Heart (comparative anatomy)

The heart beats with a lifelong, continual rhythm to supply a constant flow of oxygen and nutrients, as well as to remove metabolic waste products from every cell in the body. The interstitial fluid that surrounds each cell keeps a constant supply of these factors and prevents a buildup of waste products by communicating with the blood. However, the blood can only keep the interstitial fluid in balance by remaining in motion. If the blood flow stops, the oxygen and nutrients are quickly depleted, and harmful waste products accumulate. In adult humans, the heart beats about seventy times per minute, or one hundred thousand times per day. Each heartbeat moves about seventy milliliters of blood, which equals over seven thousand liters of blood per day, or enough blood to fill 7,400 quart-sized milk cartons.

Heart Structure

Hearts vary in their complexity, from simple pulsing vessel segments in insects to the complex four-chambered heart in birds and mammals. Hearts typically have a rhythmic contraction rate determined by specialized muscle cells and can even beat outside the body. A strong, saclike structure called the pericardium encloses, supports, and aids the heart in refilling during diastole by creating negative pressures in the atria and ventricles. Fluid is secreted into the space between the outside wall of the heart and the pericardium. This fluid lubricates and reduces friction during contraction and relaxation. When the heart contracts and ventricular pressure develops, pressure valves made from strong, thin, fibrous tissue are arranged to open and close in such a manner that blood or hemolymph flows in one direction out of the heart (unidirectional flow). When the heart relaxes, and ventricular pressure decreases, the valves close to prevent flow reversal from the arteries, and other valves open to allow flow into the heart from the tissues.

Heart chambers fill with blood during diastole and pump blood during systole. One complete heart cycle involves one phase of relaxation and one phase of contraction. The highest pressure developed in the ventricle during contraction is called the systolic pressure, and the lowest pressure in the arteries just before the next contraction is called the diastolic pressure.

The fluid pumped by the heart can be divided into two categories: blood and hemolymph. Blood has two major components: the liquid plasma and cells. The red cells contain hemoglobin for oxygen transport, while the white cells defend against invading germs and viruses. Hemolymph, on the other hand, lacks red blood cells and has hemoglobin freely dissolved in the circulating fluid. In addition, hemolymph flows directly around each cell, whereas blood exchanges nutrients and oxygen across the capillary wall to the interstitial fluid that surrounds cells.

Heart Rate

A heart will rhythmically contract even when removed from the animal. This property is called automaticity, and results from a specialized collection of cells in the upper part of the right atrium called the sinoatrial node. The sinoatrial nodal cells spontaneously generate an electrical impulse or action potential that travels throughout the heart by specialized heart muscle cells called conduction fibers. The conduction fibers provide for a uniform and effective heart contraction. Intercalated disks connect each heart muscle cell together and conduct the electrical impulse. Intercalated disks assure all the heart muscle cells will contract together and effectively pump the blood.

Heart rate varies by animal size. Elephants have a slow, thirty-five beats per minute rate, while the smallest mammal, the shrew, has a heart rate of over six hundred beats per minute. The tiny hummingbird’s heart beats at over 1,200 beats per minute while in flight.

The rate at which the sinoatrial node generates impulses can be modified by the autonomic nervous system. The sympathetic branch of the autonomic nervous system increases the heart rate, for example, during exercise or when frightened, while the parasympathetic branch slows the heart down, for example, during sleep.

Different Beats for Different Beasts

One of the simplest hearts is a rhythmically contracting muscular section in the dorsal vessel of the annelid worm, grasshopper, and other arthropods. This “heart” contracts and pumps hemolymph into open-ended sinuses that distribute the hemolymph throughout the body. The hemolymph fluid does not directly return to the heart but is drawn in through ostia or pores in the heart during the relaxation phase (open circulation).

Mollusks and octopuses have a greater diversity of heart structure, with many having two chambers. The thin-walled chamber that collects blood from venous vessels is called an auricle or atrium. The thick-walled muscular chamber that receives blood from the atrium and propels blood to the body is called a ventricle. The open circulation system in these animals delivers blood flow through several main arteries to important regions such as the kidney, head, and digestive tract.

The two-chambered fish heart has one atrium and one ventricle and represents a slight increase in complexity compared to mollusks and crustaceans. The sinus venosus collects blood at the convergence point of great veins and empties into the atrium. The bulbus arteriosus in teleosts (bony fishes), or the conus arteriosus in elasmobranchs (cartilaginous fishes), stores pressure energy from the ventricular ejection and helps to maintain a steady blood flow through the circulation during diastole.

