Week 7 – February 24, 2003 - Human Organism – Body Systems



Importance of the Cell

Priorities of the Human – Daily & Species Survival

Accomplished by Body Systems













Importance of the Cell

...... Before we look at each of the 11 human body systems, it is critical to stress the importance of the cell; i.e., to view the human organism as the working sum of its constituent cells (it is at the cellular level that we can most fully appreciate the functioning (or malfunctioning) of our body systems). Each body system is dependent upon the harmonious interplay of organs and tissues, but it is at the cellular level where we learn the roots of normal functioning as well as disease states.

......To quote from Lewis Thomas's `The Lives of a Cell' (Viking Press, 1974) – “I have been trying to think of the earth as a kind of organism, but it is a no go. I cannot think of it this way. It is too big, too complex, with too many working parts lacking visible connections. The other night, driving through a hilly, wooded part of southern New England, I wondered about this. If not like an organism, what is it like, what is it most like? Then, satisfactorily for that moment, it came to me: it is most like a single cell.” Yes, the cell is truly amazing – amazing in its complexity, yet fundamental to all forms of life including the human organism.

......By looking briefly at some of the integral parts of the cell, we can get a better grasp of its basic function as well as its overall contributions to tissues and organs as a whole. For example the cell is delimited by a lipid/protein-containing membrane; this membrane not only serves as the gate-keeper letting molecules in and out of the cell, but it also serves as critical recognition sites for hormones and the like. Within the cell is another membrane-bound site called the nucleus which houses the genetic information of the organism; within the DNA molecules are approximately 70,000 genes which provide coded instructions on how the cell is to make protein. As we'll see later, it is the proteins of the cell which give the cell it's function (viz., liver cells make liver proteins, heart cells make heart proteins, etc...). Outside of the nucleus is a complex membrane structure called the endoplasmic reticulum which serves as a work-bench to construct proteins under instructions from the genetic DNA code. Also within the cell are small, membrane organelles called mitochondria; these organelles are responsible for generating ATP energy for all of the cell's processes by metabolically breaking down glucose and fat.

......So as you can see here, the cell is somewhat a self-contained `organ' capable of directing things in and out, directing the synthesis of new protein components, as well as being able to transform molecules into useful energy. Magnify these abilities several billion-fold, and you get some idea of the over-all structure and function of the human body! Below, as you read about each human body system, periodically ask yourself this question – what are the cells doing? What specific role are they serving?

Priorities of the Human Animal – Daily as well as Species Survival

......The human body is a living animal which functions with two sets of priorities, both of which involve “survival”. On the one hand, there are the day-to-day survival mechanisms implemented by the nervous and endocrine systems working together. Whether it is maintaining a constant body temperature, or raising your blood pressure when the need arises, the neuroendocrine pathways are critical to our day-to-day, minute-by-minute adjustments to changing environmental and physiological conditions.

......On the other hand we can look at survival as a case of continuing the human species. In this instance, it is our reproductive system which insures the next generation of humanity. Hence, survival day-to-day as well as a species become overwhelming priorities within the human organism. Once these priorities are understood, it then becomes easier to see the relative roles of each body system within the scope of overall human function.


Integument – Our Body's `Hide'

......Loosely known as our `skin', this body covering is composed of three layers: outer layer called `epidermis' composed of several layers of flat, epithelial cells; middle layer called `dermis' constructed of loose connective tissue fibers, within which is contained blood vessels, hair follicles, sweat and oil glands, and various sensory nerve endings (touch, pain, heat and cold); and deep, innermost layer called `fascia' composed of connective tissue fibers, fat cells, as well as an array of blood vessels and sensory nerve endings (it is this layer that is in contact with our interior body organs and tissues (e.g., bones and muscle).

......Several of the functions of skin include:

Physical barrier of the body to resist entry of environmental particles, including bacterial, fungal, and viral pathogens;

Sensory organ which allows us to `sense' our body parts in space, painful injury, relative environmental temperature, and sexual pleasure;

Primary energy storage site through fats stored within adipose fat cells;

Excretory organ where skin secretes both water, and nitrogen waste products;

Site of the synthesis of vitamin D where sunlight converts a provitamin molecule into an active vitamin D (which in turn acts to regulate calcium in the body);

Role of skin in temperature regulation helping to maintain our core body temperature at 98.6 F or 37 C (because skin is the primary site of heat loss in the body, skin has an active role in this heat transfer- e.g., increase loss in hot conditions through sweat gland perspiration, or decrease loss in cold conditions through vasoconstriction of blood vessels within our skin).

