Anatomy and physiology of the kidneys, urinary bladder, ureters, urethra, and nephron

Anatomy and physiology of the kidneys, urinary bladder, ureters, urethra, and nephron

August 14, 2019 68 By Bertrand Dibbert


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much more. Try it free today! 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. 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 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 adult’s 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 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. The renal corpuscle is where blood filtration
starts and it includes the glomerulus – 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 the 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
such as 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
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. 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 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 – 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 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 it’s 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 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 muscles of the bladder. Once the volume of bladder is greater than
about 300-400 millilitres, basically when it’s 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 can’t 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
you’re 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 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.