What physiological change is caused by increased levels of oestrogen during heat?

Increased levels of oestrogen during heat are primarily responsible for the promotion of uterine growth. Oestrogen plays a crucial role in the reproductive cycle, particularly in the preparation of the uterus for potential pregnancy. During the heat cycle, elevated oestrogen levels stimulate the growth and thickening of the endometrial lining, which is essential for receiving a fertilized egg. This physiological change enhances the uterus's capacity to support embryo implantation and development if fertilization occurs.

The other options do not reflect the primary effect of increased oestrogen during heat. For instance, increased progesterone production typically occurs later in the cycle following ovulation, resulting from the action of luteinizing hormone rather than directly from elevated oestrogen. Decrease in FSH levels is a consequence of feedback mechanisms in the hormonal cycle, while reduction in metabolic rate is not directly influenced by oestrogen but rather would involve other metabolic and hormonal factors. Therefore, promoting uterine growth due to elevated oestrogen is the correct physiological response during the heat period.