Urinary System Anatomy

Urinary System AnatomyKidneysThe kidneys are a pair of bean-shaped organs found along the posterior wall of the abdominalcavity. The left kidney is located slightly higher than the right kidney because the right side ofthe liver is much larger than the left side. The kidneys, unlike the other organs of the abdominalcavity, are located posterior to the peritoneum and touch the muscles of the back. The kidneysare surrounded by a layer of adipose that holds them in place and protects them from physicaldamage. The kidneys filter metabolic wastes, excess ions, and chemicals from the blood to formurine.UretersThe ureters are a pair of tubes that carry urine from the kidneys to the urinary bladder. Theureters are about 10 to 12 inches long and run on the left and right sides of the body parallel tothe vertebral column. Gravity and peristalsis of smooth muscle tissue in the walls of the uretersmove urine toward the urinary bladder. The ends of the ureters extend slightly into the urinarybladder and are sealed at the point of entry to the bladder by the ureterovesical valves. Thesevalves prevent urine from flowing back towards the kidneys.Urinary BladderThe urinary bladder is a sac-like hollow organ used for the storage of urine. The urinarybladder is located along the body’s midline at the inferior end of the pelvis. Urine entering theurinary bladder from the ureters slowly fills the hollow space of the bladder and stretches itselastic walls. The walls of the bladder allow it to stretch to hold anywhere from 600 to 800milliliters of urine.Urethra

The urethra is the tube through which urine passes from the bladder to the exterior of the body.The female urethra is around 2 inches long and ends inferior to the clitoris and superior to thevaginal opening. In males, the urethra is around 8 to 10 inches long and ends at the tip ofthe penis. The urethra is also an organ of the male reproductive system as it carries sperm out ofthe body through the penis.The flow of urine through the urethra is controlled bythe internal and external urethral sphincter muscles. The internal urethral sphincter is made ofsmooth muscle and opens involuntarily when the bladder reaches a certain set level of distention.The opening of the internal sphincter results in the sensation of needing to urinate. The externalurethral sphincter is made of skeletal muscle and may be opened to allow urine to pass throughthe urethra or may be held closed to delay urination.Urinary System PhysiologyMaintenance of HomeostasisThe kidneys maintain the homeostasis of several important internal conditions by controlling theexcretion of substances out of the body.IonsThe kidney can control the excretion of potassium, sodium, calcium, magnesium, phosphate, andchloride ions into urine. In cases where these ions reach a higher than normal concentration, thekidneys can increase their excretion out of the body to return them to a normal level. Conversely,

the kidneys can conserve these ions when they are present in lower than normal levels byallowing the ions to be reabsorbed into the blood during filtration. (See more about ions.)pHThe kidneys monitor and regulate the levels of hydrogen ions (H+) and bicarbonate ions in theblood to control blood pH. H+ ions are produced as a natural byproduct of the metabolism ofdietary proteins and accumulate in the blood over time. The kidneys excrete excess H+ ions intourine for elimination from the body. The kidneys also conserve bicarbonate ions, which act asimportant pH buffers in the blood.OsmolarityThe cells of the body need to grow in an isotonic environment in order to maintain their fluid andelectrolyte balance. The kidneys maintain the body’s osmotic balance by controlling the amountof water that is filtered out of the blood and excreted into urine. When a person consumes a largeamount of water, the kidneys reduce their reabsorption of water to allow the excess water to beexcreted in urine. This results in the production of dilute, watery urine. In the case of the bodybeing dehydrated, the kidneys reabsorb as much water as possible back into the blood to producehighly concentrated urine full of excreted ions and wastes. The changes in excretion of water arecontrolled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and releasedby the posterior pituitary gland to help the body retain water.Blood PressureThe kidneys monitor the body’s blood pressure to help maintain homeostasis. When bloodpressure is elevated, the kidneys can help to reduce blood pressure by reducing the volume ofblood in the body. The kidneys are able to reduce blood volume by reducing the reabsorption ofwater into the blood and producing watery, dilute urine. When blood pressure becomes too low,the kidneys can produce the enzyme renin to constrict blood vessels and produce concentratedurine, which allows more water to remain in the blood.FiltrationInside each kidney are around a million tiny structures called nephrons. The nephron is thefunctional unit of the kidney that filters blood to produce urine. Arterioles in the kidneys deliverblood to a bundle of capillaries surrounded by a capsule called a glomerulus. As blood flowsthrough the glomerulus, much of the blood’s plasma is pushed out of the capillaries and into thecapsule, leaving the blood cells and a small amount of plasma to continue flowing through thecapillaries. The liquid filtrate in the capsule flows through a series of tubules lined with filteringcells and surrounded by capillaries. The cells surrounding the tubules selectively absorb waterand substances from the filtrate in the tubule and return it to the blood in the capillaries. At thesame time, waste products present in the blood are secreted into the filtrate. By the end of thisprocess, the filtrate in the tubule has become urine containing only water, waste products, andexcess ions. The blood exiting the capillaries has reabsorbed all of the nutrients along with mostof the water and ions that the body needs to function.

