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What is it?
The urinary system consists of two kidneys, two ureters, a urinary bladder and the urethra. The main purpose of the urinary system is to control the body’s fluid and electrolyte (ion) balance, and to eliminate waste products. The key functions of the urinary system include filtration, reabsorption and secretion, although they have a role in acid/base balance (along with the respiratory system), control of Vitamin D levels, and formation of new red blood cells by releasing erythropoietin (which causes erythropoiesis – stimulation of bone marrow to make new red blood cells).
The renal system receives approximately 1,250ml of arterial blood every minute. This is filtered by the functional unit of the kidneys, which are the nephrons. Each kidney has a million or so nephrons, of which there are two types, cortical and juxtameduallary. The differences between these nephrons are in the loops of Henlé. The loops of Henlé allow water to be reabsorbed; the longer the loop of Henlé, the more water can be reabsorbed. Most nephrons are cortical, and these have relatively short loops of Henlé.
There are five sections to the nephron, Bowman’s capsule, the Proximal Convoluted Tubule, the loop of Henlé, the Distal Convoluted Tubule and the Collecting Duct. Arterial blood enters Bowman’s capsule from a branch of the renal artery called the afferent arteriole, and it is in Bowman’s capsule where the glomerulus or filter is found. The glomerulus has very small holes in it (fenestrations), which only allow small particles to be filtered. Red blood cells, white blood cells, etc are too large to pass through the fenestrations, but water, electrolytes, glucose and acids can all pass through easily. The red cells, white cells and protein that cannot be filtered under healthy conditions move into efferent arteriole and then to the network of blood supply around the nephron, called the peritubular network.
From Bowman’s capsule, the filtered material, called filtrate, enters the proximal convoluted tubule. 80% of all material that enters the proximal convoluted tubule is reabsorbed and the reabsorbed material passes from the tubule into the peritubular network. In health, glucose is filtered but completely reabsorbed in the proximal tubule and therefore no glucose should normally be found in urine. From the proximal tubule, filtrate enters the loop of Henlé. The loop of Henlé allows water to be reabsorbed under certain conditions. If you are dehydrated the loop of Henlé opens special water channels to reabsorb as much water as possible. The water that is removed from the loop of Henlé also enters the peritubular network. The distal convoluted tubule helps to control sodium and water levels, and the collecting duct has a role in acid/base balance as well as water reabsorption. At the very end of the collecting duct, the filtrate becomes urine. Urine is not reabsorbed by the body, and passes from the collecting duct into the renal pelvis and down the ureters into the urinary bladder. From there, urine exits the body via the urethra.
The amount of water that is in the body is critical for health, but water is being lost from the body all the time. At times when the individual is not eating or drinking, i.e. adding water to their body, a series of hormones help to control water balance. The hormones act on the nephrons to either reabsorb more water (if you are dehydrated) or lose excess water (if you are over hydrated). These hormones include aldosterone (which controls sodium); atrial natriuetic peptide or ANP which traps sodium in the nephron so more urine is produced; anti-diuretic hormone/vasopressin, that helps to conserve water; and the renin-angiotensin-aldosterone system, that balances blood volume and blood pressure to ensure that the renal system works properly.
Five quick facts
- The main purpose of the urinary system is to filter blood. This involves three processes, filtration, reabsorption and secretion. In addition to this, the renal system helps to control acid/base balance (with the respiratory system); control hormone levels (e.g. Vitamin D) and releases erythropoietin (which triggers erythropoiesis).
- The kidneys filter 160 litres of fluid per day but only 1-3 litres of urine are passed out of the body
- 80% of all material that enters the beginning of the proximal convoluted tubule is reabsorbed before it reaches the beginning of the loop of Henlé
- In a healthy renal system, the urine produced is sterile, but the urea found in urine is the same substance that is used to melt ice in winter
- The colour of your urine indicates how hydrated you are. Clear or light yellow urine indicates that you are well hydrated; dark coloured urine suggests dehydration.
Hint to learning the system
The key to the understanding the renal system is to know what each section of the nephron does. Draw a nephron, starting with the afferent arteriole, and then add Bowman’s capsule; proximal convoluted tubule; loop of Henlé; distal convoluted tubule and the collecting duct. Once the sections of the nephron are known, add some key words that describe what each section does, in terms of filtration, reabsorption, and secretion.
- afferent arterioles (AFF er ent are TIER ee ohlz)
- aldosterone (al DOSS ter ohn)
- antidiuretic hormone (ADH) (AN tea dye you RET ik)
- atrial natriuretic peptide (AY tree all nat ree your RET ick PEP tide)
- cortical (CORE tih cull)
- efferent arterioles (EFF er ent are TIER ee ohlz)
- erythropoietin (eh RITH roh poy EE tin)
- erythropoiesis (eh RITH roh poy EE siss)
- fenestrations (fen EE stray shuns)
- glomerulus (glom er ROO luss)
- glomerular capsule (glom er ROO lar CAPS yule)
- juxtamedullary (JUX ta med DULL are ee)
- nephron (NEFF ron)
- peritubular (peri TUBE you lar)
- renin–angiotensin–aldosterone (REE nen–an gee oh TEN sen–al DOSS ter ohn)
- ureter (you REE ter)
- urethra (you REE thrah)
Martin Steggall is the co-author of Anatomy and Physiology for Nursing and Healthcare Professionals 2nd Ed.