

• All nutrients are useless without oxygen being available
  • Human cells can only survive a few minutes without oxygen because of the chemical reactions that release energy from foods are oxidative reactions
  • About 20% of the air we breathe contains oxygen
  • This oxygen is made available to us by the efforts of the respiratory and cardiovascular systems respectively

Water

• Water accounts for 60-80% of our body weight and is the single most abundant chemical substance in the body
  • It provides the necessary, watery environment needed for chemical reactions and the fluid base for secretions for body secretions and and excretions
  • Water is obtained through ingested food and liquids and is lost through the body by evaporation through the lungs and skin, as well as body excretions

Maintaining Normal Body Temperatures

• If chemical reactions are to take place in a normal fashion, maintaining healthy body temperatures is crucial
  • As temperatures drop below 37 degrees Celsius (98 degrees Fahrenheit), metabolic functions slow down until they stop
  • As temperatures go above 37 degrees Celsius, chemical reactions occur at a super fast pace, which results in body proteins losing their characteristic shapes, which stops their functions
• Both of these extremes cause death
• Most body heat is generated through the activities of the muscular system

Atmospheric Pressure

• Atmospheric Pressure: the force that air exerts on the surface of the body
  • Breathing and gas exchange in the lungs depends on the correct atmospheric pressure
  • In higher altitudes, where atmospheric pressure is lower, and the air is thinner, gas exchange is harder to perform and is inadequate to support the cellular metabolism
• The presence of these survival needs is simply not enough. These things must be present in the appropriate amounts necessary for survival
  • Ex. Oxygen is essential, but too much of it is toxic for body cells
• Also, while the needs listed above are considered the most crucial, it does not encompass all of the body’s needs
  • Ex. We can live without gravity, if we must, but the quality of life will suffer

Homeostasis

• Walter Cannon, an American physiologist, coined the term homeostasis
  • Cannon spoke about the “Wisdom of the Body”
• Homeostasis: A state of body equilibrium or stable internal environment of the body
• Maintaining homeostasis is more complicated than it seems
• Every organ plays a specific role in maintaining the internal environment of the body
  • Adequate blood levels of vital nutrients must be present at all times
  • Heart activity and blood pressure must be constantly monitored to see that blood is propelled to all body tissues
  • Wastes are not allowed to accumulate
  • Body temperatures must be precisely controlled

Homeostatic Control Mechanisms

• Communications within the body are necessary for homeostasis
  • This is achieved by the nervous and endocrine systems
• Variable: the factor or event being regulated
• All homeostatic control mechanisms have at least three interdependent components
  • The Receptor: the first component
  • some type of sensor that monitors the environment and responds to changes (stimuli) by sending the information to the second component
  • The Control Center: the second component
  • The information from the receptor flows through a pathway called the afferent pathway
  • Determines the set point, analyzes the input it receives and then determines the appropriate response or course of action
  • The Effector: the third component
  • Provides the means for the control center’s response output to the stimulus
  • Information flows from the control center to the effector through the efferent pathway
  • The results of the response then feed back to the influence of the stimulus, either by depressing it or enhancing it

Negative Feedback Mechanisms

• Most homeostatic control systems are negative feedback mechanisms
  • The output shuts off the original stimulus or reduces its intensity
• The mechanism causes the variable to change in the opposite direction of the initial change, returning to its ideal value
• Example of a nonbiological negative feedback mechanism: home heating system connected to a temperature rising thermostat
  • The thermostat houses the receptor and the control center
  • If the thermostat was set to 20°C, the heating system (effector) is triggered ON when the house temperature drops below that setting
  • As the furnace produces heat and warms up the air, the temperature rises and and when it reaches 20°C or slightly higher, the thermostat turns OFF
• Our body’s “thermostat” is located in a part of your brain called the hypothalamus
  • It operates in a similar manner
• Body temperature regulation is only one of the many ways the nervous system
• Another neural control mechanism is the withdrawal reflex
  • When the hand jerks away from a painful stimulus
• The endocrine system is also very important in maintaining homeostasis
• Good example of hormonal negative feedback mechanism is the control of blood glucose levels by pancreatic hormones
• To carry out normal metabolism, body cells need a continuous supply of glucose or ATP
  • Blood sugar levels are usually at 90 milligrams of glucose per 100 milliliters of blood
  • To accommodate the rising glucose levels, your pancreas releases insulin
• Glucagon, another pancreatic hormone, has the opposite effect
  • It release is triggered after blood sugar levels decline after a certain point
  • Glucagon targets the liver, causing it to release its glucose reserves from glycogen from the blood
• All negative feedback mechanisms have the same goal: preventing sudden changes from within the body
• There are many more kinds of negative feedback mechanisms in the body than just body temperature and blood sugar

