Introduction
0:09
This… is a trauma patient.
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And right now, you need to know
0:12
one thing: is there internal bleeding? So you grab the ultrasound probe and
0:17
check four spots. Right upper quadrant. Left upper quadrant. Pelvis. Pericardium.
0:22
But why those specific points? You are checking the dependent
0:26
spaces. The places where free fluid will collect first based on gravity
0:30
and anatomy. Morison’s pouch in the right upper quadrant. The perisplenic region on the left.
0:31
Specific pouches in the pelvis. The entire FAST exam is built on
0:33
an understanding of abdominal spaces. Today we are going to build the complete map. One
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video. Every abdominal space you need to know for surgery, radiology, and clinical exams.
Content
0:45
We’ll start by orienting ourselves in the torso – what we actually mean by the abdominopelvic
0:51
cavity, how the peritoneum lines it, and why it creates what we call potential spaces.
0:56
Then we’ll move into the peritoneal cavity itself – breaking down its compartments.
1:01
From there, we’ll go into the pelvis, and see how the pouch of Douglas and
1:05
the rectovesical pouch are formed. Then we’ll look behind the peritoneum,
1:09
into the retroperitoneal space, how it’s organized into three compartments,
1:13
and why pathology behaves differently here. And finally, we’ll tie everything together
1:18
clinically, how to predict where pathology spreads, and why the
1:22
FAST exam looks exactly where it does. What’s up everyone, my name is Taim. I’m
1:26
a medical doctor, and I make animated medical lectures to make different topics in medicine
1:30
visually easier to understand. If you’d like a PDF version or a quiz of this presentation, you can
1:34
find it on my website, along with organized video lectures to help with your studies.
1:38
Alright, let’s get started. Our trunk is divided into two major
Abdominopelvic Cavity
1:42
cavities. The upper part is called the thoracic cavity, and it contains the two pleural cavities
1:47
laterally and the mediastinum in the middle, which we covered in a previous video. The space
1:51
below is called the abdominopelvic cavity, which is the region we are focusing on in this video.
1:56
Now, the abdominopelvic cavity is not one empty box. It is organised by a
2:01
single membrane called the peritoneum. Now let’s back up for a moment. Here we
2:06
see the anterior view of the entire trunk with all its muscles, right?
2:10
If we gently just peel off the muscles and connective tissue on the left side,
2:14
we will eventually get to a membrane that covers all internal organs.
2:18
This is called Parietal Peritoneum. And now, if we just slice the body in half,
2:23
and look in this direction, we’ll be able to see inside the parietal peritoneum, and see all the
2:28
internal organs here, just for orientation we can see the liver, the end part of the stomach,
2:33
since we cut the whole body out, we can see the transverse colon, bunch of small intestine,
2:38
rectum, bladder, back here we got the duodenum, we can also see the pancreas,
2:42
the right kidney and of course the diaphragm here, separating the thoracic from abdominal cavity,
2:47
which contains things like the lungs, the esophagus, trachea, heart and so on. Now, the
Peritoneum
2:52
parietal peritoneum we saw earlier covers all of these abdominal organs and form kind of a pocket,
2:59
this is just an approximate highlight, but, notice one thing, there is a special membrane that goes
3:04
out from the back side of the body, and then wraps around a lot of these organs, and this special
3:10
membrane is called the visceral peritoneum. And depending on your relationship to this membrane,
3:15
you are either inside the peritoneal cavity, or behind it, in the retroperitoneal space.
3:21
This is the fundamental division. Peritoneal, inside. Retroperitoneal, behind.
3:27
Now, the space inside the peritoneum — the peritoneal cavity — is sometimes called the
3:31
greater sac. This is the main cavity we are going to explore. There is also a smaller,
3:36
hidden pocket behind the stomach called the lesser sac, which we will come to later.
3:40
But the peritoneum does more than just line walls and cover organs. What it does is,
3:45
it reaches out from the back of the abdominal wall, wraps around the organs, around small
3:50
intestine too it just don’t look like it here on this diagram, but it wraps around the organs,
3:54
and as it does that, it creates folds. And these folds have names depending on what they do.
