THE HUMAN BODY: AN ORIENTATION

 

This introductory material on the human body is similar to lifting the first of many layers of wrapping of a package. You do not see the end immediately but becomes more curious what is in the package. These many layers will include initially examining the human body from the outside to cellular reproduction and development in the end.

        This presentation examines the human body: the structures and shapes of the body parts and their functions otherwise known as anatomy and physiology. We will examine the science of anatomy and physiology; the maintenance of life (requirements); the planes of reference; body regions and cavities. This presentation will also present three concepts:

a.          structure and shapes of the body parts and their functions

b.     the hierarchy of structural organization

c.     homeostasis (maintenance of functional constancy

Although we will discuss the human body as it relates to science of anatomy and physiology; planes of reference and maintenance of life, our focus will be on the relationship of structure, shape and functions; hierarchy of organization and homeostasis.

 

I.    The Science of Anatomy and Physiology

 

Anatomy and physiology are dynamic sciences that combine morphological and structural components with functional relationships. Anatomy deals with structures and  morphology (shapes) of parts of the human body. Physiology on the other hand, deals with function; how the structures and morphology of parts of the body are related to their functions. The human body is made of different parts and shapes. These parts carryout specific functions based on their parts.  Collectively, science of anatomy and physiology attempts to explain the structure, morphology, organization and balance of molecules and their functional relationships.

        The science of anatomy and physiology has developed along with the quest for knowledge in the treatment of diseases.  During the Roman Era, it was the belief that sickness or disease was a result of the anger of gods and the Romans built many temples in which sacrifices were made to appease the gods. Religion, philosophy and magic were practiced together in most temples. The following pioneers contributed to the current understanding in anatomy and physiology disciplines.

·      Hippocrates (377BC) believed that diseases developed from natural causes, thus he separated medicine from the religion and philosophy.  Hippocrates is considered by some as the father of medicine.

·      Galen (130-201AD) was a famous Greek writer whose opinion on anatomy and medicine, was highly valued. His writings were considered to contain many fallacies because of his religious belief, which biased his scientific opinion. In addition, he did not have any practical experience dissecting human specimen; therefore, he draws conclusions from the studies of other philosophers.

·      Andreas Vesalius (1514-64) was among the first few to perform dissections on humans. His writings negated many of the established views of Galen and created controversies. Vesalius was considered the father of anatomy.

·      William Harvey, considered the father of physiology confirmed some the findings of Vesalius, but also showed that blood flow only one-way in a vessel rather than two-way as previously proposed by Galen.

·      Antony van Leeuwenhoek (1632-1723) first invested the microscope.  Robert Hooke (1635-1703) examined cell structure with the microscope and Marcello Malpighi (1628-94) used the microscope to show the motion of blood thru vessels. Together, their contribution paved the way for the development of the cell theory.

·      Mathias Schleiden and Theordor Schwan developed the cell theory

·      The development of the microscope also leads to the development of the scientific method, an organized method of scientific inquiry.

 

II.       Maintenance of Life

Certain characteristics or necessities are required of all living organisms for maintenance of life. These requirements are:

·      Organization: cells or organs in the human body are designed to carry out specific functions.

·      Responsiveness: all organisms are able to respond to changes in their environment (internal or external).

·      Locomotion: movement of ions, blood flow, electrical impulses and change in direction are necessary to maintain vital functions and survival of the organism.

·      Differentiation: cells, tissue differentiates and specializes to take on specific functions leading to growth and development.

·      Metabolism: the sum total of all of the chemical changes in the body: the process of building up (anabolism) and breaking down molecules (catabolism).

·      Digestion: Preparation of food molecules for energy production and metabolic transformation.

·      Excretion: Removal of waste materials that may be toxic in the body

·      Reproduction: cellular replication, growth and repair and sexual procreation of offspring

·      Survival instincts: An inborn instinct for survival, an essential weapon for all organisms.

