Respiration and Circulation
4. The Heart and Blood Vessels
Learning outcomes
- I can identify the major chambers of the heart.
- I can describe the pathway of blood through the heart.
- I can explain the functions of arteries, veins, and capillaries.
- I can describe how the heart acts as a pump.
- I can explain the importance of blood pressure and circulation.
Introduction
The heart is one of the hardest-working organs in the human body. It beats continuously throughout your life, pumping blood to every cell. This blood carries oxygen and nutrients to tissues while removing carbon dioxide and other waste products. Without this constant circulation, body cells would quickly run out of oxygen and die.
The heart, together with the blood vessels, forms the transport network of the circulatory system. Blood flows through the heart, travels to the lungs to collect oxygen, and is then pumped throughout the body. Understanding how blood moves through the heart and blood vessels helps explain how the body delivers essential substances to every organ.
The Heart
The heart is a muscular organ about the size of a clenched fist.
It is located:
- Slightly left of the centre of the chest.
- Between the lungs.
- Protected by the rib cage.
The heart contracts continuously, pumping blood through the circulatory system.
An average adult heart beats about 60–100 times per minute at rest.
Figure 1. The heart is a four-chambered muscular pump that circulates blood throughout the body.
The Four Chambers of the Heart
The heart has four chambers.
Right Atrium
- Receives oxygen-poor blood returning from the body.
Right Ventricle
- Pumps oxygen-poor blood to the lungs.
Left Atrium
- Receives oxygen-rich blood returning from the lungs.
Left Ventricle
- Pumps oxygen-rich blood to the rest of the body.
The left ventricle has the thickest muscular wall because it must pump blood throughout the entire body.
The Pathway of Blood Through the Heart
Blood follows a continuous pathway.
- Oxygen-poor blood returns from the body.
- It enters the right atrium.
- Blood flows into the right ventricle.
- The right ventricle pumps blood to the lungs through the pulmonary artery.
- In the lungs, carbon dioxide is removed and oxygen is absorbed.
- Oxygen-rich blood returns to the left atrium through the pulmonary veins.
- Blood flows into the left ventricle.
- The left ventricle pumps blood to the body through the aorta.
The cycle then repeats continuously.
Figure 2. Blood follows a one-way pathway through the heart, lungs, and body.
The Heart as a Pump
The heart works like two pumps joined together.
Right Side
The right side pumps oxygen-poor blood to the lungs.
This is called pulmonary circulation.
Left Side
The left side pumps oxygen-rich blood to the body.
This is called systemic circulation.
Heart valves ensure that blood flows in only one direction.
The heart contracts and relaxes in a regular rhythm to maintain continuous circulation.
Blood Vessels
Blood travels through three main types of blood vessels.
- Arteries
- Veins
- Capillaries
Each vessel has a structure adapted to its function.
Arteries
Arteries carry blood away from the heart.
Features include:
- Thick muscular walls.
- Elastic tissue.
- Small lumen.
- High blood pressure.
Most arteries carry oxygen-rich blood.
The pulmonary artery is the main exception because it carries oxygen-poor blood to the lungs.
Veins
Veins carry blood toward the heart.
Features include:
- Thinner walls.
- Larger lumen.
- Lower blood pressure.
- Valves that prevent backflow.
Most veins carry oxygen-poor blood.
The pulmonary veins are the main exception because they carry oxygen-rich blood back to the heart.
Capillaries
Capillaries are the smallest blood vessels.
Their walls are only one cell thick.
This allows:
- Oxygen to diffuse into tissues.
- Carbon dioxide to diffuse into the blood.
- Nutrients to move into cells.
- Waste products to enter the bloodstream.
Capillaries connect arteries and veins.
Figure 3. Arteries, veins, and capillaries have different structures that suit their specialised functions.
Blood Pressure
Blood pressure is the force of blood pushing against the walls of arteries.
It is created by the pumping action of the heart.
Blood pressure helps:
- Deliver oxygen.
- Transport nutrients.
- Remove wastes.
Normal blood pressure ensures that blood reaches every part of the body.
Blood pressure is usually recorded as two numbers, for example:
120/80 mmHg
- The first number is the pressure when the heart contracts (systolic pressure).
- The second number is the pressure when the heart relaxes (diastolic pressure).
Maintaining healthy blood pressure is important for protecting the heart and blood vessels.
Circulation
The movement of blood around the body is called circulation.
There are two main circulation pathways.
Pulmonary Circulation
Heart → Lungs → Heart
Purpose:
- Picks up oxygen.
- Removes carbon dioxide.
Systemic Circulation
Heart → Body → Heart
Purpose:
- Delivers oxygen and nutrients.
- Removes wastes.
Together, these two circuits ensure that every cell receives the materials it needs.
Figure 4. Blood circulates through both the lungs and the rest of the body in two connected circuits.
Why Circulation Is Important
Continuous circulation allows the body to:
- Deliver oxygen.
- Supply nutrients.
- Remove carbon dioxide.
- Transport hormones.
- Fight infection.
- Maintain homeostasis.
Without circulation, cells would quickly run out of oxygen and nutrients.
Working with Other Organ Systems
The circulatory system works closely with:
- Respiratory system – Supplies oxygen and removes carbon dioxide.
- Digestive system – Absorbs nutrients into the blood.
- Urinary system – Removes waste products from the blood.
- Endocrine system – Releases hormones carried by the blood.
- Immune system – Uses blood to transport white blood cells that fight infection.
Together, these systems maintain a healthy body.
Figure 5. The heart continuously pumps blood through the lungs and the rest of the body, supplying every cell with essential materials.
Worked Example
Question
Complete the pathway of blood through the heart.
Body → ________ → Right ventricle → Lungs → ________ → Left ventricle → Body
Solution
Body → Right atrium → Right ventricle → Lungs → Left atrium → Left ventricle → Body
The right side pumps blood to the lungs, while the left side pumps oxygen-rich blood to the rest of the body.
Real-World Connection
Doctors measure blood pressure during routine medical check-ups because it provides important information about the health of the heart and blood vessels. Regular exercise, a balanced diet, maintaining a healthy body weight, and avoiding smoking all help keep the circulatory system healthy and reduce the risk of heart disease and stroke.
Did You Know?
An average human heart beats about 100,000 times every day and pumps approximately 7,000–8,000 litres of blood in 24 hours. Over a lifetime, this remarkable organ may beat more than 2.5 billion times without ever taking a break.
Key Terms
Aorta – The largest artery in the body, carrying oxygen-rich blood from the left ventricle to the body.
Artery – A blood vessel that carries blood away from the heart.
Blood pressure – The force exerted by blood against the walls of arteries.
Capillary – A tiny blood vessel where gases, nutrients, and wastes are exchanged.
Circulation – The continuous movement of blood throughout the body.
Pulmonary circulation – The movement of blood between the heart and the lungs.
Systemic circulation – The movement of blood between the heart and the rest of the body.
Valve – A structure that prevents blood from flowing backward.
Vein – A blood vessel that carries blood toward the heart.
Key Takeaways
- The heart is a four-chambered muscular pump consisting of the right atrium, right ventricle, left atrium, and left ventricle.
- Blood follows a one-way pathway through the heart, lungs, and body.
- Arteries carry blood away from the heart, veins return blood to the heart, and capillaries allow the exchange of substances with body tissues.
- The heart generates blood pressure, which drives blood through the circulatory system.
- Blood circulates through two connected pathways: pulmonary circulation and systemic circulation.
- Efficient circulation supplies oxygen and nutrients to cells, removes wastes, and helps maintain homeostasis.