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School Biology revision notes: Human circulatory system: Part 2. The heart re-edit

The human circulatory system Part 2. How the heart works

Doc Brown's GCSE level Biology exam study revision notes

There are various sections to work through, after 1 they can be read and studied in any order.

Index of notes on human circulatory system: heart, lungs & blood vessels


2. The human heart and its pumping action - diagrams and explanation

The heart is an organ that pumps blood around the body

You need to refer to the diagram of the structure of the heart above.

Much of the wall of the heart is made from muscle tissue which continually contracts and relaxes.

The heart can act as a pump because of the physical action of a muscular wall to move the tissue of the heart chambers and one-way valves are need so that there is a continuous flow of blood in the same direction.

An involuntary, striated muscle constitutes the main tissue of the wall of the heart.

This cardiac muscle contains lots of mitochondria to provide the cells with lots of ATP for respiration - lots of energy is needed to work to contract the heart muscles.

Heart muscles need their own blood supply to continually access a source of nutrients and oxygen so they can work non-stop - a continuously beating heart to keep all of us mammals alive!

The cardiac muscles are supplied with blood from two coronary arteries which branch from the base of the aorta. They ensure a constant supply of glucose and oxygen for respiration, that can diffuse through the thick walls of the heart into the heart muscle cells .

The heart has valve systems that ensure the blood only flows in the right direction - one way only, no reverse flow allowed.

The heart has two pumps to work the double circulation system that beat together, typically 60 - 100 times per minute.

Each pump has an upper chamber (atrium) that receives blood and a lower chamber (ventricle) that pumps blood out.

Both atria and ventricles fill and pump blood at the same time.

The natural resting heart rate is controlled by a group of cells in the right atrium which act as a pacemaker and act as part of the nervous system (* see advanced note below).

Heart rate is usually measured as the number of times the pump action (sensed as a heart beat) occurs per minute.

For a healthy heart, it should normally be around 60-90 beats per minute at rest.

It will increase with more strenuous aerobic exercise OR potentially a cardiovascular problem.

(* Heart rate is controlled by the two branches of the autonomic (involuntary) nervous system. The sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The sympathetic nervous system (SNS) releases the hormones like epinephrine to accelerate the heart rate if more oxygen is needed for respiration.

Know that there are four main chambers of the heart (left and right atria and ventricles) to pump the blood around and the associated control valves which only allow flow in one direction.

See the diagram of the heart above/below where the important parts of the organ are labelled and the white arrows show the direction of flow of the two circuits for:

(i) the oxygenated blood from the lungs to the rest of the body (the organs etc.)

and (ii) the deoxygenated blood to the lungs.

The septum of the heart is a wall of tissue that separates the left and right sides of the heart and enables blood to flow as it should. The septum consists of the atrial septum and the ventricular septum.

The atrial septum separates the left and right atria, while the ventricular septum separates the left and right ventricles.  The heartís septum is critical for ensuring that blood flows properly through the heart.

Note: For visual simplification, note that oxygenated blood is shown as bright red and deoxygenated blood as blue (even though it is a dark red colour in reality!).

Repeated diagram for visual convenience

The mechanism by which the heart works

The blood flows from the heart to the organs, including the lungs, through arteries and returns through veins.

Blood is supplied to the heart by two coronary arteries which branch from the base of the aorta - the largest artery in the body.

The heart itself needs a good supply of oxygen and glucose for lots of energy to keep the heart muscles continuously working.

The four major blood vessels associated with the heart are the pulmonary artery, pulmonary vein, aorta, vena cava.

Blood is pumped away from the heart into arteries and returns to the heart in veins.

The aorta (main artery) - transports oxygenated blood from the left ventricle to the body.

The vena cava (main vein) - transports the deoxygenated blood flow from the body into the right atrium.

The pulmonary artery - transports the deoxygenated blood from the right ventricle to the lungs

The pulmonary vein - brings the oxygenated blood to the heart from the lungs into the left atrium.

gcse biology heart diagram double circulation atria ventricles veins arteries

The heart valves

The heart needs a set of valves to keep the fluid flowing in one direction.

The heart has two types of valves that keep the blood flowing in the correct direction.