 

III.                   Body Landscapes

 

The human body can be divided or group for convenience into several planes, regions, cavities and membranes they contain.

a.         Planes of reference

The anatomical position is based on three planes of reference:  a midsagittal plane passes lengthwise through the body dividing it into two equal halves. A coronal (frontal) plane also passes through the body diving the body into anterior (front) and posterior (back) position.  A transverse plane (horizontal or cross-sectional) divides the body into superior (above, upper) and inferior (lower, below) position.

 

b.       Body regions and cavities

Body regions include head and neck (facial, cranial and cervical regions); thorax (mammary, sterna, maxillary, scapula and vertebral regions); 

abdomen (umbilicus or navel, pelvic/pubic area, lumber and sacral regions; buttock or gluteal regions); upper and lower limbs (shoulders, brachium (arm) antebrachium (forearm), palm, knee, thigh, leg, patella to dorsum of the foot.

 

There are two main body cavities:

·            the posterior (dorsal) body cavity containing the cranial cavity for brain and ventral cavity for spinal cord

·            the anterior cavity contains the thoracic cavity, abdominal cavity and the pelvic cavity. The abdominal and pelvic are collectively called the abdominopelvic cavity.

 

c.         Membranes : The membranes are made of thin layers of connective and epithelia tissues that cover, support and separate viscera and lines the body cavities. The meminges for example, line the dorsal cavities of the brain and spinal cord. There are two types of body membranes: mucous and serous membranes.

·      Mucous membrane secretes a thick liquid substance called mucous which lubricates the organs. The mucous membrane lines the tubes the enter or exit the body (e.g., mouth, nasal, respiratory and digestive tracts).

·      Serous membrane covers the thoracic and abdominopelvic cavities and covers the visceral organs that secrete watery fluid called serous fluid. Other parts of the body that contain serous membrane include

 

i.                              Lungs:  pleura are a type of serous membrane that lines the interior of the lungs.  It consists of visceral pleura that cover the outside surface of the lung and parietal pleura that covers the thoracic walls and sides of the diaphragm. The space between the two linings is the pleural cavity.

 

   ii.                        Heart:  pericardial membrane is another type of serous membrane in the heart. It consists of parietal peritoneum that forms a sac surrounding the heart while the visceral peritoneum forms the outer surface covering of the heart. The fluid filled space between the two membranes is the pericardial cavity.

 

      iii.                      Abdomen:  In the abdomen, the serous membrane lining the abdominal surface is

called the peritoneal membrane. The parietal peritoneum lines the abdominal wall while the visceral peritoneum covers the surfaces of organs. A mesentery is a double layer of parietal peritoneum that connects the visceral peritoneum to the parietal peritoneum. Mesenteries also help to hold many organs in place while permitting involuntary movements.

 

IV.  Structure-Function Relationships

 

The human body is a machine that consists of many movable, slightly movable and immovable parts.  All parts of the body have different shapes that compliment other parts to carry out specific functions. The heads of the humorous and the femur for example fit the shoulder blade and the pelvic bone cavities (sockets) providing free rotational movements. The thoracic and the cranial cavities provide protection for the heart, lungs and the brain respectively. The phalanges (palm bones) are designed for grasping or holding while the tarsal (foot bones) are for weight bearing and locomotion. Parts that do not compliment may not provide maximal support and function.

 

V.       Levels of Organization

 

The human body is complex and is organized structurally and functionally.  The following hierarchy could be seen at all structural and functional levels:

·       Atomic particles: The smallest ultra structural organization of the body can be seen at the level of atomic particles: protons, electrons and neutrons that make up elements. Elements in different ratios make up chemical compound (e.g., H2S, H2O, H2SO4).

·       Chemical: the body is made of chemicals that consist of atoms and molecules. These fundamental building blocks of atoms in chemicals arrange to form molecules. The molecules combine to form macromolecules such as proteins, carbohydrates, lipids and other molecules.

 

·       Cellular level: the macromolecules in various combinations form the cell, the structural and functional unit of the body. The body consists of various types of cells including, blood, muscle and nerve.

 

·       Tissue level: cells with similar structure and morphology and functions combine to form tissues. Examples of tissues include muscle and nerve.