4:00
When a fold of peritoneum attaches an organ to the posterior abdominal wall and carries
4:05
its blood supply, we call it a mesentery. The small bowel, for example, is suspended by the
4:10
mesentery of the small intestine. The transverse colon is suspended by the transverse mesocolon.
4:15
When a fold of peritoneum connects the stomach to another organ, we call it an
4:20
omentum. The greater omentum is a large fold that hangs down from the greater curvature of
4:25
the stomach, draping over the intestines. The lesser omentum is a smaller fold that connects
4:30
the lesser curvature of the stomach to the liver. And when a fold of peritoneum connects an organ
4:36
to the abdominal wall or to another organ — but is not a mesentery or
4:40
omentum — we simply call it a peritoneal ligament. The falciform ligament, for example,
4:45
connects the liver to the anterior abdominal wall. This is generally how the peritoneum is organized,
4:52
now seeing all of this first will make the next parts of this video much easier to understand.
4:57
Because these folds divide the peritoneal cavity into compartments. They act like walls,
5:02
separating one area from another. And between these walls, they create spaces.
Potential Spaces in the Abdomen ‘
5:08
Now, the spaces we are about to cover are potential spaces. That means normally,
5:12
there is no visible gap. The peritoneum is pressed against itself, so you will
5:17
not see these spaces on a normal CT scan. But add some fluid — whether its blood, pus,
5:22
ascites — and suddenly these spaces open up. They become visible. They become clinically
5:28
relevant. This right here is the Right Subphrenic Space, which is in this region.
5:33
And this is why understanding these spaces matters. When pathology happens,
5:37
fluid does not spread randomly. It follows the specific map that the peritoneum creates.
5:42
So let us now look at how the peritoneal cavity is divided into compartments.
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Remember the transverse mesocolon — the fold that holds the transverse colon? This structure
5:52
acts like a horizontal shelf, and it divides the peritoneal cavity into two compartments.
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Everything above the shelf is called the supramesocolic compartment. Everything below
6:02
is called the inframesocolic compartment. Above the transverse mesocolon, you have
6:07
the liver, stomach, and spleen, along with all the spaces around them. Below,
6:12
you have the small bowel, the mesentery, and the associated spaces we’ll talk about.
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Now why does this division matter? Because when a small amount of fluid collects,
6:21
it tends to stay within one compartment. The transverse mesocolon acts as a barrier. But if
6:27
there is enough volume, fluid will find ways to move between them. We will see exactly how that
6:32
happens a bit later in this video, but one more thing I need to mention regarding the
Supramesocolic Compartment
6:36
supramesocolic compartment. Okay this is going to look very weird visually,
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so i’m sorry in advance. Remember I showed you that this was the parietal peritoneum?
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The parietal peritoneum is basically going to fold and anchor the liver to the anterior abdominal
6:50
wall and diaphragm. This fold is called falciform ligament. And what it does is, if we cut a part
6:57
of the peritoneum, we can see that it creates a barrier between right subphrenic space and left
7:03
subphrenic space. Here’s a better weird angle, as you see here. So naturally fluid on the right side
7:09
does not easily cross to the left, and vice versa. So when we talk about subphrenic collections,
7:14
we always have to specify: right or left. Let us start with the right side.
Right Subphrenic Space
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The right subphrenic space sits between the dome of the diaphragm above and the superior surface
7:25
of the right lobe of the liver below. Medially, it is limited by the falciform ligament. Posteriorly,
7:31
it is limited by the right coronary ligament. Just a sidenote this is called the bare area of
7:37
the liver. This is where the liver faces diaphragm directly, with no peritoneum
7:41
covering it. So the right subphrenic space cannot extend into this area since it’s extraperitoneal.
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Now, this space is not isolated. It communicates freely with the right subhepatic space below the
7:54
liver. And inferiorly, it communicates with the right paracolic gutter running down the flank.
