Introduction
This essay will consider the structure and function of the 11 systems within the human body. It will detail the interrelationship between the nervous system and the musculoskeletal system and between the circulatory system and the lymphatic system. It will then explain the roles of the circulatory and lymphatic systems in the immune response and the role of hormones in metabolism.

Human Body Systems
The human body is made up of 11 separate but interconnected systems (Sherwood, 2007). These are the skeletal, muscular, circulatory, respiratory, digestive, excretory, nervous, integumentary, immune, endocrine and reproductive systems. The success and survival of the human body is dependent on the ability of separate body systems to work together.

The skeletal system provides structure for the human body, stores minerals, produces blood cells and provides protection for delicate organs (Kelly, 2004). 206 bones are connected with ligaments, muscles and tendons, with cartilage, a softer cushion like material, providing protection in jointed areas. Body movements are controlled by the muscular system, with these muscles being connected to bones via tendons (Adams, 2004). Stimulation of these muscles by the nervous system causes contraction and the resulting movement of bones to which they are attached. A number of involuntary muscles ensure the respiratory and circulatory systems continue with contraction of the heart and lungs (Adams, 2004). The heart is central to the circulatory system and acts as to pump blood through arteries, veins and capillaries. The circulatory system is responsible for delivering nutrients and oxygen to cells as well as removing waste products and aiding the immune system through the circulation of white blood cells (Jacab, 2006). The immune system is comprised of lymph organs, such as the spleen and thymus, and the skin, all of which are responsible for protecting the body against invading pathogens (Parham, 2005).

The circulatory system and the respiratory system are closely interconnected with the latter bringing fresh oxygen into the body through the alveoli of the lungs (Johnson, 2004). The respiratory system is closely connected with the excretory system as it is responsible for the removal of carbon dioxide and other waste gases through exhalation. The excretory system eliminates both solid and liquid wastes in addition to these gaseous products, and is made up of a number of specialist tissues along with the large intestine, bladder, kidneys, rectum, lungs and skin (Sherwood, 2007). The physical and chemical breakdown of food into energy is carried out by the digestive system. This system commences with the mouth, teeth and salivary glands then passes through the oesophagus to the stomach and small intestine for digestion. The liver, pancreas and large intestine are also involved, through the production of digestive enzymes and bile and the processing of nutrients (Windelspecht, 2004).

The nervous system is responsible for sending messages to and from the brain through neurons. The nervous system controls all bodily functions by sending electrochemical signals through the neural network (Llamas, 1998). The endocrine system acts as a communication network but uses hormones as chemical messengers which travel through the bloodstream (Klosterman, 2009). The hormones have specific target organs and carry signals to start or stop performing a specific function. Finally, the reproductive system is responsible for the production of children and reproductive hormones cause our bodies to develop into sexual maturity.

Relationship between the nervous and musculoskeletal system
Muscle is a contractile tissue that can be histologically divided into three types. These are: striated or skeletal muscle, which are under direct nervous control; cardiac muscle, which is also striated but is a specialist form that is confined specifically to the heart; and smooth or visceral muscle, which is not under direct nervous control (Nair and Peate, 2013). This latter form can be found in the walls of blood vessels and the alimentary tract and in arrector pili. Smooth muscle is usually in the form of flat sheets and forms circular and longitudinal layers, or can be arranged as a sphincter in order to control passage through a tube, for example the anus (Ikebe, 1996). Skeletal muscle is usually attached to two separate bones via tendon, fleshy or aponeurosis connections.

Muscle action control is carried out by the nervous system (Stein, 1982). Contact between nerves and muscles often occurs through chemical stimulus conveyed by motor end plates, which instruct muscles to contract. Signals can also be sent through tendons via specific receptors that are able to measure the stretch of the tendon (Stein, 1982). Messages from nerves are referred to as efferent when they take a message to a specific tissue and afferent when they are taking the message to the spinal cord and brain (Craig, 2005). As such the nervous system comprises two separate but combined systems. These are the central and peripheral nervous systems, with the former being made up of the brain and spinal cord, and the latter comprising the remaining neural network (Cervero, 1988). This neural network comprises 12 pairs of head nerves connected to the brain and 31 pairs of spinal nerves connected to the spinal cord. Nerves which transfer information from receptors within the body to the central nervous system are sensoric nerves, whilst nerves that transport information from the CNS to muscle fibres are motoric nerves (Cervero, 1988). As such, the peripheral nervous system comprises collections of nerves, their insulating myelin sheaths, Schwann cells and connective tissue. The majority of these nerve cells are able to carry out efferent and afferent cell processes (Craig, 2005).

Figure 1 shows the organisation of a neuron, with the body being the axon and the smaller projections being known as dendrites. The neuron uses the dendrites to obtain and pass information from and to other neurons (Spruston, 2008). The axon passes the information to other cells particularly muscle cells. The information is then passed along the neuron through voltage changes within the cell membrane. This is known as the action potential (Bean, 2007). Information transfer between individual nerve cells occurs through chemical agents which are released when the action potential has reached the end of an axon.

Relationship between the circulatory and lymphatic systems
These systems primarily comprise of the body's blood circulating and waste removal capabilities. The two systems are significantly intertwined and work collectively to transport substances throughout the body (Sherwood, 2007). Both systems work in a similar manner as they both produce a liquid substance, either lymph or blood, which flows through a network of vessels and ducts to reach every part of the body. Both of these liquid substances are responsible for carrying nutrients or removing waste and are therefore both circulatory in nature (Sherwood, 2007).