37.3 Regulation of Body Processes

Hormones are the body's chemical messengers, regulating everything from metabolism to reproduction. They work through feedback systems to maintain homeostasis, the body's internal balance. Understanding how hormones function is crucial for grasping how our bodies respond to changes and maintain stability.

Hormonal imbalances can lead to disorders affecting metabolism, growth, and overall health. By exploring the roles of specific hormones and their effects on different body systems, you can better appreciate the intricate workings of the endocrine system.

Hormonal Regulation of Body Processes

Homeostasis and Feedback Mechanisms

Homeostasis is the maintenance of a stable internal environment in the body. Your body uses two types of feedback loops to achieve this:

• Negative feedback opposes a change to bring conditions back to a set point. Most hormonal regulation works this way. For example, when blood glucose rises after a meal, insulin is released to lower it. Once glucose drops back to normal, insulin secretion decreases. The response negates the original stimulus.
• Positive feedback amplifies a change rather than reversing it. This is less common and typically drives processes that need to reach a definitive endpoint. During childbirth, oxytocin stimulates uterine contractions, which push the baby against the cervix, which triggers more oxytocin release. The cycle only stops once the baby is delivered.

Hormonal Influence on Kidney Function

Three hormones work together to regulate how much water and sodium your kidneys retain or excrete, which directly controls blood volume and blood pressure.

• Antidiuretic hormone (ADH), also called vasopressin
  • Released by the posterior pituitary gland when blood osmolarity increases (blood is too concentrated) or blood volume decreases
  • Acts on the collecting ducts in the kidneys to increase water reabsorption, reducing urine output and conserving water
• Aldosterone
  • Secreted by the adrenal cortex when blood pressure decreases or blood potassium levels rise
  • Increases sodium reabsorption and potassium excretion in the distal tubules and collecting ducts
  • Because water follows sodium, this promotes water retention, which raises blood volume and blood pressure
• Atrial natriuretic peptide (ANP)
  • Released by the atria of the heart when blood volume or pressure gets too high
  • Inhibits sodium reabsorption in the collecting ducts, promoting sodium (and water) excretion in urine
  • This reduces blood volume and pressure, essentially counteracting the effects of aldosterone

Hormones in Reproductive Processes

Reproductive hormones are controlled through a signaling cascade that starts in the brain and ends at the gonads. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which tells the anterior pituitary to secrete two gonadotropins: FSH and LH. These then act on the ovaries or testes.

• Follicle-stimulating hormone (FSH)
  • In females: stimulates the development of ovarian follicles and estrogen production, supporting egg maturation
  • In males: promotes spermatogenesis in the testes
• Luteinizing hormone (LH)
  • In females: a surge of LH triggers ovulation and the formation of the corpus luteum, which produces progesterone to prepare the uterus for pregnancy
  • In males: stimulates Leydig cells in the testes to produce testosterone

The sex hormones produced downstream have their own distinct roles:

• Estrogen promotes the development of female secondary sexual characteristics during puberty (breast development, fat distribution) and regulates the menstrual cycle, preparing the uterine lining for implantation of a fertilized egg
• Progesterone maintains the uterine lining during pregnancy to support embryo development and inhibits further ovulation during pregnancy to prevent additional fertilizations
• Testosterone promotes male secondary sexual characteristics during puberty (muscle growth, deepening of voice), maintains libido, and supports spermatogenesis

Estrogen and progesterone also feed back to the hypothalamus and pituitary to regulate GnRH, FSH, and LH release. This is how the menstrual cycle stays on a roughly 28-day rhythm.

Key Hormones of Metabolism Regulation

Your metabolic rate, blood sugar levels, and appetite are all under hormonal control. Five hormones are especially important here:

• Thyroid hormones (T3 and T4)
  • Secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary
  • Increase basal metabolic rate, stimulate protein synthesis, and promote growth and development
  • T3 is the more biologically active form; most T4 is converted to T3 in target tissues
• Insulin
  • Secreted by pancreatic beta cells when blood glucose rises (e.g., after a meal)
  • Promotes glucose uptake by cells, stimulates glycogen synthesis in the liver and muscles, and inhibits gluconeogenesis (the production of new glucose)
  • Net effect: lowers blood sugar
• Glucagon
  • Secreted by pancreatic alpha cells when blood glucose drops (between meals or during fasting)
  • Stimulates glycogen breakdown into glucose (glycogenolysis) and promotes gluconeogenesis in the liver
  • Net effect: raises blood sugar

• Leptin
  • Produced by adipose (fat) tissue
  • Signals the hypothalamus to reduce appetite and increase energy expenditure when fat stores are sufficient
  • Regulates long-term energy balance
• Ghrelin
  • Secreted by the stomach
  • Stimulates hunger and increases appetite, promoting food intake
  • Regulates short-term energy balance (levels rise before meals and drop after eating)

Hormonal Imbalances and Endocrine Disorders

When hormone production is too high or too low, or when target cells stop responding properly, endocrine disorders result. Here are the major ones you should know:

Thyroid disorders:

• Hypothyroidism results from insufficient thyroid hormone production. Metabolism slows down, leading to weight gain, fatigue, cold intolerance, and dry skin.
• Hyperthyroidism results from excessive thyroid hormone production. Metabolism speeds up, causing weight loss, heat intolerance, rapid heartbeat, and anxiety.

Diabetes mellitus:

• Type 1 diabetes is caused by autoimmune destruction of pancreatic beta cells, leading to little or no insulin production. Blood glucose stays chronically high (hyperglycemia), and patients require exogenous insulin injections.
• Type 2 diabetes is characterized by insulin resistance, meaning target cells don't respond normally to insulin, sometimes combined with reduced insulin production. It's often associated with obesity and can be managed with lifestyle changes, medication, or insulin therapy.

Adrenal disorders:

• Cushing's syndrome results from excessive cortisol, often due to a pituitary tumor (which overproduces ACTH) or long-term glucocorticoid therapy. Symptoms include weight gain (especially in the trunk and face, producing a "moon face"), easy bruising, and muscle weakness.
• Addison's disease results from insufficient cortisol and aldosterone production due to adrenal gland dysfunction. Symptoms include fatigue, weight loss, low blood pressure (hypotension), and skin hyperpigmentation. The lack of aldosterone disrupts electrolyte balance, while the lack of cortisol impairs the stress response and metabolism.

The Endocrine System and Hormone Action

A few foundational concepts tie everything above together:

• The endocrine system consists of glands that secrete hormones directly into the bloodstream (as opposed to exocrine glands, which secrete into ducts).
• Hormones only affect target cells that have the specific receptor for that hormone. A hormone can travel throughout the entire body but will only act where its receptor is present.
• The hypothalamic-pituitary axis is the central command system of endocrine regulation. The hypothalamus links the nervous system to the endocrine system by signaling the pituitary gland, which in turn controls the thyroid, adrenal glands, and gonads. This is why the pituitary is sometimes called the "master gland," though it ultimately takes its orders from the hypothalamus.