Our Vascular System

Your vascular system is roughly 100,000 km of vessels, from large arteries down to capillaries thinner than a human hair, all responsible for delivering oxygen and nutrients to every cell in the body and carrying waste away. How well your vessels function determines how well everything else functions: your brain, your muscles, your hormones, your recovery, your skin.

What blood actually is

Blood is a tissue, a suspension of cells and proteins in a liquid called plasma. An average adult has about 5 litres of it circulating at any given time.

Plasma makes up about 55% of blood volume. It's mostly water, but it carries dissolved proteins (albumin, immunoglobulins, clotting factors, SHBG, transport proteins), hormones, glucose, lipids, electrolytes, and waste products. Plasma is the transport medium.

Red blood cells (erythrocytes) make up about 45% of blood volume. Their sole job is carrying oxygen from the lungs to tissues and carrying carbon dioxide back. They do this through haemoglobin, an iron-containing protein that binds oxygen. The percentage of blood volume occupied by red blood cells is called haematocrit (which also thickens the blood)

White blood cells (leukocytes) are the immune system's circulating workforce. They make up less than 1% of blood volume but are essential for detecting and fighting pathogens.

Platelets (thrombocytes) are small cell fragments that circulate in blood and are responsible for clotting. When a vessel is damaged, platelets aggregate at the site, forming a plug and initiating the clotting cascade to stop bleeding. Excessive platelet aggregation in the wrong context contributes to dangerous clot formation (thrombosis).

Blood vessels

Every artery has three concentric layers, each doing a different job. Working from the inside out:

The intima is the innermost layer, in direct contact with flowing blood. It's a single layer of cells called the endothelium. Despite being just one cell thick, the endothelium does most of the active work of the vascular system, regulating what passes through the wall, controlling whether the vessel dilates or constricts, and determining whether immune cells and lipid particles can enter the tissue beneath it.

The media is the thick middle layer, the bulk of the vessel wall. It's made of smooth muscle cells and elastic fibres. The smooth muscle contracts and relaxes to change vessel diameter, which is how the body regulates blood pressure and directs blood flow. In large arteries the media is rich in elastic fibres that stretch with each heartbeat and recoil between beats, keeping blood flowing smoothly even when the heart is between contractions.

The adventitia is the outermost layer, a tough fibrous wrapping that anchors the vessel to the surrounding tissue. It contains tiny blood vessels that feed the vessel wall itself (the wall is thick enough that it needs its own blood supply), and nerve fibres that tell the middle layer when to tighten or relax.

Arteries, veins, and capillaries

Arteries carry blood away from the heart. They have thick, muscular walls because they handle the highest pressure in the system, the direct output of each heartbeat. Large arteries (the aorta, the carotids, the femoral arteries) are elastic, stretching and recoiling to buffer pressure pulses. Smaller arteries (arterioles) are more muscular and are the primary regulators of blood flow distribution, constricting or dilating to direct blood toward where it's needed.

Veins carry blood back to the heart. They operate at much lower pressure and have thinner walls with less smooth muscle. Many veins contain one-way valves that prevent backflow, which is particularly important in the legs where blood has to travel upward against gravity.

Capillaries are where the actual exchange happens. They're microscopic vessels, just one endothelial cell thick, connecting arterioles to venules. Oxygen, nutrients, hormones, and immune cells pass from blood through the capillary wall into surrounding tissues. Carbon dioxide and waste products pass back the other direction. Every cell in your body sits within a few cell-widths of a capillary. The capillary network is where the entire purpose of the cardiovascular system is fulfilled.

The endothelium

The endothelium is a single layer of cells lining the inside of every blood vessel. Despite being just one cell thick, it's one of the most metabolically active tissues in the body.

Endothelial cells constantly read the blood flowing past them. They sense the speed of flow, the pressure inside the vessel, the temperature, the hormones circulating, and any inflammatory signals. Based on all of this, they release molecules that tell the surrounding muscle layer to tighten or relax, decide what's allowed to pass through the vessel wall, control whether immune cells can enter the tissue beneath, manage clotting, and signal repair.