Muscular – The Body's Pulley System

......There are 3 types of muscle in the body, cardiac muscle found in the walls of the heart, smooth muscle found within the walls of the gastrointestinal tract, blood vessels, and reproductive tract, and skeletal muscle found in association with our skeleton. The latter 2 muscle types (both under `involuntary' control) will be discussed later within the context of other organ systems, however here we will concentrate on skeletal muscle which is under voluntary nerve control. There are over 600 individual muscles within the body which together as a system allows the body parts to move (e.g., breathing, walking, writing, sitting upright, typing on a PC keyboard). Each muscle is attached to parts of our bony skeleton which if the muscle contracts (or shortens) `moves' the skeleton (e.g., lifting your arm to turn off your PC).

......How does this shortening occur? For one, our nervous system controls muscular contraction by stimulating it – motor nerves from our brain and spinal cord target each muscle set causing it to contract, hence causing our skeletal parts to move. On the other hand, muscle is composed of several `fibers' (a fiber is a grouping of fused muscle cells) which if stimulated by a nerve will shorten (hence, as the component fibers shorten, the entire muscle will shorten).

......The principal functions of the skeletal muscle system are:

......The maintenance of an erect posture whether sitting, standing, or even walking (these muscle are often referred to as our `anti-gravity muscles');

......Permits voluntary movements of our body and its parts;

......Major source of heat production in the body (when sugar or fat is burned in muscle to make ATP to serve as the energy source for contraction, heat is also produced, hence muscle becomes a major source of heat production in the body;

and lastly muscle functions in starvation conditions as a needed source of glucose (you need to keep in mind that the primary energy fuel for the `vital reflex' centers of the brain is glucose; in order to provide needed glucose to the brain under starvation conditions, muscle protein can be converted to sugar which the brain needs, minute-by-minute).

Links related to muscle: www.ultranet.com/~jkimball/BiologyPages/M/Muscles.html and www.rohan.sdsu.edu/course/ens304/public_html/section3/Muscle.htm

Skeletal – The Body's Framework

......This system is composed of approximately 200 bones (of all shapes and sizes) which together provide the body with several vital functions: the bones of the skeleton provide solid protection to the vulnerable, soft brain and spinal cord tissues; the bones act as `levers' in which to move (and support) the body and its parts (where joints between bones serve as the `fulcrum' of the lever design, muscles attached to the bones via tendons provide forceful movement of the lever construction); the marrow cavities within our bones serve as the site of blood cell formation (process known as hematopoiesis); and lastly our skeleton serves as the body's major reservoir of important minerals such as calcium and phosphorous (if our blood calcium drops, the bones are tapped for calcium in order to bring the blood level back up).

Nervous – The Internet of the Body

......As you presently sit in front of your computer screen, with a little effort you can dwell on the amazing abilities of our nervous system – its wonderful ability to receive and interpret visual signals from your pc screen, its remarkable ability to dictate instructions to muscles of your hand to type commands or scroll the screen with your mouse, and a countless array of fine adjustments to the rest of your body as you get comfortable in your chair. Furthermore, the nervous system (along with your endocrine system to be covered in the next section) is quietly working to keep just the right amount of blood circulating to your brain as you continue to work at the computer. Yes the nervous system is a marvel. In very simple terms, our nervous system is geared to perceive changes in our external as well as our internal environment; processes this information, and subsequently stores it, or better yet acts upon the information. There it is in a nutshell – our nervous system!

......This system is composed of impulse-conducting cells called neurons, and is broadly divided into two divisions: the central nervous system (CNS) and the peripheral nervous system (PNS); the PNS is further divided into sensory and motor pathways (While the PNS is anatomically divided into cranial (head) and spinal (body trunk) lateral nerve pairs, for our purpose here it is sufficient to understand PNS components as being either sensory or motor in function).

......The underlying function of the nervous system is to provide “wiring” to the body – this wiring is achieved by neurons which have the ability to transmit an impulse from one location to another. In addition to this internet-aspect, the nervous system contains groups of neurons which act like computer memory chips and processors – these cells provide capacities for relay, or reflex activity, in addition to the storage of information (like the hard-drive in your computer). Hence the nervous system provides the body with the abilities to (1) conduct impulses from one location to another, (2) sense and react to changes within and outside the body, and (3) store, interpret, and analyze sensory information, and subsequently act upon this information both voluntarily as well as involuntarily.

