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Six Different Levels of the Human Body: Level Four, Five and Six – BODY SYSTEMS

Six Different Levels of the Human Body: Level Four, Five and Six – BODY SYSTEMS

(Part three in a three part series)

In part one of my series on the 6 Levels of the human body we discussed the rolls of: atoms. molecules, and cells; and also their rolls as it relates to athlete performance.  In part two we discussed the third level: Tissues.  In the last part of the series we will discuss the ten different Body Systems.

Have you ever seen a science program about the planet earth and our solar system where they start with a close up of the earth and then pan out?  Imagine starting at the surface of the earth and moving away.  Next you see the continents, then the plant earth, then our solar system. Farther yet you see the Milky Way, then our galactic companions, then the local supercluster.  You start at the microscopic level and finish at the macroscopic level.  You can take that same concept and use it on the human body.  Starting at the cellular level and reaching the final organism we call the human body.  There are six levels of the human body, they are:

The Six Levels of the human body are:

1) The Chemical Level: consisting of atoms and molecules

2) The Cellular Level: consisting of a cell and its nucleus (organelle)

3) Tissue Level: such as muscle and connective tissue

4) Organ Level: example is the heart

5) The Body System level: There are 10 principles systems in our body.  One example is the Central Nervous System (CNS).

6) The Organism Level: the human being.

In a Nutshell How all These Systems Work Together

The atom is the smallest amount of chemical elements and they combine to form molecules.  The human body is 98 percent composed of only six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorous.  Atoms and molecules bind together to build the basic building blocks of our body.  Tissue is considered a group of closely related cells working together to perform a specific function.  The four main tissues in our body are: 1) muscle tissue, 2) nervous tissue, 3) connective tissue, and 4) epithelial tissue.  These tissues then organize themselves into organs such as the heart and the brain.  Then the organs perform specific functions in our body’s system, and all these systems combined together form our human body.

Body Systems

The human body is comprised of several independent systems that are responsible for maintaining life.  From the second part of my series I discussed the functions of tissues.  All of the tissues interact in one way or another to form functional body units called systems.  For all intensive purposes, the human body is one living system.  However, for practical purposes the human body is broken down into 10 major systems.

1) Integumentary System: the skin and structures derived from it.

2) Skeletal System: is the bodies support structure and gives the body protection.  It consists of bones and cartilage.

3) Muscle System: it consists of large skeletal muscles that allow for movement, cardiac muscles of the heart, and smooth muscles of internal organs.

4) Lymphatic System: a subsystem of the circulatory system that helps protect the body against disease.

5) Urinary System: it is the main excretory system of the body and consists of the kidneys, ureter, urinary bladder, and urethra.

6) Respiratory System: consists of the lungs and air passages which supply oxygen to the body and removes carbon dioxide.

7) Circulatory System: consists of the heart and blood vessels and serves as the transportation system.

8 ) Digestive System: consists of the digestive tract and glands that secrete digestive juices into the digestive tract and is responsible for the breakdown of foods and waste elimination.

9) Nervous System: consists of the brain, spinal cord, sensory organs and nerves.

10) Endocrine System: consists of glands and tissues that release hormones and works with the nervous system in regulating metabolic activities.

Of all these systems the ones that are of most importance to health, physical fitness, and training (other than skeletal and muscular system which I will write about later) are the: Respiratory System, Circulatory System, Digestive System, Nervous System, and Endocrine System.  Each will be discussed in detail.

The Respiratory System

The Respiratory System

The respiratory system consists of the lungs and all the air passageways leading to the from them: mouth, throat, trachea, and bronchi.  Respiration is the overall exchange of gases between the atmosphere, the blood, and the cells.  The respiratory system supplies oxygen and eliminates carbon dioxide to tissues in helping regulating the pH balance of your body.

