Test your understanding with 10 random multiple-choice questions from the question bank.
Fluid Exchange & Capillary Forces
Lymphatic Capillaries & Lymph Formation
Lymphatic Vessels & Collectors
Lymphatic Trunks (Trunci lymphatici)
Major Lymphatic Ducts
Lymph Nodes (Nodi lymphatici)
Immune Cells Inside Lymph Nodes
Primary Lymphoid Organs (Organa lymphoidea primaria)
Secondary Lymphoid Organs Spleen (Lien)
Tonsils
Lymphoid Nodules (Noduli lymphoidei)
Clinical Notes
Sources
Programs used: Complete Anatomy, Biorender, PowerPoint
0:00
Blood leaves the heart and flows into large arteries, where pressure is high.
0:04
These arteries branch into narrower and narrower arteries, and then into arterioles,
0:09
until finally becoming capillaries, which are the smallest vessels in the circulation.
0:11
Each capillary is made of a single layer of endothelial cells with tiny
0:15
pores between them. The pressure inside the capillary pushes water and small
0:20
solutes out of the blood through these pores. The fluid that leaves the capillary moves into
0:25
a tiny space between tissue cells. This area is called the interstitial space,
0:30
and is where exchange with the cells takes place. Every day, about 20 liters of fluid move out of
0:36
the capillaries and become part of the interstitial space between the
0:39
cells. About 17 liters gets quickly reabsorbed back into the capillaries,
0:45
but that leaves about 2-3 liters of fluid behind in the tissues each day. This 3 liters of fluid
0:51
needs to find a way back into the blood so that the body’s interstitial fluid volume and blood
0:56
volume both stay constant over time. That’s where the lymphatic vessels come in. They collect excess
1:02
interstitial fluid and return it into the blood. If this return system fails or becomes blocked,
1:07
fluid begins to build up in the tissues, a condition known as edema.
1:12
So in this video, we’re going to go through everything you need to understand this
1:16
system fully, starting with the four main functions of the lymphatic system. After
1:20
that we’ll follow the lymphatic vessels and see how they’re arranged in the body.
1:25
Then, we’ll look at the structures that make up the system, divided into the
1:29
primary or central lymphoid organs, which are the thymus and the bone marrow, and the
1:34
secondary or peripheral organs — the spleen, tonsils, lymph nodes, and lymphoid nodules.
1:39
And along the way, I’ll also mention a few clinical conditions related to this system.
1:44
What’s up everyone, my name is Taim. I’m a medical doctor, and I make animated medical
1:47
lectures to make different topics in medicine visually easier to understand. If you’d like a
1:52
PDF version or a quiz of this presentation, you can find it on my website, along with organized
1:56
video lectures to help with your studies. Alright, let’s get started.
Functions of the Lymphatic System
1:59
So, what are the functions of the lymphatic system? This is honestly such an interesting
2:04
system once you understand how it works. The first function is extracellular fluid drainage.
2:09
Remember: around 20 liters of fluid gets filtered out of the blood into the tissues every day.
2:15
About 17 liters gets reabsorbed straight back into the blood, and that leaves roughly 2–3 liters
2:21
behind in the interstitial space. And because we can’t just let fluid build up in the tissues,
2:23
the lymphatic system steps in, absorbs that extra fluid, and brings it back into the bloodstream.
2:23
Now, this part is really important to understand properly, so let’s take a closer
2:28
look at how this actually happens. Let’s start by zooming into this capillary here.
2:33
Blood flows from the arterial side of the capillary towards the venous side. Inside the
2:38
capillary, there is blood, and blood is a fluid. This fluid exerts a force on the capillary wall,
2:45
and this force tends to push fluid out of the vessel and into the surrounding tissue space.
2:50
Because the blood arriving at the arterial end is under higher pressure, this outward-directed
2:55
force is stronger at the arterial end. As the blood travels through the capillary toward the
3:01
venous side, this pressure gradually decreases. Outside the capillary, in the interstitial space,
3:08
there is also fluid, but the pressure here is much lower than inside the vessel,
3:12
so the net movement of fluid is outward. This outward-pushing force generated by
3:17
the fluid is called hydrostatic pressure. There is also another important force working
3:23
in the opposite direction called colloid-osmotic pressure, or oncotic pressure. The plasma inside
3:29
the capillary contains proteins, mainly albumin, which are too large to pass through the capillary
3:34
wall under normal conditions. These proteins attract water and create a pulling force,
3:40
drawing fluid back into the capillary. The interstitial space contains far fewer proteins,
3:45
so the pulling force from the outside is much weaker than the one inside the vessel. And since
3:50
the force pulling fluid inwards is higher, the net pressure along the capillary is like this.
