Mass transport is when the bulk movement of gases or liquids in one direction occurs, usually via a system of vessels in animals.
The key highlight of mass transport is the bulk movement of such substances in one direction (unidirectionality). Unidirectionality is important to maximise bulk movement efficiency and quickly bring substances from one region to another in the body. Therefore, this ensures effective cell activity.
Animals need a specialised exchange surface because of their large surface area to volume ratio. Diffusion alone would not be sufficient for an adequate exchange of substances (more on this in our surface area to volume ratio article).
Another way to look at mass transport is the transport networks of a city. With the high population density and the need to commute, roads systems for different modes of transportation are necessary to reduce congestion. In contrast, motorways are built for vehicles to travel in a specific direction to prevent road accidents: similar to organisms, whereby the unidirectionality of mass transport ensures more precise control of substance exchange.
Diffusion in mass transport in animals
You may wonder, how do animals obtain substances such as oxygen and glucose into their vessels for mass transport? Or, how do tissue metabolic wastes enter the vessels for mass transport?
Diffusion plays this significant role in mass transport, as substances travel down a concentration gradient from the external environment into mass transport systems. Capillaries of specialised exchange systems are mass transport vessels where diffusion occurs and have specific adaptations for that purpose.
A good example of diffusion’s role in mass transport is the gas exchange in the alveoli. There is an article on human gas exchange to learn about the mechanism in more detail.
Ultimately, mass transport systems help maintain the diffusion gradients at sites of substance exchange, minimising the disruption of any substance exchange processes.
What is mass flow transport like in larger animals?
Mass flow transport is mass transport where liquids move down a pressure gradient, i.e., a form of mass transport dependent on the flow of liquid. As larger animals have smaller surface area to volume ratios, they require mass flow transport systems to meet their high metabolic demands.
Circulatory systems as mass flow transport systems
You may then ask, how do the bodies of large animals supply a continuous flow of liquid necessary for mass transport? This supply is the responsibility of circulatory systems. Circulatory systems are organ systems where all vessels involved in mass flow transport are linked to a muscular, pump-like organ called the heart.
Animals with closed circulatory systems have blood transport their substances, whilst animals with open transport systems, such as insects have haemolymph.
The heart beats continuously throughout its lifetime to maintain fluid circulation for mass flow. Fluid in the circulatory system is contained in pipe-like structures called vessels. There are two major categories of vessels: arteries and veins.
Arteries carry oxygenated blood from the heart to other parts of the body, whereas veins carry deoxygenated blood from the body parts to the heart. The only exception is the pulmonary artery and vein. The pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs to the heart. The pulmonary and systemic circulation makes up the system of double circulation.
Capillaries are small vessels that link arteries and veins together.
Given how diffusion occurs in capillaries of specialised exchange systems, one can say mass flow systems work together with specialised exchange systems (e.g. lungs) for efficient substance exchange.
Role of water in mass flow transport in animals
From the definition of mass flow transport, whereby liquids move down a pressure gradient, you may wonder what liquid is involved? Blood is involved in the mass flow transport of animals. It is a mixture of blood cells, protein molecules, carbohydrates, micronutrients, fats, and electrolytes suspended in plasma and contains large amounts of water.
Water as a universal solvent allows substances to be dissolved easily for smooth mass flow around the body. Blood is contained in blood vessels and pumped by the heart around the body.
Wondering further about the composition of blood and how this composition relates to the role of blood in mass flow? The main components of blood crucial in mass transport include:
- Plasma is made of water and solutes, dissolves nutrients such as glucose transported into respiring tissues and metabolic wastes such as carbon dioxide that are rid from respiring tissues.
- Red blood cells are adapted to carry the majority of oxygen molecules around the body.
Fig. 1 - Composition of blood: blood plasma, erythrocytes (red blood cells), white blood cells (granulocytes, monocytes and lymphocytes) and platelets
More importantly, water in blood makes blood such an effective medium in mass flow because of its chemical stability. In other words, water won’t be broken down easily in the body. This property of water allows a smooth one-way flow of blood within the blood vessels and allows blood to carry essential nutrients and gases to all the cells of the body.
Mass Transport in Animals - Key takeaways
- Mass transport in animals is the bulk movement of gases or liquids in one direction via a system of vessels. Unidirectionality in mass transport is important to ensure more precise control of substance exchange.
- Diffusion is crucial in mass transport to allow substances to enter and leave the mass transport system.
- Mass flow transport requires the movement of fluids under a pressure gradient. Larger animals have circulatory systems where the heart generates the pressure gradient necessary for mass flow transport.
- Water in blood is the liquid involved in the mass flow transport in animals. Blood components - plasma and red blood cells, play an important role in the transport of substances.
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