Human Physiology

Table of contents

What is meant by Human Physiology?

Human Physiology is the science of the mechanical, physical, and biochemical functions of normal humans or human tissues or organs. It deals with organ and organ systems.

What do Physiologist do?

They analyze the function of the different organs or different organ system of the body.

What is the definition of the physiological disease?

Any disease in the body caused due to some physiological changes in the body or organ or organ system.

What is medical physiology all about?

It is the study of human biological systems and how they are interconnected or related to each other.

Digestion and Absorption

The process of breakdown of complex, water insoluble food molecules into simple water- soluble molecules is known as Digestion. It is a complex process involving various enzymes, secretion etc.

Digestive System

The human digestive system comprises of alimentary canal and digestive glands.The oral cavity leads into the pharynx, which is common passage for food as well as air. Esophagus also known as food pipe helps in passage of the food to the stomach.

Stomach is a bag-like structure located in the upper portion of the abdominal cavity.It is divided into three parts - Cardiac, Fundic and Pyloric.

Pyloric is the region which opens into the first part of the small intestine. Small intestine is divided into Duodenum, Jejunum and Ileum

Digestive SystemFig. 1. Digestive System

Large intestine is divided into caecum, colon, and rectum.

Rectum finally opens into the anus. The 3 main digestive glands are- salivary glands, liver, and pancreas. Liver is the largest gland found in the human body. Pancreas is an endocrine as well as exocrine gland. The endocrine function of the pancreas involves the secretion of insulin and glucagon which regulates the blood sugar level. Whereas endocrine function of pancreas includes secretion of pancreatic juice that participates in digestion of food.

Digestion of Food

Digestion of carbohydrates begins in the mouth. Salivary amylase helps in digestion of starch into maltose. Small amount of lipase is secreted by gastric glands where begins the process of fat digestion. Bile secreted from liver helps in emulsification of fats.

This process begins in stomach, where, pepsin breaks proteins into peptones and proteases. Trypsin, Chymotrypsin, Carboxypeptidase breakdown peptones, Proteases into dipeptides.

Digestion of food

Fig. 2. Digestion of food

Breathing and Exchange of Gases

Breathing is the process of taking in oxygen and giving out carbon-dioxide. Respiration is a catabolic process of breakdown of energy rich molecules to produce energy needed for the survival of the organism.

Humans have a pair of nostrils which leads into the nasal passage. Nasal chamber then leads to pharynx which is common passage for food as well as air. The pharynx opens through the larynx region into the trachea. Larynx is a sound box which helps in sound production.

Respiratory system Fig. 3. Respiratory system

Trachea is straight tube that divides into left and right primary bronchi. Primary bronchi further divide into secondary and tertiary bronchi and bronchioles. Each terminal bronchioles give rise thin, vascularized bag-like structure known as Alveoli.

Transport of oxygen and carbon-dioxide

Red blood cells contain iron containing red colored pigment known as Hemoglobin

Partial pressure of oxygen determines the binding of oxygen with hemoglobin. When percent saturation of hemoglobin is plotted against partial pressure of oxygen, a sigmoid curve is obtained. This is known as Oxygen Dissociation Curve. High partial pressure of oxygen, low partial pressure of carbon-dioxide, low temperature promotes oxy-hemoglobin formation whereas low partial pressure of oxygen, high partial pressure of carbon-dioxide in tissues promotes dissociation of oxygen from hemoglobin.

About 20-25% of carbon-dioxide is transported in the form of carbamino-hemoglobin

Red blood cells contain a very important enzyme known as Carbonic Anhydrase.

Body Fluids and Circulation

Blood is a fluid connective tissue consists of matrix, plasma and formed elements. Plasma is straw colored, viscous fluid which constitutes about 55% of the blood. Formed elements include erythrocytes, leucocytes, and blood platelets. Blood grouping is divided into- ABO blood group and Rh group.

Formed ElementsFig. 4. Formed Elements

ABO blood grouping is done based the presence or absence of certain antigens on the surface of RBCs

Rh is also an antigen like Rhesus monkeys. Individuals having Rh antigen on RBCs are considered as Rh positive. Those which are without Rh antigen are considered Rh negative.

There are two types of circulatory system - Open Circulatory System and Closed Circulatory System.

When blood flow in open spaces known as Lacunae and Sinuses, it is known as open circulatory system it is present in molluscs, arthropods, etc.

When blood flow in closed vessels it is known as closed circulatory system. For Example, Humans.

Human Circulatory System

It includes heart, vessels, and blood. It is located in the thoracic cavity, in between the two lungs. Double membrane that surrounds the heart are known as Pericardium. Heart is divided into two auricles and two ventricles.

Sequence of electrical and mechanical events during every heart beat is known as Cardiac Cycle. It is divided into two phases - Diastole and Systole. During diastole heart ventricles relaxes due to which it is filled with blood. During systole, ventricles contract to pump the blood into the arteries.

Human heart

Fig. 5. Human heart

Excretory Products and Elimination

Human Excretory System consists of a pair of kidneys, a pair of ureter, urinary bladder, and urethra. Kidney is divided into two zones known as - Outer Cortex and Inner Medulla. The medulla is divided into conical masses known as Medullary Pyramids. Extension of renal cortex which separates the pyramids is known as Column of Bertin.

