Renal system anatomy and physiology

Definitions & Key takeaways

The renal system, also known as the urinary system, is made up of the kidneys, ureters, bladder, and urethra. The kidneys, a pair of organs located in the back of the abdominal cavity, filter waste products from the blood in the form of urine. The urine passes down through the ureters, which are muscular tubes connecting the kidneys to the bladder. The bladder is a muscular sac that stores urine until it increases the pressure on its wall and triggers the micturition reflex. This allows the urine to flow through the urethra and out of the body.

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Introduction0:00–0:24

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.
They’re 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.

Renal anatomy0:24–1:34

The kidneys are located between the T12 and L3 vertebrae, and they’re 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 going into and coming 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, or the adipose capsule, is a fatty layer that protects the kidney from trauma. And 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.

Renal medulla & cortex1:34–2:28

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 each other.
Each renal pyramid and the renal cortex above it is called a renal lobe. So an adult’s kidneys filter about 150 liters of blood every day.

Renal irrigation2:28–4:11

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 the aorta into the left and right renal arteries. As these renal arteries enter the kidney, they divide into segmental arteries and 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.
And 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 there’s 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.

Nephron4:11–6:06

The renal corpuscle is where blood filtration starts and it includes the glomerulus, which is the tiny bed of capillaries, and the Bowman’s 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 its basement membrane, through the epithelial lining of the nephron and finally into the Bowman’s 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 like water, glucose and ionic salts to pass through while blocking large proteins and red blood cells.
As the filtrate leaves the Bowman’s capsule it flows into the renal tubule, which is surrounded by the peritubular capillaries.
Now, before we dive too far in here, let’s redraw the nephrons that the structure of the renal tubule as 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.

Juxtaglomerular complex6:06–7:26

The juxtaglomerular complex is located 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 signaling 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 and this helps raise the blood volume.
Renin also causes constriction of blood vessels which helps raise the 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 push the 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 and prevents backflow of urine.

Ureter7:26–8:05

It’s 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 tons of folds called rugae that can contract when the bladder is emptied of urine and can expand when it is filled with urine.

Bladder8:05–9:31

In the layers of the bladder wall are a mucosa layer that has a transitional epithelium, which is stretchy and allows the bladder to distend 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 a fibrous adventitia outer layer.
In people assigned female at birth, the bladder is in front of the vagina, uterus, and rectum, and in people assigned male at birth, the bladder is just in front of the rectum.
On average, the bladder can hold around 750 milliliters of urine, or about the volume of a bottle of wine; slightly less in women though because of crowding from the uterus and that’s 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 it’s 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 people assigned male at birth, the urethra first passes through the prostate, where it is called the prostatic urethra, then it passes through deep muscles of the perineum, where it’s called the intermediate urethra and finally, it passes through the penis, where it’s called the spongy urethra.

Urethra9:31–11:01

The male urethra is also used during ejaculation, except there - semen enters into the urethra via the ejaculatory ducts.
In people assigned female at birth, the urethra runs through the perineal floor of the pelvis and exits between the two labia minora, above the vaginal opening and below the clitoris in an area called the vulval vestibule.
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 isn’t full.
Additionally, there’s 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 the muscles of the bladder.
Once the volume of the bladder is greater than about 300-400 milliliters, basically when it’s half full, pressure on the bladder walls increases and sends signals to the urination or micturition center in the spinal cord, located at S2 and S3.

Urination11:01–12:05

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 the pontine micturition center are two areas in the pons part of your brain that help control urination.
When you can’t find a toilet and you want to hold your urine in, then you activate the pontine storage center and that stops the micturition reflex.
When you finally do find that toilet, and you’re ready to urinate, the pontine micturition center is active and it allows the micturition reflex to happen - and you can finally pee.All right, as a quick recap… the kidney's main function is to filter all of the body’s 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. all right as a quick recap the kidneys main function is to filter all the bodies 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 MC duration reflex is triggered This allows the urine to flow through the urethra and out the body

Review12:05–12:28