Life’s Balancing Act


Here is a concise definition of homeostasis:  Homeostasis is a dynamic state of internal equilibrium that all living things must maintain to stay alive and healthy.


In order to stay healthy our bodies need to maintain certain ranges of such things as oxygen, glucose, blood pressure, temperature, electrolytes, water, etc.  These are all part of the environment in which our cells live, and we need to keep a healthy balanced environment for those living cells.  That’s homeostasis.


Example homeostatic ranges:

  • Blood oxygen:  95-100% hemoglobin saturation is the homeostatic range.
    • Our cells need a certain amount of available oxygen at all times.  Oxygen saturations lower then 90% sturation can cause dizzyness, rapid breathing (as the body tries to maintain oxygen homeostasis), and if it gets to low we can pass out and ultimately die.  Asthma, COPD, and other respiratory disorders cause problems with oxygen homeostasis.  It’s less likely, but possible to become hyperoxic (too much oxygen in the body).  If this happens the oxydative molecules produced can damage many molecules in the cells including DNA and proteins which can cause multiple problems.
  • Body temperature:  Around 97 to 99 F (36.4 to 37.6 C) is the normal homeostatic range for oral temperature.
    • If we go above or below that level the enzymes in our cells decrease their function and this can cause widespread organ failure if it goes to far.  Oral temperatures above 104 F (40 C) is worse because not only do the enzymes stop functioning but that can be damaged.  Reviving someone with deadly hypothermia is more successful that reviving someone with deadly hyperthermia.
  • Blood sugar:  90-100 mg/dL is the homeostatic range
    • If blood sugar gets to high for long periods long term problems with small blood vessels develop (blindness, kidney failure, neuropathies, etc) and if it gets to low we can get shaky and lethargic and may pass out…  further drops can be deadly.  Diabetes mellitus and hypoglycemia are disorders involving glucose homeostasis.

After reading this you should understand that homeostasis is very important.  You should also get that there are several factors to the bodies internal environment that have to be maintained within a certain homeostatic range in order to stay healthy and alive.  It’s a balancing act that our bodies maintain 24 hours a day for our entire lives.  Our bodies have multiple ways to maintain homeostasis depending on the environmental variable, but pretty much all of them have the same basic mechanism called negative feedback…  Let’s cover that next.


Feedback loops



We need them for the “chemistry of life”

The last time you exercised you used lots of oxygen to liberate energy from food molecules like sugars and lipids.  You used that energy to make ATP that was then used to power your muscle contractions.  Every step in this required the help of our little molecular friends the enzymes (remember cellular respiration from your basic Biology class?  Cells get their energy needs met through a complicated series of enzymatic pathways–  also known as metabolic pathways).

Enzymes are biological catalysts (usually made of protein) that are usually very specific to the particular chemical reaction that they speed up or allow to happen.

What’s a catalyst you say!!!   A catalyst is a substance that speeds up a chemical reaction by lowering the activation energy of that reaction– but these catalysts don’t get used up in the reaction…  so, they can do it again and again.

What’s is activation energy you say!!!   I say go look it up!!!!  Let’s just say that enzymes allow many needed chemical reactions to occur inside of us and also allow our bodies to control those reactions so that they happen when we need them to.


The Major Parts of an Enzyme: Active Site and Allosteric Site

  • Every enzyme has what is called an active site.  This is the part of the enzyme that interacts directly with the chemicals involved in the reaction.  The active site has a specific 3 dimensional structure that allows for this interaction  (Another example of anatomy and physiology).
  • Many enzymes also have an allosteric site.  This is a site other than the active site that certain chemicals can bind to and effect the activity of the enzyme (basically, when the chemical binds to the allosteric site the shape of the active site changes leading to an increase or decrease in the activity of the enzyme).
    • Allosteric inhibition:  a chemical has bound to the allosteric site and caused the enzyme to become less effective.
    • Allosteric activation:  a chemical has bound the the allosteric site and caused the enzyme to become more effective.

What is a Metabolic Pathway?

A metabolic pathway is basically a series of enzyme mediated reactions that lead to the production of some important molecule.  Example:  Cellular respiration involves several enzymes in a step by step pathway that ultimately takes the energy from a food molecule and uses it to generate ATP molecules.


