The Principles of Circulation

Mammalian Transport System

What is diffusion?

The net movement of particles in a liquid or gas down a concentration gradient, from an area of relatively high concentration to an area of relatively low concentration.

What is a concentration gradient?

The difference in concentration of a substance between two regions; in biology this is often across a cell membrane.

Why do organisms need substances to be transported?

Cells need substances such as oxygen and nutrients delivered to them and waste products such as carbon dioxide removed.

Why is diffusion sufficient for single-celled and very small organisms?

They have short diffusion distances, a large surface area to volume ratio and relatively low metabolic demands.

How do nutrients and oxygen enter very small organisms?

They diffuse directly from the external environment into the cells.

How are waste substances removed from very small organisms?

They diffuse directly from the cells into the external environment.

Why are diffusion distances short in small organisms?

The distance from the outside of the organism to the innermost cells is very small.

What is surface area to volume ratio?

The relationship between the surface area of an organism and its volume.

Why do small organisms have a favourable surface area to volume ratio?

They have a large surface area relative to their volume, providing a large area across which substances can diffuse.

What happens to surface area to volume ratio as an organism gets larger?

It decreases because volume increases more rapidly than surface area.

Why does a decreasing surface area to volume ratio cause problems for larger organisms?

There is proportionally less surface available for exchange to supply the increasing number of cells.

How does organism size affect diffusion distance?

As organisms become larger, the distance between the surface and internal cells increases.

Why is diffusion alone insufficient in large organisms?

They have a small surface area to volume ratio, long diffusion distances and greater metabolic demands, so diffusion is too slow to supply all cells.

Why do small organisms generally have low metabolic demands?

They often do not regulate their own temperature and their cells use relatively little oxygen and food and produce relatively little carbon dioxide.

Do very small organisms usually require specialised transport systems?

No. Diffusion alone can often supply their needs.

Why do large multicellular animals need transport systems?

Their cells require continuous supplies of substances such as glucose and oxygen and need waste products such as carbon dioxide removed rapidly.

Why must carbon dioxide be removed from cells?

It is a waste product of respiration and can cause damage if it accumulates.

Why do large organisms have higher demands for oxygen and nutrients?

They contain many metabolically active cells carrying out numerous chemical reactions.

What substances may need to be transported internally in animals besides nutrients and waste?

Chemical messengers such as hormones need to be transported from one part of the body to target cells elsewhere.

What is a mass transport system?

An arrangement of structures that transports substances in the flow of a fluid, with a mechanism for moving that fluid around the body.

What is the mass transport system in vertebrates?

The heart and circulatory system, with blood acting as the transport fluid.

Why are mass transport systems needed in large organisms?

They overcome the limitations of diffusion caused by a small surface area to volume ratio and long diffusion distances.

How are substances delivered from a mass transport system to individual cells?

They leave the transport system and move to cells by processes such as diffusion, osmosis and active transport.

What are the main features of a mass transport system?

Exchange surfaces, a system of vessels, mechanisms to move substances in the correct direction, a way of moving materials rapidly, a suitable transport medium and often a way of adjusting transport rate.

Why are exchange surfaces needed in a mass transport system?

They allow substances to enter and leave the transport system efficiently.

Why are vessels needed in a mass transport system?

They carry the transport medium and substances throughout the organism.

Why must substances move in the correct direction in a mass transport system?

Useful substances such as nutrients must reach cells while waste substances must be carried away.

How can materials be moved rapidly through a mass transport system?

By mechanisms such as pumping by the heart or maintaining concentration gradients.

What is a transport medium?

A fluid that carries substances around an organism, such as blood.

Why may the rate of transport need to change?

The organism's requirements change, for example during increased activity when cells need more oxygen and nutrients.

What is a circulatory system?

A transport system in which a heart pumps blood around the body.

What is an open circulatory system?

A system in which the circulating fluid is not continuously enclosed within blood vessels and flows through body spaces.

Which animals commonly have open circulatory systems?

Insects.

What is a closed circulatory system?

A circulatory system in which blood remains contained within blood vessels.

Which animals have closed circulatory systems?

Most animals including vertebrates such as mammals.

What are the main advantages of a closed circulatory system?

Blood pressure can be increased so blood flows faster, and blood flow can be directed more precisely to organs requiring oxygen and nutrients.

