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The workhorses of the urinary system are the kidneys. Which are the twin bean shaped organs in your body that clear harmful substances by filtering your blood. Theyre like a water purification plant that helps clean the drinking water for a city they also regulate blood ph volume pressure osmolality as well as produce hormones the kidneys are located between the t12 and l3 vertebrae and theyre partially protected by ribs 11 and 12.
Which are the floating ribs. The kidneys are roughly the size of a fist and are retroperitoneal meaning. They sit behind the peritoneal membrane alongside the vertebral column the right kidney is pushed down by the liver.
So it sits slightly lower than the left kidney in the middle of each kidney. There is an indentation that forms the renal hilum. This is the entry and exit point for the ureter renal artery and renal vein lymphatics and nerves go into and come out of the kidney.
The kidney is surrounded by three layers of tissue on the outside is the renal fascia. Which is a thin layer of dense connective tissue that anchors the kidney to its surroundings. The middle layer the adipose capsule is a fatty layer that protects the kidney from trauma.
The deepest layer called the renal capsule is a smooth transparent sheet of dense connective tissue that gives the kidney its distinctive shape if you take a cross section of the kidney. There are two main parts the inner portion is the renal medulla and the outside rim is the renal cortex. The medulla is made up of 10 to 18 renal pyramids with the base of the pyramids facing the renal cortex and the tips of the pyramids called renal papilla or nipples pointing towards the center of the kidney.
The renal papilla project into minor calyces which join together to form major calyces which funnel into the renal pelvis urine collects in the renal pelvis and then heads out of the kidney through the ureter. The renal cortex can be divided into an outer cortical zone and an inner juxtamedullary zone. There are also sections of the cortex called renal columns.
Which extend down into the medulla separating the renal pyramids from one another each renal pyramid and the renal cortex above.
It is called a renal lobe. So an adults kidneys filter about 150 liters of blood every day. If we assume that there are 5 liters of blood in the body that means that the entire blood volume gets filtered about 30 times.
A day. Which is more than once every hour. Because of this the kidneys get about a quarter of the cardiac output.
Which is blood getting pumped out of the left ventricle to reach the kidneys blood flows from to aorta into the left and right renal arteries as these renal arteries enter the kidney they divide into segmental arteries then into interlobar arteries. Which pass through the renal columns. Then to arcuate arteries that go over the bases of the renal pyramids and then into cortical radiate arteries.
Which supply the cortex. The cortical radiate. Arteries continue to divide eventually forming afferent arterioles that split into a tiny bundle of capillaries called the glomerulus.
The glomerulus is the site where blood filtration starts interestingly once the blood leaves. These glomeruli. It does not enter into venules instead.
The glomerulus funnels blood into efferent arterioles. Which divide into capillaries a second time. These peritubular capillaries.
Then reunite to become the cortical radiate veins. Then the arcuate veins. Then interlobar veins and finally into the left and right renal veins.
Which connect to the inferior vena cava.
The flow of the veins are similar to the arteries. But in reverse the only difference is that theres a segmental artery. But no segmental vein within each kidney.
There are about 1 million nephrons and each nephron is made up of a renal corpuscle and a renal tubule. The renal corpuscle is where blood filtration starts. And it includes the glomerulus the tiny bed of capillaries and the bowmans capsule.
Which is made of renal cells that surround the glomerulus as blood flows into the glomerulus water and some solutes in the blood. Like sodium are able to pass through the endothelial lining of the capillary move across the basement membrane through the epithelial lining of the nephron and finally into the bowmans space of the nephron itself. At which point.
It is called filtrate. The epithelium of the nephron is made of specialized cells called podocytes. Which wrap around the basement membrane like the tentacles of an octopus between these tentacle.
Like projections are tiny gaps called filtration slits that act like a sieve allowing only small particles such as water glucose and ionic salts to pass through while blocking large proteins and red blood cells. As the filtrate leaves the bowmans capsule it flows into the renal tubule. Which is surrounded by the peritubular capillaries now before we dive too far in here.
Lets re draw the nephron so that the structure of the renal tubule is a little more accurate alright so the renal tubule itself can be divided into the proximal. Convoluted tubule. The nephron loop also known as the loop of henle which made up of the descending limb.
And the ascending limb the distal. Convoluted tubule and finally the collection ducts. Which ultimately send the urine to the minor calyces here.
The filtrate becomes fine tuned based on what the body wants to keep versus.
