Respiration and Circulation
5. Transport of Substances
Learning outcomes
- I can explain how oxygen is transported in the blood.
- I can describe how nutrients are delivered to body cells.
- I can explain how carbon dioxide and other wastes are transported for removal.
- I can describe the role of red blood cells, white blood cells, platelets, and plasma.
- I can explain how transport systems help maintain homeostasis.
Introduction
Every cell in the human body depends on a constant supply of oxygen, nutrients, and water to survive. At the same time, cells produce waste products such as carbon dioxide and urea that must be removed before they become harmful. The circulatory system acts as the body's transport network, moving essential substances to cells and carrying wastes away for removal.
Blood is the transport medium of the circulatory system. It is made up of specialised cells suspended in a liquid called plasma. Together, the components of blood transport gases, nutrients, hormones, and wastes while also helping protect the body against disease and maintaining homeostasis.
Blood as a Transport System
Blood carries many substances throughout the body, including:
- Oxygen
- Carbon dioxide
- Nutrients
- Hormones
- Water
- Mineral ions
- Waste products
- Immune cells
The heart pumps blood continuously through blood vessels so that every cell receives the substances it needs.
Figure 1. Blood transports oxygen, nutrients, hormones, wastes, and many other substances throughout the body.
The Components of Blood
Blood consists of four main components.
| Component | Main Function |
|---|---|
| Plasma | Transports dissolved substances |
| Red blood cells | Carry oxygen |
| White blood cells. | Defend against disease |
| Platelets | Help blood clot |
Each component performs a specialised role.
Red Blood Cells
Red blood cells transport oxygen.
They contain a protein called haemoglobin, which binds to oxygen in the lungs.
The oxygen-rich blood is then pumped to body tissues.
At the tissues:
- Oxygen leaves the haemoglobin.
- Oxygen diffuses into body cells.
- Cells use oxygen during cellular respiration to release energy.
Red blood cells are well adapted for this job because they:
- Have a biconcave shape, increasing surface area.
- Contain large amounts of haemoglobin.
- Lack a nucleus when mature, leaving more space for haemoglobin.
- Are flexible enough to pass through tiny capillaries.
Figure 2. Red blood cells transport oxygen by binding it to haemoglobin.
Transporting Nutrients
After digestion:
- Glucose
- Amino acids
- Vitamins
- Minerals
- Water
are absorbed into the bloodstream from the small intestine.
The circulatory system carries these nutrients to every cell.
Cells use nutrients to:
- Produce energy.
- Grow.
- Repair damaged tissues.
- Build new molecules.
The liver also processes many absorbed nutrients before they are distributed throughout the body.
Transporting Carbon Dioxide
During cellular respiration:
- Cells produce carbon dioxide as a waste product.
Carbon dioxide diffuses into nearby capillaries.
It is transported back to the lungs in three main forms:
- Dissolved in plasma.
- Bound to haemoglobin.
- Mainly as bicarbonate ions in the plasma.
In the lungs:
- Carbon dioxide diffuses into the alveoli.
- It is removed from the body during exhalation.
Figure 3. Oxygen and carbon dioxide are transported between the lungs and body tissues by the blood.
Transporting Other Waste Products
Cells also produce other wastes.
One important waste is urea.
Urea is produced in the liver when excess amino acids are broken down.
The blood transports urea to the kidneys, where it is filtered out and removed in urine.
Blood also carries excess water and mineral ions to the kidneys for regulation.
White Blood Cells
White blood cells help protect the body against disease.
Their functions include:
- Destroying bacteria and viruses.
- Producing antibodies.
- Removing damaged cells.
- Coordinating immune responses.
Unlike red blood cells, white blood cells have nuclei and are much fewer in number.
They play a vital role in maintaining health.
Platelets
Platelets are tiny cell fragments involved in blood clotting.
When a blood vessel is damaged:
- Platelets gather at the injury.
- They help form a blood clot.
- The clot reduces blood loss.
- It also helps prevent microorganisms from entering the body.
Blood clotting is an important protective mechanism.
Figure 4. The four main components of blood each have specialised functions.
Plasma
Plasma is the pale yellow liquid that makes up about 55% of blood.
It transports:
- Nutrients.
- Hormones.
- Carbon dioxide.
- Urea.
- Water.
- Mineral ions.
- Proteins.
- Blood cells.
Plasma acts as the transport medium for most dissolved substances.
Transport and Homeostasis
Transport systems help maintain homeostasis by:
- Delivering oxygen.
- Supplying nutrients.
- Removing carbon dioxide.
- Removing urea and other wastes.
- Distributing hormones.
- Regulating body temperature by transporting heat.
Without continuous transport, body cells could not survive.
Figure 5. The transport functions of blood help maintain a stable internal environment.
Working with Other Organ Systems
The circulatory system works closely with other systems.
| Organ System. | What Is Transported |
|---|---|
| Respiratory | Oxygen and carbon dioxide |
| Digestive | Nutrients and water |
| Urinary | Urea, excess water, and salts |
| Endocrine | Hormones |
| Immune | White blood cells and antibodies |
Together, these systems maintain the body's internal balance.
Worked Example
Question
Match each blood component to its main function.
| Blood Component. | Function |
|---|---|
| Red blood cells | Carry oxygen |
| White blood cells | Fight disease |
| Platelets | Form blood clots |
| Plasma | Transport dissolved substances |
Real-World Connection
After eating a meal, glucose absorbed from the small intestine enters the bloodstream. The circulatory system delivers this glucose to body cells, where it is used in cellular respiration to produce energy. At the same time, oxygen carried by red blood cells is delivered to the same cells, while carbon dioxide produced during respiration is carried back to the lungs for removal. This continuous transport keeps every cell functioning properly.
Did You Know?
Your body contains about 5 litres of blood, and your heart circulates this entire volume approximately once every minute while you are resting. During vigorous exercise, the heart pumps faster, allowing blood to circulate even more rapidly so that muscles receive extra oxygen and nutrients.
Key Terms
Antibody – A protein produced by certain white blood cells that helps destroy pathogens.
Carbon dioxide – A waste gas produced during cellular respiration.
Haemoglobin – A protein in red blood cells that binds and transports oxygen.
Homeostasis – The maintenance of a stable internal environment.
Plasma – The liquid component of blood that transports dissolved substances.
Platelet – A small cell fragment involved in blood clotting.
Red blood cell – A blood cell specialised for transporting oxygen.
Urea – A nitrogen-containing waste product produced in the liver and removed by the kidneys.
White blood cell – A blood cell that helps defend the body against disease.
Key Takeaways
- Blood transports oxygen, nutrients, hormones, water, and wastes throughout the body.
- Red blood cells carry oxygen using haemoglobin.
- Nutrients absorbed in the small intestine are transported by the blood to body cells.
- Carbon dioxide is carried from body tissues to the lungs, while urea is transported to the kidneys for removal.
- White blood cells defend the body against disease, platelets help blood clot, and plasma transports dissolved substances.
- The circulatory system works with other organ systems to maintain homeostasis by supplying cells with essential materials and removing waste products.