Under Pressure – The Vascular System

Dr. Stephen Flanagan

Alright, we’re back, and today we’re plumbing the depths — literally. We’re talking about the human vascular system, that sprawling network of highways, backroads, and scenic detours that keep blood moving everywhere it needs to go. If the heart is the pump, the vessels are the plumbing — but not the boring kind. Think of it more like a national interstate system with toll booths, scenic overlooks, shortcuts, detours, and a couple of potholes if you’re not taking care of yourself.

Keshia Rayna

Yeah, except in this system, construction never stops. The body’s constantly repairing and remodeling the roads. And unlike your commute, you can’t just take a different route — if your vessels get backed up, that’s the end of the line.

Dr. Stephen Flanagan

Exactly. And that’s why understanding blood vessels is so critical — not just for exams, but for understanding everything else that happens in the body. Every cell, every organ, every tissue is dependent on these vessels doing their job efficiently.

Dr. Stephen Flanagan

Let’s get the basics out of the way first. There are three main types of blood vessels: Arteries — carry blood away from the heart. Capillaries — those microscopic exchange points where oxygen and nutrients are delivered and wastes are picked up. Veins — carry blood back toward the heart. Between those, we’ve got some in-between players like arterioles and venules, which are like the small side streets connecting major arteries and veins to the capillary beds.

Dr. Stephen Flanagan

Before we start naming specific vessels, let’s talk about what they’re made of. Every vessel wall — arteries, veins, you name it — has three layers, or tunics. And yes, that word tunica comes from Latin for “coat” or “garment.” Which, of course, leads to the nerdiest comparison in all of anatomy — the Legend of Zelda reference. I’ve been making this reference since 2006. Link’s green tunic, his fire-resistant red tunic, his water-breathing blue tunic. Each one offers a different kind of protection — same idea with vessels.

Keshia Rayna

Except most of our students were like three years old when Ocarina of Time came out. So let’s modernize that one a little. Think Fortnite skins — or if you’re into RPGs, Elden Ring armor sets. Every layer of protection changes your stats a little bit.

Dr. Stephen Flanagan

If you say so, it seems I am a bit out of my depth with modern video games. The blood vessel’s “armor”, or "skins" I guess, have three layers: Tunica intima — the inner lining. This is like the silky base layer of athletic gear, smooth and frictionless so blood can glide right through. It’s made of simple squamous epithelium, which we call endothelium when it lines a vessel. Tunica media — the middle layer, mostly smooth muscle with some elastic fibers. This is the contractile layer. When it tightens, we get vasoconstriction; when it relaxes, vasodilation. Tunica externa — the outermost coat, made of connective tissue that helps anchor the vessel in place and protect it.

Keshia Rayna

And in the middle of all that you’ve got the lumen, the hollow core where the blood actually flows.

Dr. Stephen Flanagan

Exactly, and the balance of the layers, or tunics, varies depending on the vessel type. Arteries have a thick tunica media, because they deal with pressure. Veins have thinner walls and bigger lumens because they’re working under low pressure — they just need to get blood back, not blast it out.

Dr. Stephen Flanagan

When we start to name arteries, we start big. The biggest arteries are called the elastic arteries, like the aorta and its major branches. Their diameters range from about 2.5 centimeters down to about 1 centimeter. That’s roughly the width of your thumb down to your pinky finger. These are also sometimes called conducting arteries because they’re the main conduits from the heart. They’ve got tons of elastic fibers in their walls — imagine a reinforced garden hose that can expand and recoil with every heartbeat. That elasticity dampens the surge of blood pressure that comes with each contraction of the left ventricle. The elastic fibers running through the tunica media are shock absorbers. When the left ventricle fires off blood like a cannon, the aorta stretches to take the hit and then recoils to keep the blood flowing smoothly downstream. Without that elasticity, your organs would get hit with a pressure wave every heartbeat — not ideal.

Dr. Stephen Flanagan

As we move away from the heart, the arteries get smaller and less elastic, transitioning into muscular arteries — also called distributing arteries. These have a thick tunica media loaded with smooth muscle cells. They’re built more for control than shock absorption. They can regulate blood flow to specific organs by constricting or dilating. Want more blood to your skeletal muscles during a workout? Muscular arteries dilate. Taking a nap? They constrict and divert blood to the gut.