What it wants to discard with water and solutes getting passed back and forth between the filtrate in the lumen of the renal tubule and the blood in the peritubular capillaries. Each nephron also has a really unique region called the juxtaglomerular complex. Which is involved in the regulation of blood pressure and the glomerular filtration rate or the amount of blood that passes through the glomeruli each minute the juxtaglomerular complex is located in between the distal convoluted tubule and the afferent arteriole there are three types of cells in the juxtaglomerular complex macula densa cells juxtaglomerular cells and extraglomerular mesangial cells macula densa cells are located in the distal.
Convoluted tubule and they can sense when levels of sodium and chloride are low so in the case of hypovolemia and hypotension the macula densa cells sense the low sodium and chloride levels and send a signal over to the juxtaglomerular cells. Which are located in the wall of the afferent arteriole. The extraglomerular mesangial cells help with the signalling between macula densa cells and juxtaglomerular cells.
The juxtaglomerular cells. Then receive the signal and also independently sense the low pressure in the blood vessels and secrete an enzyme called renin which increases sodium reabsorption. Which helps raise the blood volume renin also causes constriction of blood vessels.
Which helps raise blood pressure once millions of nephrons have each made urine. It flows into the minor calyces. Then major calyces and finally into the renal pelvis from there it goes down.
The ureter. Which has a muscular lining. Which helps to push urine along the ureters insert into the bladder at the ureterovesical junction at a sideways angle.
So that when the bladder becomes full it compresses the openings to the ureters preventing the backflow of urine. Its basically a one way valve that prevents urine from refluxing backwards from the bladder into the ureters. The bladder itself is like a balloon its muscular wall has many folds called rugae that can contract when the bladder is emptied of urine and can expand when it is filled with urine in the layers of the bladder wall are a mucosa layer that has a transitional epithelium.
Which is stretchy allowing for bladder distention while maintaining a barrier between urine and the body in addition. There is a thick muscular layer called the detrusor muscle that helps with bladder contraction during urination and it has fibrous adventitia outer layer in women the bladder is in front of the vagina uterus and rectum and in men. The bladder is just in front of the rectum on average.
The bladder can hold around 750 millilitres of urine or about the volume of a bottle of wine slightly less in women because of crowding from the uterus thats especially true during pregnancy.
The floor of the bladder has a smooth triangular region called the trigone region with two corners at the ureterovesical junctions and the third corner being the internal urethral orifice. Where the bladder meets. The urethra.
The trigone region is very sensitive to expansion and once it stretches to a certain point. The bladder sends a signal to the brain that its time to pee. The urethra is a thin muscular tube that drains urine from the bladder starting from the internal urethral orifice to the external opening in males.
The urethra first passes through the prostate where it is called the prostatic urethra then it passes through deep muscles of the peritoneum where it is called the intermediate urethra and finally passes through the penis. Where its called the spongy urethra. The male urethra is also used during ejaculation.
Except there semen enters into the urethra via the ejaculatory ducts in women. The urethra runs through the perineal floor of the pelvis and exits between the two labia minora above vaginal opening and below the clitoris in an area called the vulval vestibule in men and women around the internal urethral orifice. The detrusor muscle thickens to form the internal sphincter.
This involuntary sphincter is controlled by the autonomic nervous system and keeps the urethra closed when the bladder isnt full additionally. Theres an external sphincter at the level of the urogenital diaphragm in the floor of the pelvis. Which is under voluntary control by contracting.
The skeletal muscles around the external sphincter urination can be stopped voluntarily. This is called a kegel exercise. And it can be done to strengthen the pelvic floor the act of urination involves close coordination between the nervous system and muscles of the bladder once the volume of bladder is greater than about 300 400 millilitres basically when its half full pressure on the bladder walls increase and send signals to the urination or micturition centre in the spinal cord located at s2 and s3 this sets off a reflex arc called the micturition reflex.
Which causes contraction of the bladder and relaxation of the internal sphincter and external sphincter. Now the pontine storage center and pontine micturition center. Are two areas in the pons part of the brain that help control urination.
When you cant find a toilet and want to hold urine in you activate the pontine storage center. And that stops the micturition reflex. When you finally do find that toilet and youre ready to urinate the pontine micturition center is active and it allows the the micturition reflex to happen and you can finally pee all right as a quick recap the kidneys main function is to filter all of the bodys blood about 30 times a day and to produce urine the urine passes down through the ureters and into the bladder as the urine collects in the bladder it increases the pressure on the bladder wall and the micturition reflex is triggered this allows the urine to flow through the urethra and out the body.
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