Physiology is the study of the characteristics and mechanisms of the human body. Cells are the basic unit of life and approximately 100 trillion cells make up the typical human, each specially adapted to perform one or a few particular functions. 25 trillion red blood cells function by transporting oxygen from the lungs to all tissues in the body. All cells have some basic commonalities. Oxygen reacts with carbohydrates, fat, and protein to release energy, nutrient consumption, and energy production mechanisms. Almost all cells have the ability to reproduce further similar cells.

Let us learn about the characteristics and mechanisms of a few organs in the human body.

The Endocrine System

Endocrine system

The endocrine system is an integration system that influences the metabolic activities of cells. It performs via hormones, chemical messengers produced by endocrine glands such as pineal, hypothalamus, pituitary, thyroid, parathyroid, adrenals, pancreas, gonads.

Hormones – They are the chemical substances secreted by the endocrine glands that regulate metabolic function of other cells.

  1. Steroid based hormones – cholesterol derivatives.
  2. Amino acid based hormones: amino acid derivatives, single modified amino acids, peptides, proteins, glycoproteins.
  3. Eicosanoids — derivatives of 20-carbon polyunsaturated fatty acid, arachidonic acid

Mechanisms of hormone action

Synthesised hormones effects on target cells by:

  •  Changing in membrane permeability due to opening or closing of the channels.
  •  Activation or deactivation of regulatory molecules — covalent modification.
  •  Synthesis of regulatory molecules — involves changes in gene expression.

These effects result in physiological changes:

  • Induction of secretory activity.
  •  Stimulation of mitosis
  •  Contraction or relaxation


Nervous System

Nervous System

The nervous system is a complex network of nerves and cells that perform three overlying functions of sensory input, integration, and motor output. This process is generally the same even at a very primitive level of the nervous system.

  • The sensory input is sensing the environment and changes in an organism and is carried out by sensory organs like eyes, ears, nose, tongue, and skin, some of them performing simultaneously.
  • The integration involves processing of information and is carried out by the central nervous system (CNS), which composed of the brain and the spinal cord.
  • Motoneuron output is conduction of signals from the integration center, the CNS, and is carried out by a group of effector cells, the muscle cells or gland cells, which actually carry out body’s responses to external stimuli. Both sensory input and motor output signals are carried through nerves, which are long rope-like structures made from nerve cells. Nerve cells are two types – neurons and glia.

Neurons are the cells which actually carry through signals whereas glial cells provide supporting structures and maintenance of neuronal cells. Nerves are many times are made from end to end connection between neurons, supported by the glial cells. The nerves that communicate sensor and motor signals between the central nervous system and the rest of the body are collectively referred to as the peripheral nervous system (PNS). Sensory inputs are received by receptor cells located in sensory organs. For examples, light receptor cells are located in eyes, or chemical receptor cells are located on the surface of the tongue.

Signals from these receptors are carried through sensory neurons of the PNS into the CNS, and after processing in the CNS, instructions are communicated through the motor neurons of the PNS to effector cells, such as muscles. Communication from the receptor cells to effector cells is carried in two forms – chemical and electrical. Since communication of information involves more than one cells, the communication is through special chemicals called neurotransmitters or a specialized form of electric signal called action potential.

Nerve Cells

Nerve Cells

Neurons are the functional unit of the nervous system. A neuron consists of three major parts – a cell body that contains the nucleus, dendrites which receive signals, and a long axon that carries the signal to the next cell. Length of neurons varies depending on their location. Neurons located in CNS could be a few millimeters long but some of the neurons in PNS could be more than a meter long. In a normal human body, there are about two billion neurons, approximately 1 billion in the brain, and another billion in the rest of the body. Glial cells are supporting cells provide structural and functional support to the neurons and help them carry out their functions. For example, Schwann cells provide a covering of the axons in the PNS. End of a neuron is known as synaptic terminal, which generally connects with either another neuron to continue the process of communication or to a muscle to trigger muscle action. Glial cells outnumber neurons by 10 to 50-folds.

