Final answer:
The components missing in a Mapleson D circuit are the fresh gas flow and unidirectional valves. This system has a high fresh gas flow and lacks unidirectional valves, requiring higher gas flows to prevent rebreathing.
Step-by-step explanation:
The Mapleson D circuit, also known as the Bain circuit, is one of several anesthesia breathing systems used to deliver oxygen and anesthetic gases and to remove carbon dioxide. In a Mapleson D circuit, the components absent are c) Fresh gas flow and unidirectional valves. This system consists of a fresh gas flow that enters the circuit near the patient's end, a corrugated breathing tube, a reservoir bag, and an adjustable pressure-limiting (APL) valve, also known as an expiratory valve, to control the pressure within the system. While fresh gas flow is indeed present in the circuit, unidirectional valves are not included, and these are essential in systems designed to prevent rebreathing of expired gases without reliance on high fresh gas flows. The absence of unidirectional valves in the Mapleson D circuit necessitates a higher fresh gas flow rate to prevent rebreathing.