In this blog post, we follow Red Blood Cell’s journey through the human body to deliver oxygen to cells.

In this episode, Red Blood Cell, after realising that she is not a junior cell anymore, is determined to deliver oxygen to cells by herself without getting lost or relying on other people. We follow Red Blood Cell’s journey as she travels through the veins and heart to the lungs to drop off the carbon dioxide. Picking up oxygen from the lungs, she then travels downwards through the heart and arteries to deliver oxygen to cells. Unbeknownst to her, Neutrophil follows her, ensuring that she does not get lost, drop her notes or get killed by bacteria. Thanks to Neutrophil, Red Blood Cell manages to safely go around the human body by herself which she talks about to Neutrophil.

How does blood circulation work in real-life? Join us as we learn more about the cardiovascular system and its importance in sustaining cells around the human body.

Overview of the cardiovascular system

The cardiovascular system, consisting of the heart and blood vessels, pumps blood around the body so that nutrients and oxygen are delivered to cells while carbon dioxide and wastes are taken away. Red blood cells carry oxygen which are bound to haemoglobin, but contrary to the anime they do not carry nutrients. Nutrients such as glucose, amino acids (the building blocks of proteins) and fats are carried outside cells in the plasma or liquid portion of the blood.

The cardiovascular system has two circuits:

The pulmonary circuit contains pulmonary arteries and veins which transport blood between the heart and lungs. In the lungs, carbon dioxide is exhaled out of the lungs while oxygen is inhaled into the body.

contains pulmonary arteries and veins which transport blood between the heart and lungs. In the lungs, carbon dioxide is exhaled out of the lungs while oxygen is inhaled into the body. The systemic circuit describes blood vessels that deliver blood from the heart to all parts of the human body and back again. Here, oxygen and nutrients are delivered to cells while carbon dioxide and wastes are taken away.

Diagram of the pulmonary and systemic circuits and its blood vessels with accompanying anime pictures.

Within each circuit there are three types of blood vessels:

Arteries : arteries carry blood away from the heart to other parts and organs of the human body. As the heart pumps blood, the arterial walls store the heart’s pumping energy which they dissipate to push blood through the arteries. Blood eventually drains into smaller arteries called arterioles which lead to capillaries.

: arteries carry blood away from the heart to other parts and organs of the human body. As the heart pumps blood, the arterial walls store the heart’s pumping energy which they dissipate to push blood through the arteries. Blood eventually drains into smaller arteries called which lead to capillaries. Capillaries : capillaries are small blood vessels (one cell thick, hence the very narrow alleyways in the anime) that are embedded in tissues of different organs. Here, nutrients and gases are exchanged between the blood and cells. As blood enters the capillaries, oxygen detaches from haemoglobin in red blood cells while nutrients are pushed out. As blood leaves the capillaries, wastes and carbon dioxide disposed from cells are pushed into capillaries.

: capillaries are small blood vessels (one cell thick, hence the very narrow alleyways in the anime) that are embedded in tissues of different organs. Here, nutrients and gases are exchanged between the blood and cells. As blood enters the capillaries, oxygen detaches from haemoglobin in red blood cells while nutrients are pushed out. As blood leaves the capillaries, wastes and carbon dioxide disposed from cells are pushed into capillaries. Veins: blood from capillaries are collected in venules which are pooled into veins. Veins transport blood back from cells and organs of the human body to the heart. Valves inside veins prevent backflow of blood into organs, forcing blood and red blood cells to travel towards the heart.

The moment before disaster – veins have valves to prevent backflow of blood, forcing blood to travel to the heart.

The beating of the heart: how does it work?

A diagram of the human heart. Picture traced over existing diagram with labels written by me.

The heart consists of cardiac muscle that relax and contract to pump blood to organs around the human body such as the lungs and the liver. The heart is split into two with each half containing two chambers: the atrium and the ventricle. Between the atria and ventricles are atrioventricular valves that close while the heart contracts to prevent blood flowing back from the ventricles to the atria. There are also pulmonary and aortic valves between the ventricles and the pulmonary artery and aorta respectively which prevent blood in arteries from going back into the heart.

Did you know? The walls of the left ventricle are thicker than the walls on the right. That is because the left ventricle must contract harder to pump blood over longer distances around the body compared to the short distances from the right ventricle to the lungs.

The heart pumps blood around the human body via the cardiac cycle which describes how cardiac muscle in the atria and ventricles contract in an orderly pattern. The cardiac cycle can be split into two distinct stages: diastole and systole.

A detailed look at the cardiac cycle, showing how pressures and volumes in the heart and arteries change during diastole and systole. Also described are the ECG (electrocardiogram) waves and sounds the heart produces when it is beating.

