Importance

Humans and plants are multicellular organisms. Many of the cells do not have direct contact with the environment. Thus, they may not be able to obtain useful substances or remove waste substances fast enough through simple diffusion.

An efficient transport system throughout the organism distributes useful substances and removes waste substances quickly.

Double Circulation System

Humans have a double circulation system. The first is the pulmonary circulation (towards the lungs) while the second is the systemic circulation (towards the rest of the body).

The main function of our circulatory system is to transport nutrients and oxygen to all parts of the body and the removal of metabolic waste.

Main Components

The three main components in the circulatory system are the heart, blood vessels and blood.

The Heart

The heart is a funnel-shaped, hollow and powerful muscle that pumps blood to all parts of the body.

It has 4 chambers (2 atria (singular: atrium) and 2 ventricles). The atria receive blood and pump it to the ventricles. The ventricles then pump blood to the lungs or rest of the body. The left ventricle has a thicker muscular wall than the right as it needs to pump blood to rest of body. 

Valves in the heart prevent backflow of blood, ensuring it flows in one direction only.

Blood Vessels

There are three types of blood vessels : arteries, veins and capillaries.

Arteries transport blood away from the heart. They have no valves. Arteries have thicker walls to withstand the high blood pressure.

Veins transport blood to the heart. They have very low blood pressure and thus, have valves to prevent back flow of blood due to gravity.

Capillaries have a single layer of flat endothelial cells. They facilitate the transfer of materials from blood to the body cells.

Blood Vessels - Comparison

This table summarises the main differences between the three different types of blood vessels.

Blood

Blood in our circulatory system consists mainly of : red blood cells, white blood cells, platelets and blood plasma.

Flow of Blood

This diagram illustrates the flow of blood.

How the Heart works

This video will help you visualise how the heart works in the circulatory system.

Heart Disease

The heart also needs blood for nutrients and oxygen supplies and excretion of metabolic waste. Heart disease arises when the heart is not getting enough blood, such as by blockage of coronary arteries.

An example is atherosclerosis, when fatty deposits collect on the inner surface of the coronary arteries, leading to blockage.

Heart Disease - Causes and Prevention

The table below shows the possible causes of coronary heart disease and preventive measures for each cause.

Phloem and Xylem

In a flowering plant, the transport of substances involves the phloem and xylem. They are grouped with cambium into vascular bundles.

The xylem is hollow and made of dead cells, with thick cellulose walls strengthened by lignin. It transports water and mineral salts to all parts of the plant and provides mechanical support.

The phloem consists of living cells and mainly transports food manufactured by the leaves of the plant during photosynthesis, to all parts of the plant.

The cambium produces new xylem and phloem tissues.

Arrangement of Phloem and Xylem

The positions of the xylem and phloem change at different parts of the plant. Here are some cross sections and transverse sections that show you the different positions of the xylem and phloem.

Absorption of Water

Roots are responsible for absorbing water and mineral salts. Root hairs are thin-walled, tube-like protrusions from roots. They increase surface area to volume ratio to increase absorption rates.

The root hair cells have highly concentrated cytoplasm, allowing water to enter via osmosis.

Absorption of Water - From Root to Leaf

Since water concentration in soil is higher than inside the root, water molecules enter the root hair cell via osmosis. Water molecules are transported in this manner via osmosis to the xylem.

Transpiration in the leaves creates a transpiration pull that then, along with other forces, moves water up the xylem towards the leaves.

Absorption of Mineral Salts

Since the concentration of mineral salts is usually higher in soil than in the roots, the mineral salts usually enter the roots via diffusion.