The valves between the atria and ventricles are called atrioventricular valves those at the bases of the large vessels leaving the ventricles are called semilunar valves.

When the ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria.

When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles.

Flow and valve action (simple summary)

The heart has valves to ensure the blood only flows in the one correct direction.

When the ventricles contract, the valves to the atria close and the valves to the blood vessels open.

This prevents any reverse blood flow.

 Ventricles have thicker walls than atria because they operate at a higher pressure to pump the blood further.

The left ventricle has a thicker wall than the right ventricle because the muscle tissue must be stronger because it operates at a higher blood pressure to pump blood around all of the body.

The right ventricle muscle tissue is thinner because it only has to pump the blood to the lungs.

The blood enters the two atria (right atrium and left atrium, atria) of the heart from the vena carva and the pulmonary vein.

The left atrium and ventricle pump oxygenated blood from the lungs around the body (see diagram above).

The left atrium receives the oxygenated blood from the lungs via the pulmonary vein and is pumped through the aorta to the rest of the body. It passes into the left ventricle to be pumped around the body.

The right atrium and ventricle pump deoxygenated blood

The right atrium of the heart receives the deoxygenated blood from all of the body, entering the heart by the vena cava.

The deoxygenated blood flows through the right ventricle which pumps it to the lungs via the pulmonary artery to be oxygenated.

The basic sequence of heart function is:

The heart relaxes and the blood enters both atria.

The atria contract at the same time forcing blood into the ventricles.

The ventricles contract from the bottom upwards which forces blood out of the heart through the pulmonary artery and aorta.

The atria fill up again and the whole cycle is repeated

The valves in the heart ensure that blood flows in the correct direction.

 

How is it, that normally the heart can beat at a regular rate?

In the right atrium are a group of cells that act as a pacemaker.

They control the heart beat by producing tiny electrical impulses (signals) that spread into the surrounding muscle cells causing them to contract.

If this group of cells are malfunctioning leading to an irregular, and dangerous heartbeat, an artificial pacemaker can be surgically implanted under the skin and wired up to the heart.

This device produce tiny electrical signals at the right frequency to generate a correct and regular heartbeat.

 

The pumping rate of the heart

The cardiac output is the total volume of blood pumped by a ventricle every minute.

It is calculated using the following equation:

cardiac output (cm3/min)  =  heart rate (beats/min)  x stroke volume (cm3)

Abbreviated rearrangements:

heart rate = cardiac output / stroke volume  and  stroke volume = cardiac output / heart rate

Your heart rate is the number of beats per minute e.g. when you measure your pulse rate.

The stroke volume is the volume of blood pumped by one ventricle every time it contracts.

Example questions

Q1 A person has a heart rate of 60 beats/min and average stroke volume of 70 cm3.

Calculate the cardiac output.

cardiac output = heart rate x stroke volume = 60 x 70 = 4200 cm3/min

 

Q2 An athlete sprinter heart rate is 120 beats per minute and a cardiac output of 6000 cm3/min.

Calculate the athlete's stroke volume.

stroke volume = cardiac output / heart rate = 6000 / 120 = 50 cm3

 

Q3 The cardiac output from a person is 3600 cm3/min.

If the person's stroke volume is 60 cm3, what is the pulse rate?

heart rate beat = cardiac output / stroke volume = 3600 / 60 = 60 beats/min


Summary of learning objectives and key words or phrases

Be able to interpret diagrams explaining how the heart works including the valves, cardiac muscles, aorta, pulmonary artery, pulmonary vein, vena cava, left atrium, left atrium, right ventricle and left ventricle.

Knowledge of the names of the blood vessels associated with the heart   ...

  • arteries are blood vessels that carry deoxygenated blood away from the heart to the lungs,

  • veins are blood vessels that carry oxygenated blood to the heart from the lungs and exit the heart via the aorta to the rest of the body

  • capillaries are the smallest blood vessels

  • aorta - the oxygenated blood flow exit to the organs

  • vena cava - the deoxygenated blood flow entry from the organs

  • pulmonary artery - carries the deoxygenated blood to the lungs

  • pulmonary vein - brings the oxygenated blood to the heart from the lungs


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