HUMAN BODY

HUMAN BODY

The human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs. By the time the human reaches adulthood, the body consists of close to 100 trillion cells, the basic unit of life. These cells are organised biologically to eventually form the whole body.

Constituents of the human body In a normal man weighing 60 kg
Constituent	Weight	Percent of atoms
Oxygen	38.8 kg	25.5 %
Carbon	10.9 kg	9.5 %
Hydrogen	6.0 kg	63 %
Nitrogen	1.9 kg	1.4 %
Calcium	1.2 kg	0.3 %
Phosphorus	0.6 kg	0.2 %
Potassium	0.2 kg	0.06 %

The organ systems of the body include the musculoskeletal system, cardiovascular system, digestive system, endocrine system, integumentary system, urinary system, lymphatic system, immune system, respiratory system, nervous system and reproductive system.

The cardiovascular system comprises the heart, veins, arteries and capillaries. The primary function of the heart is to circulate the blood, and through the blood, oxygen and vital minerals are transferred to the tissues and organs that comprise the body. The left side of the main organ (left ventricle and left atrium) is responsible for pumping blood to all parts of the body, while the right side (right ventricle and right atrium) pumps only to the lungs for re-oxygenation of the blood.

The digestive system provides the body's means of processing food and transforming nutrients into energy. The digestive system consists of the - buccal cavity, esophagus, stomach, small intestine, large intestine ending in the rectum and anus. These parts together are called the alimentary canal (digestive tract).

The integumentary system is the largest organ system in the human body, and is responsible for protecting the body from most physical and environmental factors. The largest organ in the body, is the skin. The integument also includes sweat  glands, hair, nails and arrectores pili (tiny muscles at the root of each hair that cause goose bumps).

The main function of the lymphatic system is to extract, transport and metabolise lymph, the fluid found in between cells.

The human musculoskeletal system consists of the human skeleton, made by bones attached to other bones with joints, and skeletal muscle attached to the skeleton by tendons.

An adult human has approximately 206 distinct bones:

The nervous system consists of cells that communicate information about an organism's surroundings and itself.

Reproductive system

Human reproduction takes place as internal fertilization by sexual intercourse. During this process, the erect penis of the male is inserted into the female's vagina until the male ejaculates semen produced by testes, which contains sperm, into the female's vagina. The sperm then travels through the vagina and cervix into the uterus or fallopian tubes for fertilization of the ovum.

The human female reproductive system contains three main parts: the vagina, which acts as the receptacle for the male's sperm, the uterus, which holds the developing foetus, and the ovaries, which produce the female's ova. The breasts are also an important reproductive organ during the parenting stage of reproduction.

At certain intervals, typically approximately every 28 days, the ovaries release an ovum, which passes through the fallopian tube into the uterus. The lining of the uterus, called the endometrium, and unfertilized ova are shed each cycle through a process known as menstruation.