An Overview: The Cardiovascular System and the Hemodynamics of Circulation 1

My fascination with blood began when I was eight years old. No, I’m not a vampire. I even hate vampire movies. I don’t mean the general horror children feel towards blood; mine was traumatic. We live close to a swampy and secluded area in Nigeria. Many are farmers, tree fellers, and firewood sellers. One day, my siblings and I went in search of firewood when we came upon a rugged hunter and tree feller. Everyone knew Yemi. He was strong, fierce and fearless. Those three words probably mean the same thing, but Yemi was that good. We stopped with our bundle of tiny sticks in our arms, staring at Yemi as he chopped a massive tree with his mighty ax.

We wished we had the strength to get bigger firewood like his. Then we wouldn’t have to search for it every three days.

As I watched the rhythmic movement of the ax rising and falling with a thud on the wood, I became hypnotized. The bundle of firewood hurt our arms, but our eyes remained glued to Yemi. Then on what looked to be the final swing, the ax went high up and landed on his leg with a sickening thud and crunch of bones I remember to this day.

My sister ran for help, but I was rooted to the spot. There was so much blood. The red liquid shone and pulsed with so much life I was equal parts fascinated and repelled.

Why did Yemi bleed? Wasn’t our blood somewhere inside us nothing could reach?

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The Homeostatic Principle.

We all have principles we live by. They define us, shape our thinking and mold our decisions. Even criminals have a code. Ironic? But true. Instinct guide animals; but humans are not. Another principle shapes the way our body behaves and responds to situations. This principle keeps us alive.

Homeostasis is the maintenance of a constant internal environment regardless of any change that occur externally.

Homeostasis gives the body a ‘goal,’ a point it must maintain at all times. For example, the cardiac output for a healthy 70kg weight is about 5L/min. The body has decided this value to be the ‘normal.’ For any variation, whether up or down, the necessary body systems are called upon to bring the value to normal. This process of striving to maintain the ‘normal’ is homeostasis. It is the inability to maintain homeostasis that leads to pathophysiology. (the abnormal)

Cardiovascular system

The cardiovascular system is made of the heart and blood vessels. The lymphatic system is also an essential part of our circulatory system, but that’s not the subject of this post. The cardiovascular system plays a vital role in homeostasis by ensuring the transport of oxygen and nutrients, play a protective role, contribute in the maintenance of blood pressure and body temperature, transports waste, serve as the body’s primary transport medium and helps to keep us alive. I agree the last one sounds trite. But it’s the truth and an understatement to say we can’t survive without our cardiovascular system.

Open and closed circulatory system.

From the ‘circu’ in circulatory system, we know it is a loop, a circle. But what kind of circle? It can be open or closed.

An open circulatory system can be found in invertebrates. Here the blood flows through spaces and washes over the body tissues. To ensure the supply and delivery of nutrients and the ‘carting away’ of waste product is done efficiently, the blood flow is slower and as you can imagine, uncontrolled.

The circulatory system eventually evolved into a closed one. Wonder why? One of the amazing aspects of the human body, there’s a reason for everything. With larger organisms like vertebrates, ‘bathing’ the tissues and organs with blood is no longer enough. It is inefficient, wasteful and irregular. In the closed circulatory system, blood is encased in blood vessels and delivered to the tissues at a well-controlled rate. Delivery and exchange of nutrients for waste is more efficient and neater, especially with the added presence of the lymphatic system. It is a closed loop; blood does not leave the vessels to bath the organs.

Heart

Whenever someone says their heart is broken, I always find it funny. Of course, I understand what the person means. But I can’t help viewing the heart in more scientific terms.

The heart is a muscular organ. The primary function of the heart is to pump blood through the blood vessels to the tissues. How does a fist-sized organ manage this?

The heart is composed of a right and left side, further subdivided into four chambers, the left atrium and left ventricle and the right atrium and right ventricle.

The atrioventricular valves separate the atrium from the ventricles. The tricuspid valve separates the right ventricle from the right atrium while the bicuspid valves separate the left ventricle from the left atrium. The semilunar valves are located at the ventricular exits. The aortic and pulmonary valves are located in the aorta and pulmonary artery respectively. The valves and septum of the heart play a unique role in ensuring heart beat is regular by contributing to the control of the spread of the depolarization wave and preventing back flow of blood. I will discuss the electrophysiological process of cardiac muscle relaxation and contraction in another post.

