HESI A2 Elsevier (HESI): HESI Admission Assessment - Set 4 - Part 2

Correct Answer: Skeletal muscle

Skeletal muscle is striated due to the regular arrangement of actin and myosin filaments into sarcomeres. It attaches to bones and is under voluntary control through motor neurons of the somatic nervous system. Option b is incorrect because Smooth muscle is not striated and is under involuntary control by the autonomic nervous system. It is found in the walls of hollow organs such as the stomach, intestines, uterus, and blood vessels. Option c is incorrect because Cardiac muscle is striated like skeletal muscle but is involuntary. It is found only in the wall of the heart and contracts rhythmically without conscious control to pump blood through the circulation. Option d is incorrect because Visceral muscle is another term for the smooth muscle of internal organs and vessels. It is not striated and is not under voluntary control, so it does not fit the description in the question.

Correct Answer: Actin and myosin

Actin and myosin are the contractile filaments in muscle cells. According to the sliding filament theory, myosin heads pull on thin actin filaments, shortening each sarcomere and producing whole-muscle contraction. Option a is incorrect because Collagen and elastin are common connective tissue proteins found throughout the body. Collagen provides tensile strength while elastin provides recoil to skin, blood vessels, and ligaments, but neither participates in active muscle contraction. Option b is incorrect because Hemoglobin carries oxygen in red blood cells, and myoglobin stores oxygen in muscle cells. They support muscle activity by supplying oxygen for ATP production but do not generate the contractile force themselves. Option c is incorrect because Tubulin forms microtubules of the cellular cytoskeleton, and keratin is a structural protein found in skin, hair, and nails. Neither protein participates in the sliding filament mechanism of muscle contraction.

Correct Answer: Acetylcholine

Acetylcholine is released by motor neurons at the neuromuscular junction. It binds to receptors on the muscle fiber, depolarizing the sarcolemma and triggering the release of calcium that initiates skeletal muscle contraction. Option a is incorrect because Norepinephrine is a neurotransmitter of the sympathetic nervous system. It acts on smooth muscle, cardiac muscle, and glands during the fight-or-flight response, but is not used at the skeletal neuromuscular junction. Option c is incorrect because Dopamine is a central nervous system neurotransmitter important in pathways involving voluntary movement, reward, and mood. It is not used as the neurotransmitter at the skeletal neuromuscular junction in peripheral nerves. Option d is incorrect because Serotonin influences mood, appetite, sleep, and gastrointestinal function within the central and enteric nervous systems. It is not the neurotransmitter that initiates skeletal muscle contraction at the somatic neuromuscular junction in huma

Correct Answer: Calcium

Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm during excitation. They bind troponin, which moves tropomyosin and exposes actin binding sites so myosin heads can form cross-bridges and contract. Option a is incorrect because Sodium ions enter the muscle cell during depolarization of the sarcolemma and T-tubule. They help propagate the action potential along the membrane but are not what binds troponin to enable contraction. Option b is incorrect because Potassium ions exit the cell during repolarization of the action potential, restoring the negative resting membrane potential. They reset the membrane for the next impulse but do not initiate cross-bridge formation. Option d is incorrect because Chloride ions help stabilize the resting membrane potential of muscle cells and support proper electrical excitability. They are not the trigger that allows actin and myosin to interact during the contraction cycle of muscle.

Correct Answer: Attaches muscle to bone

A tendon is a band of dense regular connective tissue that attaches a muscle to a bone. Tendons transmit the force of muscle contraction to the skeleton, producing movement at joints throughout the body. Option b is incorrect because Connecting bone to bone is the function of a ligament, not a tendon. Ligaments stabilize joints and limit excessive motion, while tendons join muscles to the bones they pull on during contraction. Option c is incorrect because Cushioning within a synovial joint is provided by articular cartilage on the bone ends and by synovial fluid in the joint cavity. Tendons run alongside or across joints but do not act as cushions. Option d is incorrect because A connective tissue sheath called the perimysium surrounds each bundle, or fascicle, of muscle fibers within a muscle. A tendon is a separate structure that attaches at the end of the entire muscle.

Correct Answer: Neuron

The neuron is the basic structural and functional unit of the nervous system. Neurons generate and conduct electrical impulses called action potentials and communicate with other cells at synapses using neurotransmitter molecules. Option a is incorrect because An astrocyte is a star-shaped neuroglial cell of the central nervous system. It supports neurons metabolically and helps form the blood-brain barrier, but it does not transmit nerve impulses along axons. Option b is incorrect because A Schwann cell is a type of neuroglia found in the peripheral nervous system. It forms the myelin sheath that surrounds peripheral axons but is a supporting cell, not the basic functional unit itself. Option c is incorrect because A microglial cell is a small phagocytic neuroglial cell of the central nervous system. It removes cellular debris and pathogens from nervous tissue but is not the impulse-conducting unit of the nervous system.

