How the Frog Heart Works: Anatomy and Physiology

A frog heart has three chambers, two that receive blood and one that pumps it, where a human heart has four. One shared pumping chamber might sound like a recipe for a well-stirred mix of fresh and used blood, but that is not what happens.¹ The heart’s inner shape steers the two streams so that most oxygen-rich and oxygen-poor blood still go their separate ways. The single chamber also turns out to be useful, because it lets a frog redirect blood away from its lungs when it doesn’t need them.

The Parts of a Frog Heart

Blood returning from the body first gathers in a thin-walled sac called the sinus venosus, which sits against the right atrium.² From there it passes into the right atrium. An atrium (plural atria) is a receiving chamber, and the frog has two. Oxygen-rich blood coming back from the lungs enters the left atrium.³ A solid wall between the two atria keeps them fully separate.⁴

Both atria empty through a shared opening into one muscular pumping chamber, the ventricle.⁵ Its inner wall isn’t smooth: ridges of muscle fill it with a web of small connected pockets, which gives it a spongy look.⁶ Blood leaves through a single exit, a tube called the conus arteriosus that rises from the right side of the ventricle.⁷ Inside this outflow cone runs a long twisting fold called the spiral valve. From the cone, blood heads into the arteries to the body and the arteries to the lungs and skin.⁸

The sinus venosus works as a reservoir, letting blood from the big veins pool briefly before entering the right atrium, so the atrium fills smoothly rather than in surges. It also houses the cells that set the heart’s rhythm, so this fragile-looking sac is where every heartbeat starts.

Humans have two atria and two ventricles, so each side of our heart pumps separately. The frog’s two atria match ours, but its single ventricle replaces our two. We have no separate collecting sac either; during development it became part of our right atrium.

How Blood Flows Without Mixing Much

The right atrium carries mostly oxygen-poor blood, though some of it is fresh blood from the skin, which also takes in oxygen.⁹ The left atrium carries only oxygen-rich blood from the lungs.¹⁰

Because both atria share one opening into the ventricle, some mixing does happen.¹¹ But the small pockets formed by the muscle ridges keep the streams from blending evenly.¹² The outflow cone also starts on the right side of the ventricle, where oxygen-poor blood dominates.¹³ As the heart squeezes, the spiral valve moves and directs blood into either the lung-and-skin arteries or the body arteries.¹⁴

The order of a single heartbeat shows how this plays out. The two atria contract first, squeezing their blood into the ventricle through the shared opening. The ventricle contracts a moment later, and that squeeze pushes blood into the outflow cone. Oxygen-poor blood sits nearest the cone’s opening, so it leaves first, and the spiral valve sends it toward the lung-and-skin arteries. As the squeeze continues, pressure rises and the later blood, richer in oxygen, is steered into the arteries to the body.

Why Frogs Have Three Chambers and Humans Four

Fish have two chambers, one atrium and one ventricle.¹⁵ Amphibians have three, with two atria that may be partly or fully divided depending on the species.¹⁶ Crocodiles, birds and mammals have four.¹⁷

A frog’s lifestyle fits its middle design. A fish has gills only, so one loop through the heart is enough. A frog breathes with lungs and skin, so it needs a separate route to its lungs. Because the skin also delivers oxygen, the slight mixing of blood costs the frog less than it would cost us. And since frogs are cold-blooded and use less oxygen than mammals, they can manage with less complete separation.

Sending Blood Away From the Lungs

When a frog dives and stops breathing, its heart sends less blood to the lungs.¹⁸ A lung full of stale air underwater offers little oxygen, so pumping blood there wastes effort. In a related salamander-like amphibian, about 60% of the heart’s output went to the body while it held its breath, but only about 20% while breathing.¹⁹ The vagus nerve helps control this by changing heart rate and how hard it is for blood to flow into the lung-and-skin artery.²⁰ Exactly how the brain sets this control is still poorly understood.²¹

The frog works through resistance, making it harder or easier for blood to flow into the lung-and-skin arteries. When that resistance goes up, more of the ventricle’s blood takes the easier path into the arteries to the body. When it falls, more blood flows toward the lungs and skin again. Because the outflow cone and spiral valve guide where each stream goes, a change in resistance can shift the balance of flow even though the structure stays the same.

A four-chambered heart can’t do this easily. In humans the right ventricle pumps only to the lungs, so lung flow can’t be cut sharply.

What Makes the Frog Heart Beat

Each beat begins in pacemaker cells in the sinus venosus, which fire on their own.²² No single process sets the rhythm. Several charged particles, including calcium and potassium, flowing in and out together make the cells fire repeatedly.²³ Each cell’s electrical charge drifts upward by itself after a beat until it reaches a threshold. At that point the cell fires, the signal spreads across the heart muscle, and the muscle contracts. The cell then resets and the drift begins again, producing a steady rhythm.

