Introduction to Human Body

 Chapter 1

Introduction to Human Body

Definition and Scope of Anatomy and Physiology

• The human being is a very complex multicellular organism. The study of human body is divided under two major headings, i.e. Anatomy and Physiology.

• 1) Anatomy: It is the study of structure of the body as a whole, as also the study of its

• individual parts and their relations to one another.

• 2) Physiology: It is concerned with the way in which various organs function and how

• They are integrated to produce a coordinated action of the whole body. A cell is the smallest functional unit of an organism. Groups of cells having the same physical characteristics and performing similar specialized function, are described as tissues. Various types of tissues join to form an organ, and a system consists of a number of such organs and tissues. Although each system carries out one or more of the vital functions of the body, none of the systems can exist in isolation because of specialization of cells.

• Different body organs have different structures. The variation in structures is due to the variations in the functions they perform. For example, the bony skeleton provides support to various organs; therefore, bones are rigid in structure. The heart has to pump blood through the blood vessels; hence walls of the heart are highly muscular. In the alveoli of lungs.

Level of 5tru4ural Organization and Body System

Before discussing these specialties in structures, various systems are briefly described below:

The human body consists of the following systems:

(1) Skeletal system: It consists of the bony framework. It acts as structure and offers protection to the internal organs. 

(2) Muscular system: The muscles and tendons are attached to the bones. They help to give the body its shape and power to move at will. Nervous system: It serves to control and to co-ordinate the functions of various parts of the body. 

 ( 3) It also makes an individual keenly aware of the environments in which (s)he lives. Circulatory system:

 (4)  It distributes the essential supply of oxygen and nutrient materials to all the parts of the body and removes the waste products, from the body.

(5) Respiratory system: It supplies the body with oxygen from the atmosphere and it disposes of carbon dioxide and water vapors from the body. It comprises of lungs and bronchi.

The Cell and Cellular Level of Organization

• The cell is a basic and the smallest functional, living unit of the body tissue. The cells are grouped together to form the tissues. Cytology is the branch of science which is concerned with the study of the cell. In an unicellular organism, a single cell performs all the functions while in multicellular organisms' cells get specialized to form different tissues; each tissue performs a specific function.

• Plasma or Cell Membrane The membrane which separates the internal components of a cell from extracellular material is known as plasma membrane or cell membrane.

• The plasma membrane regulates passage of the substances in and out of the cell. The plasma membrane consists of equal proportion of proteins and lipids. Lipids are made up of 75 per cent of phospholipids. Lipids normally consist of phosphorus and small amount of cholesterol and glycolipids. The phospholipids are arranged in parallel layers forming a phospholipid bilayer which forms the basic framework of the plasma membrane. Phospholipids are amphipathic in nature.

Organelles

• Exocytosis: It is a form of active transport in which a cell transports molecules out of it by expelling them in an energy using process. In this process, membrane bound secretory vesicles are carried to the cell membrane and their contents are emptied into extracellular environment.

• Inside the cell, there are specialized structures which have a specific role in growth, maintenance, repair and control. These specialized structures provide compartments inside the cells and are called Organelles.

Nucleus

• It is the largest structure in the cell and is usually spherical or oval in shape. Nucleus contains hereditary units called genes. These genes control the cellular activities and the structure. Genes are arranged in a single unit along structures known as chromosomes. Human body contains 46 chromosomes in 23 pairs. Nucleus has a double membrane known as nuclear envelope which separates the nucleus from the cytoplasm. Envelope also consists of a phospholipids bilayer which has nuclear pores. It allows ions and water-soluble molecules to pass between nucleus and cytoplasm. Inside the nucleus, there are spherical bodies called nucleoli, which are the sites for ribosome biogenesis. The chromosomes are clusters of DNA molecules. DNA molecule is a sequence of nucleotides, and each nucleotide has three components, a sugar molecule, phosphate group and nitrogen containing base. Also, there are four bases in DNA: adenine (A), thiamine (T), guanine (G) and cytosine (C). Ribosomes are essential for protein synthesis.

3. Anaphase

• It is a third state of mitosis. It is characterized by splitting and separation of centromere and movement of the two sister chromatids of each pair towards opposite poles of the cell. After separation, sister chromatids are known as daughter chromosomes. These chromosomes move due to shortening of kinetochore microtubules. Elongation of also non- kinetochore microtubules occurs, this type of process helps in increasing the distance between the two separated chromosomes.

4. Telophase

• It is the final state of mitosis. This state starts as the movement of chromosomes stop. In this state, the nuclear membrane reappears, the spindles disappear and the constriction develops round the middle of the cell body. This finally leads to two daughter cells.

Homeostasis 

• Homeostasis is the property of a system within the body of a living organism in which a variable, such as concentration of a substance in solution, is actively regulated to remain very nearly constant. The word originates from two words: homes and stasis. In Greek language, homotops means similar and stasis means standing still. Thus, the word indicates "staying the same". In human body, there are internal and external variables operating at various points of time. InSite of such variables, the body maintains a fairly constant environment due to mechanisms of homeostasis.

Homeostatic Mechanisms

• There are two kinds of feed backs in a chain like event in human body. The mechanisms are called as negative feedback and positive feedback respectively. Take a case of chain reaction in which A is converted to B; B is converted to C; C to D and D to E. Quite often every change is controlled by activity of an enzyme. The rate of every enzymatic reaction can vary. The slowest reaction is termed as rate limiting step. If any of the intermediate from A to E is over produced, then higher concentration of the intermediate slows down the reaction leading to its generation. This is termed as negative feedback. Thus, negative feedback limits higher production of an intermediate through a self-controlling mechanism. Alternatively, if production of an intermediate is reduced below certain limits, then related enzymes may be activated to restore level of the intermediate. This is called as positive feedback mechanism.

homeostasis of Volume of Body Water

• The volume of water in the body is measured by stretch receptors in the atria of heart. They are also indirectly sensed by measurement of the osmolality of plasma by the hypothalamus. Measurement of the plasma osmolality gives an indication of the water content of the body. It relies on the fact that water losses from the body through sweat, gut fluids in the form of fecal water and through vomiting/diarrhea and the exhaled air, are all hypotonic. It means that fluids like saliva, tears are less salty than plasma. Tears have almost the same salt content as that of extra cellular fluids while saliva is hypotonic with respect to plasma. Thus, taste of saliva is not salty while tears are decidedly salty. Nearly all normal and abnormal losses of body water make extra cellular fluids hyper-osmolar. Conversely, excessive water intake dilutes the extra cellular fluids making it hypo-osmolar. Only after loss of water through urine body can become isotonic. Hence excessive water intake leads to frequent urination.

Homeostatic Breakdown 

• Many diseases are the result of failure of one or more homeostat(s) in the body. The body water homeostat can be disrupted by inability to secrete ADH in response to even the normal body water losses via the exhaled air, the feces and sweating. On not receiving ADH signal, the kidneys produce huge unchanging volumes of very dilute urine, causing dehydration and even death.

• Another example is of type I diabetes mellitus. In this case, blood glucose homeostat ceases to function due to destruction of beta cells of pancreas. This means that the glucose sensor is absent and the effector pathway of insulin level in blood remains unchanged. The blood glucose concentration rises to very high levels, while the body's proteins are degraded in to amino acids and are used for generating energy via gluconeogenesis. If untreated, the condition can be fatal.