Correct Answer: Cerebellum

The cerebellum lies posterior to the brainstem beneath the cerebrum. It coordinates voluntary movement, maintains balance and posture, and refines motor skills by integrating sensory input from the body and inner ear. Option a is incorrect because The medulla oblongata is part of the brainstem and controls vital autonomic functions such as heart rate, blood pressure, and breathing. It does not coordinate fine voluntary movement, posture, and balance. Option c is incorrect because The hypothalamus regulates homeostatic functions such as body temperature, hunger, thirst, and hormone release through close links with the pituitary gland. It does not coordinate balance, posture, and voluntary motor control of the limbs. Option d is incorrect because The cerebrum initiates voluntary movement through its motor cortex and processes higher functions like thought, memory, and language. Coordination, balance, and refinement of motor skills, however, are primarily cerebellar functions.

Correct Answer: Sodium

Sodium ions rush into the neuron through voltage-gated sodium channels during the depolarization phase. The rapid influx of positive charge raises membrane potential above threshold and propagates the action potential along the axon. Option a is incorrect because Potassium exits the neuron during the repolarization phase of the action potential. This outward flow of positive charge restores the negative resting membrane potential and does not produce the initial depolarization. Option b is incorrect because Calcium ions play a key role at the synaptic terminal, where their entry triggers vesicle fusion and neurotransmitter release. They are not the main ion responsible for depolarizing the neuronal membrane during the action potential. Option d is incorrect because Chloride is an inhibitory ion that hyperpolarizes the membrane when it enters certain neurons. It does not produce the depolarizing phase of an action potential and instead tends to suppress neuronal firing.

Correct Answer: Retina

The retina lines the inner back of the eye and contains rods and cones. Rods detect dim light and motion, while cones detect color and provide sharp central vision needed for reading and detail. Option b is incorrect because The cornea is the transparent front layer of the eye that helps focus incoming light onto the retina. It contains many sensory nerve endings for protection but does not contain photoreceptors that respond to light. Option c is incorrect because The lens further focuses light onto the back of the eye by changing shape during accommodation. It is a clear, flexible structure but does not contain rods and cones, which are sensory receptor cells. Option d is incorrect because The sclera is the tough white outer layer of the eye that maintains the shape of the eyeball. It is a protective fibrous coat and does not contain photoreceptors or other light-sensitive cells.

Correct Answer: Sympathetic nervous system

The sympathetic nervous system drives the fight-or-flight response. It increases heart rate and force, dilates airways and pupils, mobilizes glucose, and shifts blood flow toward skeletal muscles to support vigorous activity. Option a is incorrect because The somatic nervous system controls voluntary movement of skeletal muscle. It is part of the peripheral nervous system but is not part of the autonomic system that drives the fight-or-flight or rest-and-digest responses. Option b is incorrect because The parasympathetic nervous system promotes rest-and-digest activities throughout the body. It slows the heart rate, constricts the pupils, and stimulates digestion, opposing rather than supporting the body's fight-or-flight response to stress. Option c is incorrect because The enteric nervous system controls the gastrointestinal tract and is sometimes called a third division of the autonomic system. It coordinates digestion locally but does not drive the systemic fight-or-flight res

Correct Answer: Through hormones released into the blood

The endocrine system communicates by releasing hormones from ductless glands directly into the bloodstream. Hormones travel to distant target cells with matching receptors and produce slow but widespread effects throughout the body. Option a is incorrect because Communication through electrical impulses along axons is characteristic of the nervous system, not the endocrine system. Neurons use rapid action potentials and synapses, while endocrine signaling relies on circulating chemicals. Option b is incorrect because Antibody binding to antigens is a feature of the immune system. Antibodies neutralize pathogens and tag them for destruction, but they are not the messengers used by the endocrine system to regulate body functions. Option d is incorrect because Muscle contractions produce movement and generate force. They are an output of the muscular system rather than a means of endocrine communication, although hormones can influence how muscles respond and contract.

Correct Answer: Insulin

Insulin is secreted by beta cells of the pancreatic islets after meals. It lowers blood glucose by promoting glucose uptake into muscle and adipose cells and by stimulating glycogen formation in the liver. Option b is incorrect because Glucagon is secreted by alpha cells of the pancreas during fasting. It raises blood glucose by stimulating glycogen breakdown and gluconeogenesis in the liver, opposing the action of insulin during the postprandial period. Option c is incorrect because Cortisol is a glucocorticoid produced by the adrenal cortex during stress. It tends to raise blood glucose by promoting gluconeogenesis and reducing peripheral glucose uptake, opposing the glucose-lowering effect of insulin. Option d is incorrect because Epinephrine is released by the adrenal medulla during stress and exercise. It increases blood glucose by stimulating glycogen breakdown and inhibits insulin release, which is the opposite of lowering blood sugar.

