NEET Biology Notes Body Fluids and Circulation Blood Pressure
Blood Pressure
Blood Pressure (BP)
The pressure created by the blood on the walls of the blood vessels due to the repeated pumping of heart is called blood pressure. It can be felt at certain places in our body viz., wrist of the hands, etc. The rate of pulsation increases during excitement.
Blood pressure is recorded as systolic/diastolic. Blood pressure in a normal person = 120 / 80 mm Hg. Factors affecting blood pressure are age, cardiac output, total peripheral resistance, etc.
Cardiac Cycle
In the beginning, all the four chambers of heart are in a relaxed state, i.e. joint diastole.
The bicuspid and tricuspid valves open and blood from pulmonary veins and vena cava flows into the left and right ventricle respectively through the left and right atria.
SAN now generates action potential leading to atrial systole. The action potential is conducted to the ventricular side by the AVN and AV bundle from where, the bundle of His transmits through the entire ventricular musculature.
This leads to ventricular systole, which increases the ventricular pressure causing the closure of tricuspid and bicuspid valves.
The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing the closure of semilunar valves, which prevents the backflow of blood into the ventricles. The ventricles and atria are now again in a relaxed (joint diastole) state as earlier.
The SAnode again generates a new action potential and the events above are repeated in sequence and the process continues.
This sequential event in heart is called cardiac cycle and it consists of systole and diastole of both atria and ventricles. Many such cardiac cycles are performed per minute.
Cardiac Output
During a cardiac cycle, each ventricle pumps out about 70 mL of blood called the stroke volume. The stroke volume multiplied by the heart rate gives the cardiac output. Cardiac output is the volume of blood pumped out of each ventricle per minute and averages 5000 mL or 5 L in a healthy individual. The cardiac output of an athlete is much higher than that of an ordinary man.
Heartbeat
It is the rhythmic contraction and expansion movement of heart. The movements of heartbeat are similar to the contraction and dilation of ventricles. The rate of heartbeat .is 72 per min in adult male and little higher in women, children, infants, during exercise, fever and excitement. A heartbeat has two components, systole and diastole. These components produce sounds like lub and dupp. Stethoscope is used to listen heartbeat on the left side of the chest.
Pacemaker
SA node generates impulses in both the atria and through AV node pass it to ventricles for their rhythmic contraction (natural pacemaker). Disturbance in any component of this impulse conducting system, results in irregularity of heart rhythm or independent contraction of atria and ventricles.
Sometimes, atrial impulses fail to pass into ventricles for a few seconds to few hours. This is called ventricular escape. In this case, artificial pacemaker is implanted.
Heart Sounds
The beating of heart produces sounds, heard by stethoscope. There are four heart sounds designated as St, S2, S3 andS4.
First sound (lub =S t) is a long and louder sound created by the closure of atrioventricular valves (tricuspid and bicuspid) at the beginning of ventricular systole.
Second sound (dupp = S2) is a short and sharper sound created by the closure of semilunar valves towards the end of ventricular systole (beginning of ventricular diastole).
The S3 and S4 may not be audible but are usually faint. These sounds are associated with the rapid ventricular filling (S3) and atrial systole (S4).
Double Circulation
The blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta.
The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs from, where the oxygenated blood is carried by the pulmonary circulation. This pathway is called pulmonary circulation.
The oxygenated blood entering the aorta is carried by a network of arteries, arterioles and capillaries to the tissues from, where the deoxygenated blood is collected by a system of venules, veins and vena cava and emptied into the right atrium. This is systemic circulation, which provides nutrients, 02 and other essential substances to the tissues and takes COz and other harmful substances for elimination.
A connection between digestive tract and liver is called hepatic portal system. The hepatic portal vein carries blood from intestine to the liver before it is delivered to systemic circulation.
Regulation of Cardiac Activity
The activities of heart are auto regulated by nodal tissue, hence, the heart is called myogenic.
A special neural cell in the medulla oblongata regulates the cardiac function through Autonomic Nervous Systems (ANSs),
Neural signals through the sympathetic nerves can – increase the rate of heartbeat, the strength of ventricular contraction and thereby cardiac output.
The parasympathetic neural signals decrease the rate of heartbeat, speed of conduction of action potential and thereby the cardiac output. Adrenal medullary hormones also increase the cardiac output.
Electrocardiograph (EGG)
It is a graphical representation of the electrical activity of the heart during a cardiac cycle. The machine used to obtain this graph is called electrocardiograph.
The P-wave in ECG represents the electrical excitation or depolarisation of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiate the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole.
The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of the T-wave marks the end of systole. By counting the QRS complex, the idea of number of heartbeat per minute can be obtained. Any deviation from the usual shape, indicates an abnormality.
- ECG gives accurate analysis about the functioning of heart and its chambers.
- It indicates the functioning of heart valves.
- It helps in identification of any damage or injury of heart.
Disorders of Circulatory System
Some disorders of circulatory system are as follows:
- High Blood Pressure
(Hypertension) .
If a person has persistent high blood pressure, then it is called hypertension. It is 150/90 mmHg. The factors responsible are over eating, fear, worry, anxiety, sorrow, etc. Hypotension is a condition of low blood pressure, i.e. persistent 100/50 mmHg. - Coronary Artery Disease (CAD)
It is also called as artherosclerosis. It affects blood vessels, caused by the deposition of calcium, fat, cholesterol and fibrous tissues, which makes the lumen of arteries narrower. - Angina
Angina pectoris is a symptom of acute chest pain, where no oxygen reaches the heart muscles. It is more common in middle-aged and elderly and affects blood flow. - Heart Failure
It is the state, when heart is not pumping enough blood to meet the body demands. It is also called congestive heart failure because of congestion of lungs (main symptom). Heart failure is different than cardiac arrest, where heart stops beating or a heart attack, when the heart muscle is suddenly damaged by an inadequate blood supply.
- Blood flow is lowest in blood capillaries.
- A thrombosis in the blood vessels of brain is called a stroke.
- Cardiomegaly-Heart enlargement.