Understanding the Structure and Function of the Kidneys
The kidneys are charged with many important tasks in water and fluid balance, blood pressure regulation, and waste elimination (Li 2011; Navar 1997; Baynes 2014). Kidneys are highly active organs, and they receive about one-quarter of cardiac output for their blood supply (Al-Awqati 2012; Baynes 2014).
The main functional unit of the kidney is the nephron; each kidney has approximately 1 million of these independently functioning units (Baynes 2014).
The nephron has two distinct parts. The glomerulus is a microscopic blood filter that allows electrolytes, small molecules, and water to pass while preventing blood cells and large proteins from leaving the bloodstream (Szczepańska-Konkel 2014; Al-Awqati 2012).
Molecules that pass through the glomerulus enter the renal tubule, a convoluted channel lined with specialized transport proteins that can either reabsorb needed electrolytes or excrete unwanted small molecules (Szczepańska-Konkel 2014; Al-Awqati 2012).
Finally, renal tubules empty into collecting ducts, which channel the unabsorbed water and electrolytes toward the ureters and eventually the bladder. Microscopic molecular pores in the collecting ducts can selectively allow water to be pulled out of the ducts and retained in the body (Baynes 2014; Khalifeh 2009; Al-Awqati 2012; Agre 2002).
Urine, the output of the collecting ducts, is a solution of released electrolytes, nitrogen compounds, and other metabolic waste. Normally, the kidneys excrete about 1–2 L of urine per day (Baynes 2014; Khalifeh 2009; Al-Awqati 2012).
The kidneys help regulate several aspects of normal physiology:
Blood pressure regulation. When blood pressure is low, a collection of cellular sensors in the kidney release the hormone renin, which initiates a series of reactions to produce the hormone angiotensin II. Angiotensin II constricts blood vessels, which increases blood pressure (Wackenfors 2004). Renin also promotes the release of the hormone aldosterone from the adrenal glands (Underwood 2013). Aldosterone causes the kidney to reabsorb sodium and water, which also increases blood volume and pressure (Epstein 2001; Genazzani 2007; Al-Awqati 2012).
Water balance (hydration) and electrolyte balance. By retaining or excreting water and electrolytes, healthy kidneys maintain blood volume and optimal hydration of tissues (Baynes 2014; Al-Awqati 2012). When dehydration is detected by the central nervous system, antidiuretic hormone causes the kidneys to retain water (Al-Awqati 2012). Likewise, when the blood contains too much water, antidiuretic hormone secretion is suppressed, and the extra water is excreted in urine (Baynes 2014).
Muscular contraction, nerve impulses, skeletal integrity, and a variety of metabolic reactions depend on very specific concentrations of electrolytes—particularly sodium, potassium, chloride, magnesium, calcium, and bicarbonate (Rizzoli 2014; Saric 2005; Prieto 1993; Turnberg 1970; Dempsher 1981; Adelstein 1987). The renal tubules selectively absorb, excrete, or exchange electrolytes to maintain the proper balance of electrolytes in the blood (Al-Awqati 2012; Baynes 2014).
Hormone secretion and activation. The kidneys are involved in the activation and secretion of several important hormones:
- Activation of vitamin D - The kidneys activate vitamin D (calcitriol or 1,25-dihydroxyvitamin D) from calcidiol or 25-hydroxyvitamin D (Al-Awqati 2012; Baynes 2014). Activated vitamin D is a hormone that acts at multiple tissue sites throughout the body (Norman 2008).
- Erythropoietin - The kidneys secrete erythropoietin, a hormone that signals bone marrow to produce new red blood cells (Baynes 2014).
- Renin - This hormone, secreted by the kidneys, is an essential part of the renin-angiotensin system that helps regulate sodium, fluid levels, and blood pressure (GHR 2015; Hawley 2015).
pH maintenance. The pH of the blood must be maintained within a very narrow range. pH is maintained by both the lungs (by retaining or removing carbon dioxide) and kidneys (by secreting acid and absorbing bicarbonate [HCO3-] from the filtrate in the renal tubules) (Al-Awqati 2012; A.D.A.M. 2013a; Seifter 2011; Walker 1990a; Walker 1990b).
Waste excretion. Nitrogen from broken down proteins is converted to a biologically inactive compound called urea in the liver and is excreted by the kidneys (Bankir 1996; Vilstrup 1980; Karsai 1982). The kidneys filter and also enable the ultimate excretion of other waste, such as excess hormones, vitamins, toxins, and drug metabolites (Al-Awqati 2012; Baynes 2014).
Drug and toxin metabolism. Cells of the renal tubules contain many of the same detoxification enzymes as liver cells (Jancova 2010). See Life Extension’s Metabolic Detoxification protocol for a detailed discussion of detoxification enzymes and their mechanisms.
Glucose production (gluconeogenesis). Only the kidneys, liver, and intestine are able to produce glucose from non-sugar compounds in humans (Baynes 2014; Mithieux 2009; Mithieux 2005; Mithieux, Andreelli 2009). After an overnight fast, the kidneys produce about one-quarter of the glucose released into the blood. Insulin suppresses kidney glucose production whereas epinephrine stimulates gluconeogenesis (Baynes 2014).