......Before we go further, let's take a closer look at the neuron – this cell is highly specialized to transmit an impulse. The impulse itself is in essence a depolarization of the cell membrane which travels along the membrane in an “all-or-nothing” manner (i.e., once the fuse is lit the impulse travels unabated the entire length of the neuron). What causes an impulse in the first place? A sufficient change in the local environment at one end of the neuron (receptor) is enough to initiate a nerve impulse (often referred to as a “threshold stimulus”). What kind of change in local environment will stimulate a neuron? There are many: chemicals like molecules in your food, in the air, or in your blood; rays of light penetrating your eye; sound waves reaching your middle ear; changes in relative temperature; pressure on and within your various organs and tissues. These and other factors are sufficient to initiate a threshold stimulus which subsequently travels along the length of our neurons (You might liken our neurons to a long fuse where one end is lit, and the other end is left free to target something – not dynamite!). Once a stimulus reaches the other end of a neuron, a chemical called a transmitter is released into a space called a junction or gap. Subsequently the transmitter binds to the membrane of a target cell – which could be any one of a variety of targets: another neuron (such as commonly found in the CNS), heart muscle (to speed or slow up), smooth muscle (to contract or dilate such as found in the walls of our blood vessels or gastro-intestinal tract), skeletal muscle (to achieve some physical activity), or one of our glands such as the pituitary or adrenal. Hence we see that our neuron provide the interconnections necessary to tie the body together as well as to sense and react to changes in and out of the body.

......Before we look at the various levels of function of the nervous system, let's look more closely at the CNS components. For one, the spinal cord is made up of bundles of nerve pathways called ascending and descending tracts. Like a two-way street, ascending tracts receive sensory information from the PNS and take it up to the brain, while descending tract take motor signals from the brain to a range of motor targets (e.g., heart, smooth, and sketetal muscles, and selected glands, like the adrenal, tear, salivary, and sweat). On the other hand, the brain is composed of 4 major divisions: (1) the brain stem which relays info from spinal cord to higher centers in the brain, as well as containing neurons which serve as vital reflex centers controlling our heart and breathing rates; (2) the midbrain which thru the thalamus relays two-way traffic to and from the higher brain centers, as well as containing the hypothalamus which acts as a “housekeeping” or homeostatic reflex center controlling day-to-day activities such as body temperature regulation; (3) the cerebellum which is composed of reflex centers to maintain and coordinate our body parts in space as we pursue our various physical activities; and (4) the cerebrum which contains billions of neurons and connections serving as sensory, association, and motor areas serving to receive, store, coordinate, and act upon sensory information either stored or newly received.

......Our nervous system can looked upon as having 3 levels of function: (1) a simple reflex level where a sensory stimulation is carried to the CNS and there is an involuntary motor response. Examples of this level range from our unconscious effort to duck our head from a baseball coming toward us, to the need of raising our blood pressure when we begin to climb a flight of stairs; (2) a simple reflex plus the targeting of other organs mainly glands. A good example of this level of function is the effort of our body to maintain our body temperature when we enter a cold room in the winter (and the window is left open!). In this case our body senses thru our PNS that it is cold (outside as well as inside our body); this sensory information is sent to the temperature-control reflex centers in the hypothalamus which subsequently acts in two ways – first the open window is shut! That is, motor nerves from the hypothalamus travel to smooth muscle targets in our blood vessels thereby constricting the flow of warm blood to our arms and legs, hence conserving the loss of heat from the body. Secondly, the hypothalamus stimulates the pituitary gland to secrete thyroid stimulating hormone which activates the release of thyroid hormone from the thyroid gland. Subsequently thyroid hormone targets our body cells to burn more fuel (sugar and fats) and hence increase our body's production of heat – in effect it as if we went to the room's thermostat and turned up the heat setting; and (3) there are the higher levels of neural function which are ascribed to our cerebrum.