Frog heart muscle gets enough calcium from outside its cells in a single beat to contract fully.²⁴ Mammal heart muscle relies more on calcium stored inside the cells. That is why it took 5 to 7 beats to recover full force after a calcium blocker wore off, while the frog recovered in one.²⁵ It also explains why a frog heart keeps beating outside the body: the pacemaker is in the heart itself, so no brain signal is needed, and a simple salt bath supplies the calcium.

Nerves turn the rate up and down. The sympathetic nerves speed the heart by making pacemaker cells reach their firing point faster.²⁶ The vagus nerve, a long nerve running from the brain to the heart and other organs, slows it.²⁷ In 1921 Otto Loewi showed with two frog hearts that the vagus nerve slows the heart by releasing a chemical, later identified as acetylcholine.²⁸

His experiment was simple. He kept two frog hearts beating in salt solution and stimulated the vagus nerve of the first, which slowed. He then moved the fluid bathing it onto the second heart, and that heart slowed too, so something in the fluid had carried the signal.

How Temperature, Meals and Freezing Change the Heart

As a cold-blooded animal warms, its heart generally beats faster. A frog’s body temperature follows its surroundings, so warm water warms the heart itself, speeding the chemical and electrical processes in the pacemaker cells so they reach their firing point sooner. The effect has limits. In bullfrogs, pulse was clearly highest at 33 °C, a hot temperature for them.²⁹

Warmth changes more than the pulse. In toads, the reflex that adjusts heart rate to blood pressure became two to three times stronger between 15 and 30 °C.³⁰ When pressure changes, the heart rate shifts to correct it, so a warm toad corrects those changes more strongly than a cool one. Temperature’s effects also mean any single “normal frog heart rate” depends heavily on temperature, unlike ours, since humans keep a steady body temperature and resting heart rate changes little with weather.

Meals matter too. After a meal, a bullfrog’s heart speeds up for more than a day because the slowing pull of the vagus nerve fades, dropping from about 26% to 6%.³¹

Wood frogs take the heart’s flexibility furthest. They survive winter with much of their body frozen, their heart stopping until thaw.³² They flood their organs, including the heart, with glucose, a sugar that acts like antifreeze.³³ Frogs given extra glucose survived freezing to -5 °C while untreated frogs did not.³⁴ Freeze-tolerant frogs also dry out their organs slightly and delay heart failure as they freeze.³⁵

As ice forms on the skin, the heart briefly beats faster and glucose pours into the blood and organs. As freezing deepens, the heart slows and eventually stops. On thawing, it restarts and the frog revives.

From Tadpole Heart to Frog Heart

The heart starts as a simple tube in the embryo, then loops and divides into compartments.³⁶ The loop brings the future atria and ventricle into their correct positions, and constrictions then split the tube into chambers. In frogs the atria divide later than the ventricle’s structure forms, so the tadpole heart is built in stages. During metamorphosis the single atrium is fully divided into right and left atria.³⁷

Early tadpoles already have two separate exits from the ventricle, ready to split blood toward the lungs before the lungs even form.³⁸ They breathe with gills and skin, so they have no use for lung blood flow yet. But the pathway is laid out, and when the lungs do develop, blood has an existing route to reach them.

Healing changes along the way. Tadpoles can regrow damaged heart muscle efficiently, but they lose this ability during metamorphosis, a change linked to thyroid hormone.³⁹ Adult frogs left with heart damage still had scar tissue nearly a year later.⁴⁰ Salamanders and zebrafish, by contrast, can regrow heart tissue as adults.⁴¹

Scientists care because human hearts also scar rather than regrow after a heart attack. Frogs show both abilities in one animal’s life, before and after metamorphosis, and comparing the two stages may reveal what switches regrowth off.

The Frog’s Extra Hearts for Lymph

Frogs have two pairs of small lymph hearts located near the back.⁴² As blood moves through the smallest vessels, some of its fluid leaks into the surrounding tissue. They pump lymph, the clear fluid that leaks out of blood vessels, back into the veins.⁴³

Frogs need these pumps because they have large lymph sacs under the skin instead of the fine lymph vessels humans have.⁴⁴ Their thin, moist skin and big sacs hold a lot of fluid. Humans move lymph by muscle squeezing and valves, without any pump. For a frog that loses water through its skin, the lymph hearts help restore blood volume after drying out or bleeding, though how well differs between species.⁴⁵

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