Why do We Care?

We care about enzymes for a few fundamental reasons– beyond the fact that we can’t live without them.

  • Genetic Diseases:  One reason we care is that they are the source of several genetic diseases.  If the gene for a specific enzyme is mutated so that we don’t get the right amount of the enzyme made we will have a disease (because too little or too much of some product will be made).
  • Drugs:  Another reason we care is that we can use drugs that interfere in various ways with enzyme function.  One example drug is ibuprofen, an anti-inflammatory drug or NSAID.  When you take an NSAID it helps to decrease inflammation by inhibiting the activity of an enzyme who’s job is to help produce signaling molecules involved with in inflammation (specifically they inhibit cyclooxygenase enzymes that catalyze one of the reactions needed to make prostaglandins).  Other drugs that interfere with enzymes include some drugs for blood pressure and some antibiotics.
  • Poisons:  Many poisons also interfere with enzyme function.  Cyanide for example interferes with one of the enzymes involved in cellular respiration.  Too much cyanide and our body can’t make the ATP needed to stay alive.


Enzyme Images and Explanations

(click on images for larger version)

enzyme diagram with words

enzyme diagram with numbers

Label explanations (for the above image with numbers):

  1.   Enzyme and substrate:  The substrate is the molecule(s) that will undergo some kind of chemical reaction.  (in the above example there is only one molecule involved as a substrate, but there may be more substrates depending on the enzyme).
  2. Enzyme substrate complex:  In this part of the image the substrate has bound to the active site of the enzyme and the chemical reaction is starting to happen.
  3. Enzyme and products: In this part of the image the reaction is over, the substrate has undergone a chemical reaction and become product(s).  Note that the enzyme is unchanged in the reaction… another substrate molecule can now bind and the process can repeat again and again (until we run out of substrate).
  4. Active site:  This is the site on the enzyme that interacts with substrate.  The active site has a specific shape that fits the substrate just right.  In fact it appears that the active site changes shape as the substrate binds…  it also changes shape as it catalyzes the reaction.
  5. Allosteric site:  This is a site different from the active site where other chemicals can interact with the enzyme and alter its activity (inhibit the enzyme or activate it).

******Both the active site and the allosteric site are potential targets for drug therapies!!

Basic functions and needs

Major Functions of Humans 

(things we need to do in order to live)

  • Organization:  The body must be organized…  a fetus that starts to develop without good organization is not likely to live long.
  • Metabolism:  This refers to all of the chemical reactions that happen inside the body, especially those involving energy usage.  If your body can’t do the chemistry you will not survive.
  • Responsiveness:  If your body can not react effectively to changes in the external or internal environment you will die.
  • Movement:  If you can’t move at all you can’t survive (your outside body needs to move so you can find food and not get hit by cars and things inside your body also need to move like your blood through vessels, urine out of body, food along the gut, etc.).
  • Development:  In order to survive humans need to be able to change from one life stage to the next (a fetus that does not develop a brain or heart will not survive).


Basic Requirements of Human Life

(environmental needs for human life)

  • Pressure:  The human body requires a certain range of atmospheric pressure in order to survive.  In a zero pressure environment, such as outer space you would stay awake for only a few seconds and would die within minutes.  You would not explode or quickly freeze however…  those are myths.
  • Levels of respiratory gasses:  There must be adequate levels of oxygen in the air we breathe (normal air is around 21% oxygen…   anything under 10% oxygen and you can easily get into trouble), and there can’t be too much carbon dioxide in the air we breathe (normal air CO2 is less than 1%…  anything above 6% can be toxic).
  • Temperature:  Humans have adapted to a wide range of temperatures here on Earth, but we do have limits.
  • Nutrients:  Humans need available nutrients.  We can live for a time without food, but only about month or a little more.  We can only live a matter of days without water.

