Nov 15, 2009

The mammalian cardiovascular system: An overview

There are two types of circulatory systems in animals : Open system and Closed system.

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

Closed System
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 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 deoxgenated blood, and the pulmonary vein is the only vein that carries oxygenated blood. The overall system is known as pulmonary circulation.

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 muslce 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 vena 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.

Regulation of the cardiac output -There are two nerves running to the heart, 1. The vagus (a.k.a. parasympathetic nerve) and 2. the sympathetic nerve. These two nerves bring impulses from the Medulla Oblongata (more precisely the cardiovascular centre from the medulla oblongtata).

The vagus nerve is used to slow down the heart. It sends its impulses to the SAN and to the AVN.

The sympathetic is used to speed up the heart. It sends its impulses to the walls of the heart. These two nerves work in opposite to each other.

Adrenaline - is a hormone and it is secreted from the adrenal glands in time of fear, stress or nervous anticipation (eg. SPM result day). Adrenaline 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 oblongta, which then reacts accordingly.

The concentration of carbon dioxode - 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.