Transport and Gas Exchange
1. Circulatory Systems
Learning outcomes
- I can explain the purpose of circulatory systems.
- I can compare open and closed circulatory systems.
- I can identify the major components of a circulatory system.
- I can describe how substances are transported throughout the body.
- I can explain how circulatory systems support homeostasis.
Why Do Organisms Need Circulatory Systems?
Every living cell requires substances from its environment and produces substances that must be transported away.
Cells may need:
- oxygen
- glucose and other nutrients
- water
- mineral ions
- hormones
Cells also produce substances such as:
- carbon dioxide
- metabolic wastes
- heat
In very small organisms, diffusion may be sufficient to move substances between cells and their surroundings.
In larger multicellular organisms, however, many cells are located far from the external environment. Diffusion alone would be too slow to supply these cells efficiently.
A circulatory system provides a mass-transport system that moves materials rapidly throughout the organism.
The Main Purpose of Circulation
The central purpose of a circulatory system is transport.
A circulatory system connects different parts of the body so that substances can move between organs and tissues.
For example:
lungs → oxygen → blood → body cells
digestive system → nutrients → blood → body cells
body cells → carbon dioxide → blood → lungs
body cells → wastes → blood → excretory organs
Circulation therefore links many different organ systems.
Why Diffusion Alone Is Not Enough
Diffusion is effective over very short distances.
However, diffusion becomes increasingly slow as distance increases.
Imagine oxygen entering the lungs and then having to diffuse all the way to a muscle in the leg without a circulatory system.
This would not provide oxygen rapidly enough to support the metabolic demands of a large, active organism.
Circulation solves this problem by moving substances rapidly over long distances and leaving diffusion to handle the final short-distance exchange between blood and cells.
Surface Area to Volume Ratio
As organisms become larger, their surface area to volume ratio decreases.
This means there is relatively less external surface available to supply a larger volume of cells.
Large organisms therefore need specialized systems for:
- gas exchange
- digestion
- transport
- excretion
The circulatory system connects these specialized exchange surfaces with cells throughout the body.
Major Components of a Circulatory System
Although circulatory systems vary among animals, they usually contain three basic components:
- a circulating fluid
- a pump
- pathways or spaces through which the fluid moves
In humans these are:
blood → circulating fluid
heart → pump
blood vessels → pathways
Together, these components form the cardiovascular system.
The Heart
The heart is a muscular organ that generates pressure to move blood through the circulatory system.
The human heart contains four chambers:
- right atrium
- right ventricle
- left atrium
- left ventricle
The right side mainly pumps blood toward the lungs.
The left side mainly pumps blood toward the rest of the body.
Repeated contraction of cardiac muscle keeps blood moving continuously.
Blood Vessels
Humans have three major types of blood vessel:
- arteries
- veins
- capillaries
Each has a structure suited to its function.
Arteries
Arteries carry blood away from the heart.
Because blood leaving the heart is generally under relatively high pressure, arteries have:
- thick walls
- muscular tissue
- elastic tissue
- relatively narrow lumens compared with similar-sized veins
Elastic tissue allows artery walls to stretch and recoil as the heart pumps.
A common misconception is that arteries always carry oxygenated blood.
They do not.
The correct definition is:
arteries carry blood away from the heart.
For example, the pulmonary artery carries deoxygenated blood from the heart toward the lungs.
Veins
Veins carry blood toward the heart.
Blood pressure is generally lower in veins than in arteries.
Veins typically have:
- thinner walls
- less muscle and elastic tissue
- relatively large lumens
- valves in many regions
Valves help prevent blood from flowing backward.
Contraction of surrounding skeletal muscles can also help push blood through veins toward the heart.
Capillaries
Capillaries are extremely small blood vessels that connect the arterial and venous sides of circulation and allow exchange with tissues.
Their walls are only about one cell thick.
This creates a short diffusion distance between the blood and surrounding tissues.
Capillary networks also provide a very large total surface area for exchange.
