Circulatory and Excretory Systems: Heart, Blood and Kidney

The correct answer is B, nephrons. The nephron is the structural and functional unit of the kidney, and each human kidney holds about one million of them. Blood is pushed into the cup shaped Bowman's capsule through a knot of capillaries called the glomerulus, and there the actual filtration happens: water, salts, glucose, amino acids and urea pass out while blood cells and large plasma proteins are held back. Option A is wrong because the ureter is only the tube that carries formed urine from the kidney down to the urinary bladder; it filters nothing. Option C is wrong because neurons are nerve cells that conduct impulses and belong to the nervous system, not the excretory system; the similar sound of nephron and neuron is exactly the trap in this question. Option D is wrong because the urethra is the last passage through which urine leaves the body. Remember the order: nephron filters, ureter carries, bladder stores, urethra releases.

The correct answer is C, the pulmonary artery. An artery is defined by direction, not by the kind of blood it holds: it carries blood away from the heart. The pulmonary artery leaves the right ventricle carrying the deoxygenated blood that has come back from the body, and takes it to the lungs to be loaded with oxygen, so it is the single exception to the rule that arteries carry oxygenated blood. Its partner exception is the pair of pulmonary veins, the only veins that carry oxygenated blood. Option A is wrong because the aorta leaves the left ventricle with fully oxygenated blood and is the largest artery of the body. Option B is wrong because the coronary arteries branch off the aorta to feed the heart muscle itself and therefore carry oxygenated blood; their blockage causes a heart attack. Option D is wrong because the carotid arteries carry oxygenated blood up to the head and brain.

The correct answer is D, group O. In the ABO system the antigens sit on the surface of the red blood cell and the antibodies float in the plasma. Group O red cells carry neither the A antigen nor the B antigen, so there is nothing on them for the recipient's antibodies to attack, and blood of this group can be given to a person of any ABO group. That is why group O is called the universal donor. Option A is wrong because group A cells carry the A antigen and would be destroyed by the anti-A antibody present in the plasma of groups B and O. Option B is wrong for the same reason with the B antigen. Option C is wrong because group AB is the opposite case: its plasma has no anti-A or anti-B antibody, so it can receive from everyone and is called the universal recipient, not the universal donor. Keep the pair straight: O gives to all, AB takes from all.

The correct answer is B, the sino-atrial node. The heartbeat is not started by a nerve from the brain; the heart generates its own rhythm, and the patch of specialised tissue that does so fastest sets the pace for the rest. That patch is the sino-atrial node, lying in the wall of the right atrium near the opening of the superior vena cava, and it is therefore called the pacemaker of the heart. Option A is wrong because the atrio-ventricular node only receives the impulse from the sino-atrial node and delays it briefly so that the atria can finish emptying; it can take over as a slower backup pacemaker but is not the normal one. Option C is wrong because the bundle of His is the conducting pathway that carries the impulse from the atrio-ventricular node into the septum between the ventricles. Option D is wrong because the Purkinje fibres are the final branches that spread the impulse through the ventricular muscle. The order to memorise is sino-atrial node, atrio-ventricular node, bundle of His, Purkinje fibres.

The correct answer is B, the spleen. A red blood cell lives about one hundred and twenty days. As it ages its membrane grows stiff, and the narrow winding channels of the spleen trap such cells so that the macrophages lining them can break the cells down and salvage the iron of their haemoglobin. Because so many worn out red cells are destroyed there, the spleen is called the graveyard of red blood cells; it is also the largest lymphoid organ of the body and acts as a blood reservoir. Option A is wrong because the liver, although it does handle the bilirubin left from broken haemoglobin and turns it into bile pigment, is not the place where the cells are trapped and destroyed. Option C is wrong because the red bone marrow is the opposite: it is the birthplace of red blood cells, not their graveyard. Option D is wrong because the pancreas makes digestive juice and the hormones insulin and glucagon and has no role with red cells at all.