3:56
At the arterial end, the hydrostatic pressure pushing fluid out is stronger
4:01
than the colloid-osmotic pressure pulling fluid in, so more fluid leaves the capillary here. As
4:07
the blood continues toward the venous end, the hydrostatic pressure gradually falls
4:12
because energy is lost due to resistance along the capillary. The colloid-osmotic pressure, however,
4:17
stays relatively constant along the capillary since the concentration of plasma proteins remains
4:22
unchanged. This means that, toward the venous end, the inward-pulling force becomes stronger
4:28
than the outward-pushing force, allowing some of the fluid to return back into the capillary.
4:34
Even with this reabsorption, the amount of fluid that enters at the arterial end
4:38
is still slightly greater than the amount that returns at the venous end. Over time,
4:43
this produces a small but continuous accumulation of fluid in the interstitial space. If this
4:50
excess fluid was not removed, it would gradually build up and cause swelling,
4:54
known as edema. This is where the lymphatic system becomes essential. It collects that extra fluid
4:59
from the interstitial space and returns it to the bloodstream to maintain normal fluid balance.
5:05
Let’s zoom into the lymphatic capillary to see how the lymphatic system actually does this. At
5:10
the very beginning of the lymphatic network, we have the lymphatic capillaries, and all
5:15
of this surrounding them is the interstitial space. Unlike the blood circulatory system,
5:20
the lymphatic system is not a closed loop. It is an open-ended system that begins in the tissues.
5:26
These lymphatic capillaries are the smallest lymphatic vessels, and they are structurally
5:31
designed to be highly permeable. Their walls are formed by a single
5:35
layer of endothelial cells that loosely overlap each other. This overlapping arrangement forms
5:41
small one-way mini-valves. When the pressure in the interstitial space becomes higher than the
5:47
pressure inside the lymphatic capillary, these mini-valves open, allowing interstitial fluid
5:52
to enter. Once the fluid moves inside and the pressure within the lymphatic capillary
5:57
rises above the surrounding tissue pressure, the mini-valves close, preventing fluid from leaking
6:03
back out. This ensures a unidirectional flow of fluid into the lymphatic system.
6:09
To help maintain their structure, lymphatic capillaries are anchored
6:13
to the surrounding connective tissue by collagen filaments. These anchoring
6:16
filaments prevent the walls from collapsing when the interstitial pressure changes, allowing the
6:20
capillaries to remain open and functional. The fluid that enters the lymphatic system
6:25
is called lymph. In most tissues, lymph is very similar to interstitial fluid,
6:30
consisting mainly of water, dissolved ions, and small amounts of plasma proteins that have
6:35
escaped from the blood capillaries. There is one important exception in the small intestine. Here,
6:41
specialized lymphatic capillaries called lacteals absorb dietary fats. These fats are packaged into
6:48
chylomicrons and enter the lymph, giving it a milky appearance known as chyle. This means that,
6:54
in addition to draining extracellular fluid, the lymphatic system also transports larger molecules
7:00
that cannot easily enter the blood capillaries, such as plasma proteins and lipids. This is the
7:05
second major function of the lymphatic system. On average, the body returns about 2–3 liters
7:11
of lymph to the bloodstream each day. All of this lymph eventually flows toward the heart and enters
7:17
the large veins at the root of the neck, bringing the recovered fluid back into circulation.
7:22
Another important function of the lymphatic system is the removal of
7:26
old or damaged cells. A key organ for this is the spleen, which filters the blood and
7:31
removes aged or abnormal red blood cells. The fourth main function of the lymphatic
7:37
system is immune surveillance. As lymph travels through the lymphatic vessels, it passes through
7:43
multiple lymph nodes. These lymph nodes contain large populations of immune cells that monitor the
7:49
lymph for pathogens such as bacteria, viruses, or abnormal cells. If anything potentially harmful is
7:55
detected, the immune system is activated. We will look this process in detail later in the video,
8:00
but for now, those are the four core functions of the lymphatic system. Let’s tick that off, and now
Lymphatic Vessels
8:06
continue with the lymphatic vessels, step by step, so the entire system becomes clear and logical.
