Development of Bodily Fluids

Developmental Changes in Fluids

The balance of body fluids that are crucial for good health begins during fetal development.

Learning Objectives

Describe developmental changes in body fluids

Key Takeaways

Key Points

  • Hormonal mechanisms are active in maintaining and regulating fetal fluids through renal excretion, reabsorption, and vasodilation.
  • Hypertension has its origin in problems with fetal fluid regulation.
  • Research has shown that the cholinergic mechanism that regulates the hormones involved in fluid homeostasis is active during the final trimester of pregnancy.

Key Terms

  • vasopressin: An antidiuretic hormone secreted by the pituitary gland.
  • arginine vasopressin: A neurohypophysial hormone responsible for increasing water absorption in the collecting ducts of the kidney nephron.
  • renin-angiotensin system: A collection of hormones involved in the regulation of the body’s blood pressure and water content.

Examples

Fetal development affects an individual’s health in later life, so it is imperative that the uterine environment be optimal for proper fetal development.

Hormonal, renal, and behavioral control of body fluids function to some extent in utero. Hormonal mechanisms including the renin-angiotensin system, aldosterone, and vasopressin are involved in modification of fetal renal excretion, reabsorption of sodium and water, and regulation of vascular volume. In utero behavioral changes, such as fetal swallowing, have been suggested to function early in development in response to dipsogens. Since diseases such as hypertension can be traced to fetal origin, it is important to understand the development of fetal regulatory mechanisms for body fluid homeostasis in this early stage of life.

This diagram indicates the hypothalamus of brain, corticotropic-releasing hormone, pituitary gland, thirst, anti-diuretic hormone, ACTH, vasoconstriction of blood vessels, angiotensin II, ACE in lungs, effective circulating volume, extracellular fluid volume, blood pressure, liver, adrenals, high plasma K, aldosterone, kidneys, excretion of NA+, H2O, and K+, angiotensinogin, renin, and angiotensin I.

Schematic diagram of renin–angiotensin–aldosterone system: Overview of the renin-angiotensin system that regulates blood pressure and water (fluid) balance.

The hypothalamic-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting arginine vasopressin (AVP) and oxytocin (OT) in response to a variety of signals, including osmotic and non-osmotic stimuli. It is well-established that central cholinergic mechanisms are critical in the regulation of cardiovascular responses and maintenance of body fluid homeostasis in adults. Recent research has demonstrated that intracerebroventricular (i.c.v.) injection of carbachol elicited an increase of blood pressure in near-term ovine fetuses. However, in utero development of brain cholinergic mechanisms in the regulation of the hypothalamic neuropeptides is largely unknown. Further research indicates that the central cholinergic mechanism is established and functional in regulation of the hypothalamic neuropeptides during the final trimester of pregnancy. This provides evidence for a functional link between the development of central cholinergic mechanisms and hypothalamic neuropeptide systems in the fetus.