Fig. 2. Major effects of anoxia on cellular energetic turnover and on intra- and extracellular environments of a turtle cardiac cell. Top: normoxic conditions; Bottom: anoxic conditions; Left: ATP production; Right: ATP consumption. Top left: 36 mol ATP are produced mol^–1 glucose and waste products in the form of CO₂ and water are lost to the blood; Top right: in the normoxic cell, energy is predominantly used for protein synthesis, contractile work and ion-motive pumps such as the Na⁺/K⁺ ATPase and the Ca²⁺ATPase that are both important for contractile performance and Ca²⁺ transport. Thus, the Na⁺ gradient established by the Na⁺/K⁺ ATPase pump is used for the extrusion of Ca²⁺ through the Na⁺/Ca²⁺ exchanger during relaxation. The energetic state is high with a high ATP/ADP ratio and low levels of free phosphate. Bottom left: Anaerobic respiration only produces 3 mol ATP mol^–1 glucose, and the waste product lactic acid accumulates intra- and extracellularly causing intra- and extracellular acidification. The turtle shell buffers a considerable part of the lactic acidosis whereby calcium- and magnesium ions are released. During anoxia, energy state, i.e. the ATP/ADP ratio, decreases and the level of free phosphate increases. Anoxia is associated with a general metabolic depression due to translational arrest, spike and channel arrest. Moreover, mechanical work is reduced. Despite channel and spike arrest, long-term anoxia in turtles is associated with a progressive leak of K⁺, which accumulates in the extracellular space and cause a depolarisation.