I'm unclear as to what you mean with your question, are we talking about white blood cells? Brain cells? Stem cells? Red blood cells? Most don't really "connect" to the system so much as make up the system itself.
A nerve cell is part of the nervous system, a network of neurons that carry signals throughout the body.
Spiracles that connect tubes in the body belong to the respiratory system of insects. These structures allow air to enter into the insect's body and reach the tissues for gas exchange.
when they all connect
The respiratory and circulatory system connect in the lungs. At the alveoli, the air sacs are wrapped closely by capillaries to facilitate gas exchange.
It belongs to the immune system.
the impulses are brought to the cell body from whre its carried by the axon to the synapse
Nerves and nervous system.
macrophage
The circulatory system and the skeletal system
The order of living things from smallest to largest in the body is: cell, tissue, organ, organ system, and organism.
Located (obviously) in the blood, a white blood cell's job is to find and eliminate potential threats to the human body. The system of eliminating threats such as these from the body is undertaken by the IMMUNE system.
Circulatory system are connected by a network of arteries, veins and capillaries. They connect the body with heart. Inside the heart, various valves connect different chambers.