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Why does this matter clinically? Because infection from the lower abdomen — for example,
8:04
a ruptured appendix, or blood — can track up the right paracolic gutter and end up in the
8:09
subhepatic or subphrenic spaces. This can for example form right subphrenic abscess.
8:14
Makes sense? Now let us move to the left
Left Subphrenic Space
8:17
side. The left subphrenic space sits between the diaphragm above and the left lobe of the liver,
8:23
the gastric fundus, and the spleen below. Medially, it is separated from the right
8:27
side by the falciform ligament, and it spreads far enough to the left to surround the spleen,
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so it forms the perisplenic region. But here is the major difference compared
8:38
to the right side: the phrenicocolic ligament. This is a fold of peritoneum that runs from
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the splenic flexure of the colon up to the diaphragm. And it partially blocks
8:48
communication between the left paracolic gutter and the left subphrenic space.
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So while fluid on the right side can travel freely from the pelvis, up the right paracolic gutter,
8:57
all the way to the right subphrenic space, on the left side, this pathway is partially obstructed.
9:02
This is why right-sided subphrenic abscesses are more common than left-sided
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ones. The anatomy on the left side is literally providing some protection.
Right Subhepatic Space (Morison’s pouch)
9:11
Now let us go below the liver, to the right subhepatic space. This space has two parts:
9:17
an anterior portion and a posterior portion. The posterior portion has a special name. We
9:22
call it Morison’s pouch – also known as the hepatorenal recess.
9:27
Morison’s pouch sits between the posterior surface of the right lobe of the liver above and the upper
9:33
pole of the right kidney below. Inferomedially, it is bordered by the hepatic flexure of the colon
9:38
and the second part of the duodenum. In this view I just cut the liver
9:42
horizontally across the right lobe so we can expose this pouch better, but keep
9:46
in mind that usually the liver covers this area. Now, here is why this space is so important: when
9:52
a patient is lying supine, Morison’s pouch is the most dependent part of the entire supramesocolic
9:59
compartment. Gravity pulls fluid here first. So when you perform a FAST exam on a trauma
10:05
patient, this is where you look. And what you would see normally is a liver, and the right
10:09
kidney without any fluid between them, but if there is free fluid. Free fluid will appear
10:14
as a dark, anechoic stripe between the liver and the kidney, that is blood in Morison’s pouch.
10:20
This space communicates superiorly with the right subphrenic space and inferiorly
10:25
with the right paracolic gutter. So fluid can arrive here from above or from below.
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Alright let’s go back. We did the right Subhepatic Space, but on the left side, there is the left
Left Subhepatic Space
10:37
subhepatic space. This sits below the left lobe of the liver, in front of the stomach.
10:42
But behind the stomach, there is a completely separate compartment called
10:46
the lesser sac, also called the omental bursa. Earlier, I mentioned that the peritoneal cavity is
Lesser Sac (Omental Bursa)
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divided into a greater sac, which is basically the whole compartment. But it also has a lesser sac,
10:57
which is a smaller pocket that sits behind the stomach, separated from the greater sac.
11:02
Now, between the stomach and the liver, there is a fold of peritoneum called the
11:07
lesser omentum. Think of it as a curtain. As you see here, the lesser sac sits behind
11:12
this curtain. And the only door connecting this hidden room to the rest of the peritoneal cavity
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is a narrow passage called the epiploic foramen. Let’s just expose this foramen a little bit more.
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This foramen sits between the hepatoduodenal ligament anteriorly — which is the right edge
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of the lesser omentum — and the inferior vena cava posteriorly,
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as it continues down. The portal triad runs within the hepatoduodenal ligament, containing the portal
11:37
vein, the hepatic artery, and the bile duct. Now, why does the lesser sac matter clinically?
11:43
Because the pancreas forms its posterior wall. In acute pancreatitis, inflammatory fluid leaks
11:50
directly into this space, forming what we call lesser sac collections. If you take a look at
11:55
this CT here, the pancreas is back here, and the stomach is in front of it. Between them,
12:01
normally you can’t see the lesser sac, but in this CT you will see fluid sitting in this region,
12:06
and with clinical history you can normally tell, is this due to perforated ulcer, or anything else.
