• Each living cell requires essential substances such as oxygen and nutrients, and expels cellular waste products such as carbon dioxide and nitrogenous wastes.
• Unicellular organisms have a small body mass.
• Therefore, the total surface area to volume ratio (TSA/V) of the organism is large.
• As such, Amoeba sp. does not require a specialised transport system to transport substances in and out of the cell.
• Large complex multicellular organisms cannot obtain essential substances and expel wastes by diffusion because their TSA/V is small.
• The distance between the external environment and the cell is too far for direct substance exchange.
• To address this problem, multicellular organisms have an internal transportation system.
• In vertebrates, the transportation system is called the blood circulatory system.

• The circulatory system in multicellular organisms is divided into two types: open circulatory system and closed circulatory system.

• The circulatory system of insects is an open circulatory system.
• This means that one or more hearts pump haemolymph through the blood vessels into the haemocoel.
• Haemolymph flows out from the heart into the haemocoel when the heart contracts.
• In the haemocoel, substance exchange between haemolymph and body cells occurs through diffusion.
• When the heart relaxes, haemolymph flows back into the heart through tiny openings called ostium.

• The heart of the fish has two chambers, that is, an atrium (plural: atria) and a ventricle.
• Blood that leaves the ventricle is pumped to the gill capillaries to enable gaseous exchange.
• The gill capillaries carry blood to the blood vessels that transport oxygenated blood to systemic capillaries.
• In the systemic capillaries, oxygen diffuses into the tissues while carbon dioxide diffuses from the tissue into the capillaries.
• The deoxygenated blood is then returned to the heart atrium through the veins.
• As the blood flows in one direction, the fish circulatory system is known as a single circulatory system.

• The heart of an amphibian has three chambers, that is, two atria and a ventricle.
• Unlike the single circulatory system of fish, blood flows in two directions: pulmocutaneous circulation and systemic circulation.
• Therefore, this system is known as a double circulatory system.
• Amphibians are said to have an incomplete double circulatory system because the deoxygenated blood and the oxygenated blood are mixed.
• Pulmocutaneous circulation transports blood to the lungs and skin, and the exchange of gases takes place here.
• Systemic circulation transports oxygenated blood to the body tissues and returns the deoxygenated blood to the right atrium through the veins.

• The human heart consists of four chambers: two atria and two ventricles that are separated completely.
• Humans have a double circulatory system.
• This means that in one complete circulatory cycle, blood flows in the blood vessels through the heart twice.
• As there are two different circulations, humans are said to have a complete double circulatory system because the deoxygenated blood and the oxygenated blood do not mix.

Pulmonary Circulation:

• Deoxygenated blood is transported through the pulmonary artery to the lungs for gaseous exchange.
• Oxygenated blood from the lungs is returned to the left atrium and flows into the left ventricle.

Systemic Circulation:

• Blood is pumped from the heart to all the body tissues through the aorta.
• Then the deoxygenated blood returns to the right atrium through vena cava.

• The circulatory system is found in all multicellular organisms, consists of a heart to pump blood or haemolymph (in insects), functions to transport nutrients and wastes.
• The heart has valves that ensure blood flows in one direction.

• Each living cell requires essential substances such as oxygen and nutrients, and expels cellular waste products such as carbon dioxide and nitrogenous wastes.
• Unicellular organisms have a small body mass.
• Therefore, the total surface area to volume ratio (TSA/V) of the organism is large.
• As such, Amoeba sp. does not require a specialised transport system to transport substances in and out of the cell.
• Large complex multicellular organisms cannot obtain essential substances and expel wastes by diffusion because their TSA/V is small.
• The distance between the external environment and the cell is too far for direct substance exchange.
• To address this problem, multicellular organisms have an internal transportation system.
• In vertebrates, the transportation system is called the blood circulatory system.

• The circulatory system in multicellular organisms is divided into two types: open circulatory system and closed circulatory system.

• The circulatory system of insects is an open circulatory system.
• This means that one or more hearts pump haemolymph through the blood vessels into the haemocoel.
• Haemolymph flows out from the heart into the haemocoel when the heart contracts.
• In the haemocoel, substance exchange between haemolymph and body cells occurs through diffusion.
• When the heart relaxes, haemolymph flows back into the heart through tiny openings called ostium.

• The heart of the fish has two chambers, that is, an atrium (plural: atria) and a ventricle.
• Blood that leaves the ventricle is pumped to the gill capillaries to enable gaseous exchange.
• The gill capillaries carry blood to the blood vessels that transport oxygenated blood to systemic capillaries.
• In the systemic capillaries, oxygen diffuses into the tissues while carbon dioxide diffuses from the tissue into the capillaries.
• The deoxygenated blood is then returned to the heart atrium through the veins.
• As the blood flows in one direction, the fish circulatory system is known as a single circulatory system.

• The heart of an amphibian has three chambers, that is, two atria and a ventricle.
• Unlike the single circulatory system of fish, blood flows in two directions: pulmocutaneous circulation and systemic circulation.
• Therefore, this system is known as a double circulatory system.
• Amphibians are said to have an incomplete double circulatory system because the deoxygenated blood and the oxygenated blood are mixed.
• Pulmocutaneous circulation transports blood to the lungs and skin, and the exchange of gases takes place here.
• Systemic circulation transports oxygenated blood to the body tissues and returns the deoxygenated blood to the right atrium through the veins.

• The human heart consists of four chambers: two atria and two ventricles that are separated completely.
• Humans have a double circulatory system.
• This means that in one complete circulatory cycle, blood flows in the blood vessels through the heart twice.
• As there are two different circulations, humans are said to have a complete double circulatory system because the deoxygenated blood and the oxygenated blood do not mix.

Pulmonary Circulation:

• Deoxygenated blood is transported through the pulmonary artery to the lungs for gaseous exchange.
• Oxygenated blood from the lungs is returned to the left atrium and flows into the left ventricle.

Systemic Circulation:

• Blood is pumped from the heart to all the body tissues through the aorta.
• Then the deoxygenated blood returns to the right atrium through vena cava.

• The circulatory system is found in all multicellular organisms, consists of a heart to pump blood or haemolymph (in insects), functions to transport nutrients and wastes.
• The heart has valves that ensure blood flows in one direction.