The mammalian heart is myogenic and it is regulated by nerves. The heartbeat originates from
- A.sinoatrial node
- B.QRS wave
- C.T wave
- D.hepatic portal system
Correct answer
Explanation
The correct answer is A, sinoatrial node. The SA node, a small patch of special muscle in the wall of the right atrium, produces the electrical impulse that starts each heartbeat, which is why it is called the pacemaker of the heart. Because the beat begins in the heart muscle itself, the mammalian heart is called myogenic; nerves only speed it up or slow it down. The impulse spreads over both atria, then reaches the atrioventricular (AV) node, the bundle of His and the Purkinje fibres, which make the ventricles contract. In a resting adult the SA node normally fires about 70 to 75 times a minute. Option B is wrong because the QRS wave is the part of an ECG that records the spread of the impulse through the ventricles; it is a reading, not a source. Option C is wrong because the T wave shows the ventricles recovering after each beat. Option D is wrong because the hepatic portal system carries blood from the intestine to the liver. Exam tip: the SA node is the natural pacemaker; an artificial pacemaker takes over when it fails.
Practice Questions
View allCertain species of which one of the following organisms are well known as cultivators of fungi?
- A.Ant
- B.Cockroach
- C.Crab
- D.Spider
Show answer
Several groups of ants, above all the leafcutter ants of the Americas, grow fungus as their food, a form of farming that began millions of years before human agriculture.
Read the full explanation →The solution of which one of the following will have pH less than 7?
- A.NaOH
- B.KCl
- C.FeCl₃
- D.NaCl
Show answer
Ferric chloride is a salt of a weak base, iron(III) hydroxide, and a strong acid, hydrochloric acid, so its solution is acidic and has a pH below 7.
Read the full explanation →Consider the following statements about a solenoid: 1. The magnetic field strength in a solenoid depends upon the number of turns per unit length in the solenoid 2. The magnetic field strength in a solenoid depends upon the current flowing in the wire of the solenoid 3. The magnetic field strength in a solenoid depends upon the diameter of the solenoid Which of the statements given above are correct?
- A.1, 2 and 3
- B.1 and 3 only
- C.2 and 3 only
- D.1 and 2 only
Show answer
The magnetic field inside a long solenoid is B = μ₀nI, where n is the number of turns per unit length and I is the current, so the field depends on these two and not on the diameter.
Read the full explanation →Which of the following are the characteristics of electromagnetic waves? 1. They are elastic waves. 2. They can also move in vacuum. 3. They have electric and magnetic components which are mutually perpendicular. 4. They move with a speed equal to 3 lakh meters per second. Select the correct answer using the code given below:
- A.1, 2, 3 and 4
- B.1, 2 and 4 only
- C.2 and 3 only
- D.3 and 4 only
Show answer
Electromagnetic waves need no medium, so they travel through vacuum, and they consist of electric and magnetic fields that oscillate at right angles to each other and to the direction of travel.
Read the full explanation →Which one of the following cell organelles may play a role in expelling excess water and wastes in case of unicellular organisms?
- A.Lysosome
- B.Vacuole
- C.Golgi body
- D.Endoplasmic reticulum
Show answer
In many single-celled organisms such as Amoeba and Paramecium, a special contractile vacuole collects excess water and some wastes from the cytoplasm and pumps them out of the cell.
Read the full explanation →