In human anatomy, anastomosis refers to a connection or opening between two things, particularly between blood vessels or other tubular structures. The term is derived from the Greek term 'anastomosis,' which means 'outlet' or 'opening.' This natural network of connections plays a critical role in the survival and functionality of the human body.

Anastomoses can be formed at arterial, venous, and lymphatic vessels.

Arterial Anastomosis: These occur between arteries. They are most common in areas of the body that require a constant and significant blood supply, such as the brain and heart. For instance, the Circle of Willis in the brain is an arterial anastomosis that provides multiple routes for blood to reach brain tissues.

Venous Anastomosis: These occur between veins and are found throughout the body. An example would be the anastomoses between veins in the forearm, which allow for the redirection of blood flow if one vein becomes blocked.

Lymphatic Anastomosis occurs between lymph vessels and plays a vital role in maintaining the body's fluid balance and overall immunity.

An artery and vein can also form an anastomosis called an arteriovenous anastomosis. The vascular shunt in the capillary bed formed between the metarteriole and thoroughfare channel, which connects the arteriole and postcapillary venule, is a prominent example.

Anastomoses can occur naturally in the body or be created surgically in procedures like bypass surgery. In both cases, they ensure that blood and other fluids can circulate effectively, even when a primary pathway is blocked or damaged.

One of the most common places where anastomosis occurs naturally is in the circulatory system. For example, the coronary arteries, which supply blood to the heart muscle, have anastomotic connections. This means that if one artery is blocked, another can often still deliver blood to the affected area, preventing heart muscle death and reducing the risk of a heart attack.

Apart from the circulatory system, anastomoses are also found in the digestive system, where they can connect different parts of the intestine, and in the respiratory system, where they link different airways.

In conclusion, anastomosis is a vital anatomical feature that helps maintain body functionality by ensuring the continuity of blood flow and the delivery of nutrients and oxygen to various tissues and organs. Whether occurring naturally or created surgically, anastomoses contribute significantly to our survival and health.