 

·       Organ level: An organ such as the heart, liver and kidney consists of several types of tissues that are related and carryout similar but specific function.

 

·       Organ system level: the organ systems of the body such as circulatory, respiratory, digestive and others are independent of each other yet carrying out their functions in an integrated fashion.

 

·       Organism level: this is the highest level of organization that makes up the entire organism. At the organism level, all the components of the organism function in coordination.

 

V.       Homeostasis

 

Homeostasis refers to maintenance of constant stable environment in the human body.

       French physiologist Claude Bernard (1813-78) made the observation that the internal environment of the body remains fairly stable even when the outside conditions are variable (i.e. hot or cold).

       American physiologist Walter Cannon (1871-1945) coined the word homeostasis to describe stable conditions in the body.

 

·       All units in the body function best when their internal environment is stable. Consequently, blood and tissues must operate at or near physiological range with narrow limits of fluctuation.

 

·       Homeostasis implies stable environment within narrow margins of variation. The body has set point for functional operation in all systems of the body. For example, the body temperature is set at 37˚C but may fluctuate between 38˚C or 35˚C. The ability of the body to fluctuate and return to the set point is referred to as dynamic equilibrium.

 

·       It is similar to a thermostat in the house.  The set point of the temperature may be 64˚C. If the temperature drops below the set point, a temperature-sensitive sensor triggers the furnace, which blows and warms the house. As soon as the temperature goes above the set point, it is deactivated. The reverse is true if the temperature is above the set point.

 

·       Although there are variations, such changes must be kept at narrow physiological range (small margins above or below the set point).

 

·       Maintenance of homeostasis requires adequate levels of nutrients to supply energy, proper functioning of the heart and monitoring of the blood volume and pressure; monitoring and disposal of waste materials and many other factors work together to maintain homeostasis.

 

Control Mechanism of homeostasis

 

The ability of the body to maintain homeostasis is dependent on two mechanisms: negative feedback and positive feedback.

·       A negative feedback is a process in which the body senses a change and activates mechanisms that reverse or negates the change.

 

·       A good example of negative feedback is the home-heating furnace and how this device maintains homeostasis, by warming or cooling the house when the internal temperature changes with respect to the outside. By reversing or negating changes in the set-point of temperature in the house, homeostasis is maintained.

 

·       In our body, all of the systems carryout such adjustments to maintain homeostasis. For example, the set-point temperature for the body is 37˚C (68.7F). The hypothalamus monitors variations in our body temperature and maintains homeostasis. If the temperature goes over the set-point (i.e. 40˚C and above), the negative feedback mechanism reverses the rising temperature and tries to bring it back with range (37˚C -38C)

·       The hypothalamus reverses this condition by causing nerves to stimulate the heart to pump faster. It causes blood vessels to dilate which brings more blood to the surface of the skin. As a result the body loses more heat, and water in the form of perspiration that cools the body. The opposite is true if the temperature falls below the set point. Blood vessels constrict, and less blood comes to the surface therefore less heat is lost to the surface, and the muscles contract more burning more fuel to supply heat to the body.

 

Positive feedback mechanism

 

·       Positive mechanism is a process that enhances or promotes a positive trend.  It is directly opposite to negative feed.

 

·       Positive mechanism attenuates or supports effectors or signals that bring out change. For example when temperatures go above the set point of the thermostat in home-heating furnace, a positive mechanism works by enhancing the increase in the temperature rather than reversing it.

 

·       An example is the secretion of the hormone oxytocin. During childbirth, the baby’s head pushes towards the birth canal. This triggers nerves in the cervical canal that stimulates the hypothalamus to secrete oxytocin. The release of this hormone further stimulates muscles in the cervix to relax thus enabling the delivery.

·       While positive mechanism is beneficial it may also be dangerous. For example fever causes a slight rise in temperature that is beneficial because it raises the metabolic rate of the body and also destroys bacteria unable to live at that temperature.  Continue increase in the temperature may also affect enzyme and other high temperature functions in the body.