Exchange at Capillaries
Capillaries are where much of the exchange between blood and tissues occurs.
Substances moving from blood toward cells can include:
- oxygen
- glucose
- amino acids
- water
- some hormones
Substances moving from tissues toward blood can include:
- carbon dioxide
- metabolic wastes
- water
- other products of cellular metabolism
The exact movement depends on concentration gradients, pressure differences, membrane properties, and the substance involved.
Blood
Blood is a specialized transport tissue.
It contains:
- plasma
- red blood cells
- white blood cells
- platelets
Each component has particular functions.
Plasma
Plasma is the liquid component of blood.
It is mostly water and carries many dissolved or suspended substances.
These can include:
- nutrients
- hormones
- carbon dioxide, much of it transported as bicarbonate
- urea
- mineral ions
- plasma proteins
- heat
Plasma allows substances from one organ to be transported to another.
Red Blood Cells
Red blood cells are specialized for oxygen transport.
They contain haemoglobin, a protein that binds oxygen.
In humans, mature red blood cells:
- have a biconcave shape
- contain large amounts of haemoglobin
- lack a nucleus
- are flexible enough to pass through narrow capillaries
In the lungs, haemoglobin binds oxygen.
In body tissues, oxygen is released and becomes available for cellular respiration.
White Blood Cells
White blood cells are involved in immune defence.
Different types can:
- destroy pathogens
- engulf microorganisms
- produce antibodies
- coordinate immune responses
Although defence is not the primary transport role of circulation, the circulatory system allows immune cells and immune molecules to reach different parts of the body.
Platelets
Platelets are involved in blood clotting.
When a blood vessel is damaged, platelets participate in processes that lead to formation of a clot.
Clotting helps:
- reduce blood loss
- seal damaged vessels
- reduce entry of microorganisms through the wound
Open Circulatory Systems
Not all animals have blood enclosed entirely within vessels.
An open circulatory system pumps circulating fluid into body spaces where it can directly bathe organs.
The circulating fluid in many animals with open systems is called hemolymph.
Open circulatory systems occur in animals including:
- insects
- many other arthropods
- most molluscs
The fluid moves through body cavities or spaces rather than remaining entirely within a continuous network of blood vessels.
Closed Circulatory Systems
In a closed circulatory system, blood remains within vessels as it circulates.
The basic route is:
heart → vessels → exchange vessels → vessels → heart
Closed circulatory systems occur in:
- vertebrates
- annelids such as earthworms
- cephalopod molluscs such as octopuses and squid
Humans have a closed circulatory system.
Open vs Closed Circulation
Open circulatory system
- circulating fluid is not continuously confined to vessels
- fluid enters body spaces
- organs can be directly bathed by hemolymph
- generally operates at lower pressures
- distribution can be less precisely controlled
Closed circulatory system
- blood remains within vessels
- usually allows higher pressure
- blood flow can be directed more precisely
- rapid transport can support high metabolic demands
Neither system should simply be described as "good" or "bad." Each evolved in organisms with different body structures, lifestyles, and metabolic requirements.
Why Insects Can Use Open Circulation
Insects can be highly active even though they have an open circulatory system.
This seems surprising until we consider their respiratory system.
Insects have a network of tracheae and tracheoles that delivers gases directly between the environment and tissues.
Therefore, insect hemolymph usually does not perform the same major oxygen-transport role that blood performs in humans.
This reduces one of the transport demands placed on their circulatory system.
Single Circulation
Some vertebrates have a single circulatory system.
Fish are a common example.
A simplified pathway is:
heart → gills → body → heart
Blood passes through the heart once during each complete circuit of the body.
Double Circulation
Humans have a double circulatory system.
During one complete circuit, blood passes through the heart twice.
There are two major circuits:
- pulmonary circulation
- systemic circulation
Pulmonary Circulation
The pulmonary circulation carries blood between the heart and lungs.
Simplified pathway:
right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium
At the lungs:
- carbon dioxide leaves the blood
- oxygen enters the blood
The blood then returns to the heart.