Positive Feedback Mechanisms

• Positive feedback mechanisms: the result or response enhances the original stimulus so that the activity is accelerated
• This mechanism is considered “positive” because the change that occurs will happen in the same direction as the initial disturbance
• Compared to negative feedback mechanisms, positive feedback mechanisms control infrequent events that do not require continuous adjustments
  • They set off a series of events that may be self-perpetuating and once initiated, have an amplified/waterfall effect
  • These mechanisms also referred to cascades because of these characteristics
• Positive feedback mechanisms are likely to race out of control, so that is why they are rarely used to promote the moment-to-moment wellbeing of the body
• Two most common types of positive feedback mechanisms
  • Enhancement of labor contractions
  • Oxytocin causes the contractions to become more frequent as the labor goes on, and the birth of the baby ends the stimulus for oxytocin release
  • Blood clotting
  • Normal response to a break in blood vessels
  • Once the injury occurs, platelets immediately start to cling to the site of injury
  • This will repeat until the final formation of a clot

Homeostatic Imbalance

• Homeostasis is regarded the most important that most diseases are the result of its disturbance
• Homeostatic Imbalance: fluctuation of homeostatic functions and the inability to maintain equilibrium
• Our body’s systems become less efficient and our internal environment becomes less stable
  • This increases our chances of illness and produces the change we associate with aging
• Another type of homeostatic imbalance occurs when the negative feedback mechanisms are overwhelmed and destructive positive feedback mechanisms take over
  • Ex. some instances of heart failure

The Language of Anatomy

Anatomical Position and Directional Terms

• We need an initial reference point to study body parts accurately
• Anatomical Position: anatomical reference point is a standard body position
  • The body is erect with the feet only slightly apart
  • The palms face forward and the thumbs face away from the body
• Most of the directional terms are referred to as if they were in this position, regardless of its actual position
• The terms “right” and “left” are to the sides of the cadavers being used, not the eyes of the observer
• Directional terms: allows us to explain where one body structure is in relation to the other
  • Ex. the relationship between the ears and nose informally by stating “the ears are located on each side of the head to the right and left of the nose” and anatomically (using anatomical terminology) “the ears are lateral to the nose”

Orientation and Directional Terms Table:

Regional Terms

• Two fundamental divisions of our body are its axial and appendicular parts
  • Axial part: makes up the main axis of our body, including the head, neck, and trunk
  • Appendicular part: consists of appendages, or limbs, which are attached to the body’s axis
• Regional terms: used to designate specific areas within the major body divisions

Commonly used regional terms

Anatomical Variability

• Even though we use common directional and regional terms to refer to all human bodies, we know that we humans differ in our external anatomy
  • This same variability also occurs in our internal environment
• Well over 90% of all structures present in any human body matches the textbook descriptions
  • Extreme anatomical variations are seldom seen because they are incompatible with life

Body Planes and Sections

• The body is often sectioned off into planes for anatomical studies
• There are three most commonly used planes
  • Sagittal
  • Frontal
  • Transverse
• A section that is cut by a plane is named for that nearby plane
  • Ex. a sagittal plane produces a sagittal section
• Sagittal Plane: vertical plane that divides the body into right and left parts
  • Midsagittal Plane: also known as a median plane, a sagittal plane that lies in the middle
  • Parasagittal Plane: all other sagittal planes offset from the midline
• Frontal Plane: vertical planes that divides the body into anterior and posterior parts
  • Also called the coronal plane
• Transverse Plane