Is the heart the same in all animals? Human hearts have four chambers and beat about 72 times a minute. A hibernating groundhog’s heart beats just five times and a hummingbird’s heart reaches 1,260 beats per minute during flight.

The pygmy shrew is the smallest known mammal—it can fit onto the thumbnail of an adult. Its heart beats at 1,300 beats per minute. The heart keeps the metabolism of the animal so high that it can starve to death in an hour without food. ’Grizzly bears hearts beat at 40 times a minute and slow down to eight beats when asleep.

The human heart weighs about 300 gms in men and 200 gms in women, but a giraffe’s weighs about 12 kgs since the organ needs to be powerful enough to pump blood up the animal’s long neck. How do giraffes get the blood all the way up that neck when the animal is standing erect and yet keep the blood pressure from blowing out the giraffe’s brains when the animal lowers its head, for example to drink water. After all, the giraffe’s heart pumps blood under the highest known pressure of any mammal—280/180 millimetres of mercury, more than twice the pressure in a healthy human. The giraffe has a system of valves in the blood vessels of its neck that regulate the flow to the head, reducing pressure when the

animal dips for a drink or any other purpose.

The cheetah’s heart can go from a resting low of 120 beats per

minute to 250 within seconds during a hard run and is relatively large, so it can really pump out blood when needed. A cheetah can accelerate from 0 to 100km per hour in 3 seconds.

Of chambers

Mammals and birds all have four-chambered hearts two atria and two ventricles. The heart keeps oxygenated blood and deoxygenated blood in separate chambers.

Frogs only have three chambers. Grooves, called trabeculae, keep the oxygenated blood separate from the deoxygenated blood in its one ventricle. Frogs get oxygen not only from their lungs, but also from their skin. The frog’s heart sends deoxygenated blood from the right atrium into the ventricle and out to the lungs and skin to get oxygen. Oxygenated blood comes back to the heart through the left atrium, then into the ventricle and out to the major organs.

The blue whale’s heart is the size of a small car and has been weighed at about 430 kgs. The walls of the aorta, the main artery, are over nine inches thick. This mass of muscle pumps the seven tonnes of blood—which is more than the weight of an adult elephant—but it beats only seven times a minute.

The octopus, squid and cuttlefish, have three hearts each. Two hearts on either side of the body oxygenate blood by pumping it through the blood vessels of the gills, and the heart in the centre of the body pumps oxygenated blood from the gills through the rest of the organism. These creatures are blue-blooded because they have copper in their blood. Human blood is red because of the iron

in haemoglobin.

Cockroach, earthworm

The cockroach’s heart does not move around oxygenated blood. Cockroaches breathe through surface openings in the bodies instead of lungs, so the blood doesn’t need to carry oxygen from one place to another. Instead, the blood carries nutrients and is white or yellow. The heart doesn’t beat by itself, either. Muscles in the cavity expand and contract to help the heart send the blood to the rest of the body.

The earthworm doesn’t have a heart. It has five pseudo-hearts that wrap around its oesophagus. These pseudo-hearts don’t pump blood, but rather squeeze vessels to help circulate blood throughout the worm’s body. It also doesn’t have lungs, but absorbs oxygen through its moist skin. Air dissolves in the skin mucous, and the oxygen is drawn into the cells and blood system where it is pumped around the body.

Heart disease kills more humans than any other disease. Once the heart is damaged it is impossible to regenerate it. Humans can only do that with their livers. But if a zebrafish has a broken heart, it can simply regrow one. A study published in 2002 in the journal Science found that zebrafish can fully regenerate heart muscle just two months after 20 percent of their heart muscle is damaged.

Fish have unique hearts. They have one atrium and one ventricle, but they also have two structures that aren’t seen in humans. The ‘sinus venosus’ is a sac that sits before the atrium and the ‘bulbusarteriosus’ is a tube located just after the ventricle.As in other animals, the heart drives blood throughout the body. Deoxygenated blood enters the sinus venosus and flows into the atrium. The atrium then pumps the blood into the ventricle.

The ventricle has thicker, more muscular walls, and pumps the blood into the bulbusarteriosus. The bulbusarteriosus regulates the pressure of the blood as it flows through the capillaries surrounding the fish’s gills. It is in the gills where there is oxygen exchange across cell membranes and into the blood. The fish need the bulbusarteriosus to regulate blood pressure because the gills are delicate and thin-walled and can be damaged if the blood pressure is too high.

Fish and bears

A tuna fish’s heart continues to beat even when the fish deep-dives to 1,000 feet below the ocean surface at temperatures that would stop a human heart. Tuna fish are warm bodied but their heart runs as all fish at ambient temperatures. When tunas dive down to cold depths, their body temperature stays warm but their heart temperature can fall by 15 degree celsius within minutes. The heart is chilled because it receives blood directly from the gills which mirrors water temperature. This clearly imposes stress upon the heart but it keeps beating, despite the temperature change. In most other animals, the heart would stop.

When black bears are sleeping, their heart rates increase whenever intruders get close. It’s as if their hearts have electronic devices that increase their detection beep sounds even if the brain is asleep. On an average their hearts beat 200 times a minute, rising to 250 when a human or a predator animal gets close to them during their sleep.For half of the year, bears are asleep and during this time their heart rate falls to as low as 14 beats per minute.

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