Glomerulus is the cluster of capillaries. The blood enters the glomerulus via afferent arteriole and leaves the glomerulus via efferent arteriole. Glomerulus is enclosed in a cup- shaped structure known as Bowman’s Capsule. Bowman’s capsule together with glomerulus is known as Malpighian Body or Renal Corpuscles.

Excretory systemFig. 6. Excretory system

Urine formation occurs in three major steps - Glomerular Filtration, Tubular Reabsorption and Secretion.

Glomerular Filtration - On an average, kidney filters about 1100 ml to 1200 ml of blood per minute. The glomerular capillary blood pressure causes filtration of blood through three layers - Endothelium that surround the glomerular blood vessels, the Epithelium of Bowman’s capsule and a basement membrane in between these two layers. The epithelial cells present in the Bowman’s capsule are known as Podocytes which are arranged in an intricate manner to leave some minute spaces called Filtration Slits or Slit Pores.

The amount of filtrate produced by the kidneys per minute is known as glomerular filtration rate.

Tubular Reabsorption - It involves the absorption of required molecules or ions such as sodium ions, glucose, amino acids etc. Some of the substances are absorbed actively and some are absorbed passively.

The last step of urine formation is secretion. Potassium ions, hydrogen ions, ammonia are secreted out to maintain the ionic and acid balance of the body fluids.

Neural Control and Coordination

The structural and functional unit of neural system are known as Neurons

Human Neural or Nervous System is divided into - Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Types of nervous systemFig. 7. Types of Nervous System

The structural and the functional unit of nervous system is neuron. It consists of three parts - Cell Body, Dendrites and Axon.

Nerve impulse transmission occurs from one neuron to another neuron through junctions known as Synapses. Synapse is formed by the membrane of pre-synaptic neuron and post-synaptic neuron separated by a gap known as Synaptic Cleft.

Synapses are of two types - Electrical Synapse and Chemical Synapse. During electrical synapse, pre- and post-synaptic neuron are in close proximity. Electric current can directly flow from one neuron to another neuron. This mode of nerve impulse transmission is faster than the chemical synapse.

The brain is divided into Forebrain, Midbrain and Hindbrain.

Forebrain consists of three parts - Cerebrum, Thalamus and Hypothalamus.

Midbrain is located between the forebrain and hindbrain

Hind brain comprises of pons, cerebellum, and medulla oblongata

Sensory Organs detect the changes in the environment and send signals to the central nervous system

Eyes are situated in the sockets of the skull known as Orbits.

It is composed of three layers - the outermost sclera (dense connective tissue), middle choroid (supplied with blood vessels) and innermost retina.

Structure of the EyeFig. 8. Structure of the Eye

Retina consists of three layers of neuronal cells - Ganglion Cells, Bipolar Cells and Photoreceptor Cells. Rods and cones are two types of photoreceptor cells.These cells contain light sensitive proteins. Cones are meant for bright light and color vision whereas rods are meant for dim light. Rods contain protein known as Rhodopsin. Cones respond to three different colors - Red, Green and Blue.

Hearing and balancing are the two main functions of ears. Ear is divided into three parts - Outer Ear, Middle Ear and External Ear

The outer ear consists of pinna and external auditory meatus.

The middle ear consists of three ossicles - Malleus, Incus and Stapes.

Structure of earFig. 9. Structure of ear

Inner ear comprises of bony labyrinth that contains three semicircular canals.

Chemical Control and Integration

Ductless glands are known as Endocrine Glands. Their secretions are known as Hormones.

Different endocrine glands are pituitary, pineal, thyroid, parathyroid, thymus, adrenal and gonads. Kidney, liver, gastrointestinal tract also produces some hormones.

List of endocrine glands with their function

Endocrine glandsFunction
Pituitary glandDivided into two parts - Anterior Pituitary and Posterior Pituitary. Anterior pituitary produces growth hormone, thyroid stimulating hormone, prolactin, adrenocorticotropic hormone, melanocyte stimulating hormone, follicle stimulating hormone and luteinizing hormone whereas posterior pituitary produces vasopressin and oxytocin.

The pineal gland


It secretes a hormone known as Melatonin. The function of melatonin is to regulate 24 hours rhythm in our body such as body temperature sleep-awake cycle etc.

Thyroid gland


Thyroid hormones control basal metabolic rate, synthesis of red blood cells, metabolism of proteins, carbohydrates, and fats
Parathyroid glandIt increases the calcium level in the blood.
ThymusThis gland secretes a hormone known as thymosin. It participates in cell mediated immunity. I
Adrenal gland

Divided into adrenal cortex and adrenal medulla. Adrenal medulla produces two hormones known as adrenaline and noradrenaline

Adrenal cortex is divided into 3 layers- zona reticularis (inner layer), zona fasciculata (middle layer) and zona glomerulosa (outer layer). Hormones of adrenal cortex are commonly known as corticoids

PancreasIt is a dual gland, that is, it is endocrine as well as exocrine in function. Endocrine part produces insulin and glucagon that controls blood glucose level.
TestesThe hormone produce by testes regulates spermatogenesis, development of beards and moustaches, as well as maturation of male accessory sex organs. The hormone produced is known as testosterone.
OvariesIt produces two hormones known as estrogen and progesterone. Estrogen helps in ovulation and during menstruation. Progesterone is a pregnancy hormone.

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