The Human Body ( SCIENCE )

The human body is composed of billions of cells to perform various functions. The cells are organised to from tissues, tissues are organised to from organs, and organs together form organ system and so on. Human body consists of many specialised organ systems such as skeletal system, muscular system, nervous system, digestive system, respiratory system, circulatory system, etc.

1. Skeletal system: It is made up of all the bones of the body and tissues such as tendons, ligaments and cartilage that connects them. In humans, the skeletal system is made up of 206 bones and few cartilages. Functions: Its 206 bones form a rigid framework to which the softer tissues and organs of the body are attached. Vital organs are protected by the skeletal system. The brain is protected by the surrounding skull and the heart and lungs are encased by the sternum and rib cage. The movement in the body is carried out by the interaction of skeletal and muscular system. Bone marrow is the site of production of red blood cells. An average of 2.6 million red blood cells is produced each second by the bone marrow to replace those worn out and destroyed by the liver. Bones serve as a storage area for minerals such as calcium and phosphorus. When an excess is present in the blood, buildup will occur within the bones. When the supply of these minerals within the blood is low, it will be withdrawn from the bones to replenish the supply.

2. Muscular system: There are several types of muscles such as cardiac, smooth and skeletal muscles that form a part of muscular system. The muscular system is composed of over 600 muscles. Differences between each muscle are recognized by location, function, structure, and the way they are contracted. Functions: The main function of muscular system is movement. Muscles in coordination with bones, joints, and some other tissues provide movement to the body. The integrated action of joints, bones, and skeletal muscles produces obvious movements such as walking and running. Skeletal muscles also produce more subtle movements that result in various facial expressions, eye movements, and respiration. In addition to movement, muscle contraction also fulfills some other important functions in the body, such as posture, joint stability, and heat production. Posture, such as sitting and standing, is maintained as a result of muscle.

3. Digestive system: The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body. The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract. Functions: The main function of digestive system is to break down the food particles into smaller and smaller particles so that they get absorbed in the blood.

4. Respiratory system: Respiration is the sequence of events that results in the exchange of oxygen and carbon dioxide between the atmosphere and the body cells. We generally associate breathing to be the only function of the Respiratory system, breathing is only one of the activities of the respiratory system. The body cells need a continuous supply of oxygen for the metabolic processes that are necessary to maintain life. The respiratory system works with the circulatory system to provide this oxygen and to remove the waste products of metabolism. It also helps to regulate pH of the blood. It consists of following organs: Nose, pharynx, larynx, trachea, bronchi, alveoli. The process of breathing, which involves inhalation and exhalation moves air into and out of the lungs. After this, there is an exchange of gases between the lungs and the blood. This is called external respiration. The blood transports the gases to and from the tissue cells. The exchange of gases between the blood and tissue cells is internal respiration. Finally, the cells utilize the oxygen for their specific activities. This is cellular metabolism, or cellular respiration. Together these activities constitute respiration.

5. Circulatory system: This system is also known as cardio-vascular system. It consists of the heart, a muscular pumping device, and a closed system of vessels called arteries, veins, and capillaries. The blood is pumped by the heart around a close circuit of vessels. The vital role of the cardiovascular system in maintaining homeostasis depends on the continuous and controlled movement of blood through the thousands of miles of capillaries that permeate every tissue and reach every cell in the body. It is in the microscopic capillaries that blood performs its ultimate transport function. Nutrients and other essential materials pass from capillary blood into fluids surrounding the cells as waste products are removed.

6. Reproductive system: In humans, male reproductive system consists of testes, epididymus, scrotum, vas deferens, penis, and glands.

Female reproductive system consists of vagina, cervix, fallopian tubes, uterus,

The reproductive system has following functions
(i) to produce sperm and egg cells
(ii) to produce hormones
(iii) to nurture the developing offspring

7. Urinary system: The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys form the urine and account for the other functions. The urinary system maintains an appropriate fluid volume by regulating the amount of water that is excreted in the urine. Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood. Although the urinary system has a major role in excretion, other organs contribute to the excretory function. The lungs in the respiratory system excrete some waste products, such as carbon dioxide and water. The skin is another excretory organ that rids the body of wastes through the sweat glands. The liver and intestines excrete bile pigments that result from the destruction of hemoglobin. The major task of excretion still belongs to the urinary system. If it fails the other organs cannot take over and compensate adequately.

