Evolution of the Human Heart: One Beat at a Time Quiz

Answer: Deoxygenated blood is pumped to the lungs, returned to the heart then oxygenated blood pumped to the rest of the body

The human heart is a fist sized organ that is divided into two. The right side of the heat pumps deoxygenated blood to the lungs where carbon dioxide (a waste product) is exchanged for oxygen and then oxygenated blood is returned to the left slide of the heart, slightly bigger than the right side, as it has more work to do.

The left side then pumps nutrient rich, oxygenated blood to the organs and tissues of the rest of the body. This is a closed-loop system.

Answer: None. External fluid environment provides nutrient flow passively

In all animals that are not single cells, there needs to be a system to transport fluids containing nutrients reach their natural end points: the cells (where cellular metabolism keeps the cells and hence the organism alive).

The simplest animals, such as the sponges (Porifera) do not need a circulatory system because they are in an aqueous environment. Diffusion provides exchange of water, nutrients, and waste products. In slightly more complex organisms such as jellyfish (Cnidaria), diffusion occurs through their epidermis (upper side) and internally through a gastrovascular section (underside). All tissues are soaked in an aqueous environment and exchange fluids by diffusion on both sides. Exchange of fluids is assisted by the pulsing of the jellyfish body in the absence of a dedicated pump such as a heart.

Answer: Blood is mixed with other body fluid, pumped into a body cavity to bathe the organs

In arthropods, blood is not enclosed in the blood vessels in an open circulatory system but is pumped by the heart into a cavity called a haemocoel. The blood mixes with the interstitial fluid and this is called haemolymph. As the heart pumps, the haemolymph circulates through the haemocoel surrounding the body organs then re-entering the heart through ostia (openings). Haemolymph movement facilitates gas and nutrient exchange. Insect blood does not carry haemoglobin and is therefore not red. As such oxygen is distributed by a complex series of air-sacs called tracheae.

In an open circulatory system, not as much energy is needed to operate it compared to a closed system. There is a cost - It is difficult to reach specific organs and tissues that require high oxygen concentrations. This is why insects with wing spans of up to 60cm /24 inches do not exist today because they were ousted by the arrival of birds around 150 million years ago. Birds have a closed circulatory system, can move more efficiently, filling food and oxygen needs faster.

Answer: 2

Fish blood circulation is a single circuit. Oxygenated blood is returned to the atrium (upper chamber) of the heart. When full, this fills the ventricle (lower chamber) with blood which is then pumped to the gills (which contain many, many smaller blood vessels to increase surface area) and the blood is re-oxygenated.

The blood then travels via two arteries and a network of capillaries to the rest of the organs and tissues and returns, oxygen depleted, to the atrium of the heart via a closed system of veins. Within this system, the tissues that receive the blood last do not receive as much oxygen as the tissues that receive the blood soon after it leaves the gills. (i.e. there is an O2 gradient in this system) and as such, this reduces the overall metabolic capacity of fish.

Answer: No

The ability for the heart to be as effective as an efficient pump depends on having two chambers. The upper chamber acts as a reservoir for blood returning to the heart. Then the valve opens, the atrium contracts and the blood fills the ventricle. This is necessary as the the main pumping mechanism is actually the lower chamber called the ventricle.

It must be allowed to fill completely so with one pumping action (a heartbeat or the pulse), all the blood is pumped out in a single action leaving the ventricle empty.

The cycle then repeats itself. The amphibian circulatory system has two circuits. A circuit for blood to be sent to the lungs for oxygenation which then returns to the heart. The other atrium fills with deoxygenated blood. Both feed into a common ventricle mixing O2-rich and O2-depleted blood.

This mixing is partially ameliorated by a ridge of tissue located within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmonary and skin circuit. Amphibians also have the capacity to absorb oxygen from the environment through the skin.

Answer: Slow its heart rate to a few beats a minute whilst underwater

Crocodilians (alligators, gharials, caimans and crocodiles) have a unique well-developed circulatory mechanism where the heart can shunt blood from the lungs toward the gastrointestinal tract and other organs during long periods of underwater. This type of animal waits for prey by staying underwater or just close enough to the surface for its nostrils to be exposed. An adaptation includes two main arteries that leave the same part of the heart normally take blood to the lungs but an additional artery provides an alternate route to the stomach and other parts of the body.

Another adaptation includes a hole in the heart between the two ventricles, called the Foramen of Panizza, which allows blood to move from one side of the heart to the other under the control of the animal unlike the three chambered amphibian heart where there is no alternative but to mix O2-rich and O2-depleted blood.

