Revision:Circulation in Animals

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There are two types of circulatory systems in animals

1. Open systems - the organs/cells are bathed in the blood. (eg. in insects such as locusts)
2. Closed systems - the organs/cells are provided with blood via vessels. (eg. in mammals such as kangaroos )

Closed Systems

Single Circulation

Single Circulation is a circulatory system in which the blood passes through the heart once, in its passage around the body. eg. Fish

Blood leaves the heart and is oxygenated in the gills.

The oxygenated blood goes to the rest of the body, and then returns back to the heart.

The heart has one atrium and one ventricle.

Remember the ventricle is at the bottom.

The Blood which arrives at the organs of the body are at a low pressure.

Double Circulation

Double Circulation is when blood flows through the heart twice during its journey around the body. eg. Mammals.

Blood leaves the right hand side of the heart, via the pulmonary artery (right hand side meaning the person's right, ie, the left hand side as you look at it in a book). The oxygenated blood returns to the heart to the left hand side, via the pulmonary vein. The pulmonary artery is the only artery in the body that carries deoxygenated blood, and the pulmonary vein is the only vein that carries oxygenated blood. The overall system is known as pulmonary circulation.

• Systematic circulation - the oxygenated blood is pumped to the rest of the body by the left hand side of the heart.

Blood reaching the organs are of high pressure due to the double circulatory system. This is a more efficient system for transporting blood.

Blood Vessels

• Tunica adventitia - the outer layer which contains collagen fibres

• Tunica Media - the middle layer. Contains elastic fibres and smooth fibres. When the ventricles contract and push the blood into the arteries the blood is evened out by the elastic fibres. The smooth fibres vary the amount of blood reaching the different organs by contracting accordingly.

• Tunica Intima - made up of thin epithelial cells, which are smooth and reduce friction between the blood and the vessel walls.

The Heart Beat

• The mammalian heart is myogenic which means the heart beat starts at the heart itself. The sinoatial node (SAN), which is located in the right atrium, sends an electrical impulse across the muscle in the right atria to another node called the atrioventricular node (AVN).

• This electric impulse causes the muscle to contract, so as it goes across the atria, they contract forcing blood through into the ventricles, via the bicuspid and the tricuspid valves.

• The AVN is located in the middle of the two ventricles in the septum, when the impulse reaches this node, it spreads along the specialised fibres (The Bundle of His). This causes the ventricles to contract forcing the blood to leave via the semi lunar valves to the arteries.

Things to remember:

• The aorta takes oxygenated blood TO the body.
• The vana cavae bring deoxygenated blood BACK to the heart.
• The pulmonary artery takes deoxygenated blood TO the lungs.
• The pulmonary vein brings oxygenated blood back to the heart.
• In a frog's heart the sinus venous controls the heart beat.

Regulation of the cardiac output

There are two nerves running to the heart:

1. The cardiac nerve, which forms part of the sympathetic division
2. The vagus nerve, which forms part of the parasympathetic division

These nerves are 'wired' between the cardiovascular centre (located in the Medulla Oblongata of the brain) and the sinoatrial (SA) node of the heart.

When we exercise, there is an increase in the level of carbon dioxide in our blood. This is detected by chemoreceptors in the aorta, which transmit information to the cardiovascular centre. The cardiovascular centre then sends impulses down the cardiac nerve (sympathetic) to the SA node. This stimulates the SA node to send impulses through the cardiac muscle more frequently, thereby increasing cardiac output.

When we rest, there is a decrease in the level of carbon dioxide in our blood. This, again, is detected by chemoreceptors which transmit the information to the cardiovascular centre. An impulse is sent down the vagus nerve (parasympathetic) and the SA node sends fewer impulses through cardiac muscle, thereby decreasing cardiac output.

Adrenalin

Adrenalin is a hormone and it is secreted from the adrenal glands in time of fear, stress or nervous anticipation (eg. A Level results day). Adrenalin speeds up the cardiac cycle so that more oxygen can be provided to muscles and cells which need it. Similar effects are seen when the sympathetic nerve is stimulated.

These two nerves can be effected by many different things:

• Blood pressure - Baroreceptors/stretch receptors in the aorta and also in the walls of the carotoid artery are sensitive to any changes in pressure. When the blood pressure changes these receptors send messages to the medulla oblangta, which then reacts accordingly.
• The concentration of carbon dioxide - If there is a low pH then there's an increase in CO₂ levels, chemoreceptors in the brain, aorta and carotoid arteries detect these changes and then send messages to the medulla oblangta. N.B. The concentration of oxygen does NOT affect the heart rate, the carbon dioxide concentration does. Look back at the locust in a test tube experiment.

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