Major Systems

The cells of similar shape and size are combined to form Tissue and billions of different kind of tissues are combined to form an organ. Then, different Organs are combined to form a system and many systems combine to form the whole body.

Nervous system



It consists of billions of Nerve cells, called Neurons and each Neuron has Nucleus, Axon & Dendrites. The MOTOR neurons (having longer Axons and smaller Dendrites) are efferent nerves (carry message from Brain to diffrent parts of body muscles) and SENSORY neurons (having smaller Axons and longer Dendrites) are afferent nerves (carry message from different parts of body muscles to Brain). The electrical and chemical (neurostructure) signals are passed from one neuron to another by Synapse.

The Nervous system is basically divided into 2 catagories viz: CNS (Brain & Spinal cord) and Peripheral Nervous System. The peripheral NS is divided into Somatic NS for voluntary actions and Autonomic NS for involuntary actions and ANS is again divided into Sympathetic, Entric & Parasympathetic NS & their function are to control quick/fast reflexes, gastrointestinal area & SLUDD (Salivation, Lacrimation, Urination, Digestion and Defecation) respectively.

The Autonomic Nervous System (ANS or visceral nervous system) acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind.
Sympathetic and parasympathetic divisions typically function in opposition to each other. But, this opposition is better understood as complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake.

Blood circulatory system

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It deals with the circulation of Blood (by means of a 4 chambered organ, called Heart_explained separately) in all body parts through Arteries and Veins.
An artery is a thicker, and deeper placed blood vessel in the body which carries the oxygenated or pure blood from the heart and supplies it to various organs of the body.

A Vein is a thinner, more superficially placed blood vessel in the body which carries the deoxygenated or impure blood from various organs back to the heart for purification.
The blood vessels, supplying blood to the heart itself, are called coronary arteries and veins.

These are mainly -Right Circumflex Artery (RCA), Left Coronary Artery (LCA)..divided into Left Anterior Descending Artery (LAD) & LCX. Other arteries/veins are named, relevant to their associated body parts like hepatic (liver), renal (Kidney), Sciatic, femoral (femur bone), ophthalmic (eyes), nasal (nose), maxicular, mandibular (upper and lower jaws respectively), iliac (intestine) etc.



It is the red fluid that circulates in our blood vessels, i.e.veins and arteries. The main function of blood is to act as the body’s transport system, but it also has a major role in the body’s defense against infection. There is no substitute for blood. It cannot be made or manufactured. Donors are the only source of blood for patients who need it. The blood is composed of mainly 4 componants viz: Red Blood Corpsules (RBC), White Blood Corpsules (WBC), PLASMA & PLATLETS.

Nearly half the volume of blood consists of cells, which include red blood cells, white blood cells, and platelets. The remainder is a fluid called Serum/Plasma.

The Red & White blood cells and Platelets are made in Bone marrow -especially the vertebrae, ribs, hips, skull and sternum. These essential blood cells fight infection, carry oxygen and help control bleeding. Plasma is the liquid portion of our blood that transports water and nutrients to your body’s tissues.

Red blood cells are disc-shaped cells containing hemoglobin, which enables the cells to pick up and deliver oxygen to all parts of the body, then pick up carbon dioxide and remove it from tissues.

White cells are the body’s primary defense against infection. They can move out of the blood stream and reach tissues to fight infection.

Lymphatic system




It is part of the circulatory system and a vital part of the immune system, comprising a network of lymphatic vessels that carry a clear fluid called lymph (from Latin, lympha meaning water) directionally towards the heart.

The lymphatic system was first described in the seventeenth century. Unlike the cardiovascular system, the lymphatic system is not a closed system.

The human circulatory system processes an average of 20 liters of blood per day through capillary filtration, which removes plasma while leaving the blood cells. Roughly 17 litres of the filtered plasma are reabsorbed directly into the blood vessels, while the remaining three litres remain in the interstitial fluid. One of the main functions of the lymph system is to provide an accessory return route to the blood for the surplus three litres.

The other main function is that of defense in the immune system. Lymph is very similar to blood plasma: it contains lymphocytes and other white blood cells.

It also contains waste products and cellular debris together with bacteria and proteins. Associated organs composed of lymphoid tissue are the sites of lymphocyte production.

Lymphocytes are concentrated in the lymph nodes. The spleen and the thymus are also lymphoid organs of the immune system. The tonsils are lymphoid organs that are also associated with the digestive system.

