8.1. TRANSPORT SYSTEMS IN MULTICELLULAR ANIMALS

Question 1

why are specialized transport systems needed?

Answer 1

• metabolic demands of most multicellular animals are high so diffusion over the long distances is not enough to supply the quantities needed
• SA:V ratio gets smaller as multicellular organisms get bigger so not only do the diffusion distances get bigger but the amount of surface area available to absorb or remove substances becomes relatively smaller
• molecules (i.e. hormones/ enzymes) may be made in one place but needed in another
• food will be digested in one organ system, but needs to be transported to every cell for use in respiration
• waste products of metabolism need to be removed from the cells and transported to excretory organs

Question 2

what features do most circulatory systems have in common?

Answer 2

• have liquid transport medium hat circulates around the system (blood)
• have vessels that carry the transport medium
• have a pumping mechanism to move the fluid around the system

Question 3

what is a mass transport system?

Answer 3

• when substances are transported in a mass of fluid with a mechanism for moving the fluid around the body

Question 4

what is an open circulatory system?

Answer 4

• there are few vessels to contain the transport medium
• blood is pumped straight from the heart into the body cavity (haemocoel) of the animal
• here it under low pressure and comes into direct contact with the tissues and the cells
• the blood then returns to the heart through an open-ended vessels

Question 5

where are open-ended circulatory systems?

Answer 5

• mainly in invertebrate animals, including most insects and some molluscs
• in insects, gas exchange takes place in the tracheal system (insect blood is called haemolymph)

Question 6

what does haemolymph carry?

Answer 6

• no oxygen or carbon dioxide
• transports food and nitrogenous waste products as well as the cells involved in defence against disease

Question 7

what is the body cavity like in insects?

Answer 7

• split by a membrane and the heart extends along the thorax and the abdomen of the insect
• haemolymph circulates but steep diffusion gradients cannot be maintained for efficient diffusion
• amount of haemolymph flowing to a particular tissue cannot be varied to meet changing demands

Question 8

what are closed circulatory systems?

Answer 8

• blood is enclosed in blood vessels and does not come into direct contact with the cells of the body
• the heart pumps the blood around the body under pressure and relatively quickly, and the blood returns quickly to the heart
• substances leave and enter the blood by diffusion through the walls of the blood vessels
• most contain a blood pigment that caries the respiratory gases

Question 9

how can the amount of blood flowing to a particular tissue be adjusted?

Answer 9

• widening of narrowing of the vessels

Question 10

where are closed circulatory systems found?

Answer 10

• different animal phyla
• incl. echinoderms, cephalopod molluscs, annelid worms, all of the vertebrate groups

Question 11

what are single circulatory systems?

Answer 11

• the blood flows through the heart and is pumped out to travel all around the body before returning to the heart
• blood travels only once through the heart for each complete circulation of the body

Question 12

what happens in a single closed circulatory system?

Answer 12

• the blood passes through two sets of capillaries before it returns to the heart
• in the first set, it exchanges oxygen and carbon dioxide
• in the second set, in the different organ systems, substances are exchanged between the blood and the cells
• the blood pressure in the system drops considerably so the blood returns to the heart slowly limiting the efficiency of the exchange processes so the activity level of these animal tends to be relatively low

Question 13

how do fish have an efficient single circulatory system?

Answer 13

• means they can be very active
• have counter current gaseous exchange mechanism that allows them to take a lot of oxygen from the water
• their body weight is supported by the water in which they live in and they do not maintain their own body temperature
• greatly reduces metabolic demands on their bodies

Question 14

what is the most efficient system for transporting substances around the body?

Answer 14

• double closed circulatory system

Question 15

what two separate circulations do double closed circulatory systems?

Answer 15

1. blood is pumped from the heart to the lungs to pick up oxygen and unload carbon dioxide, and then returns to the heart
2. blood flows through the heart and is pumped out to travel all around the body before returning to the heart again

Question 16

what happens in a double circulatory system?

Answer 16

• blood travels twice through the heart for each circuit of the body
• each circuit only passes through one capillary network
  -this means a relatively high pressure and fast flow of blood can be maintained