Now that we talked about the nephron segments,
we get to go into the blood flow supply.
So the blood flow supply in the kidney is a little bit complex,
dependent upon where we’re at.
So let’s start by breaking these down into nice little discrete portions.
The first is you have an artery that travels into the glomerular capillary.
This is known as the afferent arteriole.
The afferent arteriole then,
it mixes up into the glomerular capillaries,
and then leaves by the efferent arteriole.
The glomerular capillaries,
where filtration happens,
which was again one of our most important functions of the kidney.
Once you leave the efferent arteriole,
you enter into the peritubular capillaries.
This sets up a very unique condition of the kidney
in which we have now two capillary beds that we travel through
when we go from the arterial to venous side of the circulation.
Now, besides the peritubular capillaries,
which are located in the cortex,
meaning that these are going to be surrounding vessels or structures
the proximal convoluted tubule,
the proximal straight tubule,
the distal convoluted tubule
– all of those that are located in the cortex.
If we look at the blood vessels
that are down a little bit deeper into the inner and outer medulla,
those are oftentimes termed vasa recta vessels.
These deeper vessels are going to plunge more into that
high osmolality environment in the inner and outer medulla.
Now, how do you control those various segments of the kidney
and how do you control the blood flow?
Also, the kidney has some unique endocrine components,
meaning that it will signal distal tissues
simply by putting a substance that then can be reabsorbed into the blood,
and then travel around the circulation.
So let’s first talk about the nervous system part.
The sympathetic nervous system is the most important
with controlling blood flow
through the efferent and afferent arteriole.
This controls the blood flow through the glomerular capillaries.
You can constrict one or the other of these particular arterioles,
and this helps control both blood flow,
as well as a very important variable known as glomerular filtration rate.
It also signals small cells that are located
just close proximity to the afferent and efferent arteriole
known as juxtaglomerular cells or granular cells.
These particular cells release renin.
Renin is an enzyme that goes in a cascade
through the renin-angiotensin-aldosterone system
to help control blood sodium levels
and the sodium levels that will be within the urine.
And we’ll get into the renin-angiotensin-aldosterone system a little bit later,
but I would like to mention that right now because
renin is the first component of that system.
Finally, there are some hormonal aspects of which there are three primary ones.
The first is the renin-angiotensin-aldosterone system,
which we just talked about being controlled by the sympathetic nervous system
in terms of its release of renin.
The next is erythropoietin.
Erythropoietin is the hormone that signals cells,
especially in the bone marrow to produce more red blood cells.
So erythropoietin is a very important process
to stimulate the formation of new red blood cells.
And finally, it does have some hormonal regulation by turning a form of vitamin D into its most active form
so that you can go through vitamin D reabsorption.
Now, all of these sympathetic nervous system
and endocrine systems feed together
to help control some of the most primary bodily processes
– things like blood pressure,
electrolyte and osmolality homeostasis or control.
These sometimes work together between each other,
between the blood pressure and the osmolality component.
These allow for the whole process of controlling blood volume.
It also controls the amount of water you have within the body.
So if you have too much water,
you’ll want to urinate more of it out.
If you want to retain water,
you hold up to it tight.
You also go through blood glucose metabolism.
The kidney has the ability to utilize glucose.
It also utilizes lactate.
It can be involved in something known as gluconeogenesis or making new glucose.
Interestingly, the kidneys are also involved in oxygen-carrying capacity.
You might ask,
“Well, how does that work because it has nothing to do with the lungs, right?”
Well, remember, you need to bind oxygen to hemoglobin.
Where is hemoglobin carried?
In red blood cells.
So your erythropoietin helps stimulate red blood cell production,
also helps in oxygen-carrying capacity.
Finally, there are a lot of other parameters associated with something known as clearance.
So what is clearance?
Clearance is what is filtered out from the blood and eventually urinated out.
So if you need to clear something from the blood –
whether it be a toxin,
whether it be a substance like a metabolite that could build up
– what do you do?
You want to clear it.
And so, most of the various toxins and other substances
that might be build up to be poisonous within the body,
the kidney gets rid of by clearing it.
And finally, one of our most important topics in physiology is acid-base balance.
So acid-base balance involves two different components,
and that is sodium bicarbonate or bicarbonate,
and hydrogen ions.
So it’s the balance between bicarbonate and hydrogen ions that make acid-base balance work.
The kidney is involved with both of these,
so you can either excrete or reabsorb bicarbonate.
You can even form new bicarbonate in the level of the kidneys.
And finally, you can secrete acids or decide not to.
And those will all help the body’s acid-base balance.