Keshia Rayna

And you’ll see something cool in histology here — muscular arteries have internal and external elastic laminae, those wavy, elastic lines you can actually see separating the layers. They help the vessel maintain its structure during the pulse cycle.

Dr. Stephen Flanagan

This is the slide that looks like a pizza crust covered in dog hair?

Keshia Rayna

That's a gross way to think about an aorta histological slide. But yes, I suppose it does.

Dr. Stephen Flanagan

Alright, I'll keep my analogies a little less gross. Next stop down the size chart — the arterioles. These guys are the smallest arteries, ranging from about 0.3 millimeters down to about 10 micrometers. That’s roughly the width of a human hair to a single red blood cell.

Keshia Rayna

And they are mighty. Arterioles are the main controllers of systemic blood pressure. They’re like the valves on a garden irrigation system — a little constriction here or dilation there changes the overall flow resistance.

Dr. Stephen Flanagan

Spot on. Large arterioles still have all three tunics, but the smallest ones are just a thin layer of smooth muscle wrapped around endothelium. They respond to two main factors: Local factors — like oxygen, CO₂, and pH levels in nearby tissue. Sympathetic nervous system — which can tighten them up body-wide when you need to raise blood pressure. That’s why stress or cold can make your skin pale — the arterioles constrict to divert blood to the core.

Keshia Rayna

And that same mechanism goes the other way when you work out — the local buildup of carbon dioxide and heat in muscles makes arterioles dilate, increasing perfusion.

Dr. Stephen Flanagan

Since we’re talking about regulation, let’s hit a favorite of mine — the carotid sinus. This is a slightly enlarged area at the base of your internal carotid artery, right around where the common carotid splits into internal and external branches. Inside that little bulge are baroreceptors, specialized stretch receptors that monitor blood pressure. When the pressure rises, those receptors send a signal via the glossopharyngeal nerve (cranial nerve IX) to the brainstem — particularly the cardiovascular center in the medulla oblongata. The response? Lower heart rate and vasodilation. When pressure drops, those signals decrease, and the medulla responds by cranking up the sympathetic output — raising heart rate and constricting vessels.

Keshia Rayna

Which is why when you stand up too fast and get lightheaded, you’re actually feeling that reflex lag for a second. Your blood pools in your legs, pressure at the carotid sinus drops, and it takes a moment for the brain to catch up and tighten the vessels again.

Dr. Stephen Flanagan

Exactly. It’s an elegant system — simple feedback, instant response. And it’s also why massaging the carotid sinus can dangerously drop someone’s heart rate. Not a party trick.

Keshia Rayna

Speaking of the carotids — let’s take a detour upstairs. The Circle of Willis. You’ve probably seen that beautiful loop diagram in every anatomy textbook — a circular arterial anastomosis at the base of the brain.

Dr. Stephen Flanagan

Right — it’s formed by branches of the internal carotid and the basilar arteries. Functionally, it’s like the ultimate traffic roundabout for blood flow in the brain. If one route gets blocked — say, due to a clot or narrowing — the circle allows blood to reroute from the opposite side. And the name? It comes from Thomas Willis, a 17th-century English physician who was one of the first to describe the cerebral arterial circulation in detail. So yeah, the “Willis” in Circle of Willis is a person, not a body part.

Keshia Rayna

And you can remember it by thinking of it as a literal circle of life for your brain — or maybe, for pop-culture fans, as the “network hub.” If the brain were a city, the Circle of Willis is the central interchange where all the major highways meet.

Dr. Stephen Flanagan

Beautiful metaphor. And if you ever forget its branches, a good mnemonic is A-COP — Anterior communicating, posterior communicating, and the paired anterior and posterior cerebral arteries. Picture it like four exits on a roundabout.

Dr. Stephen Flanagan

Alright, let’s zoom down one more level — to the capillaries. These are the smallest blood vessels, only about 8 to 10 micrometers in diameter — just wide enough for a single red blood cell to pass through in single file. They’re where everything actually happens — gas exchange, nutrient delivery, waste removal, hormone pickup, you name it.

Keshia Rayna

And what’s cool is how specialized they are depending on where they’re located. In the lungs, capillaries let oxygen in and carbon dioxide out. In the intestines, they pick up digested nutrients. In endocrine glands, they collect hormones. And in the kidneys, they help filter waste into the urine.

