Why do large animals need a circulatory system?

Large animals have a small surface area to volume ratio, so diffusion alone cannot supply oxygen and nutrients to all cells quickly enough. A circulatory system uses a pump (heart) and a network of vessels to transport substances rapidly, overcoming the limitations of diffusion. It also helps remove waste products like carbon dioxide from tissues.

What is the difference between systole and diastole?

Systole is the contraction phase of the cardiac cycle, when the heart chambers pump blood out. Diastole is the relaxation phase, when the chambers fill with blood. Atrial systole forces blood into the ventricles, ventricular systole forces blood into the arteries, and diastole allows the heart to refill.

How does the heart control its own beat?

The sinoatrial node (SAN) in the right atrium generates electrical impulses spontaneously, setting the heart rate. These impulses spread across the atria, causing them to contract. The atrioventricular node (AVN) delays the impulse, then sends it down the bundle of His and Purkyne fibres to the ventricles, causing ventricular contraction from the apex upwards.

Why do arteries have thick walls?

Arteries carry blood away from the heart under high pressure. Their thick walls contain smooth muscle and elastic fibres to withstand and maintain this pressure. The elastic fibres allow arteries to stretch during systole and recoil during diastole, helping to push blood along and maintain a continuous flow.

What is the role of haemoglobin in oxygen transport?

Haemoglobin in red blood cells binds reversibly with oxygen to form oxyhaemoglobin. In the lungs, where oxygen concentration is high, haemoglobin loads oxygen. In respiring tissues, where oxygen concentration is low, haemoglobin releases oxygen. This cooperative binding allows efficient oxygen transport and release where needed.

How does exercise affect heart rate?

During exercise, muscles require more oxygen and produce more carbon dioxide. Chemoreceptors detect increased CO2 and decreased pH, while proprioceptors detect movement. The medulla oblongata increases sympathetic stimulation to the SAN, raising heart rate. This increases cardiac output to deliver more oxygen and remove waste.

Transport in animals

OCR

A-Level

This topic explores the necessity of transport systems in multicellular animals due to their size, metabolic rate, and surface area to volume ratio. It covers the structure and function of the mammalian circulatory system, including the heart, blood vessels, and the role of haemoglobin in gas transport.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Transport in animals is a core topic in OCR A-Level Biology that explores how multicellular organisms overcome the limitations of diffusion alone. You'll study the structure and function of the circulatory system, focusing on the mammalian heart, blood vessels, and the composition of blood. Understanding this topic is essential because efficient transport systems are vital for delivering oxygen and nutrients to cells and removing waste products like carbon dioxide. This knowledge also links to homeostasis, gas exchange, and the immune system.

The topic begins with the need for a transport system in larger organisms, comparing open and closed circulatory systems. You'll then dive into the mammalian double circulatory system, including the pulmonary and systemic circuits. Detailed study of the heart's structure, the cardiac cycle, and the control of heart rate follows. Blood vessels (arteries, veins, capillaries) are examined in terms of their adaptations for function. Finally, the composition of blood—red blood cells, white blood cells, platelets, and plasma—is covered, along with the transport of oxygen and carbon dioxide.

This topic is fundamental for understanding how organisms maintain internal balance and respond to changing demands. It also provides a foundation for more advanced concepts like cardiovascular disease, the effects of exercise, and the role of the lymphatic system. Mastery of transport in animals is crucial for exam success, as it frequently appears in multiple-choice, short-answer, and extended-response questions.

Key Concepts

Core ideas you must understand for this topic

→The need for a transport system: surface area to volume ratio, metabolic rate, and the limitations of diffusion in large, active organisms.
→The mammalian double circulatory system: pulmonary circulation (heart to lungs) and systemic circulation (heart to body), ensuring efficient separation of oxygenated and deoxygenated blood.
→Structure and function of the heart: four chambers (atria and ventricles), valves (atrioventricular and semilunar), and the cardiac cycle (atrial systole, ventricular systole, diastole).
→Control of heart rate: the role of the sinoatrial node (SAN), atrioventricular node (AVN), Purkyne fibres, and the autonomic nervous system (sympathetic and parasympathetic).
→Blood vessel adaptations: arteries (thick muscular walls, elastic fibres), veins (valves, thin walls), and capillaries (one-cell thick walls, large surface area) for efficient transport and exchange.