Organization of Nervous System Structurally

The nervous system is organized into two parts – the central nervous system and the peripheral nervous system. CNS The CNS is made of the brain and the spinal cord. The brain is divided into three parts – Forebrain, Midbrain, and Hindbrain. The Forebrain develops into two parts – the telencephalon which consists of the cerebrum or the cerebral hemispheres, and includes cerebral cortex, white matter, and basal nuclei; and diencephalon which consists thalamus, hypothalamus, and epithalamus. The Midbrain develops through mesencephalon into a part of the brainstem. The Hindbrain develops through two parts, the metencephalon, and myelencephalon. The metencephalon eventually develops into pons (part of the brainstem) and cerebellum. The mesencephalon develops into medulla oblongata. which is also part of the brain stem.

Spinal cord

The spinal cord which is about 45 cm in length, starts from the brain stems and stretches to the lowest end of the backbone. It comprises a nerve bundle and is protected by a series of vertebrae which divided into five regions – cervical, thoracic, lumbar, sacral, and coccygeal.  The spinal cord itself spans only about two-thirds of the vertebral column, but the rest of the space is filled with nerve fibers of spinal roots. Both the spinal cord and the brain contain fluid-filled spaces or cavities called as cerebrospinal fluid (CSF) which contains nutrients, hormones, white blood cells to maintain the CNS. Additionally, the cerebrospinal fluid acts as the shock absorber cushioning the brain and provides a direct link across the blood-brain barrier for exchanging nutrients and other essential biomolecules.

The Immune System


A human body has an inbuilt immune system comprising of different parts which work together by defending against the disease-causing microorganism. Hence it is referred to the body’s best defensive system. This system function against infringing microorganisms and keeps us healthy. Sometimes foreign materials manage to invade successfully. When this happens, the body relies on the immune system to respond. This system functions in several ways:

  • The thymus gland, the spleen, tissues of the bone marrow, and the lymph nodes produce white blood cells (WBC’s) which are specialized cells functions by destroying foreign organisms.
  • The white blood cells produce -A nonspecific response to injury or infection and proteins called antibodies which are part of a specific immune response to foreign materials.

White Blood Cells

White Blood Cells

The immune system has specialized cells called white blood cells that recognize foreign materials in the body and respond. The number of white blood cells in the blood can increase during an immune response. These cells travel through the circulatory system and the lymphatic system to an injured or infected area of the body. White blood cells leave the blood vessels and travel into the damaged tissue, where the immune response takes place.

The Lymphatic System

The lymphatic system is the network of tissues and organs which has multiple interrelated functions.The primary function of the lymphatic system is to transport lymph and pathogen-fighting white blood cells throughout the body, which is significant to the circulatory system. Lymph drifts through the lymph vessels when our skeletal muscles contract or when our body changes position. As it moves, it passes through lymph nodes, which filter out pathogens and other infections which are stored in white blood cells and causing them to swell when we get sick.  It is also responsible for the elimination of body of toxins, unwanted materials and other interstitial fluid from tissues.

The immune system responds to attack.

Certain illnesses can cause symptoms such as coughing, sneezing, and fever. These symptoms make you uncomfortable when you are sick. But in fact, most symptoms are the result of the immune system responding to foreign materials in the body. The immune system responds in two ways.

  1. At first, white blood cells (WBC’s) respond to the site of injury or infection and attack the foreign materials in a nonspecific response. Some of these cells attack pathogens by producing chemicals that help other white blood cells work better.
  2. The second part of the response is very specific to the types of pathogens invading the body. These white blood cells produce antibodies specific to each pathogen and provide our body with immunity.

Development of Immunity

After our body has destroyed a specific pathogen, B cells which fight against the pathogen remain in our system. If the same pathogen invades our body again, then our immune system will certainly destroy it before we became ill. This revolution is called Immunity.It is of two types: active immunity and passive immunity. Newborn babies get their immune defenses transferred from their mothers as their body has no ability to develop antibodies of their own. This type of immunity is called passive immunity. Antibodies are not produced by the body but it is transmitted or delivered from another source. Babies produce their own antibodies after a few months of birth. An active immunity is produced whenever our body makes its own antibodies. The significant role of immunity is fighting against any specific pathogen which our body has developed antibodies against. For example, it is most unlikely that you will get chicken pox twice.

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Practise This Question

The organ that stores bile is ___ .