Blood Circulation and the Heart

This section explores blood circulation and the heart, topics covered include pulmonary and systemic circulation, atriums and ventricles in the heart, controlling heartbeat, blood vessels, coronary heart disease and blood as a tissue. 

Blood Circulation

The circulatory system is responsible for transporting essential substances around the body, including oxygen, nutrients, and hormones, and for removing waste products such as carbon dioxide and urea. It consists of the heart, blood vessels, and blood. The blood circulates in two main circuits:

• The Pulmonary Circulation: This takes deoxygenated blood from the heart to the lungs for oxygenation and then returns oxygenated blood back to the heart.
• The Systemic Circulation: This carries oxygenated blood from the heart to the body’s tissues and returns deoxygenated blood back to the heart.

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The heart functions as a pump to maintain blood flow throughout the body. It pumps blood through two main circulations: the pulmonary circuit (heart to lungs and back) and the systemic circuit (heart to the body and back).

The Heart

The heart is a muscular organ located in the chest, slightly to the left. It is divided into four chambers:

• Right Atrium: Receives deoxygenated blood from the body.
• Right Ventricle: Pumps deoxygenated blood to the lungs for oxygenation.
• Left Atrium: Receives oxygenated blood from the lungs.
• Left Ventricle: Pumps oxygenated blood to the rest of the body.

Blood flows through the heart in a specific sequence:

1. Deoxygenated blood from the body enters the right atrium.
2. The right atrium contracts, pushing the blood into the right ventricle.
3. The right ventricle contracts and pumps the blood to the lungs via the pulmonary artery.
4. Oxygenated blood from the lungs returns to the left atrium.
5. The left atrium contracts, sending the blood into the left ventricle.
6. The left ventricle contracts, pumping oxygenated blood to the body through the aorta.

The heart’s valves, including the tricuspid valve (right side) and the bicuspid valve (left side), ensure that blood flows in the correct direction.

Controlling Heartbeat

The heartbeat is controlled by electrical impulses that originate in the sinoatrial (SA) node (also known as the pacemaker), located in the right atrium. These impulses cause the atria to contract, pushing blood into the ventricles. The impulses then travel to the atrioventricular (AV) node, which delays the impulse slightly to allow the ventricles to fill with blood before they contract.

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The electrical impulses then travel through the bundle of His and Purkinje fibres, causing the ventricles to contract and pump blood to the lungs and the rest of the body. This electrical system coordinates the rhythm of the heartbeat.

Factors that can influence heart rate include:

• Exercise: Increases heart rate to deliver more oxygen to muscles.
• Stress: Can increase heart rate due to adrenaline release.
• Rest: Heart rate slows down to conserve energy.

Blood Vessels

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Blood vessels form a network throughout the body, transporting blood between the heart and tissues. There are three main types of blood vessels:

Arteries: Carry oxygenated blood away from the heart under high pressure. Arteries have thick, elastic walls to withstand the pressure.

• Example: The aorta is the largest artery in the body, carrying oxygenated blood from the left ventricle to the rest of the body.

Veins: Carry deoxygenated blood towards the heart under lower pressure. Veins have thinner walls and larger lumens (the central space through which blood flows). They also contain valves to prevent blood from flowing backwards.

• Example: The vena cava is the large vein that carries deoxygenated blood from the body back to the right atrium.

Capillaries: Microscopic blood vessels that connect arteries to veins. Capillaries have thin walls (one cell thick) to allow the exchange of gases, nutrients, and waste products between the blood and the tissues.

Coronary Heart Disease (CHD)

Coronary heart disease occurs when the blood vessels supplying the heart muscle (coronary arteries) become narrowed or blocked due to the build-up of fatty deposits (atherosclerosis). This can lead to chest pain (angina) or even a heart attack if the blood supply to the heart is severely reduced or blocked.

Risk Factors for Coronary Heart Disease:

• Smoking: Damages the lining of blood vessels, leading to a build-up of fatty deposits.
• High blood pressure: Puts strain on the heart and blood vessels.
• High cholesterol: Increases the likelihood of fatty deposits forming in the arteries.
• Obesity: Contributes to high blood pressure and high cholesterol levels.
• Lack of exercise: Can lead to poor cardiovascular health.
• Poor diet: High-fat, high-salt diets contribute to cholesterol build-up.

Remedying Heart Disease

There are various treatments available to manage and treat coronary heart disease, including:

Lifestyle changes:

• Stopping smoking, exercising regularly, and eating a balanced diet can significantly reduce the risk of heart disease.

Medications:

• Statins: Drugs that reduce cholesterol levels in the blood.
• Aspirin: Can be prescribed to reduce the risk of blood clots forming.

Surgical interventions:

• Angioplasty: A procedure where a small balloon is inflated inside a blocked artery to widen it, improving blood flow.
• Coronary artery bypass graft (CABG): A surgical procedure where a healthy blood vessel is used to bypass a blocked coronary artery.

Heart transplant: In severe cases, a heart transplant may be necessary if the heart is no longer functioning properly.

Blood as a Tissue

Blood is a connective tissue composed of various components, each with specific functions:

• Plasma: The yellowish liquid part of the blood, which carries water, salts, enzymes, hormones, and waste products. Plasma makes up about 55% of blood volume.
• Red Blood Cells (Erythrocytes): Contain haemoglobin, which binds to oxygen, allowing the blood to transport oxygen around the body. They are biconcave discs that increase the surface area for gas exchange.
• White Blood Cells (Leukocytes): Part of the immune system, white blood cells help fight infections. There are several types, including phagocytes (which engulf pathogens) and lymphocytes (which produce antibodies).
• Platelets: Small cell fragments that help with blood clotting. When a blood vessel is damaged, platelets form a plug to prevent excessive blood loss.

Functions of Blood:

• Transporting oxygen and carbon dioxide.
• Carrying nutrients and waste products.
• Defending against pathogens.
• Involved in regulating body temperature and maintaining pH balance.

This video explains the structure of the heart in more detail.

By understanding blood circulation, the heart, blood vessels, and the management of heart disease, you gain a deeper insight into how the cardiovascular system works to maintain health and well-being.