It is the cardiac muscle that gives the heart its distinct ability to generate and transmit the electrical impulses that lead to muscular contraction and relaxation.

Blood flow through the heart.

The coronary circulation (that supplies the heart), the superior and inferior vena cava deliver deoxygenated blood to the right atrium. When the right atrium contracts, it ejects the deoxygenated blood through the tricuspid valve into the right ventricle. The right ventricle ejects the deoxygenated blood into the pulmonary trunk that divides into a right and left pulmonary artery that carries the deoxygenated blood to the lungs for oxygenation.

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Anatomy of the heart
Blood flow through the heart major chambers: Right Atrium → Right Ventricle → Left Atrium → Left Ventricle

After oxygenation in the lungs, the blood is pumped into the left atrium by the pulmonary veins. The left atrium ejects the blood into the left ventricle through the bicuspid or mitral valve. The left ventricle then ejects the oxygenated blood through the aorta into the arteries to be delivered to the body tissues. The tissues take what they need and dump waste products that are carried back by veins back to the heart for the cycle to continue.

A Two-way circulatory system

The human body has two types of circulation- a systemic and pulmonary circulation.
The heart serves as a double pump, pumping deoxygenated blood through the right heart (right atrium and right ventricle) and pumping oxygenated blood through the left heart (left atrium and left ventricle).

The pulmonary circulation sends deoxygenated blood to the lungs to be replenished with oxygen while carbon (iv) oxide is taken out. The systemic circulation pumps the oxygenated blood to the tissues.

The left ventricle has the most work to do as it expels blood through a large distance and with great force. The systemic circulation matches the pulmonary circulation.

Therefore, the systemic circulation has to work against gravity when we’re standing and move blood through a greater distance than the pulmonary circulation. The respiratory system and the cardiovascular system work in tandem to ensure the body is never starved of oxygen.

Blood vessels

The three main blood vessels are the

• Arteries
• Veins
• Capillaries

The arteries transport oxygenated, rich blood away from the heart to the tissues with the exception of the pulmonary artery that carry deoxygenated blood. The veins transport deoxygenated, 'nutrient-low' and 'waste-high' blood to the heart with the exception of the pulmonary vein. At the capillaries, exchange of nutrient for waste occurs.

Biophysics of blood circulation

Venous return is the amount of blood returned by the veins to the heart.
Cardiac output is the amount of blood pumped out by the left ventricle each minute.
It is what the heart is given (venous return) that it pumps out. (cardiac output)

Remember, the closed circulatory system is better regulated than the open? This is because the rate of blood supply is directly proportional to the tissue needs. This means that each tissue is supplied according to what it needs. For example, during exercise, blood supply to the skeletal muscles is increased because at that time those tissues need it more.

Also, the blood vessels adjust supply in relation to tissue needs in a process I will discuss later. For example, if the concentration of oxygen is low, the blood flow to that part is amped up.

Blood flow, Pressure, and Resistance

These three are connected by the formula, F= change in pressure/ resistance.
Where F is the blood flow to a tissue.
The pressure here is not the usual blood pressure but the change in pressure, P₁-P₂ the difference in pressure between two ends of a blood vessel. For example, if P₁-P₂ is unchanged, that is P₁ is 100mmHg and P₂ is 100mmHg, the rate of blood flow will be zero.
But if there exists a pressure difference, there will be blood flow.
R is the resistance or impediment to blood flow.
This law is called Ohm’s law.
When the pressure difference is high, blood flow will be high. When the resistance is high, blood flow is reduced.
This is the same law applied in physics that says current is directly proportional to voltage difference divided by resistance.

This Ohm's Law in hemodynamics is just one of the principles guiding blood flow. The others will be discussed in the next post.

^References

^{Circulatory system
Cardiac output
Difference between open and closed circulatory system
Guyton, Arthur C, and John Hall, Textbook of Medical Physiology. Elsevier, 2000.
Heart
Homeostasis}