The Circulatory System

The Circulatory System - also known as The Cardiovascular System, allows the blood to circulate the body, transporting;

• nutrients
• Oxygen
• Carbon dioxide
• Hormones
• waste
• and blood cells

to and from each cell in the body.

The Circulatory System includes the heart, blood and blood vessels.

The heart; pumps blood around the body by contracting and relaxing.

Blood; is pumped around the heart and the body by a network of vessels; arteries, veins and capillaries.

It is a double circulatory system which means the blood travels through two systems; the pulmonary and systemic. 

Arteries; carry oxygenated blood at higher pressure from the heart to parts of the body. They have a thick layer of elastic tissue in the wall.

Veins; Carry deoxygenated blood at lower pressure from the body back to the heart. This means that they have less elastic than arteries. 

Valves stop the blood from flowing backwards. They do this by opening and closing due to pressure changes in the chambers to allow the blood to flow at the right times, preventing a backflow.

Capillaries; Have thin walls that allow exchange of compounds such as nutrients, glucose, oxygen and carbon dioxide between blood and tissues.

Mammals and birds  have four chambered hearts. This system does not allow oxygenated (red) and deoxygenated (blue) blood to mix.

The aorta is the largest artery, which is very elastic and thick as it has to withstand the higher pressures as it carries the oxygenated blood away from the heart.

The vena cava transporting deoxygenated blood to the heart is made up of the superior and inferior vena cava. As it is not under as much pressure, it is not as thick.

There are two types of chambers;

Atria

Ventricles

Both have muscular walls which helps them pump blood.

Ventricles are more muscular than the atria, as the blood has to be forced out of the heart at higher pressure and around the body.

The left side of the heart is more muscular as it pumps blood around the body, while the right side just pumps blood to the lungs.

The Cardiac Cycle is the series of events that occurs when the heart beats.

This circulates blood through the pulmonary and systemic circuits that make up the double circulatory system.

A double circulatory system is more efficient in transporting oxygenated blood around the body because the oxygenated and deoxygenated blood are kept separate. 

The pulmonary circuit carries blood to the lungs to be oxygenated then back to the heart.

In the lungs, carbon dioxideis removed from the blood, and oxygen taken up by the haemoglobin in the red blood cells.

1. The systemic circuit carries blood around the body to deliver the oxygen and returns de-oxygenated blood to the heart.

2. This is broken down further into the cardiac cycle, which is the sequence of mechanical and electrical events that are repeated every heartbeat. It includes two phases:

• Diastole - relaxation phase. During this phase,blood pressure is at its lowest.
• Systole - contraction phase, this is when the blood pressure rises. 

3. The frequency of the cardiac cycle is monitored by the heart rate, which is recorded by number of beatsper minute. 

1. Blood drains into the left atrium from the lungs along the pulmonary vein

2. Raising of the blood pressure in the left atrium forces the left tricuspid valve open.

3. Contraction of the left atrial muscle (left atrial systole) forces more blood through the valve.

4. As soon as left atrial systole (muscle contraction) is over, the left ventricular muscles start to contract. This is called left ventricular systole.

5. This forces the left tricuspid valve to close and opens the valve in the mouth of the aorta (semilunar valve). Blood then leaves the left ventricle along the aorta. 

The same steps are repeated on the right side at the same time.

Cardiac distole is when the heart refills with blood.

Ventricular diastole is when the ventricles are refilling and relaxed. 

For the heart to beat, electrical signals trigger muscles to contract and relax. There is a specific pathway to allow this to happen. 

1. Sinoatrial node - this is also known as the natural pacemaker as it causes an impulse to travel through the atria causing them to contract and force blood into the ventricles. This node also sets the heart's rhythmn and rate.

2. Atrioventricular node - this detects the impulse travelling through the atria and redirects the impulse to the bundle of His.This causes a delay, slowing the spread of the electrical impulse across the heart andaloowing the atria to contract before the ventricles. 

3. Bundle of His. - also called the atrioventricular bundle, is a group of fibres in the septum which the impulse travels through to the base of the ventricles. 

4. Purkinje fibres - these fibres act like neurons and are found in the walls of the ventricles.The impulse fromthe bundle of His reaches the Purkinje fibres which causes the ventricles to contract. 

Stretch receptors (Baroreceptors) in the heart detect changes in the pressure of blood filling the atria.

These send signals to the CNS which triggers vasodilation. 

This reduces the pressure in the blood.