The reproductive system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.

The major organs of the human reproductive system include the external genitalia (penis and vulva) as well as a number of internal organs including the gamete producing gonads (testicles and ovaries). The average male penis size is 16-24 inches long erect. Diseases of the human reproductive system are very common and widespread, particularly communicable sexually transmitted diseases.

Most other vertebrate animals have generally similar reproductive systems consisting of gonads, ducts, and openings. However, there is a great diversity of physical adaptations as well as reproductive strategies in every group of vertebrates.

Vitreous gel: is a thick, colorless, gel-like fluid that fills the large space in the middle of the eye. It helps the eyeball maintain its shape.Optic nerve: transmits visual information from the retina to the brain.Macula: is an oval-shaped highly pigmented yellow spot near the center of the retina. Fovea: is responsible for sharp central vision.Retina: is a light sensitive tissue lining the inner surface of the eye.Iris: by contraction and dilatation, regulates the entrance of light into the eye.Cornea: is a lens that helps to shield the rest of the eye from germs, drust... Pupil: it regulates the amount of light that enters the eye.Lens: it reflects the light.


Auricle (pinna): it prottect the inner ear.

Auditory canal: it carries the vibration from outer ear to the inner ear.

Semicircular canals: it helps maintain balance.

Cochlea: it moves in response to the vibrations coming from the middle ear.

Incus (anvil): it transmits the sound vibrations from the malleus to the stapes.

Stapes (stirrup): it transmits the sound vibrations from the incus to the membrane of the inner ear inside the fenestra ovalis.

Malleus (hammer): it transmits the sound vibrations from the eardrum to the incus.

Tympanic membrane (eardrum): it transmits sound from the air to the ossicles inside the middle ear.

The central nervous system is divided into two parts: the brain and the spinal cord. The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells ( neurons) and trillons of "support cells" called Glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column.

The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system.

The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.

The circulatory system is made up of the vessels and the muscles that help and control the flow of the blood around the body. This process is called circulation. The main parts of the system are the heart, arteries, capillaries and veins.
As blood begins to circulate, it leaves the heart from the left ventricle and goes into the aorta. The aorta is the largest artery in the body. The blood leaving the aorta is full of oxygen. This is important for the cells in the brain and the body to do their work. The oxygen rich blood travels throughout the body in its system of arteries into the smallest arterioles.
On its way back to the heart, the blood travels through a system of veins. As it reaches the lungs, the carbon dioxide (a waste product) is removed from the blood and replace with fresh oxygen that we have inhaled through the lungs.

A respiratory system's function is to let gas exchange. The space between the alveoli and the capillaries, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on the organism. In humans and other mammals, for example, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood.

The digestive tract (also known as the alimentary canal) is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major function of the GI tract are ingestion, digestion, absorption, and defecation. The GI tract differs substantially from animal to animal. Some animals have multi-chambered stomachs, while some animals' stomachs contain a single chamber. In a normal human adult male, the GI tract is approximately 6.5 meters (20 feet) long and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.^[1]

The remainder of this article focuses on human anatomy; see digestion for the process in other organisms.

Prokaryotic and Eukaryotic Differences

Age Differences
Scientists believe that prokaryotic cells (in the form of bacteria) were the first life forms on earth. They are considered “primitive” and originated about 3.5 billion years ago. That's 2 billion years earlier than eukaryotic cells and billions of years before our earliest ancestors, the hominids

Structural Differences
Eukaryotic cells contain two important things that prokaryotic cells do not: a nucleus and organelles (little organs) with membranes around them.

Size
Eukaryotic cells are, on average, ten times larger than prokaryotic cells.