In the lungs, the air you breath is processed.  In this process oxygen is removed and then transferred into the red blood cells and sent into the bloodstream.  When the red blood cells reach tissue, the oxygen is exchanged for carbon dioxide and water and then carried back to the lungs where it is flushed out.  So you can see that your lung capacity or in other words, the amount of air that your lungs can process; is the first limiting factor on your over all physical condition.  There are two factors that limit the lungs capacity.

First Limiting Factor: the lungs have very little muscle of their own.  The expansion and contraction of the lungs are dependent on the muscles of the rib cage and of the diaphragm.  As you inhale, the muscles surrounding the lungs create a larger area in the lung cavity which creates a partial vacuum.  Now because the atmospheric pressure is lower in your lung cavity, air rushes in.  A common myth about traveling to places like Colorado (Home of the “Mile High City”) where we are at minimum over a mile above sea level is that there is less oxygen and that is why it is harder to breath.  Yes, Colorado does have slightly less oxygen that at sea level, but the underlying reason it is hard to breath the air here is because there is less atmospheric pressure, so not as much oxygen gets into your lungs.  To compensate the body produces more red blood cells.

For physical activity the amount of air you can inhale and exhale is limited.  The first limit is the size of the vacuum that your muscles can create for the air to expand into.  The second is the size of the area they can be squeezed back into.  Conditioned athletes have the capacity to inhale more air and sustain the process for longer periods of time.  This also means that the trained athlete is capable of exhaling more wastes because the muscles around the lungs are trained and are more efficient.

Second Limiting Factor: is the condition inside the lungs.  When sports performance and lung capacity are discussed, there is less concern with the size or total capacity of the lungs, but rather how much of that capacity is usable.  The usable portion of your lungs is called the vital capacity.  It is a measure of the amount of air that can be completely exhaled in one breath.

Interesting studies that have been done which show that a conditioned athlete can have a vital capacity of 75 percent of their total lung volume.  The same studies show that an decondition person can have the exact vital capacity by virtue of good genetic makeup.  So, the way to differentiate between the condition athlete and the deconditioned person is to look at their maximum minute volume.  This is the amount of air an individual can process during one minute of vigorous exercise. A conditions athlete can force as much as 20 times their vital capacity, while an deconditioned person will be lucky to force even 10 times through.

During each cycle in the breathing process your lungs do not use all the air volume inside them.  After the usable lung volume is measures the remaining air in your lungs is called the residual volume.  This volume is fixed and even conditioned athletes cannot breath it out.  Although the volume is fixed, it can increase.  Too much residual volume is unhealthy.  Inactivity or disease can case the unusable portions of the lungs to increase.  This can result in shortness of breath even with light activity.

Training and exercising can reverse this trend.  Exercising the muscles surrounding your lungs will increase their strength and efficiency and open up more usable lung space.  This will also lead to increasing your vital capacity and reducing the residual volume.  With exercise, your lung will become more efficient at extracting more oxygen.  Other than the obvious, why is this important?  At rest the oxygen supply to the body is about one cup per minute.  During extreme exercise a trained athlete may process about one gallon per minute!

If you are looking for a non-technical way to test your breathing condition of you lungs, take a deep breath and see how long you can hold it.  Most adults in moderately good physical condition and with healthy lungs should be able to hold it for about 50 seconds or longer.

The Circulatory System

The Circulatory System

The circulatory system is the transportation system of the body.  The heart, blood, arteries, and veins are all part of this system.  The circulatory system is made up of two subsystems: the cardiovascular system and the lymphatic system.  In the cardiovascular system the heart pumps blood thought the vast system of blood vessels.  The lymphatic system helps preserve the fluid balance and helps fight against disease.


Blood is made up of four main components: plasma, erythrocytes, leukocytes, and platelets.

Plasma: is the fluid portion of the blood.  It is made up of numerous chemicals including: sugars, minerals, and proteins.

Erythrocytes: contains hemoglobin, and carries oxygen to all the tissues of the body.

