What stimulates the kidney to release erythropoietin EPO?

a) Low level of iron concentration in the blood: Iron is necessary for hemoglobin synthesis, but it does not directly stimulate EPO production.

b) Low level of oxygen concentration in the blood: Hypoxia is the primary stimulus for EPO release, which then promotes red blood cell production to increase oxygen-carrying capacity.

c) Low number of red blood cells in the blood: While anemia can lead to hypoxia, it is the oxygen deficit (not RBC count itself) that directly stimulates EPO.

d) High number of red blood cells in the blood: This would decrease EPO production through negative feedback.

a) Decreased evidence of endocytosis: Endocytosis increases in hyperthyroidism as T3/T4 are actively removed from colloid.

b) Decreased evidence of lysosomal activity: Lysosomal activity increases to process thyroglobulin into active hormones.

c) A decrease in the size of the follicular cells: Follicular cells become taller (columnar) and more active, not smaller.

d) A decrease in the cross-sectional area occupied by the colloid: In hyperthyroidism (e.g., Graves' disease), the thyroid follicles are hyperactive, so colloid is used up rapidly, reducing the colloid area.

e) Decreased number of reabsorption lacunae: These increase in number as colloid is actively resorbed.