Biology: Transport Systems

What is a Transport System?

A transport system is a means by which materials are moved (‘transported‘) from an exchange surface or exchange surfaces to cells* located throughout the organism.

Not all individual cells require all of the many different types of materials carried by a transport system. For example, the blood circulation system in humans carries various substances including some hormones that have target organs. Although they are carried around the whole body via the blood system, those specific molecules are only received by and useful to specific types of cells located in the target organ of that particular hormone.

Multiple exchange surfaces?
Although gas exchange surfaces are important due to the need for respiration, they are not the only types of exchange surfaces. Another example in humans is the wall of the small intestine where digested (food) material is transported into the blood stream for distribution to cells throughout the body.

What is a mass transport system?

Explanation of terms: The terms transport system and mass transport system both appear in biology textbooks.
They are sometimes used interchangeably because the main transport systems included in introductory biology are mass transport systems:

  • In biology, mass flow is the movement of a fluid in one direction, usually through a system of tube-like vessels.
  • mass transport system is a transport system (i.e. a means by which materials are moved from exchange surface/s that form part of an organism to all other locations within the organism where the materials from the exchange surface/s are required by cells) that involves mass flow.

So … a definition of a mass transport system is:

Mass transport system is an arrangement of physical structures by which materials are moved in the form of a fluid containing particles of those materials travelling in one direction [through a system of tubes*] from one or more exchange surface(s) within an organism to cells located throughout the organism.

If the fluid is contained within a system or network of vessels it forms a “closed” transport system. If fluid is not contained within a closed system, e.g. of blood vessels + heart, the system is described as “open”, e.g. the open circulation system in insects. In that case the fluid is not constrained to flow all in the same direction, but can circulate with more freedom so may not count as a mass transport system (not if the fluid isn’t all moving in the same direction). See the table below for open vs closed systems.

Examples of mass flow in mass transport systems include the movement of blood e.g. in the blood systems present in mammals and the movement of xylem and phloem through plants.

Features of Transport Systems

Transport systems in many different (and different types of) organisms have lots of common features, such as:

  • Contains a transport medium in which materials, including various sizes and shapes of molecules, can be conveyed. This medium is usually water-based. Water acts as a solvent for a wide range of substances and flows easily at the temperatures of living organisms.  Examples of transport media in animal transport systems include blood, lymph and hemolymph.
  • A structure or ‘system’ of vessels that contain (enclose) the transport medium and extend, via a branching network, to all locations to which materials carried in the transport medium must be transported.
    Note: This doesn’t apply to ‘Open Circulatory Systems‘, which may have some vessels but the transport medium is not retained within them at all times.
  • Enables substantial volumes* of the transport medium to be moved over large distances*.
    *Volumes appropriate to the size of the organism, e.g. the average human adult body contains approx. 8-10 pints of blood. Distances are also large relative to the size of the organism, i.e. as necessary to supply materials to all parts of it, so larger distances in an elephant than in a mouse!
  • A mechanism for moving the transport medium through the system, e.g. through a network of vessels.
    Movement of a fluid through a system requires a difference in pressure between parts of the system.

    • Animal Transport Systems sometimes include a pumping organ such as the heart in mammals, birds and some other creatures. They may also use other mechanisms such as muscular contraction of muscle tissues as well as, or instead of, a heart.
    • Plant Transport Systems tend to rely on passive physical process e.g. evaporation of water.
  • Mechanism(s) to maintain the mass flow movement of the transport medium in one direction.
    The pressure difference that moves the transport medium through the system is helpful but not necessarily sufficient to prevent back-flow. Some of the vessels in the circulation systems of animals include valves that prevent black-flow of the fluid contained in the vessel.
  • Ability to control the flow of the transport medium, i.e. to adjust the flow of the transport medium (e.g. blood) according to the needs of the organism. An example of this is the blood circulation system in mammals reducing the flow of blood to extremities, such as fingers, in very cold conditions – which helps to conserve heat and blood supply to essential internal organs.