Leukocytes: combat infections.

Platelets: are the mechanism for blood clotting.

The total composition of whole blood is about 57 percent plasma, 42 percent erythrocytes (red blood cells) and less than 1 percent leukocytes (white blood cells) and platelets.

Respiration and Circulation

From your lungs, oxygen goes directly into the bloodstream.  Your lungs contain millions of tiny air sacs called alveoli which the blood flows around.  Incoming air to the lungs is forced into the alveoli, and by the process of the Law of Gaseous Diffusion, the oxygen moves from the sacs and into the red blood cells.

The limiting factor for receiving oxygen to your systems is not your lung capacity, but rather the number of red blood cells you have in your body.  Even if your lungs were able to process more oxygen, without the sufficient amount of red blood cells there is no way to transport this extra oxygen capacity of the lungs.  For athletes, exercise and training produces more red blood cells, resulting in more hemoglobin, which carries oxygen, more red blood cells, which carry the hemoglobin, more blood plasma, which carries the red blood cells, with the end result being more blood volume.  Simply put, people in good physical condition have a larger blood supply than the typical deconditioned person.  The conditioned person can increase their blood supply by nearly a quart!

Not only does having more red blood cells carry more oxygen through your system, but it also carries away more wastes.  The removal of carbon dioxide and other wastes are just as important in reducing fatigue and increasing endurance.  The process whereby the blood reached the tissue, releases its oxygen, and collects wastes is called osmosis.  The cycle is complete when the carbon dioxide and other wastes are carried away and back to the lungs via the veins.  Now the Law of Gaseous Diffusion works in reverse and the pressure of the carbon dioxide in the veins forces it to go into the alveoli, and then exhaled with the expired air.  Again, the efficiencies of this cycle and it capacity for gas exchange is all dependent on the physical fitness of the individual.  With proper exercise and training this system become more efficient.

Another effect of exercise and training is the effect it has on the conditioning on the blood vessels.  Along with the increased amount of blood that the trained person has they also begin to create new veins and capillaries to carry the blood to the working muscles.  This process is called vascularization, and is an essential factor in building endurance muscles.  Basically, you will become more efficient with specific training for your sport or activities.

Again, there is a limiting factor to vascularization and that is fat metabolism.  Fat is one of the major foodstuffs along with proteins and carbohydrates.  The metabolism of fat is important because if fat intake is not monitored one of the major factors that can occur is arteriosclerosis, basically the hardening of the arteries. Fat circulates in the bloodstream for prolonged periods following fatty meals, and the length of time it take to clear depend directly on your physical condition. A low-fat eating lifestyle and aerobic exercise help speed the process of clearing fat from your bloodstream.  Here is an interesting finding: Ideally you would want to have a low-fat eating lifestyle and exercise.  However, it has been shown that a high-fat diet and aerobic exercise is preferable to a low-fat diet and no exercise.

Exercise and training has three major effects on your blood vessels: 1) it enlarges them and make them more pliable to pressure. 2) it increases their number for more blood flow throughout your body and working muscles. and 3) it helps keep the interior linings clear of clogging materials.

The Heart

The Heart

The heart is the engine the keep the body and all of its systems going.  It takes the oxygen rich blood from the lungs and pumps it throughout the body, and takes carbon dioxide filled blood from the body and returns it to the lungs for exchange with more oxygen.  The heart actually work faster and less efficiently when you give it little to do than it does when you put more demands on it.  Athletes that are either trained and conditioned  anaerobically or aerobically will have a lower resting heart rate (usually around 60 beats per minute or less) than and deconditioned person.  A deconditioned person (a rest) who does not exercise, forces their heart to beat nearly 30,000 time more each and every day of their life.  The two main factors that determine how health your heart is are: the tissue itself, and 2) the number of times it beats during rest or exercise.