In diastole, blood collects in the atria from the pulmonary veins in the pulmonary circuit and the superior and inferior vena cava (collecting blood from the upper and lower parts of the body respectively) in the systemic circuit. In the episode, this can contain a lot of red blood cells crammed together in one small space. The cardiac muscle in the ventricles relax, opening the atrioventricular valves which allow blood to flow from the atria to the ventricles. (this is described in the episode by the marching of red blood cells through the valve into the ventricle). Towards the end of diastole, the atria contract, forcing more blood into the ventricles.

Something not described in the episode is what happens in systole. During systole, cardiac muscle in the ventricles contract. This closes the atrioventricular valves so that blood is not shunted back into the atria, forcing blood to enter the pulmonary artery or the aorta through the pulmonary and aortic valves respectively. The ventricles then relax, closing off the pulmonary and aortic valves so that blood in the arteries do not flow back into the heart. A short time later, the atrioventricular valves open which allow blood in the atria to flow into the ventricles, repeating the cardiac cycle.

A look at the beating of a real donor heart. Note that the ventricles contract harder than the atria to eject a lot of blood from the heart to the arteries (or tubes in this case).

Did you know? The heart pumps at 60-100 beats/minute in a healthy adult. This is equivalent to 3,600-6,000 beats/hour and 86,400-144,000 beats/day.

Transporting oxygen from the lungs to cells

The structure of haemoglobin. Source

Red blood cells, devoid of the nucleus and organelles, are packed with haemoglobin. Haemoglobin is a protein complex consisting of two alpha and two beta subunits attached to each other. Each subunit has a heme group containing an iron ion where oxygen binds to. Haemoglobin is the functional unit used to transport oxygen around the human body.

A summary of gas exchange of oxygen and carbon dioxide in the lungs and cells (as described in the next few sections).

In the lungs, the air breathed in is rich with oxygen. Oxygen moves from inside the alveolus to the blood vessel and enters red blood cells by passing through the cell membrane. Here, they bind to a free iron ion in haemoglobin. The oxygen stays bound to haemoglobin while the heart pumps the red blood cell from the oxygen-rich lungs to oxygen-poor tissue. As the oxygen concentration in the blood decreases, oxygen detaches from the iron ion in haemoglobin and diffuses out of the red blood cell (this is Red Blood Cell’s “oxygen delivery” to Normal Cell in the anime episode). It then enters cells where it is used to break down nutrients (typically glucose) from food to produce energy. The energy is used to run processes inside the cell such as protein production and cell movement. This reaction is called cellular respiration which is described as:

Glucose + oxygen -> carbon dioxide + water + energy

C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + energy

Did you know? Up to four oxygen molecules can be attached per haemoglobin molecule. However, in a resting person, only one oxygen molecule attaches and detaches to haemoglobin at a time. During oxygen-deprived states such as exercise; however, more oxygen molecules can be released from haemoglobin to maintain increased energy production in active cells.

Transporting carbon dioxide from cells to lungs

Carbon dioxide can be transported around the human body in a number of ways. It can be transported in blood plasma or bound to haemoglobin in red blood cells. However, most of the time, carbon dioxide is converted to bicarbonate ions (HCO 3 –) which are transported in plasma to the lungs where it is converted back to carbon dioxide and exhaled.

As described previously, carbon dioxide is produced by breaking down nutrients to produce energy. Carbon dioxide diffuses out of cells and enters red blood cells in the blood. Here, it combines with a water molecule to form carbonic acid (H 2 CO 3 ) with the help of the enzyme carbonic anhydrase. Carbonic acid then breaks down into hydrogen (H+) and bicarbonate (HCO 3 –) ions which are transported out of red blood cells and dissolved in plasma.

Carbon dioxide + water ↔ carbonic acid ↔ hydrogen ion + bicarbonate ion

CO 2 + H 2 O ↔ H 2 CO 3 ↔ H+ + HCO 3 –

As blood is pumped by the heart to the lungs, bicarbonate ions go back into the red blood cell and convert back to carbon dioxide. The resultant carbon dioxide then diffuses out of the red blood cell and enters the alveolus where it is breathed out from the human body.

Conclusion

Red Blood Cell talking happily to Neutrophil about the journey she has experienced in the episode.

In this blog post, we have followed Red Blood Cell’s journey in the cardiovascular system. Here, she carries oxygen in haemoglobin from the lungs to the cells where it is used to produce energy. In return, she converts the carbon dioxide from cells into bicarbonate ions which are transported to the lungs, where it is reverted back to carbon dioxide and exhaled. All this is powered by the pumping of the heart which follows a pattern of contractions in the atria and ventricles. Altogether, the cardiovascular system acts to pump blood around the body in order to deliver nutrients and oxygen while taking away wastes and carbon dioxide, ensuring the proper functioning of the human body.

In the next blog post, we will look at how T cells develop in the boot camp known as the thymus. See you then!

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