8. Nervous system: The nervous system is composed of organs, principally the brain, spinal cord, nerves. These, in turn, consist of various tissues, including nerve, blood, and connective tissue. Together these carry out the complex activities of the nervous system. The nervous system is the major controlling, regulatory, and communicating system in the body. It is the center of all mental activity including thought, learning, and memory.}

9. Endocrine system: The endocrine system, along with the nervous system, functions in the regulation of body activities. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities.

There are basically two types of glands— endocrine and exocrine gland. The endocrine glands are also known as ductless glands. The secretory products of endocrine glands are called hormones and are secreted directly into the blood and then carried throughout the body where they influence only those cells that have receptor sites for that hormone. There are eight major endocrine glands scattered throughout the body. These are Pituitary and pineal gland; thyroid and parathyroid gland; adrenal gland; pancreas; gonads(testes and ovaries), etc.

Some glands also have non-endocrine regions that have functions other than hormone secretion.

For example, the pancreas has a major exocrine portion that secretes digestive enzymes and an endocrine portion that secretes hormones. The ovaries and testes secrete hormones and also produce the ova and sperm.

COMPONENTS OF THE BODY

I. Structural organization of cell

The word cell is derived from a Latin word ‘cellula’, which means ‘a little room’.

Robert Hooke was the scientist who coined the term cell. He observed the cells while examining a thin slice of cork. The human body, which is made up of numerous cells, begins as a single, newly fertilized cell.

Cells constitute various components of plants and animals. A cell is the smallest unit of life and is capable of all living functions. Cells are the building blocks of life. This is the reason why cells are referred to as the basic structural and functional units of life.

The cell theory that all plants and animals are composed of cells and that cell is the basic unit of life was presented by two biologist, Schleiden (1838) and Schwann (1839).

The cell theory was further expanded by Rudolf Virchow in 1855. He suggested that all cells arise from pre-existing cells.

Principles of cell theory
A. All living organisms are composed of one or more cells.
B. Cells are the basic units of structure and function in an organism.
C. All cells come only from the reproduction of existing cells.

There are many different types, sizes, and shapes of cells in the body. For descriptive purposes, the concept of a “generalized cell” is introduced. It includes features from all cell types. A cell consists of three parts: the cell membrane, the nucleus, and between the two, the cytoplasm. Within the cytoplasm lie intricate arrangements of fine fibers and hundreds or even thousands of miniscule but distinct structures called organelles.
Plasma membrane or cell membrane:
The cell membrane (or plasma membrane or plasmalemma) is the outermost covering of the cell that separates the contents of the cell from the external environment.

Cell membrane is an extremely delicate, thin, elastic, and living membrane of the cell. It surrounds the cytoplasm of the cell and regulates the movement of substances in and out of the cell. This means that the cell membrane allows the entry of only some substances and prevents the movement of some other materials. Therefore, the cell membrane is known as selectively permeable membrane.

Functions of plasma membrane:
1. It gives a definite shape to the cell.
2. It separates the contents of the cell from its surrounding medium.
3. It provides mechanical barrier for the protection of the internal contents of cell.
4. It regulates entry and exit of substances in and out of the cell.

Cytoplasm:
It is the fluid that fills the cell and occurs between the plasma membrane and the nuclear membrane. The cell organelles are suspended in the cytoplasm.

Functions:
1. Cytoplasm helps in the exchange of material between the cell organelles.
2. It acts as a storage organelle of vital chemicals, such as amino acids, glucose, vitamins, ions etc.
3. It is the site of certain metabolic pathways, such as glycolysis, synthesis of fatty acids, nucleotides, and some amino acids.