Dr. Stephen Flanagan

Exactly — and their structure adapts to those roles. But before we dive into the different types, let’s talk about how things get through them. There are four routes for exchange: Direct diffusion through the endothelial cell membrane (for gases like oxygen and carbon dioxide). Intercellular clefts — small gaps between cells. Cytoplasmic vesicles — little transport bubbles that carry larger molecules. Fenestrations — literal pores through the endothelium.

Keshia Rayna

“Fenestrations” is such a satisfying word. Comes from the Latin fenestra, meaning “window.” It’s one of those terms that helps you visualize exactly what’s going on — they’re tiny windows that let light — or in this case, molecules — pass through.

Dr. Stephen Flanagan

And I always picture it like a skylight. You’ve got a solid roof (the endothelial cell), but with little openings that let certain things in and out while keeping others sealed off.

Keshia Rayna

Exactly. So when you hear fenestrated capillary, think “perforated window.” These are found where you need rapid exchange — kidneys, intestines, endocrine glands.

Dr. Stephen Flanagan

Whereas continuous capillaries — the most common type — have only tiny intercellular clefts and tight junctions. These are found in muscle, skin, and the brain. They’re the most selective and least leaky.

Keshia Rayna

And then there are sinusoids, the wide, leaky ones found in bone marrow, liver, and spleen — basically the blood cell factories and recycling centers.

Dr. Stephen Flanagan

Alright, so blood’s made it to the capillaries, given up its oxygen, exchanged nutrients, and lost a little plasma to the lymphatics. There are a lot of details I am glazing over to keep this more anatomy focused. Now it’s time for the blood to head home — through the veins.

Keshia Rayna

Veins are fascinating. They’re often dismissed as the passive “return line,” but they’re actually dynamic, adjustable reservoirs. About 65% of your total blood volume sits in the venous system at any given time.

Dr. Stephen Flanagan

That’s right — veins are capacitance vessels. Their walls are thin, their lumens are wide, and they can stretch to hold extra blood without much pressure change. And just like arteries, veins come in different sizes: Venules — small, post-capillary vessels that collect blood from capillary beds. Medium veins — with valves to prevent backflow. Large veins — like the superior and inferior vena cava, which dump blood back into the right atrium.

Keshia Rayna

Speaking of valves — they’re one of the coolest features of veins, especially in the limbs. These are literal folds of the tunica intima that work like trapdoors. They only open toward the heart. Combine that with skeletal muscle contractions, and you’ve got the skeletal muscle pump — every time you move, you’re squeezing veins and pushing blood upward.

Dr. Stephen Flanagan

Exactly. Kesh, you are on today! In the thorax, breathing helps too — the respiratory pump. When you inhale, pressure in the thoracic cavity drops, drawing blood toward the heart. Exhale, and pressure rises slightly, keeping it from falling back. So, between the valves, skeletal muscle pump, and respiratory pump, venous return stays efficient even against gravity.

Keshia Rayna

But if those valves fail, you get venous insufficiency — blood pools in the veins, they distend, and you end up with varicose veins.

Dr. Stephen Flanagan

Right — those ropy, twisted veins people notice especially in the legs. They’re caused by prolonged standing, pregnancy, or anything that increases venous pressure. The valves weaken, walls stretch, and it becomes a vicious cycle.

Keshia Rayna

And then there’s the more dangerous cousin — deep vein thrombosis, or DVT. That’s when a clot forms in a deep vein, usually in the calf, and can travel to the lungs as a pulmonary embolism.

Dr. Stephen Flanagan

Which is why long flights or post-surgical recovery are risk periods. Move your legs, stretch, stay hydrated — it’s not just good advice, it’s physiology.

Dr. Stephen Flanagan

Let’s not forget the special cases — like the dural venous sinuses in the brain. These are not veins in the traditional sense — they’re channels formed between layers of dura mater that drain cerebrospinal fluid and venous blood into the internal jugular veins.

Keshia Rayna

Or the hepatic portal system, another classic favorite. Instead of returning straight to the heart, blood from the digestive organs first passes through the liver.