Heart Tissue

The heart is made up of mostly muscles and unlike the lungs, does its own work.  A healthy heart depends on how well it is supplied with blood vessels, and it size.  Hearts come in three basic sizes.  A normal deconditioned heart is relatively small and weak.  Without exercise it literally begins to waste away (atrophy).  An enlarge heart usually grows to compensate for other deficiencies in the cardiovascular system.  There interior volume is very small despites it larger exterior. As such, a heart that grows large in this way is not as efficient as a heart that grows large through exercise and training.  The exercised and trained athletic heart is strong and health.  It is relatively large and highly efficient, pumping more blood with each stroke and with less effort.

We mentioned vascularization earlier, and nowhere is it more important than with the heart.  Exercising will increase the amount of veins and the blood supply surrounding the entire heart.  The health of your heart depends on healthy cardiac tissue which depends on the saturation of blood supplied to the heart.

Heart Rate

Heart rate is the second factor that determines the health of your heart.  The conditioned heart beats more slowly because it is pumping more blood with each stroke.  On the other end, training also reduces the maximum heart rate, which is just as important.  At rest the heart is conserving energy by beating slowly and during activity it has built-in protections against beating too fast and resulting in strain or failure.  Exercise and training can help lower the maximum rate, and it also can strengthen the amount of time the you can hold your near-maximum heart rate before fatigue sets in.  Without laboratory testing you can calculate your maximum heart rate by subtracting your age from 220.

The risk for the deconditioned heart is that when the heart speeds up the slowing down sometimes does not take place and the heart “takes off” leading to a heart attach.  With a conditioned heart there is a better balance where the highly potentially damaging levels are never reached.

Stroke Volume of the Heart

The stroke volume of the heart is described as how much blood is pushed out of the left ventricle with each beat.  The more blood your heart pumps out with each beat, the fewer times your heart will be required to beat.  This is described as the “left ventricle ejection fraction.”  A well trained athlete can push about 95 percent of their blood out of their left ventricle while working at about 80 percent of their capacity.  The deconditioned person is lucky to push about 75 percent working at 80 percent of their capacity.  Once the blood is pushed out of the heart it goes to your working muscles.  How efficiently the oxygen it taken into the muscles is called your VO2 uptake.

The Digestive System

The Digestive System

Both the Endocrines System and the Nervous System  play an very importation role in the digestive process.  The endocrine system tells the digestive system how to respond by the release of various hormones (discussed below), while the nervous system uses nerve impulses to direct the digestive system.

The digestive system runs about 25 feet through your body starting at the mouth and ending at the anus.  The digestive system broken down into its separate parts are know as: the alimentary canal, gastrointestinal system, and the gut.  The digestive system is the life support of the body and the connection with the external environment.  Basically, food is eaten, the body breaks it down into useful biomolecules for energy that is necessary for the building blocks of life; this process is called digestion.  Digestion is the process where food is broken down through physical and chemical means by which the nutrients can be absorbed. The nutrients are absorbed through the intestinal walls, transported by the blood to the liver, and then transported further onto the trillions of cells in your body via the bloodstream.

The functions of the digestive system include:

a) The receipt , mastication (chewing), and the transportation of ingested substances.

b) The secretions of acids, mucus, digestive enzymes, bile, and other materials to break food down.

c) The digestion of ingested foodstuff.

d) The absorption of nutrients.

e) The storage of waste products.

f) Excretion

g) Auxiliary functions

The components that make up the digestive system are: mouth, esophagus, stomach, the small intestine, the large intestine & rectum, pancreas, and the liver & gallbladder.  I will discuss each of these in detail.

The Mouth

The mouth is the mechanism that food enters the digestive system.  The mouth has four main functions when it comes to the digestion of foodstuff.

First, it physically breaks apart the food we eat by mastication (chewing).  I’m sure you here this a lot, but I’m going to say it again, “chewing your food thoroughly is very important to digestions.”  Chewing thoroughly helps the stomach perform its digestive duties more easily.