Systemic Circulation
The systemic circulation carries blood between the heart and the rest of the body.
Simplified pathway:
left ventricle → aorta → body tissues → venae cavae → right atrium
Body tissues receive oxygen and nutrients.
Wastes such as carbon dioxide enter the blood for transport away from the tissues.
Following an Oxygen Molecule
Imagine an oxygen molecule entering the lungs.
Its journey might be:
1. Oxygen enters an alveolus.
2. Oxygen diffuses across the gas-exchange surface.
3. Oxygen enters the blood.
4. Oxygen binds to haemoglobin in red blood cells.
5. Blood travels through pulmonary veins to the heart.
6. The heart pumps the oxygenated blood into systemic circulation.
7. Blood reaches capillaries near body cells.
8. Oxygen leaves the blood and diffuses into tissues.
9. Cells use oxygen during aerobic respiration.
Circulation therefore connects gas exchange with cellular respiration.
Transporting Carbon Dioxide
Cells produce carbon dioxide during aerobic respiration.
Carbon dioxide moves from tissues into the blood.
Much of it is converted to and transported as bicarbonate ions in the blood, while smaller amounts are dissolved or associated with haemoglobin.
The blood carries it toward the lungs.
At the lungs, carbon dioxide moves into the alveoli and is removed during exhalation.
Transporting Nutrients
After digestion, nutrients are absorbed from the digestive system.
Substances entering the blood can include:
- glucose
- amino acids
- mineral ions
- water-soluble vitamins
The circulatory system transports these substances to tissues throughout the body.
Some absorbed lipids initially enter the lymphatic system before eventually reaching the bloodstream.
Transporting Hormones
Hormones are chemical messengers produced by endocrine glands.
Examples include:
- insulin
- adrenaline
- thyroid hormones
Hormones enter the blood and are transported throughout the body.
Only cells with the appropriate receptors respond strongly to a particular hormone.
Circulation therefore connects endocrine glands with their target tissues.
Transporting Wastes
Metabolism produces waste substances that must be removed or processed.
For example:
carbon dioxide
is transported toward the lungs.
urea
is transported in blood plasma toward the kidneys.
The circulatory and excretory systems therefore work closely together.
Transporting Heat
Blood also distributes thermal energy around the body.
When tissues such as muscles are active, cellular respiration produces heat.
Blood carries some of this heat to other regions.
Changes in blood flow near the skin also contribute to temperature regulation.
What Is Homeostasis?
Homeostasis is the regulation of internal conditions within ranges that allow cells and enzymes to function effectively.
Examples of regulated conditions include:
- body temperature
- blood glucose concentration
- water balance
- ion concentrations
- pH
- oxygen and carbon dioxide levels
The circulatory system is essential because it connects the organs responsible for monitoring and adjusting these conditions.
Circulation and Temperature Regulation
When the body becomes too warm, blood flow near the skin can increase through vasodilation.
More warm blood passes near the body surface, increasing heat transfer to the environment.
When the body becomes cold, vasoconstriction reduces blood flow near the skin.
This helps reduce heat loss.
The circulatory system therefore plays an important role in thermoregulation.
Circulation and Blood Glucose
After a carbohydrate-containing meal, glucose enters the bloodstream.
The pancreas detects changes in blood glucose and releases hormones such as insulin.
Blood transports insulin to target tissues.
Cells then respond by changing glucose uptake and storage.
The circulatory system therefore links:
digestive system → blood → pancreas → hormones → target tissues
This helps maintain blood glucose within an appropriate range.
Circulation and Water Balance
The blood carries water and dissolved ions throughout the body.
The kidneys continuously interact with the blood and regulate how much water and various ions are removed in urine.
The circulatory system transports blood to and from the kidneys, allowing them to contribute to:
- water balance
- ion balance
- waste removal
- acid-base regulation
Circulation and pH
Cells continuously produce substances that can affect pH.
Blood contains buffer systems that help resist sudden changes in pH.
