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.

Heart

It is a four chambered organ, having 2 auricles (upper chambers) and 2 ventricles (lower chambers). All these four chambers are separated from each other by septae, which makes sure of the separation of pure and impure blood of different chambers and prevents it from mixing. The circulation of blood throughout the body takes place by means of Blood Vessels, namely Artery and Vein.

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 impure or deoxygenated blood from all parts of the body is carried to the heart by means of two veins, namely Superior Vena Cava (from the upper part of the body) and Inferior Vena Cava (from the lower half of the body). This impure blood from the Vena Cavae enters the right auricle.

From the right auricle, the blood flows downwards and fills the right ventricle. This blood in the right ventricle is prevented from flowing back (Regurgitation) to the auricle by a Tricuspid Valve.

From the right ventricle, the impure blood is carried to the lungs by the Pulmonary Artery for oxygenation. During this tranfer of blood, the prevention of backflow is maintained by the Pulmonary Valve. After oxygenation in the lungs, the pure blood is carried back from the lungs to the heart by the Pulmonary Vein and is filled in the left auricle. The Pulmonary artery and vein thus become the only artery to carry impure and only vein to carry pure blood.

From the left auricle, the blood moves downwards and fills the left ventricle, and the backflow during this flow is prevented by the Mital Valve (Bicuspid). From the left ventricle, the pure blood is supplied to the entire body by means of Aorta, and the backflow during this phase is prevented by the Aortic Valve. The arteries an veins, supplying blood to Heart it self, are called coronary arteries an veins.

The whole body thus receives the oxygenated and pure blood and the deoxygenated or impure and used blood is carried back to the heart by Vena Cavae and the cycle repeats.

Brain

It is one of the largest and most complex organs in the human body. It is made up of more than 100 billion nerves that communicate in trillions of connections called synapses. The brain is made up of many specialized areas that work together: The cortex is the outermost layer of brain cells.

The human brain is the command center for the human nervous system. It receives input from the sensory organs and sends output to the muscles. The human brain has the same basic structure as other mammal brains, but human brain is largest in relation to body size.

Facts about the human brain

• The human brain is the largest brain of all vertebrates relative to body size

• It weighs about 3.3 lbs. (1.5 kilograms)

• The brain makes up about 2 percent of a human’s body weight

• The cerebrum makes up 85 percent of the brain’s weight

• It contains about 86 billion nerve cells (neurons) — the “gray matter”

• It contains billions of nerve fibers (axons and dendrites) — the “white matter”

• These neurons are connected by trillions of connections, or synapses

Anatomy of the human brain

The largest part of the human brain is the cerebrum, which is divided into two hemispheres. Underneath lies the brainstem, and behind that sits the cerebellum.

The outermost layer of the cerebrum is the cerebral cortex, which consists of four lobes: the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe.

The Brain has three sections known as the forebrain, midbrain and hindbrain. Each of these contains fluid-filled cavities called ventricles.

The forebrain develops into the cerebrum and underlying structures; the midbrain becomes part of the brainstem; and the hindbrain gives rise to regions of the brainstem and the cerebellum.

Left brain vs. right brain

The human brain is divided into two hemispheres, the left and right, connected by a bundle of nerve fibers called the corpus callosum. The hemispheres are strongly, though not entirely, symmetrical.

The left brain controls all the muscles on the right-hand side of the body; and the right brain controls the left side. One hemisphere may be slightly dominant, as with left- or right-handedness.

The popular notions about “left brain” and “right brain” qualities are generalizations that are not well supported by evidence. Still, there are some important differences between these areas.

The left brain contains regions involved in speech and language (Broca’s area and Wernicke’s area), and is also associated with mathematical calculation and fact retrieval, Holland said.

The right brain plays a role in visual and auditory processing, spatial skills and artistic ability — more instinctive or creative things, Holland said — though these functions involve both hemispheres. “Everyone uses both halves all the time,” he said.

Nervous system

Thyroid

It is a small, butterfly-shaped organ/gland located just below the Adam’s apple. This gland plays a very important role in controlling body’s metabolism, that is, the rate at which your body uses energy. It does this by producing thyroid hormones (primarily Thyroxine, or T4, and Triiodothyronine, or T3), chemicals that travel through your blood to every part of your body. These thyroid hormones tell the cells in your body how fast to use energy and create proteins. The thyroid gland also makes Calcitonin, a hormone that helps to regulate calcium levels in the blood by inhibiting the breakdown (reabsorption) of bone and increasing calcium excretion from the kidneys.