Dr. Stephen Flanagan

Yeah. The hepatic portal vein, carrying nutrient-rich but unprocessed blood from the intestines, stomach, and spleen. The liver acts as a customs checkpoint — metabolizing nutrients, detoxifying substances, and storing glucose as glycogen before blood continues to the inferior vena cava.

Keshia Rayna

And that’s one of the few places where blood flows through two capillary beds in sequence — one in the gut, one in the liver. Same idea shows up in the hypophyseal portal system between the hypothalamus and pituitary gland.

Dr. Stephen Flanagan

Right — a two-step control relay between brain and endocrine system. It’s beautifully efficient.

Dr. Stephen Flanagan

Now, let’s talk numbers. Average venous pressure in large veins near the heart is about 0 mm Hg — sometimes even slightly negative. Compare that to the 120/80 mm Hg we see in arteries.

Keshia Rayna

So, the system’s low-pressure, but still continuous — thanks to those pumps and valves. And because veins have thin walls, they’re easily influenced by body position and gravity.

Dr. Stephen Flanagan

That’s why fainting can happen so fast when venous return drops — like standing up too quickly or dehydration. Blood momentarily pools in the legs, less returns to the heart, cardiac output drops, and boom — lights out.

Keshia Rayna

A good reflex arc saves you there too — baroreceptors trigger an increase in heart rate and vasoconstriction, restoring pressure. Your body’s built-in safety net.

Keshia Rayna

Speaking of the carotid sinus — let’s pause for a moment on that name. “Carotid” comes from the Greek karoun, meaning “to stupefy” or “put to sleep.” Ancient anatomists noticed that pressing on these arteries in the neck could cause fainting, so they named them for the effect.

Dr. Stephen Flanagan

That’s amazing — it’s like they discovered the vagus response long before neurophysiology existed.

Keshia Rayna

Exactly. And the “sinus” part just means a cavity or dilation — so the carotid sinus is literally a widened section of the carotid artery that detects pressure.

Dr. Stephen Flanagan

While that is interesting, lets move on for the sake of time. The body doesn’t distribute blood evenly. Some tissues are priority zones. Let’s highlight a few special circulations.

Dr. Stephen Flanagan

The coronary circulation — the heart’s own blood supply. The coronary arteries branch off the ascending aorta and encircle the heart like a crown — hence “coronary,” from corona, meaning “crown.”

Keshia Rayna

Nice etymology there, Flan. And remember, blood only flows through those arteries during diastole — when the heart relaxes. During systole, the contracting myocardium actually compresses them shut.

Dr. Stephen Flanagan

Then there’s cerebral circulation, where the Circle of Willis comes back in — the ring of interconnected arteries at the base of the brain that provides redundant blood flow routes.

Keshia Rayna

Ok, and just to rehash this... The circle’s formed by the internal carotids and vertebral arteries joining via communicating branches — like a safety net. If one side gets blocked, blood can reroute through the circle to maintain perfusion. So in that way the Circle of Willis is the brain’s version of an arterial anastomosis — multiple routes for one destination.

Dr. Stephen Flanagan

It’s the vascular equivalent of having Wi-Fi from three routers.

Keshia Rayna

But that redundancy also has its weak points. In places like the Circle of Willis, constant high pressure can lead to aneurysms — balloon-like dilations in vessel walls that may rupture.

Dr. Stephen Flanagan

Or arteriovenous malformations (AVMs), where arteries connect directly to veins without a capillary bed in between — causing turbulent flow and risk of rupture.

Keshia Rayna

Which is why understanding structure is crucial — the vascular system’s architecture is the blueprint for both health and pathology.

Dr. Stephen Flanagan

Before we move on, let’s wrap this section with one of my favorite physics-meets-physiology moments: the pulse. Each time the left ventricle contracts, it sends a pressure wave down the arteries. You can feel it wherever an artery runs close to the surface — radial, carotid, femoral. That’s your pulse pressure — the difference between systolic and diastolic pressure. The average mean arterial pressure (MAP), is the real measure of tissue perfusion it is a big mathematical formula but if your BP is 120/80, your MAP is about 93 mm Hg — enough to keep tissues oxygenated. Below 60, though, tissues start getting hypoxic. Above 110, long-term damage starts to accumulate. The balance is delicate, but the system’s engineering is stunning.