Transport Systems in Animals

All living organisms require oxygen and nutrients, and a method of removing carbon dioxide and waste products. However, the circulatory system is not limited to the delivery of nutrients, gas exchange, and waste removal. Hormones, too, rely on the circulatory system to reach target organs, and the immune system depends on the transport of white blood cells and antibodies. The next section discusses transport systems found in mammalian systems, with a focus on transport systems found in humans.

Key Concepts

  • There are open and closed circulatory systems. In an open circulatory system blood enters a cavity, in a closed circulatory system blood remains in vessels.
  • A double closed circulation system consists of the pulmonary and systemic circulatory systems.
  • The direction of blood flow is significant. In the systemic circulatory system oxygenated blood is transported to the body and deoxygenated returns to the heart. In the pulmonary circulatory system, deoxygenated blood is sent to the lungs, and oxygenated blood is returned to the heart.
  • Specialised cells (sinoatrial node) send signals to the atrioventricular node to cause the atria and ventricles to contact and control the cardiac cycle and heart rate.
  • The structure of blood vessels such as arteries, veins and capillaries are suited to their function.
  • The lymphatic system transports lymph around the body and returns fluid to the blood circulatory system.
  • The lymphatic system also plays an important role in immunity.
  • Conditions and diseases of the heart and circulatory system include high and low blood pressure, heart attacks and strokes. Treatments include stents, valve replacements, bypass surgery, pacemakers, and heart transplants.

Open and Closed Circulation Systems

Circulatory Systems in Animals

Transport systems are crucial to survival. Unicellular organisms rely on simple diffusion for transport of nutrients and removal of waste. Multicellular organisms have developed more complex circulatory systems.

Open and Closed Circulation Systems

There are two types of circulatory systems found in animals: open and closed circulatory systems.

Open Circulatory Systems

In an open circulatory system, blood vessels transport all fluids into a cavity. When the animal moves, the blood inside the cavity moves freely around the body in all directions. The blood bathes the organs directly, thus supplying oxygen and removing waste from the organs. Blood flows at a very slow speed due to the absence of smooth muscles, which, as you learnt previously, are responsible for contraction of blood vessels. Most invertebrates (crabs, insects, snails etc.) have an open circulatory system. The figure below shows a schematic of an open circulatory system delivering blood directly to tissues.

Open circulatory system.

Closed Circulatory Systems

Closed circulatory systems are different to open circulatory systems because blood never leaves the blood vessels. Instead, it is transferred from one blood vessel to another continuously without entering a cavity. Blood is transported in a single direction, delivering oxygen and nutrients to cells and removing waste products. Closed circulatory systems can be further divided into single circulatory systems and double circulatory systems.

Single and Double Circulation Systems

Single and Double Circulation Systems

The circulatory system is a broad term that encompasses the cardiovascular and lymphatic systems. The lymphatic system will be discussed later in this tutorial. The cardiovascular system consists of the heart (cardio) and the vessels required for transport of blood (vascular). The vascular system consists of arteries, veins and capillaries. Vertebrates (animals with backbones like fish, birds, reptiles, etc.), including most mammals, have closed cardiovascular systems. The two main circulation pathways in invertebrates are the single and double circulation pathways.

Single Circulatory Pathways

Single circulatory pathways as shown in the diagram below consist of a double chambered heart with an atrium and ventricle. Fish possess single circulation pathways. The heart pumps deoxygenated blood to the gills where it gets oxygenated. Oxygenated blood is then supplied to the entire fish body, with deoxygenated blood returned to the heart.

Single circulation system as found in a typical fish species. The red represents oxygen-rich or oxygenated blood, the blue represents oxygen-deficient or deoxygenated blood.

Double Circulatory Systems

Double circulation pathways are found in birds and mammals. Animals with this type of circulatory system have a four-chambered heart.

The right atrium receives deoxygenated from the body and the right ventricle sends it to the lungs to be oxygenated. The left atrium receives oxygenated blood from the lungs and the left ventricle sends it to the rest of the body. Most mammals, including humans, have this type of circulatory system. These circulatory systems are called ‘double’ circulatory systems because they are made up of two circuits, referred to as the pulmonary and systemic circulatory systems.