What You Need to Demonstrate

Key skills and knowledge for this topic

Need for transport systems related to SA:V ratio and metabolic rate
Differences between single, double, open, and closed circulatory systems
Structural differences between arteries, arterioles, capillaries, venules, and veins
Formation of tissue fluid from plasma via hydrostatic and oncotic pressure
Cardiac cycle stages and pressure changes
Initiation and coordination of heart action (SAN, AVN, Purkyne tissue)
Interpretation of ECG traces (tachycardia, bradycardia, fibrillation, ectopic)
Role of haemoglobin in oxygen and carbon dioxide transport (chloride shift, carbonic anhydrase)

Marking Points

Key points examiners look for in your answers

Need for transport systems related to SA:V ratio and metabolic rate
Differences between single, double, open, and closed circulatory systems
Structural differences between arteries, arterioles, capillaries, venules, and veins
Formation of tissue fluid from plasma via hydrostatic and oncotic pressure
Cardiac cycle stages and pressure changes
Initiation and coordination of heart action (SAN, AVN, Purkyne tissue)
Interpretation of ECG traces (tachycardia, bradycardia, fibrillation, ectopic)
Role of haemoglobin in oxygen and carbon dioxide transport (chloride shift, carbonic anhydrase)
Oxygen dissociation curves (fetal vs adult, Bohr effect)

Examiner Tips

Expert advice for maximising your marks

💡Ensure you can link the structure of blood vessels to their specific functions
💡Practice calculating cardiac output using the formula: cardiac output = heart rate x stroke volume
💡Be prepared to interpret ECG traces for specific heart conditions
💡Understand the significance of the shift in the oxygen dissociation curve for fetal haemoglobin
💡Use precise terminology when describing the chloride shift
💡When describing the cardiac cycle, always mention pressure changes and valve movements. Use phrases like 'pressure in the ventricles exceeds pressure in the atria, causing the AV valves to close' to show understanding.
💡For heart rate control, clearly distinguish between the roles of the SAN (pacemaker) and the autonomic nervous system. Remember that the medulla oblongata sends impulses via the sympathetic (accelerator) and parasympathetic (vagus) nerves.
💡In questions about blood vessel structure, link adaptations to function. For example, 'arteries have elastic fibres to stretch and recoil, maintaining blood pressure during diastole'.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing hydrostatic and oncotic pressure in tissue fluid formation
Incorrectly describing the sequence of the cardiac cycle
Failing to mention the myogenic nature of cardiac muscle
Misinterpreting the Bohr effect on the oxygen dissociation curve
Confusing the roles of the SAN and AVN
Misconception: The heart pumps blood when it relaxes. Correction: The heart pumps blood during contraction (systole); relaxation (diastole) allows the chambers to fill with blood.
Misconception: All arteries carry oxygenated blood. Correction: The pulmonary artery carries deoxygenated blood from the heart to the lungs; only systemic arteries carry oxygenated blood.
Misconception: Capillaries have thick walls to withstand pressure. Correction: Capillaries have walls one cell thick to allow efficient diffusion of gases and nutrients; they have low blood pressure.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Cell structure and transport mechanisms (diffusion, osmosis, active transport) – to understand why transport systems are needed.
•Gas exchange in mammals – to see how the circulatory system works with the lungs.
•Basic biochemistry of haemoglobin and oxygen dissociation curves – for understanding oxygen transport.

Likely Command Words

How questions on this topic are typically asked

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Interpret

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Practice questions tailored to this topic

Transport in animals

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

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Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Transport in animals — OCR A-Level Biology

Examiner Tips for Transport in animals

Common Mistakes in Transport in animals

Key Marking Points

Transport in animals

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Why do large animals need a circulatory system?

What is the difference between systole and diastole?

How does the heart control its own beat?

Why do arteries have thick walls?

What is the role of haemoglobin in oxygen transport?

How does exercise affect heart rate?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules