Welcome to one of the most fundamental areas of human biology! This topic covers how your body transports essential resources (like oxygen and glucose) to every single living cell, and how those cells then convert those resources into usable energy.

What you'll learn:

• How the components of your blood are perfectly adapted for transport and defense.
• How the structures of arteries, veins, and capillaries match their specific jobs.
• The anatomy of the heart and how to calculate cardiac output.
• The differences between aerobic and anaerobic respiration, and how we measure respiration rates in the lab.

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1. The Structure and Function of Blood

Blood is not just a liquid; it is a vital tissue consisting of cells suspended in a liquid medium called plasma. Each component has a highly specialized role to play.

Plasma

Plasma is a straw-colored liquid that makes up over half of your blood volume. It acts as the transport medium for:

• Carbon dioxide: carried as hydrogencarbonate ions from respiring cells to the lungs.
• Urea: a waste product made in the liver, transported to the kidneys for excretion.
• Digested food molecules: such as glucose and amino acids, carried from the small intestine to cells.
• Hormones: chemical messengers transported from endocrine glands to target organs.
• Heat energy: distributed evenly around the body to maintain a constant core temperature.

Red Blood Cells (Erythrocytes)

The sole purpose of red blood cells is to transport oxygen from the lungs to respiring tissues.

In the lungs, where oxygen concentration is high, oxygen binds to haemoglobin to form oxyhaemoglobin:

$$Haemoglobin+Oxygen\rightleftharpoons Oxyhaemoglobin$$

In respiring tissues, where oxygen concentration is low, the reaction reverses, releasing the oxygen to diffuse into the cells.

White Blood Cells

White blood cells are an essential part of the immune system, defending the body against pathogens (disease-causing microorganisms). You need to know two main types:

1. Phagocytes: These cells engulf and digest pathogens in a process called phagocytosis. They have a lobed (multi-lobed) nucleus, which allows them to easily squeeze through capillary walls to reach infected tissues.
2. Lymphocytes: These cells produce proteins called antibodies. Each antibody has a specific shape that perfectly matches and binds to the antigens (marker proteins) on the surface of a specific pathogen, clumping them together so phagocytes can destroy them easily.

Platelets

Platelets are tiny, membrane-bound cell fragments with no nucleus. When you cut yourself, they release chemicals that trigger blood clotting. This forms a scab, which serves two vital purposes:

• It prevents excessive blood loss.
• It prevents pathogens from entering the open wound.

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Higher Tier: Calculating with Standard Form

Because cells are incredibly small and exist in massive numbers, GCSE exam questions often require you to perform biological calculations using standard form ($A\times 10^n$).

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2. Structure and Function of Blood Vessels

Blood is pumped around the body through a closed network of tubes called blood vessels. There are three distinct types, each structurally adapted to its specific job:

How Structure Relates to Function

• Arteries carry blood directly from the heart. The high pressure requires thick, muscular walls to prevent the vessel from bursting. The elastic fibres allow the walls to stretch when the heart beats and recoil afterwards, which helps smooth out the blood flow.
• Veins carry blood under low pressure back to the heart. Because the pressure is low, the walls can be thin. They have a wide lumen to minimize friction and resistance to blood flow. Because blood travels slowly under low pressure, veins contain pocket valves to ensure blood only flows in one direction—preventing it from pooling due to gravity.
• Capillaries are the site of substance exchange. Their walls are only one cell thick (made of a single layer of endothelial cells). This provides an incredibly short diffusion distance for oxygen and glucose into tissues, and carbon dioxide out. Their lumen is so narrow that red blood cells must squeeze through in single file, slowing them down and bringing them as close as possible to surrounding respiring cells.

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3. The Heart and Circulatory System

Humans have a double circulatory system. This means that for every complete circuit of the body, blood passes through the heart twice:

1. The Pulmonary Circuit: The right side of the heart pumps deoxygenated blood to the lungs to pick up oxygen and release carbon dioxide.
2. The Systemic Circuit: The left side of the heart pumps oxygenated blood to the rest of the body to deliver oxygen to tissues.

Anatomy of the Heart

The heart is a muscular pump split into a left side and a right side by a wall of muscle called the septum.

The Pathway of Blood Through the Heart

Let's follow a drop of deoxygenated blood as it returns from the body:

1. Deoxygenated blood enters the Right Atrium via the Vena Cava.
2. The atrium contracts, pushing blood through the Tricuspid Valve (an atrioventricular valve) into the Right Ventricle.
3. The right ventricle contracts, forcing the tricuspid valve shut (preventing backflow) and opening the Semi-lunar Valve. Blood is pumped out through the Pulmonary Artery to the lungs.
4. In the lungs, the blood becomes oxygenated. It returns to the heart, entering the Left Atrium via the Pulmonary Vein.
5. The left atrium contracts, pushing blood through the Bicuspid (Mitral) Valve into the Left Ventricle.
6. The left ventricle contracts, forcing the bicuspid valve shut and opening the Semi-lunar Valve. Oxygenated blood is pumped out of the Aorta to the rest of the body.

