TRANSPORT IN ANIMALS

NOVEMBER 8, 2011CAPE BIOLOGYUNIT IIOMAR WRIGHT

Objectives

To define the purpose of a vascular system Definition of the terms “open circulatory system” and

“closed circulatory system” Differences between the open and closed circulatory

systems. To describe the structures of arteries, capillaries and veins

in detail relating their structures to their functions To compare the structure and function of the arteries,

capillaries and veins The roles of arterioles and venules

Blood Vascular System

The purpose of the vascular system is the creation of a flow of blood and the movement of this blood to and from the tissues. The main functions of the blood is to

transport oxygen, nutrients and hormones to the tissues(circulation) and to collect the waste products (carbon dioxide and waste metabolites) for removal from the body via the excretory system.

Open Circulatory System

In an open circulatory system, blood is pumped from the heart through blood vessels but then it leaves the blood vessels and enters body cavities, where the organs are bathed in blood, or sinuses (spaces) within the organs. Blood flows slowly in an open circulatory system because there is no blood pressure after the blood leaves the blood vessels. The animal must move its muscles to move the blood within the spaces.

The exchange of materials between the cells and the blood is done directly. There are few blood vessels but they are not extensive. The blood vessels are open-ended as they open into the common cavities called the haemocoel. For example: Insects, Arthropods and most mollusks(except cephalopods: nautilus, squid, octopus) have an open circulatory system

Closed Circulatory System

In a closed circulatory system, blood is not free in a cavity; it is contained within blood vessels. Valves prevent the backflow of blood within the blood vessels.

This type of circulatory system is found in vertebrates and several invertebrates including annelids, squids and octopuses.

The blood of animals with a closed circulatory system usually contains cells and plasma (liquid). The blood cells of vertebrates contain hemoglobin.

Differences Between Open and Closed Circulatory Systems

OPEN CIRCULATORY SYSTEM

CLOSED CIRCULATORY SYSTEM

Occurrence: In some annelids, most of mollusks and arthropods.

Position of blood: Blood does not remain confined in the blood vessels and comes in lacunae or sinuses.

Blood Pressure: Blood flows at low pressure and cannot be regulated.

Velocity of blood: Blood flows at a low velocity.

Exchange of materials: Direct exchange between blood and body cells.

Efficiency: Less efficient as blood takes more time to complete one circulation.

Occurrence: In most of annelids, cephalopod molluscs and all vertebrates.

Position of blood: Blood remains confined in the blood vessels.

Blood Pressure: Blood flows at high pressure and can be regulated.

Velocity of blood: Blood flows at high velocity.

Exchange of materials: Exchange occurs through the tissue fluid.

Efficiency: More efficient as blood circulation is completed in short period.

Arteries

Arteries are blood vessels that carry the oxygenated blood away from the heart. They vary in their size, depending on their position in the body, and how far away from the heart they are. The structure of an artery is three layers of tissue,

Tunica adventria (outer layer of fibrous tissue)

Tunica media (middle layer of smooth muscle & elastic tissue)

Tunica intima (inner lining of endothelium)

They also help to push the rapid flow of blood when the ventricles are relaxed and the heart is refilling. As the arteries become smaller the tunica media consists almost entirely of smooth muscle. These cannot stretch as much as the larger arteries. They still pass a small amount of blood through them, directing it elsewhere in the circulation. These small arteries are known as arterioles.

Arteries contain about 20% of blood at any one time. You can feel your pulse in an artery.

Veins

Veins are the blood vessels that carry deoxygenated blood back to the heart. The walls of the veins are thinner than the arteries, but they do still have the same three layers in them. You will find less muscle and elastic tissue in the tunica media. Some veins have valves in them to ensure the flow of blood travels to the heart, and not backwards..

The smallest of the veins (furthest away from the heart) are known as venules. Veins contain about 75% of blood at ant time. There is lower pressure in the vein, compared to that of the arteries, so no pulse can be felt

Capillaries

Capillaries are the smallest blood vessels in the body. Their structure consists of just a single layer of endothelial cells. Water and other small-molecule substances can pass through this wall. Capillaries act as a link between arteries and veins. No valves can be found in the capillaries. The exchange of blood and tissue takes place at the capillary bed. Capillaries contain about 5% of blood at any time, and no pulse can be felt in one.

 

Comparison of the structure and function of Arteries, Capillaries and Veins

Arteries Capillaries Veins

Carry blood away from the heart

Carry blood throughout all tissues

Carry blood towards the heart

Carry high pressure blood

Pressure decrease along length of capillaries

Carry low pressure blood

Blood moves in pulses created by ventricle contractions

Blood flow is smooth Blood flow is smooth

Blow flow is rapid Blood flows along the length of capillary

Blood flow is slow

Carry oxygenated blood (except pulmonary artery)

Blood becomes deoxygenated along the length of the capillaries

Carry deoxygenated blood (except for pulmonary vein)

No valves present No valves present Possess valves to stop backflow of low pressure slow flowing blood