Keshia Rayna

No kidding. And next, we’ll put all that theory into action — tracing the actual routes blood takes through the body, from the aorta to the fingertips and back.

Dr. Stephen Flanagan

Grab your maps, listeners — we’re about to embark on the grand tour of systemic circulation.

Dr. Stephen Flanagan

Alright, we’ve talked pressure, flow, and reflexes — now let’s get geographical. We’re going on a road trip through the systemic circulation, tracing the major arteries and veins of the body. Buckle up — this is the Anatomical Highway Tour.

Keshia Rayna

Cue the imaginary GPS voice: “In 200 millimeters of mercury, turn left onto the aortic arch.”

Dr. Stephen Flanagan

Perfect. Our journey begins with the aorta, the body’s largest artery. It starts at the left ventricle, curves like a candy cane as the aortic arch, and descends behind the heart as the thoracic aorta, eventually passing through the diaphragm to become the abdominal aorta. The ascending aorta gives rise to the coronary arteries, feeding the heart itself. Then we hit the aortic arch, which has three main branches: Brachiocephalic trunk – this guy’s the first off the line, and it quickly divides into the right common carotid and right subclavian arteries. Left common carotid artery – second branch, supplies the left side of the head and neck. Left subclavian artery – third branch, headed toward the left arm. For those Branches on the top of the aortic arch, I always just recite the letters B, C, S. It is not a mnemonic device per say, but it works for me. Actually, back before 2013, the college football post-season selection system was called the Bowl Championship Series or BCS. And the BCS was in the news every day when I was an anatomy student so that helped. But since we are more than a decade removed from that now, it might not be very helpful.

Keshia Rayna

Notice how “brachiocephalic” literally means arm and head — brachium for arm, cephalon for head. Etymology is your GPS for anatomy.

Dr. Stephen Flanagan

Totally. It’s like learning prefixes and suffixes is the cheat code to the vascular map. Once the arch curves down, the descending aorta runs posteriorly, giving off intercostal arteries to the ribs — nine pairs from the thoracic aorta, to be exact. Then it pierces the diaphragm around vertebra T12 and becomes the abdominal aorta, which splits around L4 into the right and left common iliac arteries. as far as branches of the aorta we should start in the head region. The common carotid arteries travel up each side of the neck and split into two branches: The external carotid artery, which supplies the face, scalp, and neck. The internal carotid artery, which dives deeper into the skull to supply the brain.

Keshia Rayna

And that internal carotid connects directly to our earlier friend, the Circle of Willis — the arterial traffic circle at the base of the brain.

Dr. Stephen Flanagan

Along with the vertebral arteries, branches of the subclavians, travel up the cervical vertebrae through the transverse foramina, join to form the basilar artery, and then feed into the Circle of Willis.

Dr. Stephen Flanagan

Speaking of the basilar artery — I know you think of “Basil of Baker Street” when you hear it, right?

Keshia Rayna

Yeah, but that cartoon was released in 1986. I’m updating your references, Flan. Instead of a Sherlock Holmes mouse from the 80s, let’s go with Basil Fawlty from Fawlty Towers — okay, that’s still too old. Maybe Basil the Plant Dad Influencer on TikTok. He’s probably talking about vascular networks for houseplants.

Dr. Stephen Flanagan

If you say so Kesh. Basilar — basal — it’s all about the base. The basilar artery runs at the base of the brainstem, merging the vertebral arteries. Etymology strikes again. Moving down, the subclavian arteries pass under the clavicle — sub-clavicular, literally — and once they enter the armpit, we rename them the axillary arteries.

Keshia Rayna

That’s the naming pattern students always ask about — why the name changes even though it’s one continuous tube. It’s all about regional terminology.

Dr. Stephen Flanagan

Exactly. As the vessel crosses the teres major muscle, the axillary artery becomes the brachial artery, running down the upper arm. Near the elbow, it splits into the radial and ulnar arteries, which run parallel down the forearm and rejoin at the palm to form the superficial and deep palmar arches. On our models you can use the humeral circumflex arteries as a delineation point, they are branches of the axillary artery that supply blood to the shoulder joint and surrounding muscles.

Keshia Rayna

If you’re into mnemonics, here’s one: “Subtle Arms Bring Rad Ulna Palms.” Subclavian → Axillary → Brachial → Radial → Ulnar → Palmar.

