Organism level systems Study Guide

Insulin is a hormone vital for regulating blood glucose (sugar) levels, preventing them from becoming too high. After a meal, carbohydrates are broken down into glucose, which is then absorbed into the bloodstream, causing blood glucose levels to rise. This increase is detected by specialised cells in the pancreas. In response, the pancreas releases insulin into the bloodstream. Insulin travels through the blood to target cells, primarily in the liver and muscles. It acts by making these cells more permeable to glucose, allowing them to absorb glucose from the blood at a faster rate. Inside the liver and muscle cells, insulin promotes the conversion of excess glucose into a storage molecule called glycogen, a process known as glycogenesis. Glycogen can be stored in the liver and muscle cells. Additionally, insulin encourages other body cells to take up glucose for use in respiration, effectively lowering the amount of glucose remaining in the blood. If blood glucose levels fall too low, a different hormone, glucagon, is released, which has the opposite effect, converting stored glycogen back into glucose and releasing it into the bloodstream. This fine-tuned hormonal control ensures that blood glucose levels remain within a narrow, healthy range, which is crucial for overall body function and energy supply to cells.

The menstrual cycle, typically lasting 28 days, is a complex process in females regulated by four key hormones: Follicle Stimulating Hormone (FSH), Luteinising Hormone (LH), oestrogen, and progesterone. These hormones interact in a finely tuned feedback system to prepare the uterus for potential pregnancy. FSH, secreted by the pituitary gland, stimulates the development of follicles in the ovary, each containing an egg, and also stimulates the ovaries to produce oestrogen. Rising oestrogen levels, initially, inhibit FSH production (negative feedback). However, once oestrogen reaches a high threshold, it triggers a surge in LH release from the pituitary gland (positive feedback). This LH surge causes ovulation – the release of a mature egg from the follicle. After ovulation, the ruptured follicle transforms into the corpus luteum, which then produces progesterone and some oestrogen. Progesterone maintains the uterine lining and, along with oestrogen, inhibits both FSH and LH production (negative feedback) to prevent further follicle development and ovulation. If fertilisation and implantation do not occur, the corpus luteum degenerates, leading to a drop in progesterone and oestrogen levels. This decline removes the inhibition on FSH and LH, allowing their levels to rise again, initiating a new cycle and the shedding of the uterine lining (menstruation).