AQA GCSE Biology 8461: Principles of organisation; the heart and blood vessels – Study Guide

This guide teaches two subtopics of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards: 4.2.1 Principles of organisation and 4.2.2.2 The heart and blood vessels. Both sit in Topic 2, Organisation, which is assessed on Paper 1 (1 hour 45 minutes, 100 marks), set at Foundation and Higher Tier. Neither subtopic contains content the specification marks (HT only), so everything here is for both tiers. No required practical activity is listed under these two subtopics.

For quick recall, use the revision notes for this unit. To test yourself, use the practice questions for this unit. Digestion (4.2.2.1) is covered separately in the Enzymes and the Digestive System study guide. The course hub is AQA GCSE Biology, and the printable checklist lists every specification point.

What this unit covers

Spec point	What you must be able to do	Tier
4.2.1	State that cells are the basic building blocks of all living organisms; define tissue, organ and organ system	Both
4.2.1 (MS 1c)	Understand size and scale from cells to tissues, organs and systems	Both
4.2.2.2	Describe the structure and functioning of the human heart and lungs, including how the lungs are adapted for gaseous exchange	Both
4.2.2.2	Explain the double circulatory system: right ventricle to the lungs, left ventricle to the rest of the body	Both
4.2.2.2	Name the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries	Both
4.2.2.2	Describe the natural pacemaker in the right atrium and the use of artificial pacemakers	Both
4.2.2.2	Explain how the structure of arteries, veins and capillaries relates to their functions	Both
4.2.2.2 (MS 1a, 1c)	Use simple compound measures such as rate; carry out rate calculations for blood flow	Both

Two limits from the specification help you revise efficiently. You do not need the names of the heart valves. For the lungs, you need only the trachea, bronchi, alveoli and the capillary network around the alveoli.

4.2.1 Principles of organisation

Large organisms are built in levels. Each level is made from the one below it.

1. Cells are the basic building blocks of all living organisms.
2. A tissue is a group of cells with a similar structure and function.
3. An organ is an aggregation of tissues performing specific functions.
4. Organs are organised into organ systems, which work together to form organisms.

Cardiac muscle cells form muscle tissue. The heart is an organ: it contains muscle tissue, but also other tissues, such as the tissue that lines its chambers. The heart, the blood vessels and the blood make up the circulatory system, which is an organ system. The circulatory system works with the respiratory system (the lungs) and the digestive system to keep the whole organism supplied.

Notice the key word in the organ definition: tissues, plural. A single tissue is not an organ. An exam answer that calls “muscle tissue” an organ, or calls “the heart” a tissue, loses the mark.

Size and scale (MS 1c)

The specification expects you to understand how big each level is. Cells are measured in micrometres (µm), organs in centimetres, and whole organisms in metres. Remember the conversions:

• 1 mm = 1000 µm
• 1 cm = 10 mm = 10 000 µm
• 1 m = 1000 mm = 1 000 000 µm

Worked example 1. A heart cell is about 10 µm wide. Suppose a heart is 12 cm long. How many times longer is the heart than the cell, and how many orders of magnitude is this?

Convert to the same unit:   12 cm = 12 x 10 000 um = 120 000 um
Ratio:                       120 000 / 10 = 12 000
12 000 is about 10^4, so the heart is about 4 orders of magnitude larger.

4.2.2.2 The heart

The heart is an organ that pumps blood around the body. Its walls are mostly muscle. It has four chambers:

• two atria at the top, which receive blood coming back to the heart;
• two ventricles at the bottom, which pump blood out of the heart.

The right side and the left side are separated by a wall of muscle, so oxygenated and deoxygenated blood do not mix. Valves inside the heart stop blood flowing backwards, so blood moves one way only: atria, then ventricles, then out into the arteries. You need to know what valves do, but not their names.

The route of blood through the heart

Body -> vena cava -> right atrium -> right ventricle -> pulmonary artery -> LUNGS
Lungs -> pulmonary vein -> left atrium -> left ventricle -> aorta -> BODY

Vessel	Carries blood	Oxygenated?
Vena cava	From the body into the right atrium	Deoxygenated
Pulmonary artery	From the right ventricle to the lungs	Deoxygenated
Pulmonary vein	From the lungs into the left atrium	Oxygenated
Aorta	From the left ventricle to the rest of the body	Oxygenated
Coronary arteries	To the heart muscle itself	Oxygenated

Two vessels break the “arteries carry oxygenated blood” pattern. The pulmonary artery is an artery (it carries blood away from the heart) but its blood is deoxygenated. The pulmonary vein is a vein (it carries blood towards the heart) but its blood is oxygenated. Define arteries and veins by direction, not by oxygen.

The coronary arteries branch from the aorta and run over the surface of the heart. Heart muscle cells need their own supply of oxygen and glucose for respiration. They cannot take it from the blood inside the chambers, so the coronary arteries deliver it.

The double circulatory system

The heart pumps blood in a double circulatory system. In one complete circuit of the body, blood passes through the heart twice.

• The right ventricle pumps blood to the lungs, where gas exchange takes place.
• The left ventricle pumps blood around the rest of the body.

