Human Physiology - Hormones, homeostasis and reproduction

Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream to reach target cells. Examples include Insulin ($C_{257}H_{383}N_{65}O_{77}S_{6}$) and Glucagon.

Homeostasis is the maintenance of a constant internal environment within physiological limits, such as blood glucose concentration ($4.4$ to $6.1 \text{ mmol/L}$), body temperature ($37^{\circ}C$), and water balance.

Negative feedback loops involve a stimulus being detected by a sensor, which triggers a response from an effector to counteract the stimulus and return the system to its set point.

Blood glucose is regulated by the Islets of Langerhans in the pancreas: $\alpha$ cells secrete glucagon (to increase glucose) and $\beta$ cells secrete insulin (to decrease glucose).

Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature. It requires Iodine ($I^-$) for synthesis.

Leptin is a hormone secreted by adipose (fat) tissue that inhibits appetite by targeting cells in the hypothalamus.

Melatonin is secreted by the pineal gland to control circadian rhythms; its secretion increases in the evening to induce sleep.

Sex determination in humans: The $SRY$ gene on the $Y$ chromosome leads to the development of testes and the production of testosterone. In the absence of $SRY$, ovaries develop.

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

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