Dr. Stephen Flanagan

Love it. Actually, I haven't heard that one before... But it works really well!

Dr. Stephen Flanagan

Now, back to the abdominal aorta. This vessel’s like a tree trunk with a forest of branches: Celiac trunk – supplies the stomach, spleen, and liver. Superior mesenteric artery – feeds most of the small intestine and the first half of the large intestine. Renal arteries – to the kidneys. Gonadal arteries – testicular or ovarian, depending on your setup. Inferior mesenteric artery – the second half of the large intestine. And then the common iliac arteries at the end, dividing into internal and external iliac arteries.

Keshia Rayna

The internal iliac supplies pelvic organs, while the external iliac runs down the leg, turning into the femoral artery once it passes under the inguinal ligament.

Dr. Stephen Flanagan

Then it continues behind the knee as the popliteal artery, splits into anterior and posterior tibial arteries. Now let’s follow that blood back home through the veins. Remember, the venous system is like the return bus line — slower, lower pressure, but still crucial.

Keshia Rayna

And unlike arteries, veins have a few unique design tweaks to handle that low pressure. Larger lumens – to reduce resistance. Thinner walls – because the pressure isn’t as high. Valves – to prevent backflow, especially in the limbs.

Dr. Stephen Flanagan

And this is where the skeletal muscle pump and respiratory pump kick in. The skeletal muscle pump works when your leg muscles contract — especially the soleus, sometimes called the “peripheral heart.”

Keshia Rayna

That’s such a cool nickname. The soleus muscle, in your calf, compresses deep veins every time it contracts. Each squeeze propels blood upward toward the heart.

Dr. Stephen Flanagan

And those venous valves ensure one-way traffic — so every step you take literally helps push blood from your toes back to your heart. It’s why long flights or sitting too long can lead to deep vein thrombosis (DVT) — without muscle contraction, venous blood stagnates, and clots can form. That’s why even wiggling your toes or flexing your calves on a plane is medically sound advice. The respiratory pump is the other half of the equation. Every breath creates pressure changes between your thorax and abdomen.

Keshia Rayna

When you inhale, the diaphragm descends, increasing abdominal pressure and decreasing thoracic pressure — that pushes blood upward toward the right atrium.

Dr. Stephen Flanagan

It’s such an elegant bit of physiology — even breathing helps venous return.

Keshia Rayna

And you can really feel it if you’ve ever done heavy sandbag training like Flan does. Once those intercostals and diaphragm start pumping, you become hyper-aware of your own pressure gradients.

Dr. Stephen Flanagan

Yeah, nothing like being red-faced and gasping with a big sandbag in your lap to remind you that your venous return system works overtime.

Dr. Stephen Flanagan

So, the superior vena cava drains everything above the diaphragm — head, neck, upper limbs — and the inferior vena cava handles everything below. Both empty into the right atrium.

Keshia Rayna

Let’s not forget the azygos system — the azygos, hemiazygos, and accessory hemiazygos veins — which drain the thoracic wall into the superior vena cava.

Dr. Stephen Flanagan

Good catch. the Azygous system is a very interesting return and worth memorizing. Also, you can see where the azygous vein enters the superior vena cava just above the heart, like a half inch above the heart. In fact, that vessels is observable on most heart models.

Keshia Rayna

Then we’ve got the hepatic portal system, one of the coolest detours in circulation.

Dr. Stephen Flanagan

Right! Blood from the stomach, intestines, spleen, and pancreas doesn’t go straight back to the heart. It first travels through the hepatic portal vein to the liver, where nutrients are processed and toxins are filtered. Two capillary beds in series — that’s the hallmark of a portal system. First in the gut, then in the liver’s sinusoids — those wide, leaky capillaries.

Keshia Rayna

It’s like a security checkpoint for your bloodstream. No shortcuts past the liver bouncer.

Dr. Stephen Flanagan

Now, veins anastomose — or interconnect — much more freely than arteries. It’s part of why venous circulation is so adaptable.

Keshia Rayna

But it’s also why certain conditions, like portal hypertension from liver disease, can create emergency detours — veins of the esophagus, hemorrhoidal veins, and veins of the abdominal wall can enlarge as blood finds alternate routes.

Dr. Stephen Flanagan

That’s how you get complications like varices. It’s fascinating and dangerous — all at once.