Nucleus:
The nucleus is round in shape and is the largest organelle of the cell. It is generally present in the centre of the cell, except in plant cells where the nucleus is pushed towards the periphery of the cell because of the central vacuole. The nucleus is composed of the following components: nuclear membrane, nucleoplasm, and nucleolus.

Functions of nucleus:
1. The nucleus controls all metabolic activities of the cell.
2. It regulates the cell cycle.
3. It is concerned with the transmission of hereditary traits from the parent to the offspring.
4. It plays an important role in cellular reproduction, the process by which a single cell divides or forms two new cells.

Cell organelles

Endoplasmic Reticulum:
The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER functions as a packaging system. However, it does not work alone; it works closely with Golgi apparatus and ribosomes.

There are two types of ER—rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacturing.

Functions:
1. ER serves as a channel for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
2. ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.

Ribosomes:
These are very small, round structures found either in free state suspended in the cytoplasm or attached to the surface of the endoplasmic reticulum. They are composed of ribonucleic acids and proteins.
The main function of ribosomes is to act as a site of protein synthesis.

Golgi Apparatus:
The Golgi apparatus is another packaging organelle like the endoplasmic reticulum. It was named after Camillo Golgi, an Italian biologist. It consists of a system of membrane bound vesicles arranged approximately parallel to each other in stacks called cisterns.

Functions:
1. The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
2. It helps in the storage, modification and packaging of products in vesicles.
3. The Golgi apparatus is also involved in the formation of lysosomes and peroxisomes.

Lysosomes:
Lysosomes are found in almost all animal-like eukaryotic cells. It is a membrane-bound vesicular structure that holds variety of other enzymes. The purpose of lysosome is to digest worn out cells. Lysosomes are involved in intracellular digestion of foreign food particles or microbes and are called digestive bags.

Sometimes, this organelle is also involved in the self-digestion of cells after their death, also known as autolysis. Hence they are also called as ‘suicidal bags’. Lysosomes are a kind of waste disposal system of a cell.

Mitochondria:
Mitochondria create energy for the cell and this process of creating energy for the cell is known as cellular respiration. Most of the chemical reactions involved in cellular respiration occur in mitochondria. (though cellular respiration also occur in cytoplasm but very little energy is produced) The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphosphate) molecules. This is why, mitochondria are known as the powerhouses of the cell.

Functions of Mitochondria:
1. Mitochondria are the sites of cellular respiration.
2. They provide energy (in the form of ATP) for vital activities of the living cells.
3. Mitochondria are able to make some of their own proteins, so they are regarded as semiautonomous organelles.

Plastids:
Plastids are the organelles that are present only in plant cells. These are double membrane organelles which are usually spherical or discoidal in shape.

There are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids). Chromoplasts containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.

Vacuoles:
Vacuoles are storage sacs (for solid or liquid contents) found in the cells. These are found in both plant and animal cells but are much larger in plant cells. The central vacuole of some plant cells may occupy 50-90% of the cell volume.

In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Vacuoles store food and nutrients that a cell needs to survive. These include amino acids, sugars, various organic acids and some proteins.

Differences between plant and animal cells

Animal Cell		Plant Cell
1.	Animal cells are generally small in size.	1.	Plant cells are usually larger than animal cells.
2.	Cell wall is absent.	2.	The plasma membrane of plant cells is surrounded by a rigid cell wall of cellulose.
3.	Except the protozoan Euglena, no animal cell present.	3.	Plastids (chromoplasts and leucoplasts) are possesses plastids.
4.	Vacuoles are less in number and small in size.	4.	Vacuoles are present in abundance and larger in size.
5.	Animal cells have a single highly complex and prominent golgi apparatus.	5.	Plant cells have many simpler units of golgi apparatus, called dictyosomes.

II. Blood

Blood is a highly specialised connective tissue present in human body. It is a vascular tissue that contains cells, separated by a non-living, liquid material. This liquid material or fluid is known as plasma.

Red blood cells (RBC), white blood cells (WBC), and platelets are suspended in plasma.

