The Human Circulatory System (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

The Heart

The double circulatory system

• The human heart is part of a double circulatory system

• The circulatory system consists of

  • Blood vessels

  • A pump (the heart)

  • Valves to maintain one-way blood flow

• The heart has four chambers divided into two sides

  • The right side pumps blood to the lungs for gas exchange

  • The left side pumps blood at high pressure to the body

Benefits of a double circulatory system

• Blood loses pressure as it passes through lung capillaries, slowing flow and allowing time for gas exchange

• Oxygenated blood returns to the heart so pressure can be increased before being pumped to the body

  • This allows oxygen to be delivered to body cells more quickly

Heart structure

• The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs

• Gas exchange occurs in the lungs where oxygen diffuses in and carbon dioxide diffuses out

• The left side of the heart receives oxygenated blood from the lungs and pumps it to the body

• Veins carry blood towards the heart and arteries carry blood away from the heart

• The four chambers are arranged vertically

  • Atria are the upper chambers

  • Ventricles are the lower chambers

Pathway of blood through the heart

• Deoxygenated blood enters the right atrium via the vena cava

• Blood flows through valves into the right ventricle

• The right ventricle pumps blood to the lungs via the pulmonary artery

• Oxygenated blood returns to the left atrium via the pulmonary vein

• Blood flows through valves into the left ventricle

• The left ventricle pumps blood to the body via the aorta

Adaptations of the heart

• Ventricles have thicker walls than atria to generate higher pressure

  • The left ventricle has the thickest wall to pump blood around the body

  • The right ventricle pumps blood at lower pressure to the lungs

• Valves prevent the backflow of blood

• The septum separates the two sides of the heart and prevents mixing of oxygenated and deoxygenated blood

• Coronary arteries supply the heart muscle with oxygen and glucose for aerobic respiration

Heart rate

Resting heart rate

• The natural resting heart rate is controlled by a group of cells located in the right atrium

• These cells form a structure called the pacemaker

• The role of the pacemaker is to coordinate the contraction of the heart muscle, therefore it regulates the heart rate

• Up to a point, the faster the heart contracts, the more quickly oxygenated blood can be delivered around the body

  • When a person is at rest, the oxygen demand of their cells is relatively low and so a lower heart rate is maintained

  • When a person is exercising, the oxygen demand of their muscle cells increases so a higher heart rate is necessary

• The pacemaker sends out an electrical impulse which spreads to the surrounding muscle cells of the heart, causing them to contract

  • The pacemaker does this every time the heart needs to “beat”, so if a person has a resting heart rate of 60 beats per minute (bpm), then the pacemaker will be sending out electrical impulses on average once every second

Artificial pacemakers

• Sometimes, the pacemaker of the heart stops functioning properly (this can cause an irregular heartbeat)

• Artificial pacemakers are electrical devices used to correct irregularities in the heart rate

• The device is implanted just under the skin, with a wire that delivers an electrical current to the heart to help it contract regularly

Blood vessels

Types of blood vessels

• The body contains three different types of blood vessels:

  • Arteries: transport blood away from the heart (usually at high pressure)

  • Veins: transport blood to the heart (usually at low pressure)

  • Capillaries: links arteries to veins within the tissues of the body

Blood vessels structure

• The walls of each type of blood vessel have a structure that relates to the function of the vessel

• Blood flows through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel (with arteries having a narrow lumen, and the veins a wider one)

  • The lumen of the capillaries is extremely narrow, at the smallest the width of a red blood cell!

The structure of arteries, capillaries and veins diagram

How structure relates to function

• Arteries must withstand and maintain high pressures from the contracting and relaxing heart

  • Their thick walls contain collagen, smooth muscle, and elastic fibers

  • The elastic fibers allow expansion and recoil, maintaining high blood pressure alongside a narrow lumen

• Veins receive low-pressure blood from capillaries and return it to the heart

  • They have thinner walls with fewer layers of collagen, smooth muscle, and elastic fibers, but a much larger lumen

  • Veins contain valves to prevent backflow

• Capillary walls consist of a single layer of endothelial cells, minimising the diffusion distance for oxygen and carbon dioxide

  • These walls have pores that allow blood plasma to leak out and form tissue fluid

The Lungs & the Circulatory System

• The human circulatory system transports substances around the body

• It works closely with the gaseous exchange system in the lungs

• Blood transports

  • Oxygen from the lungs to body cells

  • Carbon dioxide from body cells to the lungs

  • Dissolved food molecules (such as glucose) from the digestive system to cells

• These substances are needed for respiration or removed as waste

Structure of the lungs

• Gas exchange takes place in the lungs

• Air enters the lungs through the trachea

• The trachea splits into two bronchi, one leading to each lung

• Bronchi branch into bronchioles which end in alveoli

• Alveoli are surrounded by a dense capillary network

Alveoli and gas exchange

• Alveoli are specialised for efficient gas exchange

  • They have a large surface area

  • Their walls are very thin, giving a short diffusion distance

  • The surrounding capillary network maintains a steep concentration gradient

• Oxygen diffuses from the air in the alveoli into the blood

• Carbon dioxide diffuses from the blood into the alveoli