Storage and Excretion of WastesAfter urine has been produced by the kidneys, it is transported through the ureters to the urinarybladder. The urinary bladder fills with urine and stores it until the body is ready for its excretion.When the volume of the urinary bladder reaches anywhere from 150 to 400 milliliters, its wallsbegin to stretch and stretch receptors in its walls send signals to the brain and spinal cord.These signals result in the relaxation of the involuntary internal urethral sphincter and thesensation of needing to urinate. Urination may be delayed as long as the bladder does not exceedits maximum volume, but increasing nerve signals lead to greater discomfort and desire tourinate.Urination is the process of releasing urine from the urinary bladder through the urethra and out ofthe body. The process of urination begins when the muscles of the urethral sphincters relax,allowing urine to pass through the urethra. At the same time that the sphincters relax, the smoothmuscle in the walls of the urinary bladder contract to expel urine from the bladder.Production of HormonesThe kidneys produce and interact with several hormones that are involved in the control ofsystems outside of the urinary system.CalcitriolCalcitriol is the active form of vitamin D in the human body. It is produced by the kidneys fromprecursor molecules produced by UV radiation striking the skin. Calcitriol works together withparathyroid hormone (PTH) to raise the level of calcium ions in the bloodstream. When the levelof calcium ions in the blood drops below a threshold level, the parathyroid glands release PTH,which in turn stimulates the kidneys to release calcitriol. Calcitriol promotes the smallintestine to absorb calcium from food and deposit it into the bloodstream. It also stimulates theosteoclasts of the skeletal system to break down bone matrix to release calcium ions into theblood.ErythropoietinErythropoietin, also known as EPO, is a hormone that is produced by the kidneys to stimulate theproduction of red blood cells. The kidneys monitor the condition of the blood that passes throughtheir capillaries, including the oxygen-carrying capacity of the blood. When the blood becomeshypoxic, meaning that it is carrying deficient levels of oxygen, cells lining the capillaries beginproducing EPO and release it into the bloodstream. EPO travels through the blood to the redbone marrow, where it stimulates hematopoietic cells to increase their rate of red blood cellproduction. Red blood cells contain hemoglobin, which greatly increases the blood’s oxygen-carrying capacity and effectively ends the hypoxic conditions.Renin