Second, the mouth mixes food with saliva creating a moist mass call a bolus, which is then ready for swallowing.  Saliva also contains digestive juices that help to start to break down the starches in the food you eat. For new parents who are just experiencing feeding your baby food from a jar you may wonder why sometime when you do not use an entire jar of food that you find it liquefied the next time you go to use it.  This is because if you feed you baby straight from the jar you are transferring saliva back into the jar where it starts to digesting the food.  This is also a good reason not to drink straight out of the milk container.

Third, the mouth regulates temperature by either cooling or warming the food.  The delivery of cold food can speed the emptying of the stomach and reduce the efficiency of digestion.  There is an exception to this for us athletes. This is drinking cold fluids before and during exercise or competition.  Emptying fluids from the stomach faster will rehydrate the body more quickly.

Fourth, it consciously initiates swallowing when the food is ready.

The Esophagus

The esophagus extends between the pharynx and the stomach and is the transportation conduit for food and liquids traveling to the stomach.  When the bolus enters the esophagus, an involuntary wave of muscle contractions is triggered which propels the bolus into the stomach.  This muscle contraction is known as peristalsis.  At the end of the esophagus, the bolus reaches the esophagus sphincter, which relaxes to let the bolus enter the stomach.  At the same time that the bolus is let into the stomach the esophageal sphincter also keeps food from spurting back up from the stomach.  To reduce the chances of heart burn and to put less stress on the esophageal sphincter, it is good practice to eat sitting up and to try not to overfill our stomach.

The Stomach

The stomach is a muscular sac that is responsible for: the storage and gradual release of food into the small intestine, digestion through chemical secretions and physical activity, and the transportation of the ingested food down the gut.  The stomach secretes several substances for the breakdown of food.  Mucus is a protective layer that lubricates the stomach wall and acts as a buffer against acid secretions.  Hydrochloric acid is secreted in the stomach to keep the stomach relatively free from microorganisms while maintaining a low pH inside the stomach.  Hydrochloric acid is also the catalyst for pepsins, which begin the digestion of proteins.

Other chemical secretions that occur in the stomach are: Intrinsic Factor which binds with vitamin B12 and allows for its absorption in the small intestines,  a hormone called Gastrin which regulates stomach secretions during digestion, and the enzymes rennin, pepsin, and lipase are also secreted.  The enzymes function to breakdown several nutrients. Rennin breaks down milk proteins, Pepsin breaks down protein in the presence of hydrochloric acid, and lipase breaks down fat molecules.

Macronutrients are nutrients that are needed in large amounts in the diet such as carbohydrates, fat, and protein.   When macronutrients are taken alone they leave the stomach at different rate of time.  For the athlete, this is a very important concept to understand.  Carbohydrates will empty from the stomach the fastest.  Athletes taking pure carbohydrate drinks during exercise can get the carbohydrates into the bloodstream fast which allows for the replenishment of glucose; the body’s main energy source.  Proteins empty from the stomach next, and fats take the longest time to empty out.  When carbohydrates, fat, and proteins are taken together they mix, causing the stomach to take longer to empty.  The stomach usually takes 1 to 4 hours to empty.

Although most absorption of nutrients occurs in the small intestines, some absorption does take place in the stomach.  The stomach can absorb some water, alcohol, aspirin, glucose, and some vitamins.  As an athlete, the fact that water and glucose can be partially absorbed through the stomach is beneficial for fast replenishment of these nutrients during exercise.  Fructose is in many sports drink products, but is absorbed much slower in the stomach is is not recommended for a replenishment drink.  However, complex carbohydrates can be added to sports drinks because they release glucose at a slow rate during digestion.  Glucose ingestion can help spare glycogen stores but it must be ingested just prior to exercise (within a half hour). Otherwise, it can cause an influx of insulin which will throw off energy generation during exercise.

