blood flow

A swimmer in a pool filled with light balls pushes off them and swims faster than in clear water. That is roughly how a trypanosome, a single-celled parasite that causes sleeping sickness, behaves when it enters blood, where red cells occupy nearly half the volume.

Every heartbeat sends a wave through the cerebral arteries that not only pushes blood but also briefly inflates the vessel. The arterial wall, like a tightly stretched rubber tube, passes that motion to the neighboring brain tissue, and the brain pulses ever so slightly in time with the heartbeat.

In a quiet room of a Budapest laboratory, a bracelet lies on the table, and instead of a strap, it has a thin conductive thread stretched in a zigzag. It doesn't heat up or glow, but when pressed against the wrist, it starts to listen. Each heartbeat sends a pressure wave through the radial artery, and the thread, deforming, changes its electrical resistance.

A drop of blood from a fingertip lands on the white membrane of a cartridge. Twenty minutes later, the phone shows whether Lyme disease is present or not. No test tube, no centrifuge, no waiting for days. Usually, for such a test, blood is first spun in a centrifuge to separate the liquid part — the plasma.

Epithelium lines our organs like tiles on a floor. At rest, cells cling to each other, and the tissue is elastic. But during embryonic development, wound healing, or tumor growth, this layer begins to flow like thick honey. What exactly triggers such softening?

During surgery, an anesthesiologist watches a screen where pressure and pulse curves run. But behind these lines hide several other quantities: how much the vessels constrict, how fast the blood flows, how elastic the artery walls are. Precise instruments for measuring them are only used in the most severe cases because it's risky — a needle in the artery, a catheter in the heart.

A bacterium swims toward food. A chemical signal guides it forward, but with each movement the signal changes. A loop emerges: sensation, action, new sensation. The authors turned this loop into a measurable quantity.

At night, when we sink into deep, dreamless sleep, a tide begins in the depths of the brain. Each heartbeat and each breath sends gentle pulses through the fluid-filled cavities (the ventricles). During the day, these pulses are timid and monotonous — the fluid sloshes back and forth almost in place. But with the onset of slow-wave sleep, the rhythm changes.

A disease outbreak is like a wildfire: the fire runs through dry grass, and its speed depends on wind and humidity. But an infection has its own 'wind'—how often people infect each other. When this rate changes along with the fraction of already infected, the epidemic wave can behave differently.

A subway train races through a tunnel, yet passengers in the last car barely sway. Blood flow in the lungs works much the same way: the heart pumps blood in powerful spurts, but by the time it reaches the veins, the pulsation has almost vanished.

A camera flash that fires in a thousandth of a second doesn't heat the subject — unlike a regular flash that lasts a full second. A similar trick has been found in radiotherapy: when a radiation dose is delivered in milliseconds instead of minutes, healthy tissues suffer less. This is called the FLASH effect.

The brain self-cleans its waste: cerebrospinal fluid flows through narrow channels surrounding arteries. The channel walls are soft and ripple with each pulse, and the tissue is porous like a sponge. Every heartbeat turns arteries into tiny pumps, driving fluid through the pores.

In calm water, the route is clear. But in turbulent flows, a robot needs a different strategy. Typical approaches demand an accurate map of currents, but in reality, you can't get one. Nature does it differently: fish feel pressure and vortices right against their bodies.

Table salt is one of the most studied substances in medicine. For decades, students were taught Arthur Guyton's model: eat something salty — blood dilutes, volume temporarily increases, kidneys promptly excrete the extra water, and everything returns to square one. A neat scheme, like a float valve in a toilet tank.

There's a leak in the garden hose. You patch the hole with duct tape, but the water presses harder and harder, and sooner or later the patch gives way. A growing clot in a blood vessel behaves much the same way. When the vessel wall is damaged, tiny blood platelets rush to the scene.

Millions of cilia, thin hairs, work inside the tiny ducts of our bodies. They beat in synchrony, like the oars of a galley, pushing viscous mucus or fluid along. The authors of a new study propose viewing the ciliary carpet as an active porous medium — something like a sponge that squeezes and releases itself, driving water through.

A porous implant filled with a drug works like a sponge that slowly releases the substance into the surrounding fluid. But if the fluid flows quickly, it washes out the drug differently than in a calm environment. The authors computed on a computer how different flow rates and properties of the porous structure affect drug release.

When a person breathes, air doesn't flow in a steady stream but in waves. Previously, doctors and scientists only looked at overall numbers: how much air entered per breath, what the peak flow was. But within a single breath, there are nuances — small variations that speak to the workings of nerves and muscles. The new approach breaks a breath into parts, much like a musician breaks a chord into individual notes.

A cell about to divide first marks its waist. Beneath its membrane, in a thin gel of protein filaments, movement begins. The filaments flow like a slow river, and a belt forms in the middle of the cell. This belt tightens more and more until the cell is pinched in half. Scientists built a computer model of this process.

A drop of water on a table usually sits still. But make the table slightly softer on one edge and stiffer on the other, and the drop will crawl toward the stiffer side. This phenomenon is called durotaxis: the liquid itself chooses where to flow, without pumps or electricity. This trick works because the surface under the drop is not uniform.