Body Systems

Human Body Systems in Brief

The human body has several organ systems.  One thing to understand is that we divide the body into organ systems mostly to make it easier to study.  In reality all of the body’s parts work together.  However, if we tried to study the human body as a whole we would get lost pretty quickly because it is complicated, so we learn piece by piece.  Here is a list of the body’s organ systems including a few organs from each and the major functions of each system:

  • Integumentary system:
    • This is the skin and its associated structures (hair, nails, glands, receptors, etc.)
    • Major jobs include:  control body temperature (by deciding how much heat to let go of), protection from infection (mostly as a barrier defense) and from trauma, also helps produce vitamin D.
  • Skeletal system
    • Consists of bones (clavicle, humerus, femur, etc.), joints, and associated tissues (like ligaments and cartilage)
    • Major jobs include:  protection of vital organs (skull protects brain for example), blood cell production (in red bone marrow), mineral storage (mostly calcium and phosphorus), energy storage (yellow bone marrow is fat tissue), pH homeostasis (phosphate acts as a pH buffer)
  • Muscular system
    • Consists of skeletal muscles (pectoral muscles, biceps brachii, gluteus maximis, etc).  Also consists of three types of muscle tissue (skeletal, smooth, and cardiac muscle).
    • Major jobs include:  All body movements (includes outward movements like walking and dancing and also movements inside of the body such as pumping blood and moving food along the digestive tract), produce body heat, maintain posture (try to sit or stand up straight without contracting muscles….  actually don’t try it— you will fall over).
  • Nervous system
    • Includes structures like the brain, the spinal cord, and nerves.
    • Major jobs include “command and control” or regulation and coordination of the body (helps to keep many functions of the body stable)…  also thinking, memory, emotions, etc.
  • Sensory system
    • Includes special senses like sight, hearing, equilibrium, smell, and taste—  as well as general senses like touch, pressure, stretching, and pain.  Parts include ears, cochlea (hearing), eyes, retina (sight), lamelar corpuscles (pressure), etc.
    • Major job is to give the brain information about what is happening in the outside world as well as what is happening inside the body (Examples:  Your eyes are sending visual information to your brain now, and chemical receptors in your major arteries are detecting levels of oxygen and carbon dioxide to help decide how much you need to breathe).
  • Endocrine system
    • Includes all endocrine glands (glands that make hormones) such as the pituitary, pancreas, thyroid, etc.
    • Like the nervous system this system’s major job is “command and control” or regulation and coordination; the endocrine system  just acts more slowly, it is interested in maintaining equilibrium of such things as blood sugar, blood pressure, water and electrolyte balance, as well as being important for some major functions like growth and development, reproduction, and metabolism (rate of chemical reactions in the body, especially energy usage).
  • Cardiovascular system
    • Includes blood, the heart, and the blood vessels.
    • Major job is transportation.  The blood is pushed by the heart through all of the blood vessels.  In doing this all of the body’s tissues receive important nutrients and signals that are carried in the blood and waste products are transported away from the tissues.
  • Lymphatic system
    • Includes lymphatic vessels and lymphatic trunks as well as lymph nodes, the spleen, thymus, and red bone marrow (shared between lymphatic system and skeletal system).
    • Major jobs include taking away excess tissue fluid, filtering that fluid, and body defense (the lymphatic system houses the majority of the immune system’s cells).
  • Immune system/body defense
    • Includes defenses such as barrier defenses, chemical defenses, and defenses mediated by white blood cells (lymphocytes, macrophages, neutrophils, etc.) that defend the body from infection and rogue cells (cancers).
    • Major job of preventing and eliminating infection with foreign organisms as well as killing cells that are becoming cancer.  This is performed through non-specific and specific defenses.  Non-specific defenses target all or broad categories of potentially infections organisms.  Specific defenses (immunity proper) is mediated by certain white blood cells capable of recognizing organisms very specifically (Example:  Your body has B-lyphocytes that are capable of producing antibodies against not just viruses and not just influenza viruses but specifically against the influenza A virus).
  • Respiratory system
    • Includes organs such as the lungs, bronchi, trachea, larynx, etc.
    • Major job is gas exchange with outside air.  In performing that job the respiratory system also helps to maintain pH balance as well as oxygen and carbon dioxide levels.
  • Digestive system
    • Includes organs like the esophagus, stomach, small intestine, liver, pancreas, large intestine, etc.
    • Major jobs of ingestion, digestion, absorption of nutrients, and elimination of fecal waste (everybody poops).
  • Urinary system
    • Includes organs such as the kidneys, ureters, urinary bladder, urethra, etc.
    • Major jobs include elimination of nitrogenous wastes and many drugs, water and electrolyte balance, blood pressure control, pH balance, stimulate red blood cell production, activate vitamin D.
  • Reproductive system
    • Includes organs such as the vagina, clitoris, uterus, penis, testicles, vas deferens, etc.
    • Major job is to make more humans in order to increase or maintain human populations.  This is also the only body system that we can survive without as individuals (this system is really only required because all humans are mortal— the average human life span is less that 80 years of age).