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Alright that was the supramesocolic compartment. Now let us move below the transverse mesocolon,
Inframesocolic Compartment
12:19
into the inframesocolic compartment. Let us just go through something
12:22
again really quick. Here we see the anterior
12:25
view of the abdomen. Between the liver and the stomach, there is the Hepatogastric ligament,
12:30
which is part of the lesser omentum. It connects the liver to the lesser curvature of the stomach.
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Now, hanging down from the greater curvature of the stomach is a much larger fold called
12:40
the greater omentum. This drapes down over the intestines like an apron, and then loops
12:45
back up to attach to the transverse colon. If we look at the whole thing from the side,
12:51
you can see another important structure — the small bowel mesentery. This is a
12:56
fold of peritoneum that suspends the jejunum and ileum from the posterior abdominal wall.
13:02
Imagine for a second now, we’re looking at the anterior abdominal wall. We’ve removed the small
13:06
intestine, and exposed the mesentery. The root of the mesentery attaches diagonally — from the
13:12
duodenojejunal flexure on the upper left, down to the ileocecal junction on the lower
13:17
right. This diagonal attachment divides the inframesocolic compartment into two spaces:
Inframesocolic Spaces
13:24
a right inframesocolic space and a left inframesocolic space.
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The thing is, the right inframesocolic space is smaller. It is partially boxed in because
13:34
the mesentery attaches to the cecum. So fluid here does not easily drain into the pelvis.
13:39
The left inframesocolic space is larger, and it opens directly into the pelvis. So
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fluid on the left side has a free pathway down to the pelvic spaces.
Paracolic Gutters
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Then there’s the right and left paracolic gutters. These are on either side of the abdomen,
13:55
between the colon and the lateral abdominal wall. Think of these as highways connecting
13:59
the pelvis to the upper abdomen. The right paracolic gutter is the
14:03
major route. It is larger, deeper, and has no significant barriers. Fluid – whether blood, pus,
14:09
or ascites – can travel freely from the pelvis, up the right gutter, into Morison’s pouch, and also
14:15
all the way to the right subphrenic space. The left paracolic gutter exists too,
14:20
but remember the phrenicocolic ligament somewhere in here? It partially blocks
14:24
the superior end of the left gutter, making it a less efficient pathway for fluid to travel upward.
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Now, one more thing I wanna highlight before we move on: peritoneal fluid
14:35
is constantly moving. It is not static. The subdiaphragmatic region has negative
14:40
pressure, especially during inspiration. This creates an upward suction that pulls fluid from
14:45
the lower abdomen toward the diaphragm, mainly through the paracolic gutters.
14:50
At the same time, gravity pulls fluid to dependent areas. In a supine patient, that means Morison’s
14:56
pouch and the pelvis. In an upright patient, the pelvis becomes the primary collection site.
15:02
This is why, on a FAST exam, you check multiple locations — right upper quadrant,
15:06
left upper quadrant, and pelvis. You are checking all the dependent spaces based
15:11
on patient position and fluid dynamics. Ok, so far we have talked through what
15:16
the abdominopelvic cavity is, we’ve covered the peritoneal cavity, how it is divided,
15:21
its major spaces, and how fluid moves through it. As the peritoneum continues inferiorly,
Pelvic Space
15:27
it goes into the pelvis and covers over the pelvic organs. And as it does this, it creates pouches.
15:34
The anatomy here is different in males and females, so let us look at each.
15:38
In females, the peritoneum covers the bladder, then the uterus, and then the rectum. This
15:43
creates two pouches. Between the bladder and the uterus is the vesicouterine pouch. Between
15:48
the uterus and the rectum is the rectouterine pouch — also known as the pouch of Douglas.
15:53
The pouch of Douglas is the most dependent part of the entire peritoneal cavity in females.