Relative Thickness of the Chamber Walls

If you look closely at the heart diagram, you will notice two major differences in wall thickness:

• Atria walls vs. Ventricle walls: Atria have very thin muscular walls because they only have to pump blood a very short distance down into the ventricles. Ventricles have much thicker walls because they must pump blood out of the heart.
• Left Ventricle wall vs. Right Ventricle wall: The muscular wall of the left ventricle is significantly thicker than that of the right ventricle. The right ventricle only pumps blood a short distance to the lungs, which are delicate and close to the heart. The left ventricle must generate enough pressure to pump blood all the way around the entire body.

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Calculating Cardiac Output

To assess how well a heart is functioning, we can calculate its cardiac output—the volume of blood pumped by one ventricle per minute.

• Stroke Volume: The volume of blood pumped out of a ventricle in a single beat (measured in cubic centimetres, ${cm}^3$).
• Heart Rate: The number of times the heart beats per minute (measured in beats per minute, $bpm$).
• Cardiac Output: Measured in ${cm}^3/min$ or ${dm}^3/min$.

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4. Cellular Respiration

Many students confuse breathing with respiration. Breathing (ventilation) is a mechanical process of moving air in and out of the lungs. Respiration is a chemical reaction.

Because respiration releases energy to its surroundings, it is classified as an exothermic reaction. Cells use this released energy for:

• Contracting muscles (movement).
• Active transport of molecules against concentration gradients.
• Building large molecules from smaller ones (protein synthesis).
• Maintaining a constant body temperature (in mammals and birds).

Aerobic Respiration

Aerobic respiration occurs only when plenty of oxygen is available. It is highly efficient because glucose is completely broken down, releasing a large amount of energy. Most of the reactions in aerobic respiration take place inside the mitochondria of cells.

• Word Equation:

$$Glucose+Oxygen⟶Carbon\; Dioxide+Water(+Energy\; released)$$

• Balanced Chemical Equation:

$$C_6{H}_{12}{O}_6+6O_2⟶6{CO}_2+6H_{2}O$$

Anaerobic Respiration

Anaerobic respiration occurs in the absence of oxygen (or when oxygen levels are extremely low, such as during vigorous exercise). Because glucose is only partially broken down, it releases much less energy per glucose molecule than aerobic respiration.

In Animal Muscle Cells:

During high-intensity exercise, the blood cannot supply oxygen to muscles fast enough to keep up with the demand for energy. Muscles switch to anaerobic respiration, producing lactic acid:

• Word Equation:

$$Glucose⟶Lactic\; Acid(+Some\; energy\; released)$$

In Plants and Yeast (Fermentation):

When plants or yeast cells respire without oxygen, they produce ethanol and carbon dioxide instead of lactic acid:

• Word Equation:

$$Glucose⟶Ethanol+Carbon\; Dioxide(+Some\; energy\; released)$$

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5. Core Practical: Investigating Respiration Rate

You must be able to describe how to set up and use a respirometer to measure the rate of respiration in small, living organisms (such as germinating seeds or woodlice).

How the Respirometer Works

1. Oxygen Consumption: The living organisms in the experimental tube respire aerobically. They take in oxygen gas from the air inside the tube.
2. Carbon Dioxide Absorption: As they respire, they release carbon dioxide gas. However, the sodium hydroxide (or soda lime) at the bottom of the tube chemically absorbs this carbon dioxide instantly.
3. Pressure Drop: Because the oxygen gas is being consumed, and the produced carbon dioxide is being absorbed, the overall volume of gas inside the experimental tube decreases. This causes a decrease in gas pressure.
4. Liquid Movement: The air pressure in the control tube remains higher, which pushes the colored liquid in the capillary U-tube towards the experimental tube.
5. Measuring Rate: By measuring the distance the colored liquid moves in a set amount of time (using a ruler and stopwatch), you can calculate the rate of oxygen consumption, which directly represents the rate of respiration.

Why is the Control Tube Necessary?

The control tube contains glass beads with the exact same volume and mass as the living organisms. Since no living things are inside, it does not respire.

If the room temperature fluctuates or atmospheric pressure changes, the air in both tubes will expand or contract by the same amount. This prevents external environmental factors from pushing the liquid, ensuring that any movement of the colored liquid is solely due to the respiration of the organisms.

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