When the endothelium is healthy, the vessel responds fluidly to the body's demands: widening when more blood flow is needed, narrowing when less is needed, and maintaining a smooth, non-sticky surface that prevents unnecessary clotting. When the endothelium is damaged or chronically inflamed, the vessel becomes stiff, the surface becomes sticky (attracting immune cells and promoting clot formation), and the regulation of blood flow breaks down. This is endothelial dysfunction, and it's one of the earliest detectable stages of cardiovascular disease, often present years or decades before any clinical symptoms appear.

Nitric oxide (NO)

The most important molecule the endothelium produces is nitric oxide (NO). It's a simple gas with a half-life of only a few seconds, but it controls an enormous amount of vascular function.

The endothelium produces nitric oxide on demand using an enzyme called eNOS (endothelial nitric oxide synthase). The raw material is L-arginine, an amino acid that's present in any normal diet. Once produced, NO diffuses from the endothelium into the muscle layer wrapping the vessel and tells it to relax. The vessel widens, blood flow increases, and pressure drops. This is the primary way your body regulates vessel diameter moment to moment.

NO also:

Stops blood from clotting. Blood is constantly being told to clot at injury sites, NO is one of the main signals telling it not to clot everywhere else. Low NO means the blood becomes more prone to forming clots that shouldn't be there, which is one of the mechanisms behind heart attacks and strokes.

Keeps the endothelium non-sticky. Healthy endothelium is smooth and doesn't grab onto immune cells passing by. NO is part of what maintains that smooth state.

Stops the vessel wall from thickening. NO prevents the muscle cells in the vessel wall from multiplying inappropriately. Without that brake, those cells migrate into the inner wall and lay down collagen and matrix material.

Several different things tell endothelial cells to produce more NO.

Shear stress. When blood flows faster through a vessel, it creates physical friction against the endothelial cells. They detect this mechanical force and ramp up eNOS activity. This is one of the main reasons exercise improves vascular function. The increased cardiac output during a workout creates sustained shear stress that trains the endothelium to produce more NO over time.

Heat. Higher core temperature activates a protein called Hsp90 that stabilises and accelerates eNOS. This is part of why sauna improves vascular function.

L-arginine availability. eNOS needs L-arginine to work. The body can synthesise it, but supplementing with L-citrulline (which the kidneys convert into L-arginine more efficiently than oral arginine itself) is one of the most direct ways to support NO production.

Oestrogen. Oestradiol directly increases eNOS expression in endothelial cells. This is one of the primary mechanisms behind oestrogen's cardioprotective effect. The advantage narrows significantly after menopause when oestrogen drops.

Insulin. Insulin activates eNOS through a separate signalling pathway. This is how insulin resistance contributes to the endothelial dysfunction and elevated blood pressure that come with metabolic syndrome.

The endothelium gets damaged by the same things that damage cells everywhere else, but the consequences are particularly important because of how exposed it is to circulating blood:

Chronically elevated blood pressure physically stresses the endothelium with every heartbeat

High blood glucose glycates proteins on the endothelial surface and impairs NO production

Oxidised LDL triggers inflammatory signalling in the endothelium and starts the atherosclerotic cascade

Smoking introduces free radicals that directly damage the endothelium and inhibit eNOS

Chronic inflammation keeps the endothelium in an activated, sticky state

Sedentary behaviour reduces shear stress and slowly weakens NO production capacity

Endothelial damage is essentially the entry point for almost every cardiovascular problem. The lipid section explains what happens when LDL particles meet a damaged endothelium. The atherosclerosis section explains the downstream cascade. But the starting point in every case is the endothelium itself, either failing to produce enough NO or becoming sticky enough that immune cells and lipid particles start penetrating the wall.

