Blood pressure is generally somewhat higher in birds than in mammals. Large brachial and pectoral arteries which supply blood to flight musculature. These veins drain the posterior part of the body trunk and enter the capillary bed surrounding the kidney tubules. Lymph System is poorly developed compared to mammals.
Valves are present in lymph vessels to prevent backflow.
Lymph is moved by skeletal muscle contraction, as in mammals. Ostrich, Cassowaries, Gulls, Storks, some Passerines. To meet the high oxygen demand of flight, birds have the most efficient respiratory system of any vertebrate. It differs from the respiratory system of mammals in several aspects. Air sacs are thin sacs continuous with the lungs but not involved in gas exchange.