The Human Heart
Chambers of the heart
The heart is made up of four chambers. The chambers on the right side of the heart are the right atrium and right ventricle.
The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava (major veins).
The blood then flows to the right ventricle where it is then pumped, via the pulmonary circuit, through the pulmonary artery to the lungs for gaseous exchange (CO2 out, O2 in).
The chambers on the left side of the heart are the left atrium and left ventricle. The left atrium receives oxygenated blood from the lungs via the pulmonary veins.
The blood then flows to the left ventricle where it is then pumped, via the systemic circuit, through the aorta and then pumped out to the to the organs and tissues of the body to supply them with oxygen and nutrients, and pick up waste products for expulsion.
Follow the arrows on the diagram above as this shows the direction of blood flow through the heart.
Being a muscle which requires nutrients for fuel, the heart also has its own blood supply.
It receives the oxygenated, nutrient rich blood that it needs from the coronary arteries which branch off the aorta. The deoxygenated blood is then returned to the right atrium through the cardiac veins.
It is damage to the coronary arteries that often leads to heart problems such as heart attacks.
The ventricles of the heart have thicker muscular walls than the atria. This is because blood is pumped out of the heart at greater pressure from these chambers compared to the atria.
The left ventricle also has a thicker muscular wall than the right ventricle, as seen in the adjacent image. This is due to the higher forces needed to pump blood through the systemic circuit (around the body) compared to the pulmonary circuit.
Valves of the heart
There are four valves within the heart which serve to prevent backflow of blood as it passes through the various chambers of the heart and out through the associated arteries.
The tricuspid valve and the mitral valve are known as ‘atrioventricular’ valves as they are situated between the atria and ventricles on both sides of the heart.
The tricuspid valve is positioned between the right atrium and ventricle, and the mitral valve sits between the left atrium and ventricle, as seen in the adjacent image.
As blood is pumped out of the ventricles through the aorta and the pulmonary arteries, these valves close to ensure the blood does not get pumped back into the respective atria it came from.
The other two valves are the pulmonary and aortic valves, which are collectively known as the ‘semilunar’ valves.
The pulmonary valve sits between the right ventricle and the pulmonary artery. Its role is to prevent the backflow of blood into the right ventricle after it contracts. The aortic valve sits between the left ventricle and the aorta and prevents backflow of blood into the left ventricle after it contracts.
How the heart beats
One heart beat consists of one contraction phase and one relaxation phase.
The contraction phase, known as ‘systole’, occurs when blood is pumped out of the heart. The relaxation phase, known as ‘diastole’, occurs when the heart is relaxing and filling with blood. These phases can be seen in the following image.
There are two unique sounds associated with the heart beat (which you can hear through a stethoscope placed on a friend’s chest).
There is a low pressure ‘lub’ sound which is the closing of the atrioventricular valves, followed quickly by a higher pressure ‘dub’ sound as the semilunar valves close.
The heart beat is controlled by the sympathetic and parasympathetic branches of the autonomic nervous system.
The sympathetic system stimulates the heart to beat faster while the parasympathetic system returns the heart rate to its normal resting level.
Electrical impulses (action potentials) from both systems are sent to the sino-atrial (SA) node. The SA node, known as the hearts ‘pacemaker’, then sends out an impulse that stimulates both atria to contract and this stimulus is also sent to the atrioventricular (AV) node. This is shown on the below image.
The AV node then further distributes the impulse from the SA node to both ventricles which contract a fraction of a second after the contraction of the atria.
The rate at which the SA node sends out impulses determines the rate at which the heart beats. As well as receiving impulses from the autonomic nervous system, the rate at which the SA node sends impulses can also be influenced by hormones such as adrenaline.
When released into the system adrenaline stimulates the SA node to send impulses at a faster rate, thus increasing the rate at which the heart beats.