......Throughout our life, as well as throughout our minute-by-minute waking moments, we receive sensory information. It is the responsibility of our cerebrum to receive, interpret, catalogue, associate, store, and react. You and I refer to these activities in various forms: e.g., listening to good music, reading a book, discussing what you are going to have for dinner, deciding on your future career, remembering an answer to a question on an exam, decide on a mate for life, learning to hit a tee shot in golf, etc....Our cognitive activities involving cerebral functioning are seemingly endless. The more we learn about the human nervous system, the more we understand how really unique we are as human organism. In fact it might be worth mentioning here that in addition to the human genome project and the wonders of the immune system, the mapping of the human brain still remains as one of life science's great challenges.

Links related to nervous system: http://faculty.washington.edu/chudler/neurok.html and www.ultranet.com/~jkimball/BiologyPages/C/CNS.html and www.ultranet.com/~jkimball/BiologyPages/P/PNS.html

Endocrine – Chemical Communication Within the Body

......Distributed throughout the body are a dozen or more glands which together form the endocrine system. Each gland is composed of hormone-secreting cells; when the right 'stressor' is present, the gland is activated to release its hormone directly into the blood stream. Subsequently, a hormone will activate some other tissue so long as the tissue has a specific receptor for the hormone; if it does the target tissue can be described as being `receptored' or `hormone-sensitive'. In this instance, the hormone will either bind with a receptor at the cell surface (hormones made out of proteins) or within the cell nucleus (steroid hormones); once a hormone-receptor complex is formed the cell's function is changed in some way depending on the target tissue in question.

......The general function of the endocrine system is to work in harmony with the nervous system to maintain body functions day-by-day, to control growth and development throughout the stages of the human life-cycle, and to maintain and control our reproductive systems (hence, provide a system for survival of the human species!)

......Some of our glands and their functions are: pituitary gland which controls growth, metabolism, reproduction, reaction to normal stress, blood pressure, as well as body temperature; thyroid gland which controls the rate in which we burn sugar and fat (i.e, our metabolic rate); parathyroid gland which along with the thyroid gland controls the amount of calcium we have in our blood; pancreatic islet cells which controls our blood sugar levels; adrenal glands which controls the rate at which sugar and fat is available for use in the body, and serves as an important control of our blood pressure; and the gonads (ovaries and testes) which serve dual functions of making gametes (eggs and sperm), and making estrogen and androgen hormones (which in turn function to develop and maintain the secondary sexual characteristics of females and males).

Links related to endocrine system: www.ultranet.com/~jkimball/BiologyPages/H/Hormones.html and www.people.virginia.edu/~rjh9u/day20.html and www.people.virginia.edu/~rjh9u/day21.html

Circulatory – Lifeline for Distribution of Nutrients

......This critical system under neuroendocrine control consists of a heart pump, blood vessels, and blood. While the circulatory system services all the cells and tissues of the body, its primary objective is to continually provide nutrients to the brain. The heart made up of cardiac muscle acts as a pump to forcefully push blood through a system of arteries to all parts of the body; in turn blood returns to the heart through a system of veins. Both arteries and veins are relatively thick-walled vessels whose primary responsibility is transport blood under considerable pressure to and from the heart; however as arteries get nearer to their target tissues the walls of the arteries thin considerably becoming vessels known as capillaries – because the walls of the capillaries are only one-cell thick, there can be the free exchange of nutrients and waste products with the tissues.

......Blood consists of a watery mixture of solutes and cells. Solutes include: inorganic salts, minerals and gases (e.g., sodium chloride, potassium, calcium, carbon dioxide), and a range of organic nutrients (e.g., simple sugars, fatty acids, amino acids, nucleotides, vitamins) and waste products (urea, creatinine, uric acid). Cells found within blood include: red blood cells containing hemoglobin for the transport of oxygen; white blood cells of the body's immune system; and thrombocytes which function in the clotting process.

......Within the circulatory system there are a number of important sub-circuits. For example the most extensive one is the `systemic' circuit which consists of major arteries from the heart branching off to the head, neck and shoulder regions, and another branch supplying blood to the rest of the body (trunk, legs); the systemic circuit is completed by a series of major veins returning blood back to the heart. On the other hand, the `pulmonary' circuit consists of arteries from the heart to the lungs at which oxygen is picked up and waste carbon dioxide is given off. Because the heart is 4-chambered, there can be a clear separation of the systemic from the pulmonary circuits. Another critical circuit is the `coronary'; this small but important coronary artery supplies oxygen and nutrients on a continual basis to muscles of the heart (if blood flow is interfered for even a short period of time a life-threatening heart attack would result). Moreover there is the `hepatic portal' circuit which consists of veins draining the gastro-intestinal tract and are all joined to form the hepatic portal vein which enters the liver; in this way our liver has the first opportunity to inspect what we have eaten and digested.