There is also specialized connective tissue that slows the blood flow to the lungs. So when underwater the crocodile, staying as still as possible, with less need to send blood to the lung for oxygen replenishment can slow its heart rate to a few beats a minute (Some authors claim 1-2 beats per minute). Additionally if underwater the crocodile is probably waiting for prey, it can slow blood travelling to the digestive system, just enough to main organ viability as there is no food in its upper digestive tract at least to warrant further blood flow. All together, these adaptations have made crocodiles and alligators possess one of the most sophisticated circulatory systems across animal groups (including humans).

Answer: It is the only artery that carries deoxygenated blood

When oxygenated blood returns from the lungs via the pulmonary vein, it fills the left atrium. When full, the valve between the left atrium and the ventricle valve opens and the blood is pumped down into the left ventricle (The left ventricle is larger and surrounded by thicker musculature than the right ventricle as the former must pump blood around the whole body not just the lungs like the right ventricle). The blood is ejected into the aorta, the largest artery in the body which branches into many smaller arteries and eventually the tiny capillaries at which level nutrients and wastes are transferred at the cellular level. The deoxygenated blood is then returned to the heart via the venous system until it comes to the inferior and superior vena cava (the largest veins in the body) where it then moves into the right atrium, then right ventricle where it is pumped out into the pulmonary artery until it reaches the capillaries of the lung. Oxygen and CO2 are exchanged at the capillary / alveoli interface and the blood returns via the pulmonary vein to the left atrium.

This cycle is repeated about 70 times a minute in a human. With approximately 70 ml of blood being ejected per heartbeat, that means approximately five litres of blood are circulated every minute in the adult human heart.

Answer: Blockage of the arteries of the heart causing part of the heart to die

Because the heart is a muscle, it needs its own blood supply. The coronary arteries branch out from the aorta, surrounding the outer surface of the heart like a crown with two arteries, one of which branches out into two, before feeding the cardiac capillaries.
Myocardial infarction means "heart muscle obstruction". This can be caused by a build up of a cholesterol-like substance over many years in the cardiac arteries, a blood clot or a combination of factors. When one coronary artery becomes blocked, blood cannot reach all of the heart muscle it feeds and the pain felt is that part of the muscle dying.
Chest pain felt that is not a heart attack may be angina pectoris. This is when one or more of the cardiac arteries are partially blocked, limiting but not stopping blood flow. This is a warning sign of a possible infarction in the future, so it needs medical attention.

Answer: The sinoatrial node on the wall of the right atrium acts as the definitive pacemaker

(This explanation covers mammalian hearts as described in the question. The knowledge about conduction mechanisms in other less developed circulatory systems in other animals is fragmentary). Cardiomyocytes are specialised muscle cells that comprise the heart muscle (myocardium). Cardiomyocytes are distinct from skeletal muscle calls as they are striated but pump rhythmically and involuntarily like smooth muscle.

This mechanical pumping is used by the sinoatrial node which is a network of nervous tissue (ie capable of producing electrical signals) to initiate electrical charges which spontaneously pulse (throb) from the SA node.

This makes the two atria contract together. The pulse travels to a second node, the atrioventricular (AV) node, where the right atrium and right ventricle meet, where it pauses for 0.1 seconds before radiating to the walls of the ventricles.

The pause facilitates the blood in the atria to empty completely into the ventricles prior to the ventricles pumping out the blood. From the AV node, the electrical impulse travels down an electrical conduit to the bundle of His, then up through interventricular septum (Middle heart wall) to the Purkinje fibres conduct the impulse from the apex of the heart up the ventricular myocardium, causing the ventricles to contract and empty.

This ejection is the heartbeat we can sometimes feel in our chest.

Answer: Detecting a pulse at the neck or the wrist

An ECG (or EKG in some countries), is a representation of the electrical activity of the heart. It forms a characteristic wave pattern when recorded and this pattern is predictable. When this pattern is altered, the new pattern is diagnostic of cardiac pathology in some cases. However you would not use it to detect a heart beat. An EEG is a recording of the electrical activity of the brain.

When the 70ml of blood is ejected from the left ventricle it travels along the arteries like waves, the capillaries slow down the flow and the returning veins return the blood to the heart in a more uniform manner. This wave of blood in the arteries is the pulse or the heartbeat. In the wrist or neck, the arteries are very close to the surface of the skin so you can feel a heartbeat. This is what first responders are trying to ascertain. If there is no pulse the heart is not beating and if not restored, brain damage will occur as the brain can only operate for a few minutes in the absence of a heart beat. When the head has stopped beating, one option is to defibrillate the patient. This is to apply a high electric charge to the body hoping it will kick start the electrical activity of the heart to make it start beating on its own accord again.