Have a Heart

 

Figure 1. Shark Heart (author's original image)

Hearts have been on my mind lately. 

 

We, humans that is, of course, have a four-chambered heart. We have two atria and two ventricles. The right side carries blood low in oxygen, and the left side, having just returned from the lungs, carries blood high in oxygen. We share this heart anatomy with the other mammals, birds, and crocodiles. 

 

Found in most amphibians and some reptiles is a three-chambered heart with two atria and one ventricle, which allows mixing of oxygenated and deoxygenated blood. That is a bit freaky in my view: oxygenated blood and deoxygenated blood mix in the central chamber; whoever thought this was a good design? 

 

Most animals in the kingdom do not have hearts with 3 or 4 chambers. The most speciose vertebrates, scaly fishes, have a two-chambered heart. Suppose we follow the blood flow in 18-spined sculpin (AKA long-horned sculpin), Myoxocephalus octodecimspinosus. In that case, we find that blood returning from a trip around the body, after dropping off oxygen to the muscles and organs, enters the atrium via the sinus venosus, a bulbous vein. Blood then flows into the ventricle, and when this thick-walled chamber contracts, blood flows into another bulb, the bulbus arteriosus, as it enters the arteries. Blood then, and only then, flows to the gills. So, blood in a fish's heart is low in oxygen. After getting reoxygenated, the blood leaving the gills does not increase its pressure via another heart chamber. The pressure already conveyed by the ventricle before the gills must suffice. 

Figure 2. Blood flow in fish. (Stolen from a textbook).

This design gives fish a much lower blood pressure than humans. We have a target pressure of 120/80. The 120 is the high-pressure, systolic pressure, brought on by the contraction of the left ventricle. In rainbow trout, a systolic pressure of about 40, well, 38.7 mm Hg was found. While in the bowfin, Amia calva (a super cool fish), it was observed to be even lower at 27.7 mmHg. AND the very chill Greenland Shark had a blood pressure of 17-20 mm Hg. I need to learn something from this chill shark because my blood pressure runs high, around 150.

Figure 3. Smooth dogfish heart. (Author's photo).

Side note: Don’t get me started on the silliness of still measuring blood pressure in mm Hg, a leftover from when air pressure was measured with a bowl of mercury and a tube. The air pressure pushed down on the mercury in the bowl and up the tube, presumably with ticks at each mm. First off, who goes around carrying a bowl of mercury? Second off, …wait, didn’t I say don’t get me started? :-) 

 

Back to hearts

If we go further afield, we find an array of hearts that are less similar. Tunicates (AKA sea squirts) are invertebrate chordates with a notocord but no vertebral column. They are, no doubt, similar to the ancestral organism that gave rise to vertebrates. The study of the tunicate heart is essential for understanding the evolution of the vertebrate cardiovascular system. Although the adult heart has a different structure, its fundamental tubular design shares similarities with the early embryonic hearts of vertebrates. This highlights the deep evolutionary relationship between tunicates and vertebrates. 

 

The tunicate heart is a simple, tubular organ with a unique ability to periodically reverse the direction of its beat, pumping blood throughout its body in alternating directions. The tunicate heart will beat for several minutes in one direction, pumping blood toward the branchial basket (its filter-feeding organ that also captures oxygen from the water), and then pause briefly before reversing to pump blood toward the body's organs (viscera). Why does it do this? Its system is semi-open, with vessels of sorts, but they do not connect back to the other vessels, so it does not have a complete circulatory system. 

 

Tunicates appear to have two pacemakers, one at each end of the tube-like heart. These pacemakers work in tandem, alternating pumping, to initiate and control the wavelike (peristaltic) muscle contractions that propel blood. In humans, there is one pacemaker in the left atrium.

 

The whole mechanisms and functions of the heart in tunicates are not textbook material. Most of this information is recent. Some work in 2020 modeled heart reversals, and by 2024, clarity on how pacemakers function was added to the wealth of scientific knowledge. As is the case, some older works, for example, Krijgsman's 1956 work, initiated questions and investigations into tunicate hearts.

 

Do other animals have reversible hearts?