Links related to cardiovascular system: www.people.virginia.edu/~rjh9u/day11.html and www.people.virginia.edu/~rjh9u/day12.html and www.ultranet.com/~jkimball/BiologyPages/C/Circulation.html and www.ultranet.com/~jkimball/BiologyPages/C/Circulation2.html

Lymphatic/Immune – Body's Defense Against Microbes

......This system has several inter-connecting components and functions. The lymphatic system is composed of a network of vein-like vessels (lymphatics) and associated glands and nodes; interlaced within this system is a network of white blood cells which comprise what is called the immune system.

......The lymphatic vessels function in returning about 40% of our tissue fluids back into the general circulation; in this way it assists the veins of the circulatory system. In addition, lymphatic vessels within our gastro-intestinal tract (called lacteals) absorb the fat we eat and digest, and transport it to the general circulation for usage or storage. Interspersed throughout the network of lymphatic vessels are glands and nodes which serve two primary functions: firstly, potentially harmful particles and cells (e.g., viruses, and bacteria) are filtered from the lymphatic fluid (lymph); and secondly, nodes and glands serve as sites for the proliferation of white blood cells comprising the immune system.

......The immune system is the third tier of our body defenses to fight infections and disease. The first tier is comprised of physical barriers such as our outer skin and mucous membranes (found lining our mouth, G/I tract, and urogenital system). The second tier is our inflammatory response in which the body attempts to heal a wound, or internal infection. For example, if you cut your skin and bleeding occurs, the following inflammatory response normally occurs. Firstly, cells within our skin called mast cells are activated from the initial wound event; in turn these activated mast cells release several chemicals called `cytokines' which will now attempt to repair the damage. How do they do it? There is a cytokine which stops the bleeding by initiating clot formation. There is a cytokine which arouses our pain receptors giving us a sense of pain at the site (`caution – ground under repair!). There is a cytokine which swells blood vessels allowing more blood to reach the region (site feels warm and looks red). There is a cytokine which attracts white blood cells to the site so that viruses and bacteria (and dead tissue) can be destroyed. And there is a cytokine which activates fibroblast cells to make scar tissue so that the site can be patched up. This is the inflammatory response in a nutshell – i.e., body's attempt to heal a wounded area.

......The third tier of our body defenses is the immune system which is comprised of a range of white blood cells; once activated into action by a foreign substance or antigen (e.g., virus, bacteria) the white blood cells destroy them either directly or indirectly. There is the `humoral response' of the immune system where white cells make specific antibodies against the invader; in this way the antibody binds to the antigenic cell or particle rendering it harmless. On the other hand there is the `cellular response' of the immune system where white blood cells attack the antigen directly and kill it. Both the humoral and cellular response make up the immune system of the body, and are carried out by the white blood cells which are located in both the circulatory and lymphatic systems.

Links related to immune system: www.people.virginia.edu/~rjh9u/day38.html and www.people.virginia.edu/~rjh9u/day39.html

Respiratory – Oxygen Pick Up and Delivery/Energy Transformation

......This system is primarily responsible for getting vital oxygen to our cells and tissues, and for getting rid of waste carbon dioxide. The major components of this system are the lungs (and its air passages), and the pulmonary and systemic circulatory circuits whose functions are to pick up oxygen from the lungs and deliver it to our body tissues, and to off-load carbon dioxide waste from these same tissues.

......Why is oxygen so important? Like most animals, we humans are aerobic organisms who depend on a steady input of atmospheric oxygen for survival; i.e., we need oxygen in order to drive all our metabolic processes. More specifically, in order for our cells to capture the energy from the food we eat (converted to a usable form called ATP), we need to sequentially break down food nutrients in an aerobic process called `energy metabolism'. In this process, simple molecules like glucose and fatty acids are metabolically broken down in a 3-step process which yields carbon dioxide, water, and energy in the form of heat and ATP molecules; it is the ATP that the cell uses to do work – whether it is making a protein like an antibody or hormone, or contracting like that found in our muscle tissues. All functions of our cells and tissues need ATP energy.