8:13
Now that we’ve seen how lymph is collected at the tissue level, let’s place this structure in
8:18
context. At the very beginning of the lymphatic system, we have these blind-ended lymphatic
8:23
capillaries that contain one-way valves and take in fluid based on pressure differences. They are
8:29
located right here. Once they take up fluid, these capillaries merge and gradually form
8:35
larger and larger lymphatic vessels as they move toward the venous system. So the progression is:
8:40
lymphatic capillaries, then lymphatic vessels, then lymphatic trunks, and finally the lymphatic
8:46
ducts, which return lymph to the bloodstream. Lymphatic capillaries are blind-ended tubes,
8:52
and this open-ended structure ensures a unidirectional flow of extracellular fluid. They
8:57
collect fluid along with metabolic substances, antigens such as bacteria, viruses, and even tumor
9:04
or pre-tumor cells, as well as antigen-presenting cells. These capillaries form interconnected
9:10
networks that run alongside blood capillaries and venules. In the small intestine, the lymphatic
9:16
capillaries run within the core of each intestinal villus to allow absorption of dietary fats.
9:21
From the lymphatic capillaries, lymph drains into larger lymphatic vessels. These vessels
9:27
are thin-walled and contain valves that maintain forward flow, preventing back-flow of lymph.
9:33
Because they collect lymph from the tissues, these are often called the “collectors.”
9:38
Within organs, we call them subcapsular and deep lymphatic networks, but in the limbs,
9:43
lymphatic vessels are divided into superficial and deep collectors that follow specific veins. In the
9:49
upper limb, the superficial collectors are grouped into three main pathways. The lateral collectors
9:55
run along the cephalic vein and drain into the axillary lymph nodes. The medial collectors run
10:00
along the basilic vein and also drain into the axillary lymph nodes. The anterior collectors
10:06
run along the median antebrachial vein and drain into the superficial cubital lymph nodes.
10:12
In the lower limb, the superficial collectors are grouped as the medial collectors, which
10:17
follow the great saphenous vein and drain into the superficial inguinal lymph nodes;
10:22
the lateral collectors, which follow the tributaries of the great saphenous vein
10:26
and also drain into the superficial inguinal lymph nodes; and the posterior collectors, which
10:32
accompany the small saphenous vein behind the knee and drain into the superficial popliteal lymph
10:38
nodes. The deep lymphatic vessels run alongside the deep veins of the lower limb and drain into
10:44
the deep popliteal and deep inguinal lymph nodes. So, lymphatic capillaries drain into lymphatic
10:50
vessels, and these then drain into the lymphatic trunks. The lymphatic trunks are larger channels
10:56
that drain lymph from specific anatomical regions. Most of these are paired, meaning there is a right
11:02
and left trunk. The jugular trunks drain lymph from each half of the head and neck,
11:07
the subclavian trunks drain lymph from the upper limbs, and the bronchomediastinal trunks
11:12
drain lymph from each half of the thorax. On the right side, these three trunks unite to
11:19
form the right lymphatic duct, which is a short duct that collects lymph from the right side of
11:24
the head and neck, the right upper limb, and the right half of the thorax, and drains into
11:29
the venous system at the junction of the right subclavian and right internal jugular veins.
11:34
In the abdomen, we have the lumbar trunks, which drain lymph from the lower limbs and
11:39
pelvic region. We also have a group of unpaired trunks that drain lymph from the
11:44
abdominal organs. These trunks converge into a dilated sac called the cisterna chyli. From here,
11:51
lymph travels upward through the thoracic duct. The thoracic duct is the main lymphatic channel
11:56
in the body. It receives lymph from the rest of the body and drains into the
12:00
venous circulation at the junction of the left subclavian and left internal jugular veins.
12:07
So, as you see here, lymph flows from the lymphatic vessels, passes through several
12:11
lymph nodes, and as it continues toward the heart, these vessels unite to form lymphatic trunks,
12:17
which then merge into the larger lymphatic ducts. That was the lymphatic vessels,
12:22
so let’s tick this one off as well. Now what I want to do is skip all of these topics for now,
Lymphatic Nodes
12:27
and start with lymph nodes, what they are, how they work, and why we have them in the first
12:32
place. We’ll then continue with the primary and secondary lymphatic organs so it becomes logical.
12:38
As lymph flows through the lymphatic vessels, it eventually passes through small, specialized
12:43
organs called lymph nodes. Imagine for a second that a pathogen, like a virus or a bacterium,
12:49
gets absorbed into the lymphatic capillaries. It then travels with the lymph through the vessels
12:54
and eventually enters a lymph node, where it will be examined and filtered, let’s see how.