The correct answer is B, the liver. When the body breaks down more protein than it needs, the amino acids are stripped of their amino group and the ammonia so released is highly poisonous. The liver converts this ammonia into urea, a far less toxic compound, through a series of reactions known as the urea cycle or the ornithine cycle. The urea then travels in the blood to the kidney, which only filters it out and concentrates it in the urine. Option A is wrong for exactly that reason: the kidney removes urea but does not make it, and this separation of the site of manufacture from the site of removal is what the question tests. Option C is wrong because the urinary bladder is only a muscular sac that stores urine until it is passed out. Option D is wrong because the spleen destroys worn out red blood cells and stores blood. Because human beings pass out mainly urea they are described as ureotelic animals, unlike ammonotelic bony fishes and uricotelic birds and reptiles.

The correct answer is B, the proximal convoluted tubule. Filtration at the glomerulus is blind: it pushes out useful glucose and amino acids along with the waste. The tubule that immediately follows the Bowman's capsule, the proximal convoluted tubule, has a brush border of microvilli that gives it an enormous surface, and it takes back practically all the glucose and amino acids and about seventy to eighty per cent of the water and salts. Option A is wrong because the Bowman's capsule is the site of ultrafiltration, where the filtrate is formed rather than reabsorbed. Option C is wrong because the loop of Henle does something different: by its counter-current arrangement it builds a salt gradient in the medulla so that concentrated urine can be produced, and it reabsorbs water and sodium, not glucose. Option D is wrong because the collecting duct carries out the last adjustment of water under the influence of antidiuretic hormone. Note the clinical link: glucose appears in the urine only when the blood sugar exceeds what the proximal tubule can carry back.

The correct answer is B, antidiuretic hormone. Antidiuretic hormone, also called vasopressin, is released by the posterior lobe of the pituitary gland and makes the distal tubule and the collecting duct of the nephron pull water back into the blood. If it is lacking, that water is not recovered and the patient passes litres of very dilute urine and is constantly thirsty, which is the disease diabetes insipidus. Option A is wrong because a shortage of insulin causes diabetes mellitus, an entirely different disorder in which blood glucose rises and sugar spills into the urine; sharing the word diabetes is the trap here. Option C is wrong because aldosterone, from the adrenal cortex, controls the reabsorption of sodium and the excretion of potassium, and its lack disturbs salt balance rather than causing floods of dilute urine. Option D is wrong because thyroxine from the thyroid gland regulates the rate of metabolism, and its deficiency causes cretinism in children and myxoedema in adults. Remember that insipidus means tasteless and mellitus means honey sweet.

The correct answer is B, the bicuspid valve. The opening between the left atrium and the left ventricle is guarded by a valve with two flaps or cusps, which is why it is called the bicuspid valve; it is also known as the mitral valve because its shape resembles a bishop's mitre. Its job is to stop blood from being pushed back into the atrium when the powerful left ventricle contracts. Option A is wrong because the tricuspid valve, with three cusps, sits on the other side of the heart, between the right atrium and the right ventricle; swapping the two sides is the commonest error in this question. Option C is wrong because the pulmonary semilunar valve stands at the mouth of the pulmonary artery, where blood leaves the right ventricle for the lungs. Option D is wrong because the aortic semilunar valve stands at the mouth of the aorta, where blood leaves the left ventricle, that is after the bicuspid valve and not between the chambers. A useful memory aid is that the left side has two cusps and the left lung has two lobes.

The correct answer is A, William Harvey. The English physician William Harvey showed by experiment and measurement that blood is not made afresh and used up in the tissues, as was then believed, but moves in a closed circuit, driven by the heart acting as a pump, and he published this in 1628 in his book on the motion of the heart and blood. Option B is wrong because Karl Landsteiner worked out the ABO blood groups at the beginning of the twentieth century and later helped identify the Rh factor; he explained the kinds of blood, not its movement. Option C is wrong because Edward Jenner introduced vaccination against smallpox using cowpox material and is called the father of immunology. Option D is wrong because Robert Koch identified the bacteria causing tuberculosis and cholera and gave the postulates that link a germ to a disease. A related name worth remembering is Christiaan Barnard, who performed the first human heart transplant.