......The 3-step process of energy metabolism involves the following: The first step in the process is called `glycolysis' in which the 6-carbon glucose molecule is broken down by enzymes into 2 molecules of the 3-carbon pyruvic acid (this process does not require oxygen, and only 2 ATP molecules are produced); the second step called the `kreb's cycle' involves the initial removal of carbon dioxide from each pyruvic acid molecule to yield a 2-carbon acetic acid molecule, in turn this 2-carbon molecule joins up with a 4-carbon molecule to form a 6-carbon molecule; then sequentially this 6-carbon molecule loses a carbon dioxide to form a 5-carbon molecule, and again the 5-carbon molecule loses a carbon dioxide yielding a 4-carbon molecule (to complete the kreb's cycle this 4-carbon molecule is rearranged by enzymes to yield the exact same 4-carbon molecule that originally began the kreb's cycle). At this stage in the metabolic breakdown of a single glucose molecule the cell has been able to form the following: 6 molecules of carbon dioxide which now needs to be off-loaded from the cell to the blood and subsequently cleared by the lungs; 4 ATP molecules which as we'll see is not enough energy to support the needs of any cell; and lastly, a large accumulation of hydrogen ions (which are removed from some of the molecules in what are called `dehydrogenation' or `oxidation' steps). In several intermediate steps within glycolysis and the kreb's cycle there are `oxidation' reactions or in chemical terms, a molecule is said to be oxidized if it loses electrons (or in this case hydrogen ions which carry electrons). So where is the energy to be found for cell use, and what role does oxygen play? This takes us to the third step in the energy metabolic process called the 'electron transport system'.

......Most of the ATP needed by our cells is made within the electron transport system. In simple terms, we can look at the electron transport system as being a hydroelectric facility located at Niagara Falls in which the power of falling water is used to spin turbines to make electricity. The potential energy of the rushing water is partially transformed into electric energy now capable of giving power to light bulbs, air conditioners, and PCs. The same principle applies to the electron transport system – potential energy from the hydrogen ions is sequentially lost to form ATP molecules. How? After the hydrogen ions have lost most of their energy in the electron transport system, hydrogen is joined to the oxygen we inhale to form water; in fact, 6 molecules of water are formed from each starting glucose molecule (to now join the 6 molecules of carbon dioxide formed earlier – hence all of the carbon, hydrogen and oxygen is accounted for in the complete break down of the 6-carbon glucose molecule). Importantly, in the process of hydrogen losing energy and uniting with oxygen to form water, 38 ATP molecules are formed from a single molecule of glucose! To put the value of oxygen in perspective, a cell without oxygen makes only 2 ATPs, while with oxygen 38 ATPs are formed – providing sufficient energy to drive all of our body functions. Hence, our respiratory system has the prominent role of delivering oxygen to our body's cells and tissues so ATP can be efficiently formed to do metabolic work. If there is no oxygen in the system there is metabolic `gridlock' – there is the buildup of hydrogen ions with all its energy, but there is nowhere for it to go; oxygen is necessary to relieve the gridlock serving as the final hydrogen-acceptor molecule to form water.

Links related to lungs and energy metabolism: www.ultranet.com/~jkimball/BiologyPages/P/Pulmonary.html and www.ultranet.com/~jkimball/BiologyPages/I/IntermediaryMetabolism.html and www.ultranet.com/~jkimball/BiologyPages/M/Metabolism.html

Digestive – Breakdown of Food into Usable Molecules

......The overall role of this system is to physically and chemically break down the food we eat and make the nutrient components available to the body's cells and tissues. Our digestive system is basically composed of a long tube (from mouth to anus) and accessory organs such as teeth, salivary glands, pancreas and liver. How does the digestive tract break down complex molecules in our food and drink, and prepare them for absorption into our blood stream so that they can be carried to our cells? The answer is - `enzymes'. These chemical catalysts are able to split chemical bonds holding subunits together making up a larger macromolecule.

......Beginning in the mouth, complex carbohydrates are acted upon by digestive enzymes in saliva creating simple sugars. Complex carbohydrates such as starch are composed of several hundred subunits of glucose units chemically-bonded together – enzymes are necessary to break these bonds. Later in the stomach, food proteins are acted upon by pepsin and other enzymes breaking them down to constituent amino acids. Proteins such as found in meat and dairy are composed of thousands of amino acid subunits held together by chemical links called peptide bonds – again enzymes are necessary to break these bonds. Subsequently within the long and twisting small intestine (with the aid of digestive enzymes from the pancreas and bile from the liver to solubilize fat), all remaining complex carbohydrates are converted into simple sugars, fats which are mainly in triglyceride form are broken down into its fatty acid subunits, and the remaining proteins are further broken down into amino acid subunits. Once these simple molecules are formed, they are now capable of being absorbed into the bloodstream, and transported to our body's cells and tissues for usage.

