Human Physiology - Hormones, Homeostasis, and Reproduction

Homeostasis involves maintaining the internal environment within narrow limits, such as blood glucose concentration ($5 \text{ mmol dm}^{-3}$), blood $pH$ ($\approx 7.4$), and body temperature ($37^\circ\text{C}$).

Negative Feedback: A mechanism where a change in a physiological variable triggers a response that counteracts the initial fluctuation, such as the regulation of $C_6H_{12}O_6$ levels by the pancreas.

Blood Glucose Regulation: $\beta$-cells in the Islets of Langerhans secrete Insulin to lower glucose (promoting glycogenesis), while $\alpha$-cells secrete Glucagon to raise glucose (promoting glycogenolysis).

Thyroxin: Secreted by the thyroid gland to regulate the metabolic rate and help control body temperature. It contains four iodine atoms ($T_4$).

Leptin: A hormone secreted by cells in adipose (fat) tissue that acts on the hypothalamus of the brain to inhibit appetite. Resistance to leptin is a common factor in clinical obesity.

Melatonin: Secreted by the pineal gland to control circadian rhythms. Secretion increases in the evening and decreases at dawn.

Sex Determination: The $SRY$ gene on the $Y$ chromosome causes embryonic gonads to develop as testes and secrete testosterone. In the absence of $SRY$, ovaries develop.

The Menstrual Cycle: Controlled by negative and positive feedback involving pituitary hormones ($FSH$ and $LH$) and ovarian hormones (estrogen and progesterone).

IVF (In Vitro Fertilization): Involves 'down-regulation' of the natural cycle, 'superovulation' using high doses of $FSH$, fertilization in a petri dish, and implantation of embryos.