The circulatory system also transports carbon dioxide to the lungs, where it can be removed.
The kidneys regulate acids, bases, and bicarbonate over longer timescales.
Together, circulation, respiration, and kidney function contribute to maintaining appropriate blood pH.
Circulation During Exercise
During exercise, muscle cells require more energy.
Their rate of cellular respiration increases.
Therefore, muscles generally require:
- more oxygen
- more glucose and other fuels
They also produce:
- more carbon dioxide
- more heat
The cardiovascular system responds by increasing cardiac output and redistributing blood flow.
This allows transport to better match the increased metabolic demand.
Why Heart Rate Increases During Exercise
During exercise:
muscle activity increases
↓
energy demand increases
↓
respiration rate in muscle cells increases
↓
greater oxygen and nutrient delivery is required
↓
more carbon dioxide and heat must be removed
↓
cardiovascular activity increases
This is a good example of different body systems working together.
Structure and Function
Circulatory systems demonstrate an important biological principle:
structure is related to function.
For example:
heart
muscular structure → produces pressure → moves blood
arteries
thick elastic walls → withstand and maintain pulsatile high-pressure flow
veins
large lumen + valves → support low-pressure return to heart
capillaries
thin walls + large total surface area → efficient exchange
red blood cells
haemoglobin + specialized shape → oxygen transport
Understanding these relationships allows us to explain why each part of the system has its particular structure.
Example 1: Why Are Capillaries Thin?
Question:
Why do capillaries have walls only about one cell thick?
Answer:
Thin walls produce a short diffusion distance between blood and surrounding tissues.
This allows substances such as oxygen and glucose to move efficiently between the blood and cells.
Example 2: Why Are Arteries Thick?
Question:
Why do arteries have relatively thick, strong, elastic walls?
Answer:
Blood enters arteries under relatively high pressure produced by the heart.
Their walls must withstand this pressure.
Elastic tissue also allows them to stretch and recoil as blood is pumped.
Example 3: Why Do Many Veins Have Valves?
Blood in veins is generally under relatively low pressure.
In some regions, particularly the limbs, blood must move against gravity.
Valves help prevent backward movement.
Therefore:
low-pressure flow + risk of backflow → valves → improved one-way return
Example 4: Comparing Open and Closed Systems
Animal A has circulating fluid that leaves vessels and directly bathes organs.
Animal B has blood that remains inside vessels.
Animal A has:
an open circulatory system
Animal B has:
a closed circulatory system
The closed system generally allows more controlled distribution and higher-pressure transport.
Example 5: Circulation and Homeostasis
Suppose body temperature rises during exercise.
The circulatory system can help by increasing blood flow near the skin.
More thermal energy can then be transferred toward the environment.
Therefore, circulation contributes to maintaining a relatively stable internal temperature.
The Circulatory System Does Not Work Alone
Circulation interacts with many other systems.
Respiratory system
supplies oxygen and removes carbon dioxide.
Digestive system
supplies absorbed nutrients.
Excretory system
removes metabolic wastes and regulates water and ions.
Endocrine system
releases hormones into the blood.
Immune system
uses blood to distribute cells and molecules involved in defence.
Muscular system
requires oxygen and nutrients and produces carbon dioxide and heat.
This coordination is essential for maintaining the internal environment.
Common Mistakes
Mistake 1: Saying arteries always carry oxygenated blood
Arteries are defined by carrying blood away from the heart.
Mistake 2: Saying veins always carry deoxygenated blood
Veins carry blood toward the heart. Pulmonary veins carry oxygenated blood.
Mistake 3: Saying capillaries pump blood
The heart provides the main pumping force. Capillaries are exchange vessels.
Mistake 4: Saying open circulation means there is no heart
Animals with open circulatory systems can still have pumping structures.
Mistake 5: Saying blood directly touches every body cell in humans
Human blood remains inside vessels. Exchange occurs across capillary walls and through tissue fluid.
Mistake 6: Saying all oxygen is dissolved in plasma
Most oxygen is transported bound to haemoglobin in red blood cells.
