Answer:
in the wall of the right atrium of the heart.
Explanation:
The sinoatrial node (SA), consisting of spindle-shaped cells, initiates the electrical activity of the heart. From its location in the right atrium in proximity to the superior vena cava, the electrical activity spreads to the atria, whose cells are larger than those of the SA.
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone.
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The nucleus contains the cells DNA(genetic material) which is basically the recipe for making proteins. mRNA is first transcribed off of DNA and then in translated(by ribosomes) to polypeptides(proteins).
During the exercise period (10-15min) the blood lactic acid concentration increases to about 13.2 mmol/dL (same units as on graph) as the individual is having problems keeping up their aerobic respiration. After 15min, they stop exercising and the lactic acid concentration starts to return to normal as their body is able to take in enough oxygen and catches up with the excess lactic acid, metabolizing it into CO2 and H2O. The period between 15-20 min shows the fastest reduction in concentration.