Blood returning from the lungs comes back to the heart and is pumped again, so it leaves for the body under higher pressure. This is why the left ventricle has a thicker, more muscular wall than the right ventricle: it pumps blood all the way around the body, while the right ventricle pumps blood only to the lungs, which are close by.

Controlling the heart rate

The natural resting heart rate is controlled by a group of cells located in the right atrium that act as a pacemaker. These cells set the rhythm at which the heart muscle contracts.

If the natural pacemaker does not work properly, the heart rate can become irregular. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate. A small device is placed under the skin, with a wire to the heart, and it sends electrical impulses to keep the heart beating at a regular rate.

4.2.2.2 The lungs and gas exchange

You need four structures only:

Structure	Role
Trachea	Carries air from the mouth and nose down into the chest
Bronchi	Two tubes that branch from the trachea, one into each lung
Alveoli	Tiny air sacs where gas exchange takes place
Capillary network	Surrounds the alveoli; blood picks up oxygen and gives up carbon dioxide

In the alveoli, oxygen diffuses from the air into the blood. Carbon dioxide diffuses from the blood into the air and is breathed out. Deoxygenated blood arrives from the pulmonary artery, and oxygenated blood leaves in the pulmonary vein.

How the lungs are adapted for gaseous exchange

Feature	Why it helps diffusion
Millions of alveoli	A very large surface area for gases to cross
Alveolus walls are very thin	A short diffusion distance between air and blood
A dense capillary network	Blood constantly carries oxygen away and brings carbon dioxide, keeping steep concentration gradients
Breathing (ventilation)	Brings fresh air in and removes stale air, so the gradients stay steep

When you explain an adaptation, always finish the sentence with the effect on diffusion: “large surface area, so more oxygen can diffuse into the blood at once”.

4.2.2.2 Arteries, veins and capillaries

The body contains three different types of blood vessel. The specification asks you to explain how the structure of each relates to its function.

	Artery	Vein	Capillary
Direction	Away from the heart	Towards the heart	Links arteries to veins, through tissues
Pressure	High	Low	Low
Wall	Thick, with layers of muscle and elastic fibres	Thinner, less muscle and elastic tissue	Only one cell thick
Lumen (space inside)	Narrow compared with wall thickness	Large	Very narrow, just wide enough for red blood cells
Valves	None along their length	Yes, to prevent backflow	None

Arteries. Blood leaves the ventricles at high pressure. The thick muscular wall withstands that pressure. Elastic fibres let the wall stretch as each surge of blood arrives and then recoil.

Veins. By the time blood reaches the veins the pressure is low. The large lumen lets blood flow back easily, and valves stop it flowing backwards.

Capillaries. These are where substances are exchanged with body cells. Their walls are only one cell thick, so the diffusion distance is short. Oxygen and glucose diffuse out to the cells, and carbon dioxide and other wastes diffuse in. Huge numbers of capillaries mean every cell is close to one.

Rate calculations for blood flow (MS 1a, 1c)

A rate is an amount divided by time. Write the formula, substitute, and give the unit.

rate = amount / time

Worked example 2: heart rate from a pulse count. A student counts 21 pulse beats in 15 seconds. Calculate the heart rate in beats per minute.

Beats per second = 21 / 15
Beats per minute = 21 / 15 x 60 = 84 beats per minute

Worked example 3: volume of blood pumped per minute. A ventricle pumps 70 cm³ of blood per beat. The heart rate is 72 beats per minute. Calculate the volume pumped per minute in dm³.

Volume per minute = volume per beat x beats per minute
                  = 70 x 72 = 5040 cm3 per minute
1 dm3 = 1000 cm3, so 5040 cm3 = 5.04 dm3 per minute

Worked example 4: rate of blood flow through a vessel. 300 cm³ of blood flows through a vessel in 25 seconds. Calculate the rate of flow in cm³ per second and in cm³ per minute.

Rate = 300 / 25 = 12 cm3/s
Per minute: 12 x 60 = 720 cm3/min

Worked example 5: speed of blood in a capillary. Under a microscope, a red blood cell moves 0.6 mm along a capillary in 2 seconds. Calculate the speed.

Speed = distance / time = 0.6 / 2 = 0.3 mm/s

Blood moves slowly through capillaries. This gives more time for substances to diffuse between the blood and the cells.

Common errors

• Calling the heart a tissue, or muscle tissue an organ. An organ contains several tissues.
• Saying the left ventricle pumps blood to the lungs. It is the right ventricle.
• Writing that all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood.
• Saying the pacemaker cells are in the left atrium, or in a ventricle. They are in the right atrium.
• Explaining the thicker left ventricle wall without saying it pumps blood around the whole body.
• Saying capillaries have “thin walls” without saying one cell thick and short diffusion distance.
• Giving veins thick walls, or arteries valves along their length.
• Dividing time by amount in a rate calculation, or leaving out the unit.

Where to go next

• Revision notes for this unit
• Practice questions for this unit
• Enzymes and the Digestive System revision notes for 4.2.2.1
• All free 10-minute diagnostics

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.2 Organisation: 4.2.1 Principles of organisation and 4.2.2.2 The heart and blood vessels.