Lymphoid tissues contain lymphocytes, and also contain other types of cells for support.
The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which also includes the bone marrow, and the lymphoid tissue associated with the digestive system.

The blood does not come into direct contact with the parenchymal cells and tissues in the body (except in case of an injury causing rupture of one or more blood vessels), but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body.

Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces -the contractions of skeletal muscles.

The organization of lymph nodes and drainage follows the organization of the body into external and internal regions; therefore, the lymphatic drainage of the head, limbs, and body cavity walls follows an external route, and the lymphatic drainage of the thorax, abdomen, and pelvic cavities follows an internal route. Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins, near their junction with the internal jugular veins.

Musculoskeletal system

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The musculoskeletal system provides form, support, stability, and movement to the body. It is made up of the bones of the skeleton, muscles, cartilage, tendons, ligaments, joints, and other connective tissue that supports and binds tissues and organs together.

The human musculoskeletal system (also known as the locomotor system, is an organ system that gives humans the ability to move using their muscular and skeletal systems.
It is made up of the bones of the skeleton, muscles, cartilage, tendons, ligaments, joints, and other connective tissue that supports and binds tissues and organs together.

The skeletal portion of the system serves as the main storage system for calcium and phosphorus and contains critical components of the hematopoietic system.

This system describes how bones are connected to other bones and muscle fibers via connective tissue such as tendons and ligaments. The bones provide stability to the body. Muscles keep bones in place and also play a role in the movement of bones. To allow motion, different bones are connected by joints. Cartilage prevents the bone ends from rubbing directly onto each other. Muscles contract to move the bone attached at the joint.
The musculoskeletal system refers to the system having its muscles attached to an internal skeletal system and is necessary for humans to move to a more favorable position.

Complex issues and injuries involving the musculoskeletal system are usually handled by a physiatrist (specialist in physical medicine and rehabilitation) or an orthopaedic surgeon.
Humans are born with over 300 bones; however, many bones fuse together between birth and maturity. As a result, an average adult skeleton consists of 206 bones. The number of bones varies according to the method used to derive the count. While some consider certain structures to be a single bone with multiple parts, others may see it as a single part with multiple bones.

There are five general classifications of bones. These are long bones, short bones, flat bones, irregular bones, and sesamoid bones.

The human skeleton is composed of both fused and individual bones supported by ligaments, tendons, muscles and cartilage. It is a complex structure with two distinct divisions; the axial skeleton, which includes the vertebral column, and the appendicular skeleton.

The skeletal system serves as a framework for tissues and organs to attach themselves to. This system acts as a protective structure for vital organs. Major examples of this are the brain being protected by the skull and the lungs being protected by the rib cage.

Located in long bones are two distinctions of bone marrow (yellow and red). The yellow marrow has fatty connective tissue and is found in the marrow cavity. During starvation, the body uses the fat in yellow marrow for energy. The red marrow of some bones is an important site for blood cell production, approximately 2.6 million red blood cells per second in order to replace existing cells that have been destroyed by the liver. Here all erythrocytes, platelets, and most leukocytes form in adults. From the red marrow, erythrocytes, platelets, and leukocytes migrate to the blood to do their special tasks.

Another function of bones is the storage of certain minerals. Calcium and phosphorus are among the main minerals being stored. The importance of this storage “device” helps to regulate mineral balance in the bloodstream. When the fluctuation of minerals is high, these minerals are stored in bone; when it is low it will be withdrawn from the bone.

There are three types of muscles—cardiac, skeletal, and smooth. Smooth muscles are used to control the flow of substances within the lumens of hollow organs, and are not consciously controlled.

Skeletal muscles are attached to bones and arranged in opposing groups around joints. Muscles are innervated, to communicate nervous energy by nerves, which conduct electrical currents from the central nervous system and cause the muscles to contract.
Contraction initiation

In our body, when a muscle contracts, a series of reactions occur. Muscle contraction is stimulated by the motor neuron sending a message to the muscles from the somatic nervous system. Depolarization of the motor neuron results in neurotransmitters being released from the nerve terminal. The space between the nerve terminal and the muscle cell is called the neuromuscular junction.

These neurotransmitters diffuse across the synapse and bind to specific receptor sites on the cell membrane of the muscle fiber. When enough receptors are stimulated, an action potential is generated and the permeability of the sarcolemma is altered. This process is known as initiation.


A tendon is a tough, flexible band of fibrous connective tissue that connects muscles to bones. The extra-cellular connective tissue between muscle fibers binds to tendons at the distal and proximal ends, and the tendon binds to the periosteum of individual bones at the muscle’s origin and insertion. As muscles contract, tendons transmit the forces to the relatively rigid bones, pulling on them and causing movement. Tendons can stretch substantially, allowing them to function as springs during locomotion, thereby saving energy.