Yes, some insects. In the geneticist’s favorite animal, the fruit fly Drosophila, the tubular heart reverses its beat to circulate hemolymph (the insect equivalent of blood) throughout the body. The reversal can occur regularly or in response to factors like back pressure.

 

Yummy leeches have two muscular tubes called hearts running along their length. At any given moment, the two hearts beat differently, with one showing a backward-to-forward peristaltic motion and the other near-synchronous contractions. They switch roles every few hundred beats.

 

Horseshoe worms (Phoronids) have a closed circulatory system with reversed blood flow in their vessels. Having a closed system is unusual for a small, inactive animal. Another remarkable thing about phoronids is that they have no heart. They move blood by peristalsis, waves of muscular contraction that run through their blood vessels. 

 

Some embryos, such as those of the snail species Limax and Planorbis, have been observed to have reversibly circulating blood. In a few cases, it has been induced in the embryonic hearts of some fish. Evidence that this is the stem evolutionary condition?

 

Cephalopods like the octopus do not have a reversing heartbeat in the same sense as tunicates, but one of their hearts does temporarily stop. An octopus has three hearts: two branchial hearts that pump blood through the gills and one systemic heart that pumps oxygenated blood to the body. When the octopus uses jet propulsion to swim quickly, the muscular contractions in the mantle are so powerful that they compress the veins returning blood to the systemic heart, causing it to pause temporarily.

Figure 4. Octopus hearts. (Modified from Frontiers for Young Minds).

Other animals have multiple hearts besides Octopuses and their relatives, the squids, which also have three hearts; two pump blood to the gills, while a third circulates oxygenated blood to the body. Earthworms have five pairs of serial hearts, known as aortic arches, which function as primitive hearts, pumping blood through their bodies. 

Heartless Animals. Cockroaches have 13 tube-like hearts located along the length of their back. The heart pumps hemolymph (the insect equivalent of blood) forward through open body cavities. 

 

Many animals, like jellyfish, starfish, and corals, do not have hearts at all. They lack blood and do not have a circulatory system. Jellyfish circulate water and extract oxygen into the body cells. Seastars do something similar, using cilia (small, hair-like structures) to move seawater through their bodies. Is it just seawater? Maybe we should call it enhanced seawater since it contains dissolved gases for exchange, along with some cells, proteins, and waste products. Officially, it’s called coelomic fluid, but it is mostly seawater, so enhanced seawater works as well.

 

 

Interesting Heart Facts to end:

 

A giraffe has a particularly long, muscular heart, about 2 feet long and weighing about 25 pounds, to pump blood up its long neck to its brain. The largest heart is that of the blue whale; of course, it weighs around 400 pounds. At just 0.2 mm long, the tiny fairyfly has the world's smallest heart, visible only under a microscope.

 

The fastest heartbeat is from a hummingbird. While flying, a hummingbird's heart can reach a breathtaking 1,260 beats per minute. For comparison, a human heart beats between 60 and 80 times per minute at rest. The slowest heartbeat is the groundhog: When hibernating, a groundhog's heart slows to as few as five beats per minute.

 

Zebrafish are notable for their hearts' ability to regenerate, a subject of intense research for potential treatments of human heart conditions. 

Figure 5. Human heart. (Stolen from a textbook).

Sources and Further Readings:

 

Anderson, H.E. & Christiaen, L., 2016. Ciona as a simple chordate model for heart development and regeneration. Journal of Cardiovascular Development and Disease, 3(3), p.25. doi:10.3390/jcdd3030025.

 

Burighel, P. & Brunetti, R., 1971. The circulatory system in the blastozooid of the colonial ascidian Botryllus schlosseri (Pallas). Italian Journal of Zoology, 38, pp.273–289.

 

Delsuc, F., Brinkmann, H., Chourrout, D. & Philippe, H., 2006. Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature, 439(7079), pp.965–968.

 

Konrad, M.W., 2015. Blood circulation in the tunicate Corella inflata (Corellidae). bioRxiv [preprint]. doi:10.1101/029322.

 

Kriebel, M.E., 1968. Studies on cardiovascular physiology of tunicates. Biological Bulletin, 134(3), pp.434–455.