Note
Studied by 6 people
G.1
The human heart has four well-developed muscular chambers arranged so that there is an atrium (pl. atria) and a ventricle on each of its right and left side.
A muscular wall between the ventricles, called the septum, keeps the blood on each side separate.
The atria are the receiving chambers. Veins collect and transport deoxygenated blood from the body to the right atrium.
The final veins that deliver the blood are anterior and posterior vena cavae.
The left atrium receives oxygenated blood from the lungs via the pulmonary veins.
Large cylindrical flaps of muscle tissue, known as the atrioventricular valves (AV valves), lie between the atria and the ventricles.
When the atrial muscles contract, blood is directed from the atria through the AV valves into the ventricles.
Once the ventricles are full, they contract and propel the blood out of the heart and into the arteries.
The deoxygenated blood in the right ventricle is pumped through the pulmonary arteries to the lungs where it is oxygenated.
This blood returns to the left atrium, completing what is known as the pulmonary circuit .
In contrast, the systemic circuit pumps oxygenated blood from the left ventricle into the aorta and on to the rest of the body from where it returns to the heart through the vena cavae.
There are extensive capillary beds that interconnect these two circuits; both in the lungs and in body tissues.
Blood returning to the heart from either circuit has very low pressure and is moving slowly because it has passed through a capillary.The heart has to pump the blood again for it to enter the next circuit.
This pattern of blood flow, where blood is pumped twice for it to completely
circulate the body, is known as a double circuit .The flaps of the large AV valves are attached to the interior walls of the ventricles by
tiny tendons called chordae tendineae that ensure the valves do not invert (flip up) and allow blood to pass back into the atria when the ventricles contract.Another set of valves, the semilunar valves , exists at the beginning of the arteries. The one at the beginning of the aorta is the aortic valve , where the valve at the beginning of the pulmonary trunk is the pulmonary valve.
During ventricular contraction, blood is forced past these valves into the arteries of each circuit. When the ventricles relax, as they do between contractions, pressure is lowered for a moment, allowing blood to settle against these valves, which forces them to close.
In this way, the semi-lunar valves prevent the blood from draining back into the ventricles. The system is ready for the next contraction.
BLOOD PRESSURE
Blood pressure, the force of blood against the blood vessel walls, is not constant.
It is greatest when the ventricles are contracting – actively forcing blood through the arteries.
Between these contractions, blood pressure is less, as if blood were resting, waiting to be pushed farther along the artery.
The term systolic pressure (systole) refers to the pressure when the ventricles are contracting.
Diastolic pressure (diastole) refers to the blood pressure when the ventricles are not contracting.
The period of time associated with diastole includes atrial contraction and ventricular relaxation and recovery.
Blood pressure is normally measured using the brachial artery of the arm.
A reading of 120/80 (systolic/diastolic) is fairly average.
The units of this measurement are “mmHg” – an old unit for pressure. (Using this unit, normal air pressure at sea level is 760 mmHg).
When one has a higher blood pressure than normal, a condition known as hypertension, additional stresses are put on the tissues being fed by the blood.
This condition may also be an indication that the heart is working too hard, further straining the blood delivery system.
The potential for tissue damage is greater the longer the blood pressure remains high.
Diet, lifestyle, and anxiety are often to blame for sustained high blood pressure.
For example, ingesting an excessive amount of salt will cause the body to retain water – greater fluid volume leads to greater blood pressure.
During physical activity, temporary elevated blood pressure is normal.
Substances like cholesterol can accumulate on the inside walls of arteries like arterioles, causing them to lose their elasticity, which restricts the flow of blood into peripheral arteries.
Legs may be painful and suffer numbness as a result.
Deposits like these can continue to accumulate and harden into plaques.
When this happens, blood pressure increases because the normal volume of blood is being forced through a narrower space.
Complete blockage of the vessels will result in tissue starvation and tissue death.
Reduced flow of blood in the coronary arteries feeding the heart can cause a heart attack when part of the heart muscle dies.
Insufficient blood delivery to the brain can result in arteriole blockage or bleeding, and cause cell death. Death of brain cells in this manner is a stroke, which can impair other body functions with potentially fatal consequences.
Low blood pressure, hypotension, can be detrimental as well, but for very different reasons.