The heart

The heart is a muscular pump about the size of your fist, sitting in the centre of the chest. It beats roughly 100,000 times per day, pumping about 7,500 litres of blood through your vascular system every 24 hours.

The heart has four chambers stacked into two parallel pumps:

The right side (right atrium and right ventricle) collects oxygen-depleted blood returning from the body and pumps it to the lungs to be reoxygenated

The left side (left atrium and left ventricle) collects oxygen-rich blood returning from the lungs and pumps it to the entire body

The left ventricle does the heaviest work. It has to generate enough pressure to push blood through the entire body, every artery, every capillary, every tissue. This is why it has the thickest muscular wall of any heart chamber. When that wall thickens beyond what's healthy (from chronic high blood pressure, untreated hypertension, or supraphysiological androgen use), it's called left ventricular hypertrophy, and it's one of the main cardiac concerns for enhanced athletes and anyone with long-term untreated high blood pressure.

The heart muscle (myocardium) is one of the most metabolically active tissues in the body, and it can't tolerate any interruption of its blood supply. It gets that supply from the coronary arteries, which branch off the aorta immediately after it leaves the left ventricle. The heart is the first organ to receive oxygen-rich blood with every beat.

When a coronary artery becomes blocked (almost always from a ruptured atherosclerotic plaque and the clot that forms at the rupture site), the heart muscle downstream of the blockage is starved of oxygen and begins to die within minutes. This is a heart attack, formally called a myocardial infarction. The longer the blockage lasts, the more muscle dies, and the more permanent the damage.

Blood pressure

Blood pressure is the force blood exerts against the inside of your vessel walls. It's reported as two numbers: systolic (the peak pressure when your heart contracts and pushes blood out) over diastolic (the resting pressure between beats, when the heart is refilling). A typical healthy reading is around 120/80 mmHg.

Blood pressure is determined by two variables:

How much blood the heart pumps per minute (cardiac output)

How constricted the vessels are throughout the body (vascular resistance)

If either rises, blood pressure rises. Most of the moment-to-moment regulation happens at the vascular resistance side, the body widens or narrows vessels to match whatever's needed. The nitric oxide system covered above is the primary mechanism for widening. When NO production drops, vessels stay more constricted, resistance climbs, and pressure climbs with it.

Several systems work simultaneously to keep blood pressure in the right range:

The autonomic nervous system. Sympathetic activation (stress, exercise, threat) raises blood pressure by constricting vessels and speeding up the heart. Parasympathetic activation (rest, sleep, vagal tone) lowers it.

Nitric oxide, the main signal for vasodilation

The kidneys, which control blood pressure by adjusting how much fluid and salt the body keeps. When blood pressure drops, the kidneys trigger a chain of hormones (the RAAS system) that does two things at once: narrows vessels to raise pressure immediately, and tells the body to hold onto more salt and water to raise pressure over the longer term. Most blood pressure medications work by blocking some step in this chain.

Circulating hormones including adrenaline (raises it sharply), cortisol (raises it gradually under chronic stress), and vasopressin (tells the kidneys to retain water)

The danger of high blood pressure

A single elevated blood pressure reading doesn't damage anything. Sustained high blood pressure over years does enormous damage, and it does it through several mechanisms simultaneously:

The endothelium gets physically damaged with every heartbeat as blood slams against the vessel walls harder than they're designed for.

The arteries stiffen because the constant pressure overload triggers structural changes in the vessel walls, replacing flexible elastin with stiffer collagen.

The heart hypertrophies because the left ventricle has to work harder against the increased resistance. Over time the heart muscle thickens and can no longer relax properly between beats.

The smaller vessels in the brain, kidneys, and eyes get progressively damaged by the increased pressure pulses reaching their delicate networks. Long-term untreated hypertension causes kidney disease, vascular dementia, and retinal damage

Most people with high blood pressure have no symptoms at all until they have a stroke, heart attack, or kidney problem. This is why it gets called "the silent killer". But blood pressure is one of the most modifiable cardiovascular risk factors. The biggest levers are body composition (visceral fat in particular drives hypertension), sodium intake relative to potassium intake, sleep, stress management, regular aerobic exercise, and limiting alcohol.

