The menstrual cycle is a natural process in females of reproductive age, typically lasting 28 days, though it can vary. It involves a series of changes in the ovaries and uterus, regulated by hormones, to prepare for a potential pregnancy. The cycle is divided into four main phases: Menstrual, Follicular, Ovulation, and Luteal.

Step 1: Menstrual Phase (Days 1-5)
This phase marks the start of the cycle. The thickened lining of the uterus (endometrium) is shed, resulting in menstrual bleeding. This occurs due to low levels of estrogen and progesterone.

Step 2: Follicular Phase (Days 1-13)
Overlapping with the menstrual phase, this phase begins on day 1. The pituitary gland releases Follicle Stimulating Hormone (FSH), which stimulates the ovaries to develop follicles. One follicle becomes dominant and matures, producing estrogen. Estrogen helps rebuild the endometrium.

Step 3: Ovulation (Around Day 14)
A surge in Luteinizing Hormone (LH) from the pituitary gland causes the mature follicle to rupture and release an egg (ovum) from the ovary. This is ovulation. Estrogen levels peak just before this.

Step 4: Luteal Phase (Days 15-28)
After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone. Progesterone further thickens and maintains the uterine lining, preparing it for implantation of a fertilized egg. If pregnancy does not occur, the corpus luteum degenerates, leading to a drop in progesterone and estrogen, triggering menstruation and the start of a new cycle.

Key Hormones:
The cycle is governed by the interplay of hormones from the hypothalamus, pituitary gland, and ovaries.
- GnRH (Gonadotropin-Releasing Hormone): From hypothalamus, stimulates pituitary.
- FSH (Follicle Stimulating Hormone): From pituitary, stimulates follicle growth.
- LH (Luteinizing Hormone): From pituitary, triggers ovulation.
- Estrogen: From ovaries, builds endometrium.
- Progesterone: From corpus luteum, maintains endometrium.

Labeled Diagram Description:
A typical labeled diagram of the menstrual cycle would have two main graphs: one for hormone levels and one for ovarian/uterine changes, both plotted against the days of the cycle (1-28).

1. Hormone Level Graph:
- The x-axis represents "Days of the Menstrual Cycle".
- The y-axis represents "Hormone Level".
- FSH: A line that starts moderately high, dips slightly, and has a small peak just before the mid-cycle LH surge.
- LH: A line that remains low for most of the cycle and shows a very sharp, tall peak at day 14 (ovulation).
- Estrogen: A line that gradually rises during the follicular phase, peaks just before ovulation, dips briefly, rises again slightly in the luteal phase, and then falls.
- Progesterone: A line that remains very low during the follicular phase, rises sharply after ovulation to form a broad peak during the luteal phase, and then falls rapidly just before menstruation.

2. Ovarian and Uterine Changes:
Below the hormone graph, the diagram often shows illustrations or labels for each phase:
- Ovarian Cycle: Shows development of follicle, ovulation, and formation/degeneration of corpus luteum.
- Uterine Cycle (Endometrium): Shows the lining thickness as low during menses, gradually thickening (proliferative phase) during the follicular phase, and becoming thick and secretory during the luteal phase before shedding.

Final Answer: The menstrual cycle is a hormonally regulated process consisting of the menstrual, follicular, ovulatory, and luteal phases, which coordinate changes in the ovaries and uterus to facilitate reproduction.

Related Topics and Formulae

Related Topics: The menstrual cycle is a key part of the female reproductive system. Understanding it is crucial for topics like fertility, contraception, and reproductive health disorders.

Key Hormonal Relationships: While there are no simple algebraic formulae, the cycle is defined by feedback loops:
- Positive Feedback: High estrogen levels trigger the LH surge.
- Negative Feedback: High progesterone and estrogen levels inhibit the release of GnRH, FSH, and LH.