12:59
If we isolate one lymph node and look at its internal structure, we can see that each node
13:05
is a small, oval organ, usually between 1 and 25 millimeters in size. The human body contains
13:12
roughly 600 lymph nodes in total. Each lymph node is enclosed by a dense connective tissue capsule
13:18
that extends inward as thin partitions called trabeculae. Beneath the capsule lies the cortex,
13:24
which contains lymphatic nodules rich in B lymphocytes. Deeper to that is the paracortex,
13:29
which is dominated by T lymphocytes, and at the center lies the medulla,
13:34
containing medullary cords and sinuses. On the concave side of the lymph node is the hilum,
13:39
a region where blood vessels enter and exit, and where one or sometimes more efferent lymphatic
13:44
vessels carry lymph away from the node. Now, how does lymph actually circulate
13:49
inside a lymph node? Lymph enters through multiple small vessels called afferent lymphatic vessels.
13:55
Once inside, it flows into a space just beneath the capsule known as the subcapsular or marginal
14:01
sinus. From there, lymph moves deeper through the cortical and paracortical
14:06
regions via smaller internodular sinuses, then into the medullary sinuses located in
14:12
the center of the node. After being filtered, the lymph exits the node
14:16
through the efferent lymphatic vessel at the hilum and continues along its pathway toward
14:21
the larger lymphatic trunks and ducts. Remember, one of the lymph node’s main
14:26
functions is immune surveillance. And it’s able to do that because inside the node,
14:31
we will find three major types of immune cells: macrophages, dendritic cells, and lymphocytes.
14:37
Macrophages are found in abundance in the subcapsular sinus, and they are also present
14:42
in the medullary sinuses. When lymph enters the lymph node, it often carries with it bacteria,
14:48
viruses, or small foreign particles from the tissues. As the lymph first arrives in
14:53
the subcapsular sinus, macrophages act almost like a filter. They trap, engulf, and destroy
15:00
many of the microbes right at the entry point. Dendritic cells are another key player. Some of
15:05
these dendritic cells picked up pathogens out in the tissue before entering the lymphatic vessel,
15:10
and they arrive in the node carrying those antigens with them. Their job is not mainly
15:15
to destroy the pathogen, but to process it and present small fragments of it, called antigens,
15:20
to lymphocytes deeper inside the node. The third group of cells here are the
15:25
lymphocytes. In lymph nodes, lymphocytes exist mainly as B cells and T cells. B cells are
15:32
found primarily in the cortex inside structures called lymphatic nodules or follicles, while T
15:38
cells are located mainly in the paracortex, just beneath the cortex, as you see here.
15:43
Now, this is where the immune response begins. After dendritic cells arrive, they migrate into
15:49
the paracortex. Here, they display those antigen fragments on their surface to T cells. This is why
15:55
macrophages and dendritic cells are referred to as antigen-presenting cells, because they “present”
16:01
antigens to lymphocytes to activate them. When a naïve T cell encounters a dendritic
16:07
cell presenting an antigen that matches its receptor, that T cell becomes activated.
16:12
It then begins to divide and produce many identical copies of itself. Some of these
16:18
activated T cells differentiate into helper T cells, which will assist other immune cells,
16:23
and others become cytotoxic T cells, which can directly kill infected or abnormal cells.
16:29
At the same time, B cells in the cortex can bind directly to antigens that happen to be present in
16:35
the lymph. They internalize the antigen, process it, and then present it to helper T cells at the
16:41
border between the cortex and paracortex. With help from these T cells, the B cells
16:46
also become activated and start to multiply. Some of the activated B cells move deeper into
16:52
the lymph node and form circular structures called germinal centers. From here, some B
16:57
cells become plasma cells, which start producing antibodies and releasing them into circulation,
17:03
while others become memory B cells that remain in the body long-term, ready to respond quickly
17:08
if the same pathogen returns in the future. So that’s the general idea of how a lymph node
17:14
filters lymph and activates the immune response. Now, let’s talk about B and T cells for a moment,
17:20
because understanding where they come from and where they mature makes the rest of the lymphatic
17:24
system much easier to understand. Our lymphatic system contains two
Primary Lymphatic Organs
17:29
categories of lymphoid organs: primary, or central lymphoid organs, and secondary,
17:34
or peripheral lymphoid organs. Primary organs are the places where lymphocytes are
17:39
produced and where they mature before being exposed to any antigens for the first time.
17:45
These organs are the bone marrow and the thymus. B cells are produced in the bone marrow, and they
17:50
also mature there. T cells, however, are produced in the bone marrow and then travel to the thymus,
17:56
where they undergo their maturation process. So B for bone marrow, and T for thymus.
Secondary Lymphatic Organs
18:02
Once B and T cells have matured, they enter the bloodstream and circulate toward the secondary
18:08
lymphoid organs, which are the spleen, the tonsils, the lymph nodes, and the lymph nodules
18:12
found in mucosal tissues. These are the places where lymphocytes finally meet antigens for the
18:18
first time, and this is where they get activated and actually start doing their immune functions.