Mistake 7: Saying the circulatory system only transports oxygen
It also transports nutrients, hormones, wastes, heat, immune components, water, and ions.
Mistake 8: Saying open circulatory systems cannot support active animals
Insects can be highly active because their tracheal respiratory system delivers gases directly to tissues.
Mistake 9: Saying homeostasis means conditions never change
Homeostasis maintains internal conditions within acceptable ranges rather than keeping everything perfectly constant.
Mistake 10: Treating body systems as independent
Circulation works closely with respiratory, digestive, excretory, endocrine, immune, and other systems.
Did You Know?
A human circulatory system contains an enormous network of blood vessels, from large arteries down to microscopic capillaries.
This branching network solves a major biological challenge:
The heart moves blood efficiently over long distances, while tiny capillaries bring the circulating blood close enough to cells for rapid exchange.
The system therefore combines:
bulk flow over long distances
with:
diffusion over short distances
This combination makes transport in large multicellular organisms highly effective.
Key Terms
- Circulatory system: Organ system responsible for transporting substances around an organism.
- Cardiovascular system: Heart, blood, and blood vessels.
- Heart: Muscular organ that pumps blood.
- Blood: Specialized transport tissue.
- Artery: Blood vessel carrying blood away from the heart.
- Vein: Blood vessel carrying blood toward the heart.
- Capillary: Microscopic vessel specialized for exchange between blood and tissues.
- Plasma: Liquid component of blood.
- Haemoglobin: Oxygen-binding protein in red blood cells.
- Open circulatory system: System in which circulating fluid is not continuously confined to vessels.
- Closed circulatory system: System in which blood remains within vessels.
- Hemolymph: Circulating fluid found in many animals with open circulatory systems.
- Pulmonary circulation: Blood flow between the heart and lungs.
- Systemic circulation: Blood flow between the heart and the rest of the body.
- Homeostasis: Regulation of internal conditions within ranges suitable for normal function.
- Vasodilation: Widening of blood vessels.
- Vasoconstriction: Narrowing of blood vessels.
- Cardiac output: Volume of blood pumped by the heart per unit time.
Transport Summary
Oxygen
lungs → blood → cells
Carbon dioxide
cells → blood → lungs
Nutrients
digestive system → blood → cells
Urea
tissues/liver → blood → kidneys
Hormones
endocrine glands → blood → target cells
Heat
active tissues → blood → other tissues and skin
Water and ions
transported between organs and tissues as part of homeostatic regulation
Key Takeaways
- Circulatory systems provide rapid transport throughout multicellular organisms.
- Large organisms need transport systems because diffusion alone is too slow over large distances.
- A circulatory system generally requires a circulating fluid, a pump, and pathways or spaces through which the fluid moves.
- In humans, these components are blood, the heart, and blood vessels.
- Arteries carry blood away from the heart.
- Veins carry blood toward the heart.
- Capillaries allow exchange between blood and tissues.
- Blood transports oxygen, nutrients, hormones, wastes, water, ions, and heat.
- Red blood cells transport most oxygen using haemoglobin.
- Open circulatory systems allow circulating fluid to enter spaces around organs.
- Closed circulatory systems keep blood within vessels.
- Humans have a closed, double circulatory system.
- Pulmonary circulation connects the heart and lungs.
- Systemic circulation connects the heart with the rest of the body.
- Circulation connects the respiratory, digestive, excretory, endocrine, immune, and other body systems.
- Circulatory systems contribute to homeostasis by supporting regulation of temperature, blood glucose, water balance, pH, gases, and other internal conditions.
- During exercise, circulation adjusts to meet increased metabolic demands.
- Structure and function are closely related throughout the circulatory system.
- A useful way to understand circulation is:
body cells need materials and produce wastes → diffusion alone is insufficient over long distances → a pump creates bulk flow → circulating fluid transports substances → exchange occurs near tissues → organs regulate the blood's composition → stable internal conditions support homeostasis.