Summary of Double Circulation.


Humans, birds, and mammals have a four-chambered heart. Fish have a two-chambered heart, one atrium and one ventricle. Amphibians have a three-chambered heart with two atria and one ventricle. The advantage of a four chambered heart is that there is no mixture of the oxygenated and deoxygenated blood.

Human Circulatory Systems

The human circulatory system involves the pulmonary and systemic circulatory systems. The pulmonary circulatory system consists of blood vessels that transport deoxygenated blood from the heart to the lungs and return oxygenated blood from the lungs to the heart. In the systemic circulatory system, blood vessels transport oxygenated blood from the heart to various organs in the body and return deoxygenated blood to the heart.

Pulmonary Circulation System

In the pulmonary circulation system, deoxygenated blood leaves the heart through the right ventricle and is transported to the lungs via the pulmonary artery. The pulmonary artery is the only artery that carries deoxygenated blood. It carries blood to the capillaries where carbon dioxide diffuses out of the blood into the alveoli (lung cells) and then into the lungs, where it is exhaled. At the same time, oxygen diffuses into the alveoli, and then enters the blood and is returned to the left atrium of the heart via the pulmonary vein.

Pulmonary circulation system. Oxygen rich blood is shown in red; oxygen-depleted blood is shown in blue.

Systemic Circulation

Systemic circulation refers to the part of the circulation system that leaves the heart, carrying oxygenated blood to the body’s cells, and returning deoxygenated blood to the heart. Blood leaves through the left ventricle into the aorta, the body’s largest artery. The aorta leads to smaller arteries that supply all organs of the body. These arteries finally branch into capillaries. In the capillaries, oxygen diffuses from the blood into the cells, and waste and carbon dioxide diffuse out of cells and into blood. Deoxygenated blood in capillaries then moves into venules that merge into veins, and the blood is transported back to the heart. These veins merge into two major veins, namely the superior vena cava and the inferior vena cava. The movement of blood is indicated by arrows on the diagram. The deoxygenated blood enters the right atrium via the the superior vena cava. Major arteries supply blood to the brain, small intestine, liver and kidneys. However, systemic circulation also reaches the other organs, including the muscles and skin. The following diagram shows the circulatory system in humans.

The systemic circulatory system supplies blood to the entire body.

Oxygen’s Surprisingly Complex Journey Through Your Body

Oxygen forms about 21% of the air around us. In your body, oxygen forms a vital role in the production of energy in most cells. But if gases can only efficiently diffuse across tiny distances, how does oxygen reach the cells deep inside your body?

The Heart and Associated Blood Vessels

External Structure of the Heart

The heart is a large muscle, about the size of your clenched fist, that pumps blood through repeated rhythmic contractions. The heart is situated in your thorax, just behind your breastbone, in a space called the pericardial cavity. The heart is enclosed by a double protective membrane, called the pericardium. The region between the two pericardium layers is filled with pericardial fluid which protects the heart from shock and enables the heart to contract without friction.

The heart is a muscle (myocardium) and consists of four chambers. The upper two chambers of the heart are called atria (singular= atrium). The two atria are separated by the inter-atrial septum. The lower two chambers of the heart are known as ventricles and are separated from each other by the interventricular septum. The ventricles have more muscular walls than the atria, and the walls of the right ventricle, which supplies blood to the lungs is less muscular than the walls of the left ventricle, which must pump blood to the whole body.


Clench your fist – the size of your fist is more or less the size of your heart.

In addition, there are a number of large blood vessels that carry blood towards and away from the heart. The terms artery’andvein’ are not determined by what the vessel transports (oxygenated blood or deoxygenated) but by whether the vessel flows to or from the heart. Arteries take blood away from the heart and generally carry oxygenated blood, with the exception of the pulmonary artery. Veins transport blood towards the heart and generally carry deoxygenated blood, except the pulmonary vein.