The fish atrium receives deoxygenated blood, and the ventricle pumps blood to gills, where the blood releases carbon dioxide and picks up oxygen from the water. The oxygenated blood then leaves the gills and circulates through the rest of the body. A significant amount of pressure is lost as the blood travels through the small capillaries in the gills. So, movements of the fish’s body are important to help move the blood through the body and back to the heart.

Fish have a single path of circulation, from the ventricle through the gills and then to other body organs and tissues. In contrast, air-breathing animals have two separate circulations, one through the lungs (pulmonary circulation) and another to the rest of the body (systemic circulation). Lungfish (dipnoan lungfish) have the first indications of two separate circulations where oxygenated and deoxygenated blood are separated by a ridge of tissue in the ventricle. This diverts deoxygenated blood to the lungs and oxygenated blood to the rest of the body or systemic circulation.

The largest heart on Earth belongs to the blue whale. A blue whale heart can be 5 feet tall (1.5 meters) and weigh over 400 pounds (200 kilograms). Like other mammals' hearts, the blue whale's heart has four chambers with two auricles and two ventricles. It can pump around sixty gallons of blood with each beat. This beat can vary, though. If they are diving, a whale's heart rate decreases to conserve oxygen. If they are heading to the surface of the water, that rate rises in order to replenish oxygen.

Amphibians, Reptiles, Birds, and Mammals

Amphibians have an increased distinction between systemic and pulmonary circulations, with two separate atria. The right atrium receives deoxygenated blood from the systemic circulation, and the left atrium receives oxygenated blood from the lung or pulmonary circulation. The ventricle is partially divided into a right and left side by a muscular wall called a septum. The septum keeps the oxygenated blood coming in from the lungs separate from the deoxygenated blood returning from systemic circulation. Differential flow timings also help to separate deoxygenated and oxygenated blood. The ventricle delivers blood into a divided artery, with the pulmonary circuit leading to the lungs and skin (frogs receive some oxygen from the skin circulation on their backs), and the systemic circuit leading to all organs except the lungs. The amphibian heart is relatively weak, with spongy, poorly muscularized ventricles compared to mammals.

Most reptiles also have two separate atria, similar to amphibians; however, the sinus venosus is no longer a distinct structure and is incorporated into the right atrium. In contrast, adult crocodiles have a completely divided ventricle, thus having completely separate pulmonary and systemic circulations, similar to birds and mammals.

Birds and mammals are characterized by two completely separate atria and ventricles. The right atrium receives deoxygenated venous blood from the systemic veins, and the right ventricle pumps blood through the pulmonary circulation to pick up oxygen. The left atrium receives the oxygenated blood from the pulmonary veins, and the left ventricle pumps this blood throughout the systemic circulation (all organs except the lungs).

With the complete separation of the right and left ventricles, the pressures between the two can be different. The ventricles are highly muscularized, with the left ventricle being approximately six times thicker than the right ventricle, and this reflects the greater pressures needed to circulate blood through the systemic and pulmonary circulations. However, the volume of blood pumped by each ventricle needs to be the same because each ventricle returns blood to the other. For example, the right ventricle pumps blood through the pulmonary circulation that returns to the left ventricle. If the right ventricle pumps a larger volume of blood than the left, blood will accumulate in the pulmonary circulation. In humans, a heart attack involving the left ventricle weakens the muscle, which decreases the volume of blood pumped. The right ventricle pumps normally, and blood accumulates in the pulmonary circulation, causing breathing problems.

During flight, birds can have an increase in oxygen demand of ten- to fifteen-fold to power their flight muscles. Birds have very powerful hearts that can deliver five to seven times more blood to active muscles compared to similarly sized mammals.

Principal Terms

Atrium (auricle): A thin-walled muscular heart chamber that receives blood returning to the heart

Closed Circulation: A circulation system made of arteries, capillaries, and veins that returns blood flow back to the heart

Diastole: Period of heart muscle relaxation and declining pressure

Heart: General term for a muscular vessel segment or organ that contracts and provides for unidirectional movement of blood or hemolymph

Open Circulation: An open-ended sinus or arterial vessel system in which the circulation system does not return blood or hemolymph directly back to the heart

Systole: Period of heart muscle contraction and peak pressure

Ventricle: A thick-walled muscular heart chamber that receives blood from the atrium and pumps blood into the circulation

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