H1 Functional Organization of the human body and control of internal environment
Human physiology= the science of human physiology attempts to explain the specific characteristics
and mechanism of the human body that make it a living being. We are alive because of control
systems, like hunger→ seek for food, fear→ seek refuge, sensing, feeling, reproduce. These
attributes allow us to exist under varying conditions.
Cells are the living units of the body
Basic living unit of the body is the cell. Each organ consists of many different cells held together by
intercellular supporting structures. Each cell type is specially adapted to perform one or a few
particular functions. Red blood cells are most abundant in our body (about 25 trillion (next to 75
trillion other cells, so in total 100 trillion cells)).
Cells are different, but have the same basic characteristics:
- Oxygen reacts with carbohydrates, fat, and protein to release energy required for all cells to
function.
- General chemical mechanisms for changing nutrients into energy are the same.
- All cells deliver products of their chemical reactions into the surrounding fluids.
- Almost all cells have the ability to reproduce additional cells of their own kind. When cells of
a particular type are destroyed the remaining cells of that type can generate new ones until
the supply is replenished.
Extracellular fluid- the ‘internal environment’
About 60% of the body is fluid, mainly a water solution of ions and other substances. Most of the
fluid is inside the cells and is called intracellular fluid, about 1/3 is in the spaces outside of the cells
and is called extracellular fluid. The extracellular fluid is in constant motion throughout the body. It is
transported rapidly in the circulating blood and then mixed between the blood and tissue fluids by
diffusion through the capillary walls. In the extracellular fluid are the ions and nutrients needed by
the cells to maintain life. Thus, all cells live essentially in the same environment, therefore the
extracellular fluid is called the internal environment of the body (milieu intérieur). Cells are capable
of living and performing their special functions as long as there are proper concentrations of oxygen,
glucose, different ions, amino acids, fatty substances, and other constituents available in the internal
environment.
Differences between extracellular and intracellular fluids
The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate (HCO3-) plus
nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. Also, carbon dioxide
(goes to the lungs for excretion) plus other cellular waste products that are being transported to the
kidneys for excretion. The intracellular fluid differs from the extracellular fluid. It contains large
amounts of potassium, magnesium, and phosphate ions instead of sodium and chloride. Special
mechanism for transporting ions through cell membranes maintain the ion concentration differences
between the extracellular and intracellular fluids.
Homeostasis- maintenance of a nearly constant internal environment
Homeostasis is the maintenance of nearly constant conditions in the internal environment. All
organs and tissues perform functions that help maintain these relatively constant conditions, like the
lungs provide oxygen to the extracellular fluid to replenish oxygen used by cells, the GI-tract provides
nutrients, and the kidneys maintain constant ion concentrations. The constituents of the body are
regulated within a range of values rather than fixed values. For some constituents this range is
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,extremely small (blood hydrogen ion→ less than 5 nanomoles per liter, blood sodium
concentration→ few millimoles per liter). Powerful control systems exist for maintaining the
concentrations of the constituents in the body that permit the cells, tissues, and organs to perform
their normal functions despite wide environmental variations and challenges form injury and
diseases. Normal body functions require the integrated actions of cells, tissues, organs, and the
multiple nervous, hormonal, and local control systems that together contribute to homeostasis and
good health.
Disease is the state of disrupted homeostasis. However, even in the presence of disease homeostatic
mechanisms continue to operate and maintain vital functions through multiple compensations.
These compensations may lead to major deviations of the body’s functions from normal range,
making it difficult to distinguish the primary cause of the disease from the compensatory responses.
Like, diseases that impair the kidney’s ability to excrete salt and water may lead to high blood
pressure→ initially helps to return excretion to normal ranges, but over long periods high blood
pressure can damage organs, including the kidneys, causing even greater increases of blood pressure
and more renal damage. Thus, homeostatic compensations that ensue after injury, disease or
environmental challenges may in the long term contribute to additional abnormalities of body
function. Pathophysiology seeks to explain how the various physiological processes are altered in
diseases or injury.
Extracellular fluid transport and mixing systems- the blood
circulatory system
ECF is transported through the body in two stages:
1. Stage 1: movement of blood through the body in
blood vessels.
2. Stage 2: movement of fluid between the blood
capillaries in the intracellular spaces between the
tissue cells.
This figure shows the overall circulation of blood. All the
blood passes the entire circulatory circuit an average of
once each minute when the body is at rest and six times
each minute when a person is extremely active. In the next
figure it is shown that when the blood passes through the
capillaries, continual exchange of extracellular fluid occurs
between plasma portion of the blood and the interstitial
fluid that fills in the intercellular spaces. Walls of capillaries
are permeable to most molecules in the plasma of blood
(amino acids are too large). Therefore, large amounts of
fluid and its dissolved constituents diffuse back and forth
between blood and tissue spaces. This process of diffusion
is caused by kinetic motion of molecules in both the
plasma and the interstitial fluid. The fluid and dissolved
molecules are continually moving and bouncing in all
directions within the plasma and the fluid in the
intercellular spaces, as well as through the capillary pores.