On the right side of the heart, the superior vena cava transports deoxygenated blood from the head and arms and the inferior vena cava transports deoxygenated blood from the lower part of the body back to the heart, where it enters the right atrium. The pulmonary artery carries deoxygenated blood away from the right ventricle of the heart towards the lungs to be oxygenated. On the left side of the heart, the pulmonary vein brings oxygenated blood from the lungs towards the left atrium of the heart and the oxygenated blood exits the left ventricle via the aorta and is transported to all parts of the body.

Since the heart is a muscle, and therefore requires oxygen and nutrients itself to keep beating, it receives blood from the coronary arteries, and returns deoxygenated blood via the coronary veins.

Did You Know?

In humans, the left lung is smaller than the right lung to make room in the chest cavity for the heart.

The external structure of the heart: the major part of the heart consists of muscles and is known as the myocardium. The region in which the heart is found is known as the pericardial cavity, which is enclosed by the pericardium.

Internal Structure of the Heart

As previously mentioned, the heart is made up of four chambers. There are two atria at the top of the heart which receive blood and two ventricles at the bottom of the heart which pump blood out of the heart. The septum divides the left and right sides of the heart. In order to make sure that blood flows in only one direction (forward), and to prevent backflow of blood, there are valves between the atria and ventricles (atrioventricular valves). These valves only open in one direction, to let blood into the ventricles, and are flapped shut by the pressure of the blood when the ventricles contract.

The tricuspid valve is situated between the right atrium and the right ventricle while the bicuspidmitral valve is found between the left atrium and the left ventricle. Strong tendinous cords (chordae tendineae) attached to valves prevent them from turning inside out when they close. The semi-lunar valves are located at the bottom of the aorta and pulmonary artery, and prevent blood from re-entering the ventricles after it has been pumped out of the heart.

The internal structure of the mammalian heart.

In the previous topics, we have discussed pulmonary and systemic circulation, and we have described the four chamber structure of the heart as well as some of the major arteries and veins that transport blood towards and away from the heart. In order to summarise all this information, study the flow diagram below which describes the passage of deoxygenated blood through one full cycle.

Flow diagram depicting movement of blood from the heart through the circulatory system. The blue boxes represent deoxygenated blood, the purple boxes represent capillary networks where gaseous exchange occurs and the red boxes represent stages at which the blood is oxygenated.


Memory trick: the tRI cuspid valve is found on the RIght side of the heart.

How the Heart Actually Pumps Blood

For most of history, scientists weren’t quite sure why our hearts were beating or even what purpose they served. Eventually, we realized that these thumping organs serve the vital task of pumping clean blood throughout the body. But how? Edmond Hui investigates how it all works by taking a closer look at the heart’s highly efficient ventricle system.

Major Organs and Systemic Circulation

All the organs of the body are supplied with blood. This is necessary so that the cells can obtain oxygen, which is required for cellular respiration, as well as essential nutrients. Each organ has an artery that supplies it with blood from the heart. Metabolic wastes, including carbon dioxide, need to be removed from cells and returned to the heart. These move into the capillaries which enter into veins that eventually enters either the superior or inferior vena cava which then enters the right atrium.

Arteries and veins have been named according to the organ to which they supply blood. The liver receives oxygenated blood from the heart via the hepatic artery. This artery runs alongside the hepatic portal vein. The hepatic portal vein contains nutrients that have been absorbed by the digestive system. This nutrient-rich blood must first pass through the liver, so that the nutrient composition of the blood can be controlled. Blood passes from the liver to the heart through the hepatic vein. Metabolic waste is circulated in the blood, and if allowed to accumulate, would eventually reach toxic levels. The kidneys are supplied with blood (which contain waste) via the renal arteries. The kidneys filter metabolic waste from the blood, passing it to urine to be excreted safely. Blood leaves the kidney via the renal vein.

