The Heart

The Heart

The heart is a cone shaped muscular pump which pushes blood around the body, it is divided into 4 chambers the atria which are the upper chambers they receive blood that is returning to the heart, and the ventricles which eject blood into the arteries.

The four main chambers of the heart are

• The right atrium (RA)
  
• The right ventricle (RV)
  
• The left atrium (LA)
  
• The left ventricle (LV)
  

It is essential that blood flows round the body in the right direction so the heart includes a series of valves.

• The Tricuspid valve this separates the right atrium from the left ventricle
  
• The Pulmonary valve separates the right ventricle from the pulmonary artery
  
• The Bicuspid valve this separates the left atrium and the left ventricle
  
• The Aortic valve separates the right ventricle from the ascending aorta
  

The wall of the heart consists of three layers

• Epicardium a thin outer layer which makes the surface of the heart smooth and slippery in texture
  
• Endocardium the smooth inner lining of the heart
  
• Myocardium makes up the bulk of the heart and is responsible for the heart pumping it is made of strong cardiac muscle fibres which are connected by electrical synapses that allow muscle actions to spread from fibre to fibre.

the cardiac cycle

The cardiac cycle describes the complete round of cardiac systole and diastole with intervals between,
Systole is a period of ventricular contraction
Diastole is a period of ventricular relaxation
Blood flows from an area of high pressure to an area of low pressure

Atrial systole
The heart is full of blood and the ventricles are relaxed
Both the atria contract and blood passes down the ventricles
The atrio ventricular valves open due to blood pressure
70% of the blood flows passively down to the ventricles so the atria do not contract a great amount.

Ventricular systole
The atria relax
The ventricle walls contract forcing the blood out
The pressure of the blood forces the atrio-ventricular valves to close which produces the lub sound
The pressure of blood opens the semi lunar valves
Blood passes into the aorta and pulmonary arteries

Diastole
The ventricles relax
Pressure in the ventricles falls below that in the arteries
Blood under high pressure in the arteries causes the semi lunar valve to close which produces the second heart sound ‘dub’
During diastole all the muscles in the heart relax
Blood from the vena cava and pulmonary veins enter the atria
The whole cycle begins again

electrical activity in the heart

The heart constantly generates a sequence of electrical activity with every single heart beat, it has a natural pacemaker that regulates the pace and rate of the heart, this sits in the upper portion of the right atrium and is a collection of specialised electrical cells known as the sinus or sino-atrial (SA) node
As the SA node fires, each electrical impulse travels through the left atrium this electrical activity causes two upper chambers of the heart to contract, the electrical impulse then moves to the area known as the atrio-ventricular (AV) node this sits just above the ventricles. The electric impulse is held here for a brief period to allow the right and left atrium to empty its blood contents into the two ventricles. The electrical impulse then travels through both. The electrically stimulated ventricles then contact and blood is pumped into the pulmonary artery and aorta.

the cardiovascular system and exercise
exercise places an increased demand on the cardiovascular system, oxygen demand by the muscles increases greatly, more nutrients are used and temperature rises. When exercising muscle cells need more oxygen so the cardiac output increases to supply the demand. Heart rate is controlled by the cardiovascular centre in the medulla of the brain. nerve impulses are sent from the cardiac centre to the SAN these nerve impulses speed up or slow down our heart rate. When exercising If blood pressure becomes too high the pressure receptors send impulses to the cardiovascular centre this then sends impulses to the SAN to slow down the heart rate. If the pressure becomes too low the pressure receptors send impulses to the SAN to speed the heart rate up.