The stomach only starts the process of digestion and the breakdown of complex molecules.  Complete digestion happens farther down the digestive tract where complex molecules like proteins are broken down into amino acids.  This breakdown process is called hydrolysis and continues in the intestines when the partially digested material leave the stomach and enters the small intestines.

The Small Intestines

The small intestine run about 12 feet long and are divided into three regions: duodenum, jejunum, and the ileum.  The duodenum is connected to the stomach and its function is for storage and the continued breakdown of food with some absorption being done here.  The majority of nutrient absorption is done in the jejunum and the ileum.  The entire inner surface of the small intestines is lined with very small finger like cells called villi. They greatly increase the surface area of the small intestine, and each villi has its own blood vessel.  When nutrients pass through the cells of the villi the nutrients are transported into the blood vessels and then transported to the liver.  Also present in the villi is another transportation system called the lymphatic system.  The lymphatic systems main function is to transport fat.

The Large Intestine and Rectum

The large intestine is about three feet long.  The large intestine is attached to the small intestine at the ilium and this area is called the cecum.  Some final absorption of water, minerals, and vitamins take place in the large intestine.  Bactria are present in the large intestine and through their metabolism produce vitamins that are absorbed.  The large intestine also stores the waste products of digestion.  Upon the proper stimulus occurs, the colon will empty its contents into the rectum, triggering defecation.  The more fiber you have in your diet, the easier it is to defecate.

The Pancreas

The pancreas produces several secretions that are important for digestion and absorption of nutrients that secrete into the small intestine.  Insulin and glucagon are two secretions in the pancreas that  helps control carbohydrate metabolism.  Insulin is secreted into the bloodstream during a meal.  Insulin functions to mediate the transportation of glucose and amino acids across cell membranes.  Insulin has a anabolic function in the body, which means that it takes simpler molecules and crates complex molecule.  Later the complex molecules are broken down and energy is release.

The Liver and Gallbladder

The liver is the largest gland in our body weighing in at about 4 pounds.  Digestion is complete when the nutrients are delivered to the liver and then released into the bloodstream.  The intestines are directly connected to the liver by the portal vein.  The liver acts as the control and release mechanism for nutrients into general circulation.  The liver cells process the digested nutrients changing them into substances that the body will need, and stores them until they are required.  In a digestive capacity, the liver produces and secretes about one pint of bile per days.  The bile secretion helps aid in the emulsification of fats.  In other words, the liver acts as a processing organ that is responsible for maintaining nutrient balance and storing nutrients such as glycogen (glucose) for energy.

The gallbladder is a storage sac for a digestive mixture called bile.  Bile is secreted into the small intestines and is necessary for: the action of lipase, the digestion and absorption of fats, and the absorption of fat-soluble vitamins.

A Final Note on Digestion

As athletes we should not take eating for granted.  To be at the top of our game, no matter what sport or activity we participate in; we must develop good eating habits to enhance the proper digestion of the foods you eat.  Here are a few considerations to to get the most out of every meal:

  • Eat slow and chew your food thoroughly.
  • Eat in an upright position, not laying down.
  • Eat several smaller meals throughout the day rather than a few large meals.
  • Do not eat when you are nervous.
  • Avoid strenuous activities after eating and allow some time for digestion.
  • Avoid foods that irritate your stomach.
  • Consult a doctor if your feel that you have some sort of digestive disorder.

The Nervous System

The Nervous System

The Nervous system is the control center of the body and the network for internal communication.  Your nervous system is made up of two major parts: the central nervous system (CNS) the the peripheral nervous system PNS).  The CNS is comprised of your brain and spinal column, and should be thought of as one organ.  The CNS receives messages and after interpreting them sends instructions back to the body.  The peripheral nervous system relays messages from the CNS to the body (the efferent system) and it relays messages to the CNS (the afferent system) from the body.