15:59
Whether the patient is supine or upright, fluid will collect here first. This is why,
16:04
on ultrasound or CT, this is where you look for free fluid in the female pelvis. It is
16:08
also why a ruptured ectopic pregnancy presents with blood pooling in the pouch of Douglas.
16:14
In males, there is no uterus, so the peritoneum covers the bladder and then
16:19
the rectum directly. This creates a single pouch called rectovesical pouch. This is the
16:24
most dependent peritoneal space in males. Both of these are the lowest points of
16:29
the peritoneal cavity, and both are very important locations to check for free fluid.
16:35
Now, we have covered everything inside the peritoneum. Let’s now go behind it,
Retroperitoneal Space
16:40
into the retroperitoneal space. The retroperitoneal space is the
16:45
area between the posterior parietal peritoneum in front and the posterior abdominal wall behind. It
16:51
sits on either side of the vertebral column and contains structures that are not covered
16:56
by peritoneum, or at least not fully. Let’s just remove the posterior abdominal wall and
17:01
expose it fully. So we’re now looking at the abdominal cavity from behind,
17:05
after removing the bones and muscles, right? Some organs are primarily retroperitoneal.
17:11
They developed here and stayed here. These are organs like the kidneys, the adrenal glands, the
17:16
ureters, the aorta, and the inferior vena cava. Other organs are secondarily retroperitoneal.
17:22
They originally had a mesentery during development but later fused to the posterior abdominal wall,
17:28
like the duodenum, the pancreas, and the ascending colon and descending colon. Because these organs
17:33
are only partially covered by peritoneum, some literature also refers to them as mesoperitoneal.
17:40
Now, the retroperitoneal space is not one open area. It is divided by
17:45
layers of fascia into three compartments. And to see those compartments better,
17:49
let’s take this guy, and just slice him in half, and look from this direction.
17:54
It’s a little bit narrow, this region. So I’ve faded the liver to be able to see them better,
Anterior Pararenal Space
17:58
but the first is the anterior pararenal space. This sits between the peritoneum
18:03
in front and the anterior renal fascia behind. It contains the secondarily retroperitoneal organs,
18:10
the pancreas, the duodenum, and the ascending and descending colon.
18:14
Why does this matter? Because in for example acute pancreatitis, inflammatory fluid can
18:19
spread in two directions. Anteriorly, it can leak into the lesser sac as we saw earlier. But it can
18:25
also spread posteriorly and laterally within the anterior pararenal space, spreading along
18:30
the colon, crossing the midline, and extending toward the pelvis. The anterior pararenal space
18:36
has no barrier in the middle, so there is free communication between the right and left sides.
Perirenal Space (Gerota’s Space)
18:41
The second compartment is the perirenal space, also called Gerota’s space. This sits between the
18:47
anterior renal fascia in front and the posterior renal fascia behind. It contains the kidneys,
18:54
the adrenal glands, the proximal ureters, and the renal vessels, all surrounded by
18:58
perinephric fat. Clinically, this is where you see perinephric abscesses, haematomas
19:03
around the kidney, and urinomas after trauma. The third compartment is the posterior pararenal
Posterior Pararenal Space
19:10
space. This sits between the posterior renal fascia in front and the transversalis fascia
19:15
behind. It contains only fat — no organs. This space communicates with the fat of the
19:20
flank and extends down toward the pelvis. In trauma, blood from a ruptured abdominal aortic
19:26
aneurysm can spread into this space, appearing as haematoma in the posterior pararenal fat.
FAST exam (Clinical Application)
19:33
Ok we have now gone through the major anatomy of the abdominal spaces. Let us see how it
19:38
applies in real clinical scenarios. Let’s go back to the FAST exam.
19:43
FAST stands for Focused Assessment with Sonography in Trauma. It is a rapid bedside ultrasound used
19:49
to detect free fluid in trauma patients, which in the acute trauma setting is assumed to be blood.
19:55
The exam evaluates four predefined locations: The first one is the right upper quadrant.