Digestive system


It is the one of the major systems of our body. The role of Digestive system is to provide useful energy & nutrients, required for the survival of our body. This system explains how the food is chewed, digested, assimilated and defecated from our body.

The entire path from mouth to anus is called Gastrointestinal Tract (GI) or, Alimentary canal. The major organs which participate in digestion are Mouth, Oesophagus, Stomach, Duodenum, Small Intestine (also called Ileum), Large intestine, Rectum and Anus.

There are other organs also which although, do not participate directly in digestion but, their contribution is equally important and more significant in digestive process because they secrete those substances in absence of which, the digestion can not be completed and these components are Saliva through Salivary gland in mouth & tounge, Pancreatic juice from Pancreas, Bile from Liver, Gastric enzymes through Stomach etc.

The digestion is completed in many stages. The first of which starts in the oral cavity (Mouth) where digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The secretion of Saliva (containing Amylase & Lipase) helps to produce a Bolus which can be swallowed to pass down in Oesophagus and then, in Stomach where the churning of food by means of Peristaltic waves occurs. The Peristalsis is a series of wave-like muscle contractions that moves food to different processing stations in the digestive tract. The process of peristalsis is rhythmic & begins in the Oesophagus when a bolus of food is swallowed.

The downward terminal part of Stomach is called Duodenum where the food is converted into Chyme which is absorbed (assimilation) as Chyle in small intestine. Assimilation occurs in every cell of the body to help develop new cells. Most of the digestion of food takes place in the small intestine.

Water and some minerals are reabsorbed back into the blood in the Colon (part of the large intestine). The waste products (faecus) are defecated from the Anus via Rectum.

Endocrine system

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The endocrine system is the collection of ductless glands (whose hormones are secreted directly in blood circulatory system) The major endocrine glands include the Pineal, Pituitary, Pancreas, Ovaries, Testes, Thyroid, Parathyroid, Hypothalamus & Adrenal glands.

The endocrine system is in contrast to the exocrine system, which secretes its hormones to the outside of the body using ducts. The endocrine system’s effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. Special features of endocrine glands are their ductless nature, their vascularity, and commonly the presence of intracellular vacuoles or granules that store their hormones. Some of the exocrine glands are Salivary glands, Sweat glands and some glands within GI and they tend to have ducts or a hollow lumen.

Some organs like Bone, Kidney, Liver, Heart and Gonads, have secondary endocrine functions (kidney secretes endocrine hormones such as erythropoietin and Renin. The Hormones are consist of either amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins. 

Excretory system



The role of Excretory system is to remove the waste, excessive & useless products in form of solids, fluids, gases by means of Urination, Defecation, Exhalation & Sweating etc from our body in order to maintain internal chemical homeostasis and prevent damage to the body.

The major organs specifically used for the excretion are Kidney (Urination), GI Tract (Defecation), Lungs (Exhalation) & Skin (Sweating). Urinary Tract system is considered as main part of the excretory system.

Main components, used for excretory system:

The liver detoxifies and breaks down chemicals, poisons and other toxins that enter the body. The liver transforms ammonia (which is poisonous) into Urea in our body which is later filtered by the kidney into urine. The liver also produces Bile and the body uses bile to break down fats into usable fats and unusable waste.

After Bile is produced in liver, it is stored in the Gall Bladder. It is then secreted within the small intestine where it helps to break down ethanol, fats and other acidic wastes including ammonia, into harmless substances.

Large intestine
The large intestine’s main function is to transport food particles through the body and expel the indigestible parts at the other end, but it also collects waste from throughout the body. The typical brown colour of waste (Fecus) is due to Bilirubin. The lower part of the large intestine (about 10 feet) also extracts any remaining usable water and then removes solid waste

The skin excretes sweat through sweat glands throughout the body. The sweat, helped by salt, evaporates and helps to keep the body cool when it is warm.

Like sweat glands, Eccrine glands allow excess water to leave the body and majority of these glands are located mainly on the forehead, the bottoms of the feet, and the palms, although the glands are everywhere throughout the body. They help the body to maintain temperature control. Eccrine glands in the skin are unique feature of our body.