The efferent system is divided into two parts: the somatic system, which are your voluntary actions; and the autonomic system, which are your involuntary actions.  In the PNS your afferent system  sends messages to the CNS in through three classes of receptors: 1) Proprioceptors, are located in joints, muscles, tendons, and the inner ear and are responsible for picking up messages about the position of the body. 2) Exteroceptors, which are located near the surface of the skin and relay information about your external environment, and 3) Interceptors, which are located in your blood vessels and viscera.  They keep tabs on inners body sensations such as hunger, thirst, pain, pressure, and fatigue.

In layman’s terms, the nervous system senses changes inside and outside of our body.  It interprets those changes, and then it responds to the interpretation by initiating action in the form of muscular contractions or glandular secretions.

As an athlete there are ways to take advantage in the modification of your nervous system, and the rewards for the modifications can be significant.  An athlete can improve strength output, develop better mental concentration, be able to handle greater training intensity, manage pain, and glandular secretions.  All these areas are modifiable to a certain measurable degree.

The Endocrine System

The Endocrine System

The endocrine system works with the nervous system to maintain the steady state of the body.  The endocrine system helps to regulate: growth, reproduction, use of nutrients by cells, salt and fluid balances, and your metabolic rate.  The endocrine system consists of tissues and glands that secrete chemical messengers called hormones.


There are various glands within the endocrine system that secrete hormones.  Two major hormones that athletes should be aware of are steroids and polypeptides.  They diffuse into the blood and course through your body an eventually act upon a target organ.  For the athlete, hormones are extremely important.  Every bite of food you eat, every supplement you take, and every act you do during your training and exercise routine is in some way modified by the hormonal interactions each act instigates.

Hormones are made up of amino acids and can be classified into three groups: 1) amino acid derivatives, 2) Peptides/proteins, and 3) Steroids.  Hormones regulate nearly all of your bodily functions such as: growth, the ability to cope with physical and mental stress, training responses including protein metabolism, fat mobilization, and energy production.  The endocrine system does not function independently of the nervous system.  These two systems act together which means that fright, pain, cold, and all other senses of both the external environment and your bodily functions will activate hormonal responses in very complex ways.

Hormones have three basic affects on the body: 1) They alter the rate of synthesis of cellular protein. 2) They change the rate of enzyme activity, and 3) They change the rate of transportation of nutrients through the cell wall.

Within the GI Tract there are a host of hormones that regulate digestion, let’s take a look at the major hormones.

1) Gastrin: When the food we eat enters the stomach the hormone gastrin is triggered and released into the blood.  This release helps to start the digestive process.  For that contain proteins seem activate the release gastrin the most.

2) Secretin: During the digestive process acids are produced in the stomach to start the digestive process.  When these acids are transported into the small intestine the hormone secretin is secreted.  Its purpose is to help neutralize the acids in the small intestine.  Another major role that secretine plays is that it slows down the production of gastrin and enhances the hormone CCK.

3) Cholecystokinin (CCK): When partially digested food enters into the small intestines, the hormones CCK is secreted.  The role of CCK is to let the pancreas know that the small intestine is ready to receive pancreatic enzymes to aid in digestion.  CCK also stimulates the gallbladder to contract which releases bile into the small intestine.  CCK also can send signals to your brain that it is time to stop eating because you are full.

4) Gastric Inhibitory Polypeptide (GIP): GIP is released into the small intestine while CCK is at work.  GIP’s role is to enhance the release of insulin which helps prepare the body for the glucose that is about to appear in the blood with the further process of digestion and absorption.

5) Motilin: When bicarbonate is released into the small intestine to create a less acidic environment, motilin is also released.  Motilin’s function is to contract the smooth muscles of the GI tract moving the chyme through the intestines for future breakdown.

6) Somatostatin: This hormone is secreted by the stomach, intestines, and pancreas.  Released in the stomach it acts to suppress the release of all the hormones listed above.  It is responsible for the slowing down of gastric emptying, reducing contractions of the smooth muscles in the intestine, reducing blood flow, and reducing enzyme release.