If the brain does not get enough oxygen to function properly due to low blood pressure, it temporarily shuts down causing dizzy spells, and perhaps, fainting.
These reactions, however, could be an indication of other circulatory disorders restricting blood delivery to the brain.
The kidneys depend on blood pressure to filter wastes out of plasma, and can only function properly if there is sufficient pressure to maintain filtration.
Luckily, the body has some abilities to adjust blood pressure. Monitored by a specialization at the base of the brain called the medulla oblongata, the nervous system can signal small arteries to dilate (widen) thus allowing increased blood flow coupled with a lowered blood pressure.
It can also signal for these blood vessels to constrict (narrow), thus increasing the blood pressure while reducing the volume of blood that passes through them.
CONTROL OF HEART FUNCTION AND THE CARDIAC CYCLE
The heart contains two spots of a specialized tissue called nodes :
The sino-atria node (SA node) and the
atrio-ventricular node (AV node).
Both are located in the right atrium.
The SA node is along the wall of the chamber itself, where the AV node is deeper, closer to the AV valve.
Nodal tissue is a unique combination of muscle and nerve tissue, which has the ability to contract intrinsically, independent of other stimuli.
These nodes stimulate the contractions of the heart.
A heart, even removed from a living organism, will continue to beat until it dehydrates, suffers a lack of ATP or some external stimulus causes a cardiac arrest.
Blood moves slowly and steadily through veins into the atria.
When these chambers are full and begin to stretch, the SA node sends out nerve impulses to stimulate their simultaneous contraction as well as an impulse to the AV node, causing it to respond.
On the average in a resting heart, the SA node initiates this action every 0.85 seconds (= 72 times per minute).
The contraction of the atria causes the blood pressure in them to exceed that of the empty ventricles, which forces the AV valves open, allowing the blood to enter the ventricles.
The filling time of the ventricles corresponds to the arrival time of the impulse from the SA node. When the ventricles are full, the AV node is activated and causes them to contract but, because the muscle tissue of the ventricles is so massive, their contraction is coordinated through a set of nerves called the Purkinje fibres.
These nerves start in a bundle at the AV node and extend throughout the ventricles, conducting impulses that cause both ventricles to contract in unison to send blood out of the heart.
The heightened blood pressure in the ventricles during their contraction snaps the AV valves closed.
Blood in the left ventricle is forced through the aortic valve into the aorta, while blood in the right ventricle is forced through the pulmonary trunk branches to form the pulmonary arteries that take the blood to the lungs.
The ventricles then relax, allowing the aortic and pulmonary valves to close as blood rests against them.
A split second later, this series of events comprising one complete heartbeat, the cardiac cycle, repeats itself.
Each heartbeat makes a double sound caused by the closing of the two sets of valves.
The significant role of the SA node plays in initiating the contraction of the heart has given it the nickname “pacemaker”.
People with irregular heartbeats may require an artificial pacemaker, which is a small device that stimulates a specific region of the heart to contract.
An electrocardiogram (ECG) is a representation of the electrical activity of a person’s heartbeat.
The cardiac cycle begins with the contraction of the atria, which is identifiable on the ECG as the P wave.
It is followed by a short interval before the ventricles tense up and contract (the QRS complex).
This interval relates to the length of time required to stimulate the muscle mass of the ventricles.
The T wave represents the electrical changes that occur when the ventricles recover from their contraction.
The atrial recovery occurs during the QRS complex, so it is not observed on the graph. A small U wave is sometimes visible. It is part of the ventricular recovery.
Blood flows to all parts of the body, including the central nervous system.
The medulla oblongata is sensitive to the speed and pressure of the blood as it passes through the capillary beds.
When it is perceived that blood is getting delivered too slowly, or if blood pressure is low, the medulla oblongata will signal the SA node to contract faster by sending an impulse along the vagus nerve that interconnects the two.
Similarly, the medulla oblongata can slow down the rate of heart contractions.
This regulation of the heartbeat is a function of the autonomic nervous system and not under conscious control.
Additionally, the aorta is equipped with aortic bodies which contain nerve receptors sensitive to the oxygen and carbon dioxide content in blood.
These important homeostatic links to the nervous system promote changes in heart rate and therefore blood pressure required to maintain the functioning of other organs like the kidneys.