The Urinary System
As I go about my day I frequently consider a rumble or pain in my stomach or the deep beating of my heart but how often do I give my kidneys a second thought? They are forgotten organs tucked away at the back of the abdominal cavity. Yet the kidneys play a vital role in maintaining the constancy of my internal environment, with urine as their primary product. Urine itself serves no purpose, it is simply the end product of the regulation of the internal environment, the waste that is discarded. The Composition of Urine Urination may not seem like a glamorous topic. Why is it so important to your body? Urine removes waste products from the body, helps maintain healthy levels of water and it helps maintain healthy levels of salt and other solutes. Urine is essentially water and solutes. Among the solutes excreted in urine are excess elements and ions, drugs, vitamins, toxic chemicals, and waste products produced by the liver or by cellular metabolism. Some substances, such as water and sodium chloride (salt) are excreted to regulate body fluid balance and salt levels. About the only major solutes not excreted by the kidneys under normal circumstances are the three classes of nutrients (carbohydrates, lipids, and proteins). The kidneys keep these nutrients in the body for other organs to regulate. The Kidneys Kidneys - the main organ of the urinary system! They’re located on either side of the vertebrae column, near the posterior body wall. Each kidney is a dark reddish-brown organ about the size of your fist and shaped like a kidney bean. A renal artery and a renal vein connect each kidney to the aorta and inferior vena cava. Each kidney consists of inner-pyramid-shaped zones of dense tissue (called renal pyramids) that constitute the medulla, and an outer zone called the cortex. At the center of the kidney is a hollow space, the renal pelvis, where urine collects after it is formed. A closer look reveals that the renal cortex and medulla contain long, thin, tubular structures called nephrons. Nephrons share a common final section called the collecting duct, through which urine produced by the nephrons is delivered to the renal pelvis. Each kidney contains approximately a million small functional units of nephrons. An individual nephron consists of a thin hollow tube of epithelial cells, called a tubule, plus the blood vessels that supply the tubule. The function of the nephron is to produce urine. Urine Formation: Nephrons don’t just pick the waste molecules out of blood and excrete them. They remove about 180 liters of fluid from the blood every day (about 2.5 times your body weight) and then return almost all of it to the blood, leaving just a small amount of fluid behind in the tubule to be excreted as urine. An analogy would be: cleaning your room by taking everything out of the room and then putting it all back except for the dust and waste you no longer want. A nephron tubule consists of Bowman’s capsule, where fluid is filtered, and four regions in which the filtrate is modified before it becomes urine: proximal tubule, loop of Henle, distal tubule, and collecting duct. Blood flows to Bowman’s capsule via the renal artery and afferent arterioles. Peritubular capillaries carry the blood to the proximal and distal tubules, and vasa recta supply the loops of Henle and collecting ducts. Glomerular filtration separates plasma fluid and small solutes from larger proteins and blood cells. High blood pressure in the glomerular capillaries drives this process. During tubular reabsorption, nearly all the filtered water and sodium and all the major nutrients are reabsorbed from the nephron tubule. The process beings with the active transport of sodium across the cell membrane located on the capillary side of the tubular cell. Tubular secretion removes toxic, foreign and excess substances from the capillaries. It is essential to the regulation of acid-base balance, potassium balance, and the excretion of certain wastes. Blood balance Blood volume is determined in part by how we retain or excrete water. Increases in blood volume raise blood pressure and decreases in blood volume lower it, so it is critical that the blood volume (and water volume) be maintained. The maintenance of water volume is a function shared by the kidneys, the hypothalamus of the brain and the posterior pituitary gland of the endocrine system. Water balance is achieved by a negative feedback loop that regulates the solute concentration of blood. The control of blood volume also depends on maintaining the the body’s salt balance, which in turn depends on three hormones: aldosterone, renin, and atrial natriuretic hormone (ANH). Aldosterone, a steroid hormone from the adrenal gland, increases the reabsorption of Na+ across the distal tubule, and collecting duct. High concentrations of aldosterone cause nearly all of the filtered sodium to be reabsorbed, so that less than 50 milligrams per day appears in the urine. Low levels of the hormone allow as much as 20-25 grams of sodium to be excreted each day. When we consider that the average North American consumes about 10 grams of sodium daily, we see that aldosterone provides more than enough control over sodium excretion. The Body’s Main Fluid Compartments The fluids of the various tissues of the body are divided into fluid compartments. Intercellular fluid is the fluid found within cells. It is separated into compartments by membranes which encircle the various organelles of the cell. Extracellular fluid usually denotes all the body fluid outside of cells. The extracellular fluid can be divided into two major sub compartments, interstitial fluid and blood plasma. Blood plasma is the straw-colored/pale-yellow liquid component of blood that normally holds the blood cells in suspension. It makes up 55% of total blood volume. Interstitial fluid is a solution that bathes and surrounds the cells. On average, a person has about 11 liters of interstitial fluid, providing the cells of the body with nutrients and a means of waste removal. Transcellular fluid is the portion of total body water contained within epithelial lined spaces. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. Fluids, Electrolytes, Acids and Bases… Plasma osmolality measures the body’s electrolyte-water balance. Buffers are substances that prevent changes in the pH by removing or releasing hydrogen ions. Buffer systems reduce the effect of an abrupt change in hydrogen ion concentration by converting a strong acid or base into a weak acid or base. Buffer systems that help maintain acid-base balance include: sodium bicarbonate, phosphate, and protein. Kidneys and Lungs! The protein buffer system changes the pH of the blood in three minutes or less by changing the breathing rate. A decreased respiration rate decreases the exchange and release of CO2 while there is also less hydrogen and the pH rises. Respiration places a crucial role in controlling pH. In addition to excreting various acid waste products, the kidneys help manage acid-base balance by regulating the blood bicarbonate concentration. They do this by permitting bicarbonate reabsorption from tubular filtrate and by forming additional bicarbonate to replace that used in buffering acids. How this all relates to herbal medicine: For general kidney support it’s important to consider hydration and diuretics. Together they help flush out toxins through kidneys and urine. Some of the classic diuretics that I like include parsley leaf, celery stalks, dandelion leaf, burdock root and nettle leaf. The best way to deliver this medicine to the kidneys is through water. Tea, broth or juice. Or just eating these lovely herbs with your food. Another action to consider when working with kidneys are demulcents. With their soothing, slimy, healing, slippery properties, demulcent herbs are a welcome addition to an herbal formula for the kidneys. In addition to the classics like marshmallow root and slippery elm, there’s also corn silk, a simple safe demulcent for the kidney-urinary system.
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AuthorThe adventures, studies, and musings of a student at the Vermont Center for Integrative Herbalism.
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