7) PYY 3-36:  This hormone is released hours after we eat and its main function is thought to be that of an appetite suppressant.  It is released in both the small and large intestines.

8 ) Ghrelin: Ghrelin is released by cells of many different organs such as: the stomach, pancreas, placenta, kidney, pituitary, and hypothalamus. It is released generally in response to low food intake and the hormone is released to encourage appetite.

Other hormones that are released int he digestive process are:


Insulin is a hormone that is released by the pancreas.  It increases the cellular uptake of glucose, which in turn causes increased synthesis of muscle glycogen.  This leads to a decrease in blood-borne glucose, which then causes a decrease in the insulin production.  For the athlete, during training and prolonged workouts, blood glucose reduction along with deceased insulin production increases the mobilization of fat stores.


Glucagon plays the exact opposite role of insulin.  Its role is to get more glucose into the blood when needed by stimulating both glycogenolysis and gluconeogenesis in the liver.  The glucose is released into the bloodstream and raises the insulin levels.  Gluconeogenesis is the production of glycogen from non-carbohydrate sources.

The human body has about 10 grams of blood-borne glucose circulating continuously. If blood sugar levels increase, then the pancreas release insulin.  If blood sugars levels drop too low, then glucagon is released.

Muscle Growth and Hormonal Regulation

The most abundant hormone in our body is growth hormone (HGH or hGH) which is produced by the pituitary gland.  HGH is the largest and the most complex hormone produced by the pituitary gland.  It is made up of 191 amino acids.  Releasing hormones secreted by the hypothalamus control growth hormones secretion.  Growth hormone stimulates tissue uptake of amino acids, the synthesis of new protein, and long bone growth.  Growth hormone secretion reaches its peak in the body during adolescence, but does not stop after adolescence.  After we turn 20, our bodies produce less and less growth hormone.  At age 20 we produce about 500 micrograms per day.  By the time we are 80 that level will drop to about 25 micrograms per day.

Thyroid Hormones

The anterior pituitary is referred to as the “master gland” because of all the different hormones it produces.  One of them is thyroid stimulating hormone (TSH).  The thyroid gland located in your neck releases two hormones: 1) thyroxin (T4) and 2) triiodothyronine (T3).  The T4 hormone raises the metabolic rate of cells by as much as four times, greatly facilitating carbohydrate and fat metabolism.  There is evidence that a healthy eating life style along with training and exercising can increase your metabolic rate.

Adrenal Hormones

The adrenal glands are made up of two parts: 1) the cortex (outer layer) and 2) the medulla (inner).  During exercise there is a dramatic increase in the output of epinephrine.  Epinephrine causes increased blood flow to muscles, enhances cardiac output, the mobilization of energy substrate, glycogenolysis, fat mobilization, and other “flight or fight” functions.

In your brain the cortex releases a group of hormones called the adrenocortical hormones (mineralocorticoids, glucocorticoids, and androgens).  There are three different mineralocorticoids and are mostly comprised of aldosterone.  Aldosterone regulates the reabsorption of sodium in the distal tubules of the kidneys.  Aldosterone is responsible for controlling the sodium balance in your body.  High levels of aldosterone causes high blood pressure.


Cortisol breaks down the proteins in muscles (catabolic).  High cortisol levels can be dangerous and lead to ketosis.  This condition is common in people who are on a carbohydrate restrictive diet.

What Does it all Mean?

Exercise has both psychological and physiological benefits.  The mind can benefit as much as the body from training and exercise.  Improved endurance makes the body less susceptible to fatigue, which will lead to less errors both mental and physical.  Your personal performance at your job will benefits because the healthy person can be sustained longer without having to take frequent breaks.  People who are physically fit in general have a better outlook on life, more self-confidence, and often excel in whatever they set their minds to.

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