Lipids in the blood

Cholesterol is a waxy, fat-like molecule that every cell in your body needs. It's a structural component of cell membranes, the raw material for steroid hormones (testosterone, oestrogen, cortisol), the precursor to bile acids, and essential for vitamin D synthesis. Your body makes most of its own cholesterol in the liver, because it's too important to rely on diet alone. Dietary cholesterol matters less than people used to think, the liver adjusts its own production to compensate. The bigger dietary drivers of blood cholesterol are saturated fat (which slows LDL clearance) and trans fats (which raise LDL and lower HDL).

Triglycerides are the body's primary form of stored energy: three fatty acid chains attached to a glycerol backbone. The liver packages excess calories into triglycerides for storage in fat tissue, and fat cells release them back into the bloodstream when energy is needed between meals.

Neither cholesterol nor triglycerides dissolve in blood. Blood is water-based, and lipids are oily. They can't just float around freely. The body solves this with a transport system.

Lipoproteins

Since lipids can't dissolve in blood, the liver packages them into spherical particles called lipoproteins. Think of each lipoprotein as a shipping container: a water-friendly shell on the outside, with the hydrophobic cargo (cholesterol and triglycerides) packed inside. The proteins embedded in the shell are called apolipoproteins, and they act as identification tags telling cells what to do with the particle.

There are several types of lipoprotein, defined by their size, density, and cargo ratio. More lipid makes the particle larger and less dense, more protein makes it smaller and more dense.

Chylomicrons are the largest and least dense. They're assembled in the intestinal wall after you eat fat and carry dietary triglycerides into the bloodstream via the lymphatic system. Once they've delivered most of their triglyceride cargo to tissues, the remnants get cleared by the liver.

VLDL (very low-density lipoprotein) is assembled by the liver and is its primary vehicle for exporting triglycerides made internally (from excess carbohydrates via de novo lipogenesis, or from recirculated fatty acids). VLDL doesn't carry only triglycerides though, the liver packages both triglycerides and cholesterol into each particle from the start. The particle is triglyceride-dominant but the cholesterol is in there from the beginning. As VLDL travels through the bloodstream, it gradually unloads its triglyceride cargo to tissues that need fuel (capillaries have enzymes that strip the triglycerides out and hand the fatty acids to nearby cells). The cholesterol stays inside the particle because the cells at those capillary beds are taking fatty acids, not cholesterol. So the particle slowly shrinks as it loses triglycerides, eventually arriving at its final form: LDL.

LDL (low-density lipoprotein) is what VLDL becomes after most of its triglycerides have been delivered. Nothing new has entered the particle. The cholesterol that's now the dominant cargo was always there, it just becomes the majority of what's left. The particle is now smaller, denser, and cholesterol-rich. LDL's job is to deliver cholesterol to cells throughout the body that need it. Cells take it up by expressing LDL receptors on their surface, pulling LDL particles out of the blood, and using the cholesterol inside.

HDL (high-density lipoprotein) runs the transport system in the opposite direction. Where LDL delivers cholesterol outward to tissues, HDL collects excess cholesterol from tissues and returns it to the liver for recycling or excretion. This process is called reverse cholesterol transport, and it's the main reason HDL is considered protective. HDL particles start small and lipid-poor when they're released from the liver and intestine. As they circulate, they pick up free cholesterol from cell membranes throughout the body. The particle grows larger and more spherical as it loads up, eventually returning to the liver to deliver its cargo.

How LDL gets cleared from the blood

The liver is the primary organ that clears LDL from circulation. Liver cells express large numbers of LDL receptors that continuously pull LDL out of the blood. How many receptors the liver puts on its surface depends on how much cholesterol it already has inside.