The body has an elaborate feedback system to control the amount of T4 and T3 in the blood. When blood levels decrease, the hypothalamus releases Thyrotopin-Releasing Hormone (TRH), which in turn causes the pituitary gland (a tiny gland located below the hypothalamus and almost in the center of the head) to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce and secrete thyroid hormones. When there is sufficient thyroid hormone in the blood, the amount of TSH decreases to maintain constant amounts of thyroid hormones, T4 and T3.

Inside the thyroid, most of the T4 is stored bound to a protein called thyroglobulin. When the need arises, the thyroid gland creates more T4 and/or releases some of what is stored. In the bloodstream, most T4 is bound to a protein called thyroxine-binding globulin (TBG) and is relatively inactive. T4 is converted to T3 by the liver and in many other tissues. T3 is primarily responsible for controlling the rate of body functions.

Thyroid diseases are primarily conditions that affect the amount of thyroid hormones being produced. Some create too few, leading to hypothyroidism and a slowing of body functions. This hypothyroidism causes symptoms such as weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women. Severe untreated hypothyroidism, called myxedema, can lead to heart failure, seizures, and coma.

In children, hypothyroidism can stunt growth and delay sexual development. If a thyroid disorder creates excessive amounts of thyroid hormone, the result is hyperthyroidism and the acceleration of body functions. This can lead to symptoms such as increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, and sometimes diarrhea. There may be puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.

Liver

It is a gland and plays a major role in metabolism with numerous functions in the human body, including regulation of glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification.

The liver is a large, meaty organ that sits on the right side of the belly. Weighing about 3 pounds, the liver is reddish-brown in color and feels rubbery to the touch. The liver is protected by the rib cage.

The liver has two large sections, called the right and the left lobes. The gallbladder sits under the liver, along with parts of the pancreas and intestines. The liver and these organs work together to digest, absorb, and process food.

The liver’s main job is to filter the blood coming from the digestive tract, before passing it to the rest of the body. The liver also detoxifies chemicals and metabolizes drugs. As it does so, the liver secretes bile that ends up back in the intestines. The liver also makes proteins important for blood clotting and other functions.

Lungs

Our body have two lungs at right and left side in Thoracic cavity of chest. The right lung is bigger than the left, which shares space in the chest with the Heart. The lungs together weigh approximately 1.3 kgs and right lung is heavier than left lung.

The lungs are part of lower respiratory tract which begans at Trachea and branches spreads into the Bronghi and Bronchioles. The Bronchioles receive Air via conducting zone. These zones divide until air reaches microscopic Alveoli, where the process of gas exchange takes place. Together, the lungs contain approximately 2,400 kilometres (1,500 ml) of airways and 300 to 500 million alveoli.

The lungs are enclosed within a sac called the Pleural sac which allows the inner and outer walls to slide over each other whilst breathing occures, without much friction. This sac encloses each lung and also divides each lung into sections called lobes. The right lung has three lobes and the left has two.

The lobes are further divided into bronchopulmonary segments and lobules. The lungs have a unique blood supply, receiving deoxygenated blood sent from the heart for the purposes of receiving oxygen (the pulmonary circulation) and a separate supply of oxygenated blood (the bronchial circulation).

The total surface area of lungs vary from 50 to 75 square metres (540 to 810 sq ft) i.e roughly the same area as one side of a tennis court.

The bronchi in the conducting zone are reinforced with hyaline cartilage in order to hold open the airways. The bronchioles have no cartilage and are surrounded instead by smooth muscle.

Kidney

These are two bean-shaped organs and are located just below the rib cage, one on each side of the spine. Every day, the two kidneys filter about 120 to 150 quarts of blood to produce about 1 to 2 quarts of urine, composed of wastes and extra fluid.

The kidneys serve several essential regulatory roles in our body. Their main function is to regulate the balance of electrolytes in the blood, along with maintaining pH level. They also remove excess organic molecules from the blood, and it is by this action that their best-known function is performed: the removal of waste products of metabolism.

Kidneys are essential to the urinary system and also serve homeostatic functions such as the regulation of electrolytes (including salts), maintenance of acid–base balance, maintenance of fluid balance, and regulation of blood pressure (via the salt and water balance).

They serve the body as a natural filter of the blood, and remove water-soluble wastes which are diverted to the bladder. In producing urine, the kidneys excrete nitrogenous wastes such as urea and ammonium.

They are also responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol and erythropoietin.

An important enzyme, renin, is also produced in the kidneys.

Located at the rear of the abdominal cavity in the retroperitoneal space, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins.

Each kidney excretes urine into a ureter which empties into the bladder.

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