......What are the metabolic fates of the newly digested food nutrients? The answer is related to a number of factors such as the specific cell or tissue involved, the degree of physical activity of the person, or the particular stage in the human life-cycle. If we first look at the carbohydrate picture, simple sugars reach the cell and 3 pathways are possible – 1. metabolically break it down into ATP energy for immediate use by the cell (active cells like brain neurons, and muscle continually burn sugar for ATP energy); 2. store glucose by converting it into a complex carbohydrate called glycogen (in this manner energy is stored up for future use; or 3. if the storage capacity for glycogen is at its maximum within a cell, the simple sugars can be converted into fatty acids and stored within adipose tissue widely-distributed within the body.

......The fate of fatty acids is somewhat similar to simple sugars in that fatty acids can be metabolically broken down for ATP energy, or it can be converted into a triglyceride storage form available for future use. Amino acids on the other hand are used primarily to make new cellular proteins (under the specific directions of the DNA code). However, if the cell has a surplus of amino acids (which is often the case in protein-rich western diets), amino acids can be converted into glucose and fatty acids (and thereby either broken down to make ATP energy or stored as glycogen/triglyceride). It should not be surprising that active, growing periods of the life-cycle need more food protein (per unit body weight) than the non-growing stages of adulthood. Likewise, it should come as no surprise that physically active individuals require more food energy sources like carbohydrates and fats than sedentary individuals (the demands of muscle for ATP generation from food sources is proportionately greater).

Links related to digestion and nutrients: www.people.virginia.edu/~rjh9u/day19.html and www.ultranet.com/~jkimball/BiologyPages/G/GITract.html

Excretory – Waste Removal from the Body

......The human body produces several waste products which need to be eliminated; otherwise serious health risks can result from the build up of certain waste products. There are a number of organs involved in the excretion process: the skin is a major site of heat and water loss from the body (and as such skin plays an important role in helping to maintain a constant body temperature by balancing heat gain and loss mechanisms); the lung is the site for the off-loading of carbon dioxide waste generated from cellular energy metabolism (lungs also provide an additional site for heat and water loss as well); the digestive tract generates waste in the form of fecal matter (in addition to non-digestible fiber made of the carbohydrate cellulose, feces also contains water, heat, bacteria, discarded hemoglobin pigments, as well as dead cells sloughed off from the digestive tract wall); and the kidney which produces watery urine (in addition to water, urine contains many nitrogen compounds which result from the normal breakdown of proteins within the body; for example, amino acids which are not recycled by our cells lose their amino groups which is then fused with carbon dioxide to form a soluble, non-toxic waste product called urea; the average person excretes 20-25 grams of urea per day).

Reproductive – Continuation of the Human Species

......Directly tied to survival of the human species, the reproductive systems of females and males provide the gametes (eggs and sperm) necessary for the next generation, as well as the secondary sex organs which permits copulation, internal fertilization and development, and post-uterine nutrition. On the female side, the pituitary gland during adolescence secretes both follicle stimulating hormone (FSH) which stimulates the ovaries to produce eggs, and luteinizing hormone (LH) which targets the ovaries to produce estrogen steroid hormones. Subsequently, estrogens target various tissues of the female reproductive tract causing them to mature and become physiologically functional (e.g., uterus, mammary glands). On the male side, FSH and LH are also secreted during adolescence which together they target the testes to produce sperm and androgen steroid hormones, respectively. Subsequently, the androgens target various tissues of the male reproductive tract causing them to mature and become physiologically functional (e.g., epididymus, penis). While further details will be covered in greater depth later this semester; it can be stated here that the reproductive system provides the mechanism for generational survival of the human species, as well as insuring that offspring are genetically different from their parents (through the process of meiosis carried on within the ovaries and testes in which eggs and sperm are produced).

Links related to human reproduction: www.people.virginia.edu/~rjh9u/day30.html and www.people.virginia.edu/~rjh9u/day31.html and www.people.virginia.edu/~rjh9u/day34.html and www.ultranet.com/~jkimball/BiologyPages/S/Sexual_Reproduction