The blood is the medium through which the entire body is nourished and supported carrying nutrient molecules from digested foods, as well as carrying away harmful waste products such as carbon dioxide. It circulates the needed oxygen from the lungs. It also picks up hormones and uses custom distribution to deliver chemical messages to the organs it comes into contact with. As other organs interact with the blood it is in a constant flux of updating and extending its chemical composition.

Functions of blood:
• It transports hormones, nutrients, waste material to different parts of the body.
• Red blood cells transport respiratory gases such as oxygen and carbon dioxide.
• White blood cells fight diseases by producing antibodies.

III. Human brain

The brain is the main coordinating centre of the body. It is a part of the nervous system that controls and monitors every organ of the body.

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS consists of nerves that connect the central nervous system to different parts of the body. The central nervous system receives information from all parts of the body and also sends information to the muscles. The communication between the CNS and the body parts is facilitated by the nerves of the PNS. The brain is divisible into three main regions—forebrain, midbrain, and hindbrain.

Forebrain: It consists of the cerebrum, thalamus, and hypothalamus. It is the main thinking part of the brain. Cerebrum is the largest part of the brain and constitutes four-fifth of its weight. The forebrain has sensory regions that receive sensory impulses from various receptors, as well as motor regions that control the movement of various muscles such as, the leg muscles. There are separate areas in the forebrain that are specialized for hearing, smelling, sight, and general sensations such as pain, touch, taste, etc. A certain part of the cerebrum primarily controls intelligence, learning, memory, thinking, and speech. The forebrain is also known as the main thinking part of the brain.

The hypothalamus contains many areas that control things such as body temperature, urge for eating and drinking, etc. Some regions of the cerebrum, along with the hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional reactions such as, excitement, pleasure, fear, etc.

Midbrain: It has regions that are concerned with the sense of sight and hearing. Some regions of the midbrain transmit motor impulses to the limbs.

Hindbrain: Most involuntary actions such as heartbeat, blood pressure, movement of food in the alimentary canal, salivation, etc., are controlled by the midbrain and hindbrain. All these involuntary actions are controlled by the medulla of the hindbrain.

The cerebellum, which is a part of the hindbrain, is responsible for maintaining posture and equilibrium of the body. It also coordinates the contraction of voluntary muscles according to the directions of the cerebrum.

The brain, as you have learnt, is an important organ for the maintenance of a variety of activities. As an important organ, it needs protection. Hence, it is enclosed by a bony box called the cranium. The spinal cord is protected by a bony, curved, vertical rod called the vertebrae or vertebral column.

EYE

Our paired eyes are located in sockets of the skull called orbits. A brief account of structure and functions of the human eye is given in the following sections.

Parts of an eye
The adult human eye ball is nearly a spherical structure. The wall of the
eye ball is composed of three layers.

•	The external layer is composed of a dense connective tissue and is called the sclera. The anterior portion of this layer is called the cornea.
•	The middle layer, choroid, contains many blood vessels and looks bluish in colour.
•	The choroid layer is thin over the posterior two-thirds of the eye ball, but it becomes thick in the anterior part to form the ciliary body.
•	The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.
•	The eye ball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body.
•	In front of the lens, the aperture surrounded by the iris is called the pupil. The diameter of the pupil is regulated by the muscle fibres of iris.
•	The inner layer is the retina and it contains three layers of cells – from inside to outside – ganglion cells, bipolar cells and photoreceptor cells.

DEFECTS OF VISION

Myopia
Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.

This defect can be corrected by using a concave lens of suitable power.

Hypermetropia
Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.

This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power.

Presbyopia
The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortable and distinctly without corrective eye-glasses. This defect is called Presbyopia.

EAR

The ears perform two sensory functions, hearing and maintenance of body balance. Anatomically, the ear can be divided into three major sections called the outer ear, the middle ear and the inner ear.

•	The outer ear consists of the pinna and external auditory meatus (canal). The pinna collects the vibrations in the air which produce sound.
•	The external auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum).
•	There are very fine hairs and wax- secreting sebaceous glands in the skin of the pinna and the meatus. The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.
•	The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion.
•	The malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear.
•	An Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalising the pressures on either sides of the ear drum.