What is a single circulation system?

A circulation in which blood passes through the heart once during one complete circuit of the body.

Which animals have a single circulatory system?

Fish.

Describe the route of blood in a fish's single circulation.

Heart → gills → body tissues → heart.

What does the heart pump to the gills in a fish?

Deoxygenated blood.

What happens to blood at the gills?

It takes up oxygen and releases carbon dioxide.

What happens to oxygenated blood after leaving the gills of a fish?

It travels around the body, supplies oxygen to cells and then returns to the heart.

Why can a single circulation operate at relatively low pressure?

Animals such as fish generally have lower metabolic demands than mammals and do not maintain a constant body temperature independently of their surroundings.

What is a double circulatory system?

A circulation consisting of two separate circuits in which blood passes through the heart twice during one complete circulation of the body.

Which animals have a double circulatory system?

Birds and mammals.

Why do birds and mammals require efficient circulation?

They have high oxygen demands and maintain a relatively constant body temperature, requiring high rates of respiration and metabolism.

What are the two circuits in a mammalian double circulation?

The pulmonary circulation and systemic circulation.

What is systemic circulation?

The circuit carrying oxygenated blood from the heart to body tissues and returning deoxygenated blood to the heart.

Describe the route of systemic circulation.

Heart → body tissues → heart.

What is pulmonary circulation?

The circuit carrying deoxygenated blood from the heart to the lungs and returning oxygenated blood to the heart.

Describe the route of pulmonary circulation.

Heart → lungs → heart.

What is oxygenated blood?

Blood carrying a high concentration of oxygen.

What is deoxygenated blood?

Blood that has given up much of its oxygen to body cells.

Why are oxygenated and deoxygenated blood kept separate in a double circulation?

This ensures tissues receive blood containing as much oxygen as possible.

What is an advantage of double circulation over single circulation?

Oxygenated blood can be delivered rapidly to body tissues at high pressure.

Why is blood sent through the lungs at relatively low pressure?

Low pressure protects the delicate lung blood vessels and allows efficient gas exchange.

Why would it be a disadvantage if blood leaving the lungs went directly around the body without returning to the heart?

Its pressure would be low, so it would move slowly around the body.

What happens to oxygenated blood when it returns from the lungs to the heart?

The heart pumps it strongly around the body at high pressure.

Why is high-pressure systemic circulation useful?

It allows oxygenated blood to reach capillaries around body cells rapidly, supplying oxygen quickly for respiration.

Why is double circulation particularly useful for active animals?

It supports high metabolic and respiratory rates by delivering oxygen and nutrients rapidly to tissues.

What is the role of the heart in a mass transport system?

It acts as a pump that generates pressure to move blood around the circulatory system.

What is the role of blood in a mammalian mass transport system?

It acts as the transport medium carrying substances such as oxygen, nutrients, hormones and waste products.

What is the relationship between surface area and volume as an organism increases in size?

Volume increases faster than surface area, causing surface area to volume ratio to decrease.

Why can't a large animal obtain all the substances it needs through its skin?

Its small surface area to volume ratio and long diffusion distances mean diffusion across the body surface cannot supply internal cells rapidly enough.

Why is a large surface area to volume ratio beneficial for exchange?

It provides a large area for diffusion relative to the amount of tissue requiring substances.

Why is a short diffusion distance beneficial?

Particles reach cells more rapidly because they have a shorter distance to travel.

Why does a high metabolic rate increase the need for a mass transport system?

Cells consume oxygen and nutrients and produce waste rapidly, so substances must be transported quickly.

Define vertebrate.

An animal with a backbone or spinal column; vertebrates include mammals, birds, reptiles, amphibians and fish.

Define mass transport system.

An arrangement of structures through which substances are transported in the flow of a fluid, with a mechanism for moving the fluid around the body.

Define single circulation system.

A circulatory system in which the heart pumps blood to the gas-exchange organs, then around the body, before it returns to the heart.

Define double circulation system.

A circulatory system with two separate circuits, so blood passes through the heart twice during one complete circulation.

Define systemic circulation.

The circulation carrying oxygenated blood from the heart to body cells and returning deoxygenated blood to the heart.

Define pulmonary circulation.

The circulation carrying deoxygenated blood from the heart to the lungs and returning oxygenated blood to the heart.