It’s Professor Dave, let’s look at the urinary system.
We just looked at the digestive system, so we know of one way in which the body eliminates waste.
But it is also the case that metabolic waste and other unwanted substances are constantly
circulating in the bloodstream.
The body has a way of filtering out this waste and expelling it from the body, and the components
that work together to achieve this are called the urinary stem, so let’s see how this
works now.
The chief organ in the urinary system is the kidneys.
These perform an impressive array of tasks, including the regulation of the volume of
water, the concentration of various solutes in that water, blood pH, as well as the production
of erythropoietin.
But the functions that will be important for the urinary system will be the constant filtering
of the bloodstream, removing toxins and waste, while returning necessary components back
to the blood.
The urinary system also includes ureters, the urinary bladder, and the urethra, and
we will get to these in a moment, but first, let’s spend some time looking at the kidneys.
We all have two bean-shaped kidneys, and they sit in the lumbar region, receiving some protection
from the ribcage.
Each has a vertical cleft called the renal hilum that leads to a renal sinus within the kidney.
It is in here that a ureter adjoins, along with blood vessels, lymphatic vessels, and nerves.
On top of each kidney we can find an adrenal gland, though its function is not directly related.
Supportive tissue surrounds each kidney, and this is made of three layers.
First, the outermost renal fascia, made of dense fibrous connective tissue that secures
the kidneys in place.
Then the perirenal fat capsule that cushions the kidney.
And then the innermost fibrous capsule that prevents infections from spreading to the kidney.
Once inside, we continue to find the renal cortex, and then the renal medulla, which
consists of renal pyramids, each with a papilla that points inwards.
These are separated by renal columns made of cortical tissue.
At the center we see the renal pelvis which connects to the ureter, although it also has
branching extensions called calyces.
These are the areas that collect urine from the papillae, which then drains into the ureter.
Before we follow that path, let’s also notice these renal arteries, which split into segmental
arteries, and then interlobar arteries, and then arcuate arteries, and finally cortical
radiate arteries.
These supply the blood that the kidneys will filter once it enters the renal cortex.
Once this process is complete, the cleansed blood leaves through cortical radiate veins,
which feed into arcuate veins, then interlobar veins, and then renal veins.
These exit the kidneys and empty into the vena cava.
The filtration occurs in tiny units called nephrons, each of which consists of a renal
corpuscle that sits in the renal cortex, and a renal tubule that extends from there into
the medulla, and then back into the cortex.
The corpuscle contains a bunch of especially porous capillaries called a glomerulus, which
sits in the glomerular capsule.
Fluid from the bloodstream, or filtrate, will enter into the capsule, and then into the
renal tubule.
This tubule has three sections, a proximal convoluted tubule with many turns, a long
nephron loop, and then a distal convoluted tubule, which again has many turns.
Filtrate will pass through all of these while being processed, reabsorbing water and solutes
to go back to the bloodstream, and what is left after this will empty into a collecting
duct, each of which collects filtrate from many tubules.
These run down the medullary pyramids, fusing as they go, and finally deliver urine in the calyces.
So how exactly is urine produced?
This is a three-step process.
It begins with glomerular filtration, which takes place in the renal corpuscle.
Blood is forced through a filtration membrane into the glomerular capsule.
This membrane allows water and small solutes to pass through, but not proteins or anything
larger, especially entire cells.
The next step is tubular reabsorption.
Here, most of the filtrate makes it back into the blood, like glucose, amino acids, and
most of the water.
Whatever doesn’t get back in will become urine.
The third step is tubular secretion.
This is the opposite of tubular reabsorption, in that some substances from blood make their
way to the filtrate, like drugs and metabolites bound to proteins, excess potassium, and more.
So urine will be a combination of substances that were filtered and substances that were secreted.
In the end, urine is almost entirely water, but contains lots of urea, which is the result
of the breakdown of amino acids, as well as other nitrogenous wastes like uric acid and
creatinine.
There are also various ionic solutes like sodium, potassium, phosphate, and sulfate.
So once the urine has been produced, it has to leave the body.
Ureters are slender tubes that transport urine from the kidneys to the bladder, where it
is stored until its immediate release from the body.
The walls of the ureters are made of three layers, a mucosa made of traditional epithelium,
a muscularis made of smooth muscle, and an adventitia around the surface, made of fibrous
connective tissue.
The bladder is a highly flexible, muscular sac with a region called the trigone which
continues on to the urethra.
This is a muscular tube that conveys urine out of the body.
After this introduction to the urinary system, you may have a newfound appreciation for your
kidneys, which are often glossed over as far as organs go.
And as we saw, this system ends with the urethra, which is part of the male and female genitalia.
These structures are much more notably involved in the reproductive system, so let’s wrap
things up by looking at this next.