Levels of organization of the human body

Levels of Organization to the human body


The human body has several levels of organization.  You should be able to put these in order and describe each a bit.  One niffty thing to notice is that with each level of organization things become more complicated (and more fun!).


Here is a simple list from the least complex level to the most complex…

  • Chemical level
  • Cellular level
  • Tissue level
  • Organ level
  • Organ system level
  • Organism level

For most classes you will have to put the above in order and maybe give some explanation of the different levels…


Here is my expanded version with some details about each level:


  • Subatomic level — electrons, protons, neutrons (and even smaller things like quarks)
  • Chemical level
    • Atoms — Atoms are the smallest form of a given element (elements like carbon, oxygen, sulfur, etc).  There are only so many natural elements, and thus only so many types of atoms.
    • Molecules — Molecules form when two or more atoms bind together.  Since molecules can be made of many different types and numbers of atoms the possibilities are endless…
    • Macromolecules — Some textbooks mention macromolecules…  these are very large molecules like proteins and DNA chromosomes (many of these are large enough to see under a simple microscope—  for a molecule that’s HUGE!!!).
  • Cellular level
    • Organelles — these are specialized structures within our cells that perform a specific function (Examples:  mitochondria that make ATP, ribosomes that help make proteins)
    • Cells — You could say that “LIFE” begins at this level.  A single cell performs all of the functions that we consider necessary to call something “alive.”  Cells are also the building blocks for human life.  Everything that you are is made of or made by cells (your hair and fingernails for example are made of dead cells that filled with a tough protein called keratin before they died).
  • Tissue level — “When cells get together to perform specific jobs” — A tissue is a relatively uniform collection of cells (and their products) that perform some specific function.  Examples:  muscle tissue (for movement), nervous tissue (for thinking).
  • Organ level — “When tissues get together to perform jobs”  —  An organ is a collection of a few tissues that together perform some important tasks for the body.  Example:  Skin is made up of at least two major tissue types–  stratified squamous epithelial tissue and dense irregular connective tissue.
  • Organ system level — “When organs get together to perform jobs” — An organ system is a group of related organs that perform some important jobs for the body.  Example:  The nervous system is made up of the brain, spinal cord, and nerves.
  • Organism level — “When all of the organ systems work together we have a human” —  You are an organism!!   The complete human body is a single organism.


I’ll keep going a bit for my peeps that major in BIOLOGY!!!

  • Population level —  When lots of individual organisms of the same species get together we call this a population.  The planet Earth has a large human population.
  • Community level — All species living and interacting in a given area.
  • Ecosystem —  All living and non-living things in a particular environment including animals, rocks, dirt, rivers, lakes, etc.
  • Biosphere —  All ecosystems on the planet.

What is Science

Since our present knowledge of human anatomy and physiology is Science based it’s a good idea to know what Science is.

What is Science?

An incredibly simple but powerful way to explore and understand our universe.
Science is basically a huge collection of knowledge as well as scientific methods that we use to gain that knowledge.  Scientific methods are basically simple but powerful steps or rules for gaining knowledge about our universe.  The specific steps or rules vary depending on the investigation being done, but here is a basic outline of these rules.

  1. Find something that interests you, a “problem” or something you want to know more about, and ask one simple question about it–  the more specific the better.
  2. Get creative and formulate a hypothesis (an educated guess) that might be the answer to your question…  make it a guess based on what you already know—  keep it simple, testable, and falsifiable (Any idea that can’t be tested or that can’t be proven wrong is not scientific).
  3. Find a way to test your idea.  This might be a laboratory test, or a prediction based test, or some other observation/evidence based test to see if your hypothesis is either supported or not supported by evidence.
  4. The findings from your testing should get you closer to understanding the original thing you were curious about and will often lead to other questions that you might want to investigate.  If so, go back to the beginning.
  5. Keep track of what you learn and share it with others so that knowledge is gained by all of us.