Kidney’s main function is to eliminate wastes from the bloodstream by production of urine. They perform several homeostatic functions such as Maintaining the -volume of extracellular fluid, -ionic balance in extracellular fluid, -pH and osmotic concentration of the extracellular fluid & -Excreting toxic metabolic by-products like urea, ammonia, and uric acid. There are over 1 million Nephrons which are present in Kidney for filteration work.

These are muscular ducts (25-30cms) that propel urine from the kidneys to Urinary Bladder.

Urinary bladder
It collects the waste, excreted by the kidneys prior to disposal by urination. It is a hollow muscular and elastic organ, and sits on the pelvic floor. Urine enters the bladder via the ureters and exits via Urethra.

It is a muscular tube which is connected with the urinary bladder at one end and used for urination.

Reproductive system

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This is a major system of our body. Its main role is to reproduce offsprings.

The human body is unisexual means both male and female bodies are created differently.

The Reproductive system in our body usually involves internal fertilization by sexual intercourse. During this process, the male inserts his erected penis into the female’s vagina and ejaculates semen, which contains sperm. The sperm then travels through the vagina and cervix into the uterus or fallopian tubes for fertilization of the ovum.

Upon successful fertilization and implantation, gestation of the fetus then occurs within the female’s uterus for approximately nine months, this process is known as pregnancy. Gestation ends with birth, the process of birth is known as Labor which consists of the muscles of the uterus contracting, the cervix dilating, and the baby passing out the vagina (the female genital organ). One important type of parental care is the use of the mammary glands in the female breasts to nurse the baby.

The female reproductive system has two functions: The first is to produce egg cells, and the second is to protect and nourish the offspring until birth. The male reproductive system has one function, and it is to produce and deposit sperm. Human body has high level of sexual differentiation.

The male reproductive system is located outside of the body around the pelvis region. The function of the male reproductive system is to provide the male sperm for fertilization of the ovum.

The major reproductive organs of male can be grouped into three categories.

The first category is sperm production and its storage. The production of sperm takes place in the Testes which are housed in the temperature regulating Scrotum, the immature sperm then travel to the epididymis for development and storage.

The second category are the ejaculatory fluid producing glands which include the seminal vesicles, prostate, and the vas deferens.

The final category are those used for copulation, and deposition of the sperm (spermatozoa) within the male body. These include the penis, urethra, vas deferens, and Cowper’s gland (also called Bulbourethral gland). These two pea-shaped glands add fluids to semen during the process of ejaculation. They are located beneath the Prostate gland at the beginning of the internal portion of the Penis.

The main role of male sexual hormones (Testesterone & Androgen) is to develop secondary sexual characteristics includes larger and more muscular stature, deepened voice, facial and body hair, broad shoulders, and development of an adam’s apple. The testes release a hormone that controls the development of sperm.

The female reproductive system is located inside of the body around Pelvic region. The female reproductive system contains three main parts: the Vagina, which leads from the vulva, the vaginal opening, to the uterus; the Uterus, which holds the developing fetus; and the Ovaries, which produce the female’s ova. The Breasts are involved during the parenting stage of reproduction. They are not considered to be part of the female reproductive system.

The vagina meets outside at the vulva, which also includes the labia, clitoris and urethra. During the intercourse, this area is lubricated by mucus secreted by the Bartholin’s glands. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes. Each ovary contains hundreds of egg cells or ova (singular ovum).

Approximately every 28 days, the pituitary gland releases a hormone, called Follicle Stimulating Hormone (FSH) that stimulates some of the ova to develop and grow. One ovum is released and it passes through the fallopian tube into the uterus. Hormones produced by the ovaries prepare the uterus to receive the ovum. The lining of the uterus, called the endometrium, and unfertilized ova are shed each cycle through the process of menstruation by secretion of Luteinizing hormone (LH).

It is also called Lutropin, produced by Pitutory gland. In females, an acute rise of LH triggers Ovulation and development of the Corpus Luteum. If the ovum is fertilized by sperm, it attaches to the endometrium and the fetus develops.

The Bartholin’s glands are (pea sized) located slightly posterior and to the left and right of the opening of the vagina. They secrete mucus to lubricate the vagina and are homologous to Bulbourethral glands in males.

The G-spot, also called the Gräfenberg spot (German gynaecologist) characterized as an erogenous area of vagina that, when stimulated, may lead to strong sexual arousal, powerful orgasm and potential female ejaculation. It is located 5–8 cms up the front (anterior) vaginal wall between the vaginal opening and the urethra and is a sensitive area that may be part of female prostate.

It is believed that the G-spot is an extension of the clitoris and that this is the cause of orgasms experienced vaginally.