Dr. Stephen Flanagan

Ok, moving around a bit. Upper limb veins — we’ve have the cephalic, basilic, and median cubital. As a quick trivia bit, and I am getting ahead of myself here: that cephalic vein running up the lateral arm — its name confuses everyone. “Cephalic” means head, so why is it in the arm? It’s thought to refer to the head of the humerus — the uppermost part of the bone it crosses. So, not so misplaced after all.

Keshia Rayna

And we can thank that vein for every blood draw and IV insertion, since its cousin — the median cubital vein — connects the cephalic and basilic veins right in the antecubital fossa.

Dr. Stephen Flanagan

In the lower limbs, the two main superficial veins are the great saphenous vein — running from the ankle up to the femoral vein — and the small saphenous vein, which empties into the popliteal vein. And those names go way back — “saphenous” comes from the Greek saphenes, meaning “visible.” They’re literally the visible veins under the skin.

Keshia Rayna

Perfect etymology moment!

Dr. Stephen Flanagan

Always. And while those veins make convenient grafts in bypass surgery, they’re also prone to varicose dilation when valves fail.

Dr. Stephen Flanagan

Ok now we’ve built the full vascular highway — arteries out, veins back, detours through the liver and lungs — and now it’s time to talk about what happens when traffic jams hit.

Keshia Rayna

The dreaded vascular bottlenecks — blockages, leaks, and blowouts. Let’s start with the big one: atherosclerosis.

Dr. Stephen Flanagan

Exactly. Atherosclerosis is the chronic buildup of fatty plaques — primarily in the tunica intima, the innermost layer of an artery. Those plaques narrow the lumen, restrict blood flow, and increase the risk of thrombosis or embolism.

Keshia Rayna

And it’s a slow process. It starts with damage to the endothelium — smoking, hypertension, diabetes — all those risk factors trigger inflammation, then LDL cholesterol sneaks under the surface.

Dr. Stephen Flanagan

Right, the immune system freaks out, macrophages move in, eat the LDL, turn into foam cells, and form fatty streaks. Over time, the streaks harden into plaques, stiffening the vessel — that’s arteriosclerosis, literally “hardening of the arteries.”

Keshia Rayna

And that’s what makes blood pressure spike — the vessel loses elasticity, so it can’t buffer the heart’s output anymore.

Dr. Stephen Flanagan

Yep. Normally, arteries like the aorta stretch with each systole and recoil during diastole, maintaining continuous flow. But atherosclerosis turns that elastic garden hose into rigid PVC pipe.

Keshia Rayna

Flan, you’ve used that garden hose analogy for years — but now that you’re into strongman training, I feel like you need to update it.

Dr. Stephen Flanagan

True. Okay — imagine trying to pressurize one of those old, stiff lifting sandbags. It doesn’t flex or give, right? That’s your calcified artery. All pressure, no compliance.

Keshia Rayna

And if a plaque ruptures, the clot that forms can completely block flow — that’s what causes a myocardial infarction in the coronary arteries or a stroke in cerebral circulation.

Dr. Stephen Flanagan

Which is why the carotid arteries are such a critical site. That’s why clinicians listen for bruits — turbulent sounds from narrowed vessels — with a stethoscope.

Keshia Rayna

Let’s switch from narrowing to bulging — aneurysms.

Dr. Stephen Flanagan

Yes! An aneurysm is a balloon-like dilation of a vessel wall, usually due to weakening of the tunica media. Common spots include the abdominal aorta, circle of Willis, and popliteal artery.

Keshia Rayna

Those are terrifying because they’re silent until they rupture — then it’s game over. The mortality rate for a ruptured abdominal aortic aneurysm is over 80%.

Dr. Stephen Flanagan

It’s like a ticking biological time bomb. When I was in college I borrowed my dads truck. I immediately noticed there was a problem. It felt like I would drive over a 2 by 4 ever 10 feet or so. So I turned around and parked it in the driveway, got out, then the tire exploded! Sounded like a bomb went off! I hit the ground thinking I was under fire. Turns out it was the tire equivalent to an aneurysm, a separated tread. and that end result, an explosion in the pressurized tube, is exactly what happens to blood vessels. Usually the wall of the vessel should be quite thin for this to happen, So, students always ask why the aorta, of all vessels, gets them — it’s because it deals with constant high pressure and massive pulsatile stress that can overcome the incredibly think walls.