20:00
In the right upper quadrant, we are examining the space between the liver and right kidney, the
20:05
hepatorenal recess, also known as Morison’s pouch. On this normal ultrasound, you can see there is no
20:12
visible space between the liver and the kidney. They appear right next to each other. That is
20:17
because Morison’s pouch is a potential space — normally, it does not exist as an open gap.
20:22
But in trauma, when there is bleeding into the peritoneal cavity, fluid collects here. And on
20:27
ultrasound, fluid appears dark — we call this anechoic. So instead of the liver and kidney
20:33
sitting directly next to each other, you see a dark stripe between them. That dark stripe
20:37
is blood in Morison’s pouch. In an unstable trauma patient,
20:41
free fluid seen here on FAST usually represents significant intra-abdominal
20:46
bleeding. That finding alone is often enough to justify immediate operative management.
20:51
In the left upper quadrant, we are looking at the space around the spleen, called perisplenic
20:56
region. On this normal ultrasound, you can see the diaphragm — this bright white curved line.
21:01
Right next to it is the spleen, and below that, the left kidney. And you can clearly see there
21:06
is no visible space between them. They are sitting directly against each other.
21:11
But in trauma, when there is bleeding — most commonly from a splenic injury — fluid
21:15
collects in this region. It appears between the spleen and the diaphragm,
21:19
and can also flow around the inferior and lateral surfaces of the spleen. Again, on ultrasound,
21:24
this fluid appears as a dark, anechoic stripe. This view is technically more difficult than the
21:29
right side and can be slightly less sensitive. But when free fluid is identified here, it is still
21:34
highly specific for intra-abdominal bleeding. In the pelvis, we are checking for
21:39
fluid in the most dependent spaces. In males, here is a normal pelvic ultrasound.
21:44
Ideally, we want the bladder to be full to get a better view, but in a trauma situation that is not
21:49
always possible when we doing a FAST exam. Here we can see the bladder, so this is a short axis view,
21:54
the probe is in a transverse position, and down here the prostate. What we’re interested in is
22:00
to see if there’s fluid around the bladder, which in this case there’s no fluid around the bladder.
22:05
In females, we have two potential spaces to consider — the vesicouterine
22:10
pouch between the bladder and the uterus, and the rectouterine pouch behind the uterus,
22:15
also known as the pouch of Douglas. Here on this suprapubic view,
22:19
we can see the bladder here, and behind it the uterus. You can see it is anteverted and
22:24
anteflexed. And down here, this bright line is actually called the vaginal stripe. So we
22:29
can assess the potential space behind the uterus — the pouch of Douglas — which is the most dependent
22:34
part of the entire peritoneal cavity in females. In trauma, even small volumes of blood — as little
22:40
as 100 to 200 millilitres — can accumulate in these pelvic spaces. A positive pelvic FAST in
22:46
an unstable patient is strongly associated with the need for operative intervention.
22:51
Finally, in this subxyphoid view, we can again see the liver here, and we can actually see the
22:56
heart. The slightly hyperechoic outline here is actually the pericardium. And the main thing
23:02
we’re evaluating here is the space between the heart and the pericardium. In penetrating trauma,
23:08
detection of a pericardial effusion on FAST has been shown to significantly reduce mortality by
23:13
enabling early recognition of cardiac injury, even before classic clinical signs of tamponade appear.
23:18
So clinically, the FAST exam is a targeted assessment of anatomical spaces,
23:23
selected because free fluid appears there early and helps to predict when
23:27
life-threatening injury is present. That logic – where we look, and why – comes directly
23:32
from the abdominal spaces we just covered. And so that was everything I had for the abdominal
Ending
23:36
spaces, in anatomy, surgery and radiology. I really hope you found this video helpful.
23:41
I’ve made free courses for other topics here on YouTube if you wanna keep learning,
23:44
otherwise if you want a handmade PDF version of this lecture or take a quiz
23:46
to test your knowledge, or access an organized list of all my videos,
23:49
you can find everything on my website. Thanks for watching! See you in the next one.