The brain is supplied with blood via the carotid arteries and the vertebral arteries. The blood from the brain is drained via the jugular veins. The brain is supplied with 15% of the total amount of blood pumped by the heart. The heart is also a muscle (myocardium) that requires blood flow to work. Blood is supplied to the heart via two coronary arteries, and leaves the heart via four cardiac veins.

Optional Class Investigation: Dissecting a Mammalian Heart


To dissect a mammalian heart (sheep or ox heart).


  • your teacher will give each group a heart to dissect
  • a scalpel handle with a blade or a sharp non-serrated knife
  • a sharp pair of scissors
  • a pair of forceps
  • gloves
  • paper towel
  • pictures of the external and internal views of the heart


  1. Work in groups of four.
  2. Place the heart on the dissecting board with the atria at the top and the ventricles facing downwards.
  3. Carefully examine the external view of the heart. Try identify the vertical and horizontal groves on the heart. This is the position of the internal walls between the chambers of the heart.
  4. Examine and note the difference in the walls of the ventricles and atria. Also note the difference in appearance between the walls of the ventricles and atria.
  5. With the scalpel or sharp knife carefully cut the heart open across the left atrium.
  6. Compare the thickness and the size of the right ventricle and atrium.
  7. Identify the valves and examine the tendinous cords which are attached to the valves.
  8. Identify the semi-lunar valves at the bottom of the pulmonary artery.
  9. Now cut through the left side of the heart in the same way as you did the right side of the heart.
  10. Carefully cut through the septum of the heart so that you have two halves.


  1. What is the smooth outer layer of the heart called?
  2. Did you notice any fat around the heart?
  3. Did you notice a difference between the atria and ventricles externally?
  4. Name the blood vessels visible on the outside of the heart.
  5. Compare the thickness of the walls of the atria and ventricles. Explain why they are different.
  6. Explain the difference between the left and right ventricular walls.


  1. Pericardium
  2. Yes – fat should be present in some places, especially in the grooves.
  3. Yes – the atria are much smaller than the ventricles, they have thinner muscle walls and are at the top of the heart, whereas ventricles are at the bottom.
  4. Coronary arteries and veins
  5. Atria have thin, flexible walls and ventricles have much thicker, stronger walls. This is because atria only have to pump blood down to the ventricles (short distance), so they do not have to be as strong as ventricles, that pump blood much further (to the lungs or the entire body).
  6. The wall of the left ventricle is much thicker than that of the right ventricle, since it needs to exert greater force / be stronger. The left ventricle pumps blood to the entire body, which requires much more force than simply pumping blood from the right ventricle to the lungs, which are also in the thoracic cavity.

The Cardiac Cycle

A cardiac cycle refers to the sequence of events that happens in the heart from the start of one heartbeat to the start of the subsequent heartbeat. During a cardiac cycle the atria and the ventricles work separately. The sinoatrial node (pacemaker) is located in the right atrium and regulates the contraction and relaxing of the atria.

  • At rest, each heartbeat takes approximately 0.8 seconds.
  • The normal heart rate at rest is approximately 72 beats per minute.
  • During systole the heart muscle contracts.
  • During diastole the heart muscle relaxes.

The phases of the cardiac cycle will be broken down and explained in the following section:

Phase 1: Atrial systole (Atrium contracts)

  • Blood from the superior and inferior vena cava flows into the right atrium.
  • Blood from the pulmonary veins flows into the left atrium.
  • The atria contract at the same time.
  • This contraction lasts for about 0.1 seconds.
  • Blood is forced through the tricuspid and bicuspid valves into the ventricles.

Phase 2: Ventricular systole (Ventricle contracts)

  • Ventricles relax and fill with blood.
  • The ventricles contract for 0.3 seconds.
  • Blood is forced upwards, closing the bicuspid and tricuspid valves (lubb sound).
  • The blood travels up into the pulmonary artery (on the right) and the aorta (on the left).
  • The atria are relaxed during ventricular systole.

Phase 3: General diastole: (General relaxation of the heart)

  • The ventricles relax, thus decreasing the flow from the ventricles.
  • Once there is no pressure the blood flow closes the semi-lunar valves in the aorta and the pulmonary artery (dubb sound).
  • General diastole lasts for about 0.4 seconds.