18:24
Let’s go through the main secondary lymphoid organs one by one, starting with the spleen.
Spleen
18:29
The spleen is the largest secondary lymphoid organ. Its main job is to filter the blood,
18:34
not the lymph. So unlike lymph nodes, which monitor lymph, the spleen monitors the blood
18:40
directly. One of its important functions is removing old or damaged red blood cells from
18:45
circulation. But in addition to that, it also plays a major role in starting immune responses
18:51
against antigens that appear in the bloodstream. I have a detailed video on spleen anatomy if you
18:57
wanna know more, but the spleen sits in the upper left part of the abdomen, under the rib cage,
19:02
and it is covered by a layer of peritoneum. If we take the spleen, cut it open and look inside,
19:08
we will see a thick fibrous capsule on the outside. This capsule sends inward
19:13
extensions called splenic trabeculae, which divide and support the inner tissue. Inside,
19:19
we can clearly see two main types of tissue: the red pulp and the white pulp.
19:24
The red pulp is involved in filtering the blood and removing old or damaged red blood
19:29
cells. The white pulp, on the other hand, is rich in lymphocytes and is responsible
19:34
for initiating immune responses against any blood-borne pathogens that enter the spleen.
19:40
So that was the spleen. Now let’s move to the tonsils.
Tonsils
19:44
The tonsils form a protective ring of lymphoid tissue at the entrance of both the respiratory
19:50
and digestive tracts. This means they act as one of the very first immune barriers for anything
19:56
we breathe in or swallow. Tonsils are made up of lymphoid nodules, and many of these nodules
20:02
contain germinal centers where active immune responses take place. We have several tonsils
20:07
in the throat region. The one most people are familiar with is the palatine tonsil, which sits
20:12
on each side at the back of the mouth. We also have the lingual tonsil at the base of the tongue,
20:18
the tubal tonsils located near the openings of the auditory tubes, and the pharyngeal tonsil,
20:23
also known as the adenoid, located in the roof of the nasopharynx. When tonsils become inflamed or
20:30
enlarged, such as during infection, they can become tender and swollen,
20:34
which is a sign that immune cells inside them are actively fighting off pathogens.
20:39
So that was the tonsils. Earlier, we already talked in detail about how lymph nodes function,
Lymph Nodes Clinical View
20:45
but now let’s briefly look at them from a clinical perspective. Lymph nodes in the body
20:50
are grouped according to their regions. This is important because during a physical examination,
20:55
doctors often check specific areas where lymph nodes lie close to the surface. If
21:00
lymph nodes in a particular region are enlarged, it often indicates that there is an infection,
21:06
inflammation, or sometimes something more serious in the area that these lymph nodes drain.
21:11
For example, in the head region, we have facial nodes, parotid nodes, mastoid nodes, and occipital
21:17
nodes. In the neck, we have submandibular nodes and the deep lateral cervical nodes. Under the
21:23
armpit, we find the axillary nodes, and so on throughout the body. I will go through these in
21:28
more detail in the written PDF version of this video so you can learn which areas they drain.
Lymph Nodules
21:34
Now, sometimes lymphoid tissue does not form a large organ like a lymph node or spleen. Instead,
21:41
it appears as small, scattered clusters. These are called lymphoid nodules. Lymphoid nodules
21:47
are small “islands” of immune tissue that sit within the mucosa of organs. Together,
21:53
they form what we call the mucosal immune system, or MALT – mucosa-associated lymphoid tissue. This
22:00
system is extremely important because our mucosal surfaces, like the gut and respiratory tract,
22:06
are major entry points for pathogens. MALT can be found in two general forms.
22:11
There is diffuse MALT, which is made of scattered immune cells within the mucosa,
22:17
and then there is organized MALT, which forms more visible and structured nodules.
22:22
Organized MALT includes regions such as GALT, or gut-associated lymphoid tissue, found in the
22:28
intestines, and BALT, or bronchus-associated lymphoid tissue, found in the airways.
22:34
There are also others, such as those found in the urinary tract, the conjunctiva, and even the skin.
22:40
So those were the secondary lymphoid organs. These are the places where B and T cells
Secondary Lymphatic Organs
22:45
finally encounter pathogens, interact with each other, and become activated to start the immune
22:50
response. And that wraps up our detailed overview of the lymphatic system. I really hope this helped
Ending
22:56
make everything clearer and more visual for you. I’ve made free courses for other topics here on
23:00
YouTube if you wanna keep learning, otherwise if you want a handmade PDF version of this
23:04
lecture or take a quiz to test your knowledge, or access an organized list of all my videos,
23:08
you can find everything on my website. Thanks for watching! See you in the next one.
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