Renin is not a hormone itself, but an enzyme that the kidneys produce to start the renin-angiotensin system (RAS). The RAS increases blood volume and blood pressure in response tolow blood pressure, blood loss, or dehydration. Renin is released into the blood where itcatalyzes angiotensinogen from the liver into angiotensin I. Angiotensin I is further catalyzed byanother enzyme into Angiotensin II.Angiotensin II stimulates several processes, including stimulating the adrenal cortex to producethe hormone aldosterone. Aldosterone then changes the function of the kidneys to increase thereabsorption of water and sodium ions into the blood, increasing blood volume and raising bloodpressure. Negative feedback from increased blood pressure finally turns off the RAS to maintainhealthy blood pressure levels.Prepared by Tim Taylor, Anatomy and Physiology InstructorUrinary BladderThe urinary bladder is a hollow elastic organ that functions as the body’s urine storage tank.Urine produced by the kidneys flows through the ureters to the urinary bladder, where is it storedbefore passing into the urethra and exiting the body. The urinary bladder plays an important rolein delaying and controlling urination so that the average person only has to urinate several timeseach day instead of constantly leaking small amounts of urine.The urinary bladder is roughly spherical in shape, although its shape and size vary amongindividuals and depends greatly upon the volume of urine that it contains. CONTINUESCROLLING TO READ MORE BELOW... Click To View Large Image Related Anatomy: Kidneys Ureter Urethra Urinary Bladder

 VaginaCONTINUED FROM ABOVE... It is located in the pelvic cavity anterior to the rectum and superior tothe reproductive organs of the pelvis. In females the urinary bladder is somewhat reduced in size andmust share the limited space of the pelvic cavity with the uterus that rests superior and posterior to it.During pregnancy the uterus takes up significantly more space and severely limits the expansion of theurinary bladder.Many tiny wrinkles, known as rugae, line the inner surface of the urinary bladder and allow it tostretch as it fills with urine. A pair of ureteral openings on the inferior end of the posterior wallof the urinary bladder allow urine from the left and right ureters to enter the hollow lumen. Asmall funnel forms at the inferior end of the urinary bladder leading into the urethra, the tube thatcarries urine out of the body during urination.The urinary bladder is made of several distinct tissue layers:  The innermost layer of the bladder is the mucosa layer that lines the hollow lumen. Unlike the mucosa of other hollow organs, the urinary bladder is lined with transitional epithelial tissue that is able to stretch significantly to accommodate large volumes of urine. The transitional epithelium also provides protection to the underlying tissues from acidic or alkaline urine.  Surrounding the mucosal layer is the submucosa, a layer of connective tissue with blood vessels and nervous tissue that supports and controls the surrounding tissue layers.  The visceral muscles of the muscularis layer surround the submucosa and provide the urinary bladder with its ability to expand and contract. The muscularis is commonly referred to as the detrusor muscle and contracts during urination to expel urine from the body. The muscularis also forms the internal urethral sphincter, a ring of muscle that surrounds the urethral opening and holds urine in the urinary bladde. During urination, the sphincter relaxes to allow urine to flow into the urethra.The urinary bladder sits in a unique position inferior to the peritoneum, a membrane that linesmost of the abdominopelvic cavity. Due to its position, the outermost layer of the superiorurinary bladder is made of serous membrane continuous with the peritoneum. Serous membraneprovides protection to the bladder from friction between organs in the abdominopelvic cavity.The surface of the lateral and inferior sides of the urinary bladder forms a layer of looseconnective tissue known as the adventitia. The adventitia loosely connects the urinary bladder tothe surrounding tissues of the pelvis.The urinary bladder functions as a storage vessel for urine to delay the frequency of urination. Itis one of the most elastic organs of the body and is able to increase its volume greatly toaccommodate between 600 to 800 ml of urine at maximum capacity. Transitional epithelium,elastic fibers, and visceral muscle tissue in the walls of the urinary bladder contribute to itsdistensibility and elasticity, allowing it to easily stretch and return to its original size severaltimes each day.It also helps to expel urine from the body during urination by contraction of the detrusor muscleand the relaxation of the internal urethral sphincter. Another separate muscle, the externalurethral sphincter, surrounds the urethra just inferior to the bladder and helps to control and delay

urination through its contraction. The external urethral sphincter is a skeletal muscle andtherefore allows for the voluntary control of the urination reflex.Prepared by Tim Taylor, Anatomy and Physiology Instructor