The liver produces its own cholesterol through an internal pathway, controlled by an enzyme called HMG-CoA reductase. When the liver has plenty of internal cholesterol, it doesn't need to pull much from the blood, so it puts fewer LDL receptors on its surface. When the liver's internal cholesterol pool is depleted, it compensates by upregulating LDL receptors to grab more from circulation.

This is why so many different interventions converge on the same outcome (faster LDL clearance):

Statins (atorvastatin, rosuvastatin) block HMG-CoA reductase directly, reducing the liver's internal cholesterol production and forcing it to upregulate LDL receptors

Soluble fibre (
Psyllium Husk) binds bile acids in the gut, preventing their recycling and forcing the liver to use its cholesterol to make replacement bile acids. Same end result as statins.

Saturated fat works in the opposite direction, increasing the liver's internal cholesterol supply, which reduces LDL receptor expression and slows clearance

Thyroid hormones independently upregulate LDL receptor expression, meaning hypothyroidism can show up as elevated LDL

PCSK9 inhibitors preserve LDL receptors on the cell surface by blocking the protein that normally tags them for degradation

Why LDL becomes dangerous

LDL isn't inherently harmful while it's circulating. The problem starts when LDL particles penetrate a damaged endothelium and get trapped in the artery wall, where they oxidise and trigger the inflammatory cascade that builds plaque.

The progressive buildup of plaques inside artery walls is called Atheroscleros. It's the underlying process behind most heart attacks and strokes, and it develops silently over decades before it produces symptoms.

Two factors determine the danger: how many LDL particles are circulating (particle count, which is what ApoB measures), and how long they've been circulating (years and decades of cumulative exposure).

You can reduce how many LDL particles are circulating (through diet, body composition, statins, PCSK9 inhibitors), which lowers the supply of particles available to penetrate damaged endothelium. And you can reduce how much damage and oxidation is happening in the wall (through sleep, stress management, body composition, omega-3 intake, controlling blood pressure and blood sugar), which lowers how much of the LDL that does get in actually becomes a problem.

ApoB > LDL on bloodwork

Every LDL particle has exactly one molecule of apolipoprotein B-100 (ApoB) on its surface. Every VLDL particle also has one. Every IDL and Lp(a) particle has one. So ApoB is a direct count of the total number of atherogenic particles in your blood.

Standard LDL cholesterol (LDL-C) on a blood test measures the total amount of cholesterol carried inside LDL particles. But LDL particles vary enormously in size and cholesterol content. A person with many small, cholesterol-poor LDL particles can have a "normal" LDL-C reading but a high ApoB, meaning they have more atherogenic particles than the cholesterol number suggests. A person with fewer but larger, cholesterol-rich LDL particles can have a higher LDL-C reading but a lower ApoB. Since it's the particles themselves that penetrate the artery wall, particle count (ApoB) is a more accurate predictor of cardiovascular risk than cholesterol content (LDL-C).

This discordance between LDL-C and ApoB is common in people with metabolic syndrome, insulin resistance, and high triglycerides. These conditions favour the production of small, dense LDL particles. The reader often gets told their LDL is "fine" based on standard testing while carrying a significantly elevated atherogenic particle burden.

The reverse pattern, a high LDL-C reading with normal ApoB, can happen in people who are metabolically healthy, lean, insulin-sensitive, and eating higher-fat diets. Saturated fat tends to increase LDL particle size, so a high-fat-diet metabolically-healthy person can end up with fewer but larger, cholesterol-rich LDL particles. The cholesterol number looks high, but the particle count is fine.

For most people, if LDL-C goes up, ApoB goes up too. But if your lipid panel comes back with numbers you want to act on, getting ApoB tested before making changes gives you a clearer picture of whether the particle burden actually matches what the cholesterol number suggests.

The same logic applies to HDL. HDL-C measures how much cholesterol HDL particles are carrying. ApoA-I would count the actual particles (each HDL has one ApoA-I on its surface). ApoA-I testing exists but never got the same clinical adoption. The more important issue is that HDL particle quality matters more than quantity, whether the HDL is actually performing reverse cholesterol transport, carrying active PON1, and suppressing endothelial inflammation. You can have high HDL-C with dysfunctional HDL that isn't doing its job.

