Human Transport » Heart Function and Pumping

What you'll learn this session

Study time: 30 minutes

The structure of the human heart and its chambers
How the heart functions as a double pump
The cardiac cycle and how blood flows through the heart
Heart valves and their role in preventing backflow
The cardiac conduction system and how heartbeats are coordinated
Heart rate regulation and factors affecting it

The Human Heart: Structure and Function

The heart is an amazing muscular organ about the size of your fist that beats around 100,000 times a day to pump blood throughout your body. It's essentially a biological pump that keeps you alive by delivering oxygen and nutrients to your cells while removing waste products.

Key Definitions:

Cardiac muscle: A specialised type of muscle tissue found only in the heart that can contract rhythmically without nervous stimulation.
Double circulation: The system where blood passes through the heart twice in one complete circuit around the body.
Cardiac cycle: The sequence of events that occurs during one heartbeat.

❤️ Heart Structure

The heart has four chambers:

Right atrium: Receives deoxygenated blood from the body via the vena cava
Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery
Left atrium: Receives oxygenated blood from the lungs via the pulmonary vein
Left ventricle: Pumps oxygenated blood to the body via the aorta

The left ventricle has a thicker muscular wall than the right ventricle because it needs to generate more pressure to pump blood around the entire body, while the right ventricle only needs to pump blood to the nearby lungs.

💓 Double Pump System

The heart functions as a double pump:

Right side (pulmonary circulation): Pumps deoxygenated blood to the lungs where it picks up oxygen and releases carbon dioxide
Left side (systemic circulation): Pumps oxygenated blood to the rest of the body to deliver oxygen and nutrients

This double pump system ensures that oxygenated and deoxygenated blood don't mix, maintaining efficient oxygen delivery to tissues.

The Cardiac Cycle

The cardiac cycle describes the sequence of events that occur during one complete heartbeat. Each cycle consists of three main phases: atrial systole, ventricular systole and diastole.

Phases of the Cardiac Cycle

🕐 Atrial Systole

What happens: Both atria contract

Result: Blood is pushed from the atria into the ventricles

Duration: About 0.1 seconds

🕑 Ventricular Systole

What happens: Both ventricles contract

Result: Blood is pushed out of the heart into the arteries

Duration: About 0.3 seconds

🕒 Diastole

What happens: All chambers relax

Result: The heart refills with blood

Duration: About 0.4 seconds

A complete cardiac cycle takes about 0.8 seconds when your heart is beating at a normal resting rate of 75 beats per minute. During exercise, this cycle speeds up to deliver more oxygen to your working muscles.

Heart Valves: Preventing Backflow

The heart contains four valves that ensure blood flows in one direction only. These valves open and close in response to pressure changes during the cardiac cycle.

🚪 Atrioventricular Valves

Located between the atria and ventricles:

Tricuspid valve: Between right atrium and right ventricle
Bicuspid (mitral) valve: Between left atrium and left ventricle

These valves prevent blood from flowing back into the atria when the ventricles contract. They're held in place by strong fibres called chordae tendineae.

🚫 Semilunar Valves

Located at the exits of the ventricles:

Pulmonary valve: Between right ventricle and pulmonary artery
Aortic valve: Between left ventricle and aorta

These valves prevent blood from flowing back into the ventricles when they relax. They snap shut when the pressure in the arteries exceeds the pressure in the ventricles.

Case Study Focus: Heart Valve Disease

Sometimes heart valves can become damaged or diseased, causing them to either not open fully (stenosis) or not close properly (regurgitation). This forces the heart to work harder, which can lead to heart failure over time. In severe cases, damaged valves may need to be repaired or replaced with artificial valves. The "lub-dub" sound heard through a stethoscope is actually the sound of heart valves closing!

The Cardiac Conduction System

For the heart to pump effectively, its chambers must contract in a coordinated sequence. This coordination is controlled by the heart's own electrical system.

How the Heart Beats

The heart can generate and conduct electrical impulses without external nervous input, making it myogenic (self-exciting). The key components of this system include:

Sinoatrial node (SAN): The heart's natural pacemaker located in the right atrium. It initiates each heartbeat by generating electrical impulses.
Atrioventricular node (AVN): Located between the atria and ventricles, it delays the electrical signal slightly to ensure the atria contract before the ventricles.
Bundle of His and Purkinje fibres: These conduct the electrical impulse rapidly throughout the ventricular walls, ensuring coordinated contraction.

This electrical activity can be recorded as an electrocardiogram (ECG), which shows the timing of the different phases of the cardiac cycle.

Heart Rate Regulation

While the heart can beat on its own, its rate is regulated by the nervous system and hormones to match the body's changing needs.

🚀 Factors Increasing Heart Rate

Exercise: Muscles need more oxygen during physical activity
Stress or fear: Prepares the body for 'fight or flight'
Adrenaline: Hormone released during excitement or stress
Fever: Higher body temperature speeds up metabolic reactions
Stimulants: Substances like caffeine can increase heart rate

💫 Factors Decreasing Heart Rate

Rest or sleep: Lower oxygen demands when inactive
Relaxation techniques: Meditation, deep breathing
Physical fitness: Athletes have stronger hearts that can pump more blood per beat
Certain medications: Beta-blockers prescribed for heart conditions

Case Study Focus: Athlete's Heart

Elite endurance athletes often have resting heart rates as low as 40 beats per minute (compared to the average 60-80 bpm). This condition, called "athlete's heart," is a beneficial adaptation to regular training. Their hearts become larger and stronger, pumping more blood with each beat, which means fewer beats are needed. Olympic swimmer Michael Phelps reportedly had a resting heart rate of just 38-40 bpm during his competitive years!

Heart Function and Health

Maintaining heart health is crucial for overall wellbeing. Common heart problems include:

Coronary heart disease: Narrowing of the coronary arteries that supply the heart muscle with blood
Arrhythmias: Irregular heartbeats caused by problems in the electrical conduction system
Heart failure: When the heart cannot pump blood effectively enough to meet the body's needs

You can protect your heart by maintaining a healthy diet, exercising regularly, avoiding smoking, limiting alcohol consumption and managing stress effectively.

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