Keshia Rayna

And then there’s arteriovenous malformations (AVMs) — direct connections between arteries and veins that bypass the capillary bed. That means high pressure arterial blood shoots straight into veins not built for it.

Dr. Stephen Flanagan

Exactly. That’s like connecting a fire hose to a garden sprinkler — it’s not going to end well.

Keshia Rayna

We can’t leave the veins out of this disaster tour. Let’s talk varicose veins, deep vein thrombosis, and pulmonary embolism.

Dr. Stephen Flanagan

Yeah, the venous side is also pretty prone to disease. Varicose veins — superficial veins that become dilated due to incompetent valves. You’ll often see them in the great saphenous vein or small saphenous vein, especially from prolonged standing or genetic predisposition.

Keshia Rayna

Right, and the physiology is all about pressure. Once valves fail, gravity wins — blood pools, the vein walls stretch, and the twisted, rope-like appearance forms.

Dr. Stephen Flanagan

Then there’s DVT, where a thrombus forms in a deep vein, typically the legs. If that clot dislodges and travels to the lungs — boom, pulmonary embolism.

Keshia Rayna

That’s why every hospital patient who’s immobilized gets compression stockings or anticoagulant shots. Venous return needs muscle movement, or it stagnates.

Dr. Stephen Flanagan

And remember: those clots almost never come from arteries — it’s the slow, low-pressure venous environment that fosters them.

Keshia Rayna

Alright, before we close out — we have some rapid-fire fun facts written down here in the studio. Lets go through the list.

Dr. Stephen Flanagan

The aorta is about the diameter of a garden hose in adults — around 2.5 cm.

Keshia Rayna

The total length of blood vessels in your body? Over 60,000 miles — enough to circle the Earth twice.

Dr. Stephen Flanagan

The capillaries alone account for 80% of that length. They’re only about the diameter of a single red blood cell — 8 micrometers.

Keshia Rayna

And your veins contain about 65% of your total blood volume at any given moment. That’s your body’s reservoir.

Dr. Stephen Flanagan

Here’s one for the gym crowd: the femoral artery can deliver over 400 mL of blood per minute to a single leg during heavy lifting. So yes — you’re literally moving gallons of blood when you squat.

Keshia Rayna

Okay, that’s the pump science we can all get behind.

Dr. Stephen Flanagan

Let’s wrap with a few study hacks for mastering the vascular system. Use directional mnemonics. For arteries we should use your: “Subtle Arms Bring Rad Ulna Palms.” For veins: “Cephalic Sees the Ceiling, Basilic by the Body.” Color-code your notes. Red for arteries, blue for veins, purple for portal systems. It’s visual reinforcement. Draw the flow. Literally sketch it. Blood movement is spatial — once you map it, it sticks. Pair it with physiology. Don’t memorize vessels in isolation — always ask, “what happens if this gets blocked?” or “what’s downstream?” Feel your own anatomy. Pulse points, veins on your wrist, that faint carotid pulse — connect theory to tactile experience. Categorize vessels as oxygenated or deoxygenated, it is not as straight-forward as it might seem.

Keshia Rayna

And my bonus tip — if you can teach it, you’ve learned it. So try explaining the blood flow path to a friend, or your dog.

Dr. Stephen Flanagan

My dog definitely knows the difference between systemic and pulmonary circulation by now.

Keshia Rayna

So, to wrap it all up — the vascular system isn’t just plumbing. It’s a living, responsive, dynamic network that keeps every tissue supplied and synchronized.

Dr. Stephen Flanagan

And the pressure — that “under pressure” theme — it’s not the enemy. It’s the reason blood moves, nutrients flow, and homeostasis holds.

Keshia Rayna

Beautifully said. So next time you feel your pulse, that’s not just your heart beating — that’s your entire vascular orchestra keeping time. speaking of time... oh my goodness! is that the time? we went a bit over!

Keshia Rayna

We better wrap this up Flan.

Dr. Stephen Flanagan

Stay curious, stay circulating, and we’ll see you next time on Flanatomy.

Dr. Stephen Flanagan

Bye, y’all — and hydrate! Always hydrate.