Low HDL isn't dangerous in the same direct way that high LDL is. LDL particles physically penetrate the artery wall and start the atherosclerotic cascade. HDL doesn't have an equivalent damage mechanism. Instead, low HDL means several protective systems are weakened simultaneously.

Less reverse cholesterol transport. HDL is what pulls excess cholesterol back out of tissues, including out of the macrophages in artery walls that are filling up with oxidised LDL. With less HDL doing this job, foam cells form faster because the cholesterol accumulating in macrophages isn't being extracted as efficiently.

Less antioxidant protection. HDL carries PON1, the enzyme that protects LDL from oxidation. Less HDL means less PON1 circulating, which means more of the LDL that gets trapped in the artery wall becomes oxidised and triggers the immune response.

Less endothelial protection. HDL suppresses the sticky proteins on the endothelium that recruit monocytes into the vessel wall. With less HDL, the endothelium becomes stickier, more immune cells get recruited, and the inflammatory cycle accelerates.

HDL is best interpreted as context rather than a standalone target. Low HDL alongside high triglycerides and high ApoB is the metabolic syndrome pattern, and it's genuinely dangerous because everything is moving in the wrong direction. Low HDL in isolation, with good ApoB and low triglycerides, is a very different and much less concerning picture.

Triglycerides

Triglycerides in the bloodstream come from two sources: dietary fat carried by chylomicrons, and triglycerides made by the liver and exported via VLDL. The liver makes more triglycerides when it's receiving more energy substrate than the body is burning, whether from excess carbohydrates (de novo lipogenesis), from recirculated fatty acids from fat tissue, or from insulin resistance impairing the normal clearance pathways.

High triglyceride levels don't damage arteries directly. The reason they matter for cardiovascular risk is what they do to the rest of the lipoprotein system. When the liver is producing lots of triglyceride-rich VLDL, two things happen that make the entire lipid picture worse.

First, lipid particles trade cargo with each other.

When the liver is producing a lot of triglyceride-rich VLDL, those VLDL particles are floating in the bloodstream alongside LDL and HDL particles full of cholesterol. A protein called CETP moves between these particles and physically swaps cargo: it takes triglycerides out of VLDL and dumps them into nearby LDL or HDL particles, while pulling cholesterol out of those particles and putting it into the VLDL in return.

The LDL and HDL particles that received triglycerides from the swap now have something inside them they're not built to carry for long. When they pass through the liver, an enzyme called hepatic lipase breaks the triglycerides down. The particles are left smaller and emptier than they started.

For LDL, "smaller and emptier" means small, dense LDL forms, the dangerous variant that's better at slipping through the endothelium, more easily oxidised, and more aggressive at building plaque.

For HDL, "smaller and emptier" means the kidneys clear it from the bloodstream faster. HDL levels drop as a result.

This is why high triglycerides, small dense LDL, and low HDL almost always show up together. They're three downstream effects of the same upstream cause: too much VLDL leaving the liver.

This is the mechanistic link between high triglycerides, low HDL, and small dense LDL. They look like three separate problems but they're actually one.

Second, triglyceride-rich remnant particles are themselves atherogenic. The leftovers from partially metabolised VLDL and chylomicrons can penetrate the endothelium and contribute to plaque formation directly, without needing to be oxidised first, because they're small enough to enter the artery wall but too large for the macrophage cleanup systems to handle efficiently.

The condition this whole pattern describes is atherogenic dyslipidaemia, the lipoprotein profile of metabolic dysfunction.

Lipoprotein(a)

Lp(a) is a variant of LDL with an additional protein called apolipoprotein(a) attached to the ApoB on its surface. It's genetically determined, your Lp(a) level is set almost entirely by your genes, and it doesn't respond meaningfully to diet, exercise, or most medications. You're largely born with whatever level you have.