There are a few things that good science and good scientists have in common.

  • Confidence:  We need to have the confidence to question what is known and to be explorers of the not yet known.
  • Objectivity:  All good scientific knowledge is observation based in one form or another and good researchers take pains to make sure that observations are done without bias (controls, double blind experimentation, and peer review help here).
  • Humility:  Basically as scientists we have to be ready to be wrong.  If our observations show us that we were wrong we have to accept that we were wrong and then update our knowledge accordingly.


click for a longer version with terms:  What is science

What is Human Anatomy & Physiology

What is Human Anatomy & Physiology and how are they related?

(Anatomy = structure)
(Physiology = function).

 (video:  What is Anatomy & Physiology)

Human Anatomy & Physiology is the study of the structures and functions of the human body.

Anatomy and Physiology are related in that “form follows function and function follows form”— in other words, the anatomy/structure of a given part allows for its physiology/function and the functions required from a given part dictate its structure (without the right structure that part won’t do its job).

Example: The anatomy of the human hand (4 fingers, 1 opposable thumb, various bones, muscles, tendons, and ligaments) allows for its functions (grasping objects and manipulating them in intricate ways). In order for the hand to perform its functions it must have the anatomy that it has.

Body regions, terminology, etc

Here is a video that will take you through many of the body regions:  surface regions of the human body

These images can be used for practice/study
(Click for a printable pdf of practice images Introduction images)

This image shows a human body in the anatomical position– use this image to practice labeling body regions, body planes, and directional terms (click on image for larger version).

01.04 body regions--sections practice

Directional terms: Here are some directional terms that most Anatomy students need to know:

  • proximal means closer to the body along a limb.
  • distal means farther from the body along a limb.
  • anterior or ventral means towards the front of the human body.
  • posterior or dorsal or caudal all mean towards the rear of the body.
  • superior means upwards towards the head
  • inferior means downwards towards the feet
  • medial means towards the middle or mid-line of the body
  • lateral means away from the mid-line of the body

Body sections/planes: These are the major planes of the body (most useful when looking at MRI or CT scan images)

  • sagittal section — A section down the body from the front or back. If you were cut in half on a sagittal plane there would be a left and a right half left behind.
  • frontal or coronal section — a section down through the body from the side. If you were cut in a frontal section there would be a front and rear half left behind.
  • transverse section — a horizontal section from the back, front, or side. If you were cut through your middle on a horizontal plane there would be a top and bottom half left behind.
  • oblique section — any diagonal section through the body.


Body Cavities

(click on images for larger version)


01.02 body cavities side labeled

01.01 body cavities front labeled

Notice how I’ve listed these cavities and how they are seen in the image above (for example:  The pericardial cavity is inside of the thoracic cavity, and the thoracic cavity is inside of the ventral cavity)

  • Ventral cavity
    • thoracic cavity
      • left pleural cavity
      • pericardial cavity
      • right pleural cavity
    • abdominopelvic cavity
      • abdominal cavity
      • pelvic cavity


Major organs

These are the organs easily seen from the front of a torso model.

01.05 major organs front labeled 01.07 major organs front 02 labeled

Here is a list of these organs that includes the cavities they are found in starting from the most specific or smallest cavity.

  • Brain — found in cranial cavity and dorsal cavity.
  • Right lung — found in right pleural cavity, thoracic cavity, and ventral cavity.
  • Heart — found in pericardial cavity, thoracic cavity, and ventral cavity.
  • Liver — found in abdominal cavity, abdominopelvic cavity, and ventral cavity.
  • Stomach — found in abdominal cavity, abdominopelvic cavity, and ventral cavity.
  • Large intestine — found in abdominal cavity, abdominopelvic cavity, and ventral cavity.
  • Small intestine — found in abdominal cavity, abdominopelvic cavity, and ventral cavity.


A&P to the point