The sound the heart makes

The heart makes two beating sounds. One is loud and one is soft. We call this the lubb dubb sound. The lubb sound is caused by the pressure of the ventricles contracting, forcing the atrioventricular valves shut. The dubb sound is caused by the lack of pressure in the ventricles which causes the blood to flow back and close the semi-lunar valves in the pulmonary artery and aorta.

A doctor uses a stethoscope to listen to the heartbeats. Alternatively, a person’s pulse can be measured by pressing a finger (other than the thumb which already has a pulse) against the brachial artery in the wrist or the carotid artery next to the trachea. The pulse of the heart allows us to measure the heart rate which is the number of heartbeats per unit time.

Mechanisms for controlling cardiac cycle and heart rate (pulse)

The cardiac cycle is controlled by nerve fibres extending from nodes of nerve bundles through the heart muscle. There are two nodes, namely the sinoatrial node (SA node) and the atrioventricular node (AV node). The SA node is located within the wall of the right atrium while the AV node is located between the atria and the ventricles. Electrical impulses generated in the SA node cause the right and left atria to contract first, initiating the cardiac cycle.

The electrical signal reaches the AV node, where the signal pauses, before spreading through conductive tissues called the bundles of His and Purkinje fibres. These fibres branch into pathways which supply the right and left ventricles, causing the ventricles to contract. The SA node is the pacemaker of the heart since electrical signals are normally generated there – without any stimulation from the nervous system (automaticity). However, although the heart rate is automatic, it changes during exercise or when experiencing intense emotions like fear, anger and excitement. This is as a result of added stimulation from the nervous system and hormones, such as adrenaline.

Simple simulation of how electrical activity spreads over the heart


Principle of ECG formation. Credit: Wikimedia Commons, CC BY-SA 3.0

Electrical activity

The electrical activity in the heart is so strong that it can be measured from the surface of the body as an electrocardiogram (ECG). A normal heart has a very regular rhythm. Arrhythmia is a condition where the heart has an abnormal rhythm, as shown in the figures. Tachycardia is when the resting heart rate is too fast (more than 100 beats per minute), and bradycardia is when the heart rate is too slow (less than 60 beats per minute).

Electrocardiogram depicting different heart rhythms.

Stroke Volume

The stroke volume is the amount of blood pumped through the heart during each cardiac cycle. The stroke volume can change depending on the needs of the body. During exercise, muscles need more oxygen and glucose in order to produce energy in the form of ATP. Therefore the heart increases its stroke volume and stroke rate to meet this demand. This is a temporary change to maintain homeostasis, and after exercise the heart rate and stroke volume return to normal.

When a person exercises regularly, and is fit, the heart undergoes certain long-term adaptations. The heart muscle gets stronger, and expels more blood with each contraction. There is therefore a greater stroke volume with each heartbeat. Since the heart expels more blood with each stroke, the heart has to beat less often in order to maintain the same volume of blood flow. Therefore, fit people often have lower resting heart rates.


Cardiac output is the volume of blood that is pumped by the heart in one-minute. Cardiac output is equal to the stroke volume (SV) multiplied by the heart rate (HR).

Blood Pressure

Blood pressure refers to the force that the blood exerts on the blood vessel walls. Blood pressure is determined by the size of the blood vessels and ensures that blood flows to all the parts of the body. Normal blood pressure is 120/80 (120 over 80) measured in units of mercury (mm Hg). The 120 represents the systolic pressure, which is when the ventricles contract. The 80 represents the diastolic pressure, which is when general diastole occurs.

Blood pressure can be increased by smoking, stress, adrenalin surges, water retention, high cholesterol, obesity and lack of exercise. High blood pressure (hypertension) is dangerous and increases the risk of an aneurysm, stroke or heart attack. Low blood pressure (hypotension) can lead to light-headedness and fainting because of insufficient blood supply to the brain.


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