Lp(a) is harmful for the same reason LDL is, it carries cholesterol into the artery wall and contributes to plaque buildup. But it has a second problem on top of that: Lp(a) interferes with the body's ability to dissolve blood clots. Once a clot forms (for example, at a ruptured plaque), Lp(a) makes that clot harder to break down. So the same particle that builds the plaque also stabilises the clot that forms when it ruptures. That's what makes Lp(a) particularly dangerous, it's working on both sides of a heart attack at once.

Lp(a) is worth knowing about because roughly 20% of the global population has Lp(a) levels high enough to meaningfully increase cardiovascular risk, and most of them don't know it because Lp(a) isn't included in standard lipid panels. It only needs to be tested once in a lifetime (since it doesn't change), and it can explain why some people with otherwise clean lipid panels develop cardiovascular disease.

How hormones affect lipids

Several hormones directly influence the lipoprotein system, which is why hormonal changes show up on lipid panels:

Testosterone increases hepatic lipase activity (which breaks down HDL), raises VLDL production, and reduces LDL receptor expression.

Oestrogen does roughly the opposite. It increases LDL receptor expression (faster clearance, lower LDL), stimulates HDL production, and has direct anti-inflammatory effects on the vessel wall.

Thyroid hormones upregulate LDL receptor expression. Hypothyroidism slows LDL clearance.

Insulin resistance drives the entire atherogenic dyslipidaemia pattern described in the triglyceride section above. The liver overproduces VLDL, triglycerides rise, CETP-mediated swapping accelerates, small dense LDL accumulates, and HDL clearance speeds up. The classic metabolic syndrome lipid pattern (high triglycerides, low HDL, small dense LDL, high ApoB despite normal LDL-C) is fundamentally an insulin resistance pattern.

Arterial stiffness

Healthy arteries are elastic. They expand with each heartbeat to absorb the pressure wave from the left ventricle, then recoil between beats to push blood forward. This elasticity keeps blood flowing smoothly between contractions, and it buffers the rest of the cardiovascular system from the peak pressure of each heartbeat.

With age, arteries gradually stiffen. The elastin in the vessel wall degrades and gets replaced by stiffer collagen, and that collagen accumulates damage of its own from glycation, oxidative stress, and inappropriate cross-linking.

The vascular consequences are significant. Stiffer arteries can't absorb the pressure wave from each heartbeat. The wave travels faster, reaches the smaller vessels in the brain and kidneys with more force, and reflects back to the heart sooner, adding to the next beat's pressure instead of being absorbed. The result is rising systolic blood pressure, widening pulse pressure (the gap between systolic and diastolic), and progressive damage to the delicate microvasculature in the brain, kidneys, and eyes.

Stiffness is measured by pulse wave velocity (PWV), the speed at which a pressure wave travels along an artery. Higher PWV means stiffer vessels. In some long-term studies, PWV predicts cardiovascular events better than blood pressure alone, because it captures vascular aging directly rather than the resulting pressure.

Arterial stiffness is partially modifiable. The elastin you've lost doesn't really come back, but the rate at which you lose more is responsive to blood pressure control, blood sugar control, anti-inflammatory inputs, and regular aerobic exercise. People who train consistently into their 50s and 60s have measurably more compliant arteries than sedentary peers.

The lymphatic system

The lymphatic system runs alongside the venous system and handles the fluid that escapes from capillaries into surrounding tissues. About 20 litres of plasma is filtered through capillary walls daily; roughly 17 litres gets reabsorbed back into the bloodstream. The remaining 3 litres is collected by the lymphatic vessels and returned to circulation, eventually draining into large veins near the heart.

Without functioning lymphatics, fluid would accumulate in tissues (oedema), and the immune system would lose one of its primary surveillance networks. Lymph nodes scattered through the system filter the fluid and house immune cells that monitor for pathogens, which is why nodes swell during infections.