Thus, the ECF in the body, both that of the plasma and that
of the interstitial fluid is continually being mixed, thereby
maintaining homogeneity of the ECF throughout the body.
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,Origin of nutrients in the extracellular fluid
Each time blood passes through the body, it also flows through the lungs. The blood picks up oxygen
in the alveoli, this is the oxygen needed by the cells. The membrane between the alveoli and the
lumen of the pulmonary capillaries (alveolar membrane) is 0.4-2.0 mm thick and oxygen rapidly
diffuses by molecular motion through this membrane into the blood.
Gastrointestinal tract
A large portion of the blood also passes through the walls of the GI-tract. Here dissolved nutrients,
including carbohydrates, fatty acids and amino acids are absorbed from the ingested food into the
extracellular fluid of the blood.
Liver and other organs that perform primarily metabolic functions
Not all substances absorbed from the GI-tract can be used in their absorbed form. The liver changes
the chemical composition to a more usable form and other tissues of the body, fat cells, GI-mucosa,
kidneys, and endocrine glands help modify the absorbed substances or store them until they are
needed. The liver also eliminates certain waste products produced in the body and toxic substances
that are ingested.
Musculoskeletal system
Were it not for the muscles, the body could not move to obtain the foods required for nutrition and
therefore the musculoskeletal system contributes also to homeostasis. It also provides motility for
protection against adverse surroundings, without which the entire body, along with its homeostatic
mechanisms, could be destroyed.
Removal of metabolic end products
- Removal of carbon dioxide (CO2) by the lungs: at the same time that blood picks up oxygen
in the lungs, CO2 is released from the blood into the alveoli. The respiratory movement of air
into and out of the lungs carries the CO2 to the atmosphere. CO2 is most abundant of all the
metabolic products.
- Kidneys: blood passes through the kidneys and the kidneys remove from the plasma most of
the other substances besides CO2 that are not needed by the cells. The substances are other
end products of cellular metabolism, like urea, excesses of ions, water, and uric acid. The
kidneys filter large quantities of plasma through the glomerular capillaries into the tubules
and then reabsorbing into the blood the substances needed by the body, such as glucose,
amino acid, appropriate amounts of water and many of the ions. Metabolic waste products
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, and other substances that are not needed, are reabsorbed poorly, and pass through the
renal tubules into the urine.
- GI-tract: undigested material and some waste products of metabolism are eliminated in the
feces.
- Liver: detoxification or removal of many drugs and chemicals that are ingested. The liver
secretes many of these wastes into the bile to be eventually eliminated in the feces.
Regulation of body functions
- Nervous system: the nervous system is composed of three major parts: the sensory input
portion, the central nervous system (the integrative portion), and the motor output portion.
Sensory receptors detect the state of the body or the state of the surroundings. The eyes, the
skin, the ears are all sensory organs. The central nervous system is composed of the brain
and the spinal cord. It can store information, generate thoughts, create ambition, and
determine reactions that the body performs in response to the sensations. Signals are then
transmitted through the motor output portion of the nervous system to carry out one’s
desires. An important segment of the nervous system is called the autonomic system. It
operates at a subconscious level and controls many functions of internal organs, like
pumping by the heart, movements of the GI-tract and secretion of body glands.
- Hormone system: in the body are 8 major endocrine glands and several organs and tissues
that secrete chemical substances called hormones. Hormones are transported in the ECF to
other parts of the body to help regulate cellular function.
o Thyroid hormone increases the rate of most chemical reactions in all cells→
increases the tempo of bodily activity.
o Insulin controls glucose metabolism.
o Adrenocortical hormones control sodium and potassium ions and protein
metabolism.
o Parathyroid hormone controls bone calcium and phosphate.
Hormones provide a system for regulation that complements the nervous system. The
nervous system regulates many muscular and secretory activities, and the hormonal system
regulates many metabolic functions. Both systems work together in a coordinated manner to
control essentially all the organ systems of the body.
Protection of the body
- Immune system: it consists of the white blood cells, tissue cells derived from the white blood
cells, the thymus, lymph nodes and lymph vessels that protect the body from pathogens. The
immune system provides a mechanism for the body to distinguish its own cells from foreign
cells and substances and destroy the invader by phagocytosis or by producing sensitized
lymphocytes or specialized proteins (antibodies) that destroy or neutralize the invader.
- Integumentary system: the skin and its various appendages (hair, nails, glands, and other
structures) cover, cushion, and protect the deeper tissues and organs of the body and
provide a boundary between the body’s internal environment and the outside world. The
integumentary system is also important for temperature regulation and excretion of wastes,
and it provides a sensory interface between the body and the external environment. The skin
comprises about 12-15% of the body weight.
Reproduction
Reproduction helps maintain homeostasis by generating new beings to take the place of those that
are dying. In the final analysis, essentially all body structures are organized such that they help
maintain the automaticity and continuity of life.
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