Chapter 20

Blood vessels are composed primarily of smooth muscle.
Smooth Muscle:
- Not yellow; described as smooth in texture.
- Smooth muscle in blood vessels is crucial for their function.

Regulation:
- Smooth muscle is regulated by the autonomic nervous system.
- Key neurotransmitters involved:
- Acetylcholine: Released from the parasympathetic nervous system.
- Nitric Oxide: Also part of the parasympathetic system.
- Function:
- In order to dilate blood vessels, smooth muscles must relax rather than contract.

Calcium ions play a pivotal role in the function of vascular smooth muscle.
Important pathways involving calcium need to be understood to address clinical questions.
Example of a clinical scenario:
- A drug inhibits the breakdown of cyclic GMP in a patient, and an agonist or antagonist is administered.

M3 Muscarinic Receptor:
- Important for smooth muscle relaxation in the penis, thus allowing for erections.
- When an agonist binds to M3: results in an increase in erectile capacity due to smooth muscle relaxation.
Questions arise:
- Are we increasing or decreasing the erection with dilation?
- If we dilate blood vessels, we are increasing the erection as this allows for increased blood flow.

Cyclic GMP's role in erections:
- When a drug inhibits its breakdown, there will be more cyclic GMP available in the cell, promoting erection.
- This mechanism helps the erection to last longer.

An example of a drug related to this is Sildenafil.
- While commonly known for erectile dysfunction, it also aids in conditions concerning the lungs, specifically:
- Pulmonary Hypertension:
  - It works by dilating pulmonary blood vessels, improving blood flow to tissues.
  - It is effective in patients on mechanical ventilation facing pulmonary complications.

Luteinizing Hormone (LH):
- Targets the gonads, primarily ovaries in females.
- Ovary Function:
  - Produces the corpus luteum which is vital for releasing hormones necessary for pregnancy preparation.
- Three stages of the ovarian cycle:
- Follicular Phase: Day 1 to Day 13; involved in egg production.
  - Proliferation occurs in the endometrium during this time.
- Ovulation: Day 14, the egg is released.
- Luteal Phase: Post-ovulation, where the corpus luteum functions to support potential pregnancy.

Endometrial Phases:
- Proliferative Phase:
- Happens during the follicular phase (Days 5-13) where endometrial tissue thickens in preparation for possible implantation.
- Secretory Phase:
- Corresponds with the luteal phase of the ovarian cycle.
- If fertilization does not occur, the endometrium is shed, marking the onset of menstruation.

Follicular Phase (Day 1-13):
- FSH (Follicle Stimulating Hormone): Low initially, begins to rise towards day 13.
- Estrogen: Gradually increases during the follicular phase.
Ovulation (Day 14):
- Surge in LH, with respective increases in FSH leading to the release of the egg.
- Estrogen levels peak before dropping sharply post-ovulation.
Luteal Phase (Day 15-28):
- After ovulation, progesterone rises significantly, supporting endometrial preparation.
- Estrogen also sees a rise due to corpus luteum activities.

Estrogen plays a dual role:
- Positive feedback on LH to trigger ovulation.
- Supportive role in the growth of the endometrium and follicle development.
The lack of fertilization leads to drops in progesterone and estrogen, prompting menstruation.

The menstrual cycle can be divided into:
- Days 1-5: Menstruation and shedding of the endometrium.
- Days 5-13: Follicular growth and thickening of the endometrium (proliferative phase).
- Day 14: Ovulation.
- Days 15-28: Luteal phase promoting potential pregnancy.

Roles of FSH:
- Stimulates follicular growth and the secretion of estrogen during the first half of the cycle.
Roles of LH:
- Triggers ovulation and the formation of the corpus luteum, leading to progesterone production.