Your finger is a living sensor: beneath the skin, arteries pulse in time with your heart. Traditional blood pressure cuffs squeeze your arm and give a single number, but this ring uses eight electrodes to pass a weak current through the tissue and map blood flow.

In porous materials (soil, rocks, filters) fluids move through tiny, winding channels. When two immiscible fluids, such as water and air, are present simultaneously, their interface constantly shifts: air bubbles push water out of some pores and open paths through others.

A thrombus is not just a clump of cells. When a vessel wall is damaged, platelets stick together into a plug, but it is still fragile. What makes it strong is fibrin — a protein that forms a network of fibers, like rebar in concrete. The model showed that the plug's density decides where this network appears.

Inside the kidneys, liver, and other organs, fluid flows through channels that squeeze and release like a hose underfoot. The walls of these channels are not rigid: they are porous, elastic, and soaked with moisture.

A powder avalanche isn't just a pile of snow sliding down a mountain. It's a cloud racing downhill at car speed. Inside, snow doesn't lie still: it flies: thousands of tiny particles are caught up in air vortices. Until now, no one had seen what actually happens to each snowflake in this chaos. Now scientists have the first footage.

Lymphatic vessels are one-way streets. Through them, fluid flows only forward thanks to tiny flap valves. These valves open in just one direction, creating asymmetry, directionality, without which the flow would stop. In the lab, scientists modeled a vessel and sent compression waves along it, like a parade of gentle nudges.

Each breath slightly speeds up the heart, and each exhale slows it down. This rhythm, visible on an ECG, is called respiratory sinus arrhythmia. For a long time, it was dismissed as a mere side effect of the nervous system. But new research shows that this coordination is not a coincidence: it's a way to save energy.

A cell is a city with gates. Each gate is a protein channel in the membrane: through it, ions, sugars, and signaling molecules pass in and out. Some gates are almost always open, while others open and close randomly, like subway doors that let a passenger through one moment and slam shut in their face the next.

Cilia are tiny hairs on cell surfaces. They beat like oars, moving fluid. In the lungs, they push dust and microbes out. But their lengths vary: in humans, they are thousandths of a millimeter; in comb jellies, several millimeters. How do such different cilia work together? Scientists built a machine that jerks fluid sharply, as if a cilium had struck. They filmed it with a high-speed camera.

The heart doesn't contract all at once but as a wave: an electrical signal travels through the muscle, and behind it, like ripples spreading on water, a contraction rolls through. In arrhythmia, this wave goes awry: it stalls in one spot, loops in another, and the pump starts working in a disorganized way.

The bladder is like a balloon: as it fills, the wall stretches and the shape changes. Doctors usually monitor the pressure inside and how much fluid has entered, but they rarely look at the geometry itself — how the surface curves. Yet irregularities can appear before the usual metrics deteriorate.

Water in a narrow gap crawls on its own: that's the capillary effect that pulls sap up a stem. But if it hits a step, the liquid can freeze, like a traveler before a high threshold. Scientists built tiny glass channels and watched when a drop steps over the ledge and when it gets stuck.

The lymphatic system is like a city's network of canals, through which water slowly moves, carrying away waste from tissues. But it has no single pump-heart. Instead, along the vessels are placed small station-locks — lymphangions. Each such lock squeezes itself like a fist and pushes fluid onward. How they coordinate their work has long remained a mystery.

A lung scan reveals a tree: the trunk is the trachea, branches are the bronchi, and leaves are the alveoli. But how does air spread through these branches? Scientists built a computer model that replicates the lung's shape from images obtained via computed tomography.

Fluid flows in the narrow gaps between brain cells. It doesn't stay still: each heartbeat pushes it forward, and breathing adds its own waves. This was seen in images ten years ago, but only now have researchers calculated how this pulsing accelerates molecules. If you drop ink into a stream, it drifts with the current. But if the stream speeds up and slows down, the ink spreads wider.

Water in a circular hose, pumped around by a pump, won't stop even if you pinch one section: the flow finds a detour and keeps going around. That's roughly how some parts of our circulatory system are arranged — not as single pipes but as closed loops. Scientists built a computer model of blood flow and compared two scenarios: an open vessel and a closed loop.

A thin plate stands across a flow of water or air. Behind it, a wake forms — a region where the flow becomes tangled and mixed. Usually, such a wake becomes turbulent when the flow speed is high enough, quantified by the Reynolds number.

A bacterium swims like a tiny motorboat with a propeller — except its propeller is a long flagellum that rotates and pushes the body forward. But the body doesn't move in a straight line; it sways like a bobber on a wave. This swaying isn't random: it's part of the mechanics of motion. Until now, such swaying was studied in still water.

Water swirls into a ring, and that ring flies through the bulk without breaking apart. Such vortex rings, from smoke rings to underwater currents, follow laws that are hard to capture on paper. Usually, the flow is divided into millions of cubic cells, and each one is tracked.

Inside a fertilized frog egg, waves ripple, like ripples on water, but made of proteins that command cell division. These waves travel millimeters, even though the egg itself is less than a millimeter across. Previously, it wasn't understood why they didn't fall out of sync. It turns out the waves clear their own path.