Correct Answer: Pituitary gland

The pituitary gland is called the master gland because it secretes hormones that regulate the thyroid, adrenal cortex, gonads, and other endocrine targets. The hypothalamus, in turn, controls the release of pituitary hormones. Option a is incorrect because The thyroid gland produces hormones that regulate metabolism, growth, and development. It is itself regulated by thyroid-stimulating hormone from the pituitary, so it is not considered the master gland of the endocrine system. Option b is incorrect because The adrenal gland produces cortisol, aldosterone, sex hormones, and catecholamines such as epinephrine. It is regulated by pituitary hormones like adrenocorticotropic hormone, so it follows rather than directs the master gland. Option c is incorrect because The thymus produces hormones involved in T-lymphocyte maturation and is important for immune development, especially in childhood. It does not control other endocrine glands or release a wide range of regulatory hormones.

Correct Answer: Antidiuretic hormone

Antidiuretic hormone, released from the posterior pituitary, increases water reabsorption in the collecting ducts of the kidney. This concentrates urine, reduces urine volume, and conserves body water during dehydration or high blood osmolarity. Option a is incorrect because Aldosterone is produced by the adrenal cortex and primarily promotes sodium reabsorption and potassium excretion in the kidney's distal tubules. Water tends to follow sodium, but aldosterone is not the direct water-conserving hormone. Option c is incorrect because Atrial natriuretic peptide is released by stretched cardiac atria when blood volume is high. It promotes sodium and water loss in the urine, lowering blood volume and pressure, opposing antidiuretic hormone in its action. Option d is incorrect because Oxytocin causes uterine contractions during labor and milk ejection during breastfeeding. It is also released from the posterior pituitary but does not regulate water reabsorption or urine volume produc

Correct Answer: Negative feedback

Negative feedback is the most common form of endocrine control. Rising hormone levels or downstream effects inhibit further secretion, which keeps the regulated variable within a narrow range and supports stable homeostasis. Option a is incorrect because Positive feedback amplifies a process and is used in events like childbirth, blood clotting, and the luteinizing hormone surge. It is not the typical mechanism for keeping hormone levels and other variables stable. Option b is incorrect because Open-loop control occurs when output does not feed back to influence the controller. Endocrine regulation depends on sensing hormone effects in the blood, so it is not an open-loop system in normal physiology. Option d is incorrect because Feedforward regulation anticipates a change before it actually occurs, such as releasing insulin in response to the smell of food. It exists in physiology but is not the predominant control system for hormone secretion.

Correct Answer: Left ventricle

The left ventricle pumps oxygenated blood through the aortic valve into the aorta and onward to the systemic circulation. Its thick muscular wall generates the high pressure needed to perfuse the entire body. Option b is incorrect because The right ventricle pumps deoxygenated blood through the pulmonary valve and pulmonary artery into the lungs. Its wall is thinner because pulmonary resistance is much lower than systemic vascular resistance during normal circulation. Option c is incorrect because The left atrium receives oxygenated blood from the pulmonary veins and pumps it into the left ventricle through the mitral valve. It primes the left ventricle but does not pump blood to systemic arteries. Option d is incorrect because The right atrium receives deoxygenated blood from the body through the superior and inferior venae cavae. It pumps blood into the right ventricle, not directly into systemic arteries that carry oxygen-rich blood.

Correct Answer: Sinoatrial node

The sinoatrial node, located in the wall of the right atrium near the entry of the superior vena cava, is the natural pacemaker. It spontaneously generates impulses that determine the resting heart rate. Option a is incorrect because The atrioventricular node delays the impulse before it passes from the atria to the ventricles, giving the atria time to empty. It can act as a backup pacemaker but does not normally initiate each heartbeat. Option b is incorrect because The bundle of His conducts impulses from the atrioventricular node into the interventricular septum and then to bundle branches. It transmits signals quickly through the heart but does not normally generate them on its own. Option c is incorrect because Purkinje fibers spread the impulse rapidly through the ventricular walls, causing coordinated and forceful ventricular contraction. They distribute the signal but do not normally generate the rhythm that determines the heart rate.

Correct Answer: Artery

Arteries carry blood away from the heart to body tissues. They have thick muscular and elastic walls that withstand the high pressures generated by ventricular contraction and help propel blood forward during diastole. Option a is incorrect because Veins carry blood toward the heart from peripheral tissues. They have thinner walls than arteries and contain valves that help prevent backflow of blood, especially in the limbs against the pull of gravity. Option c is incorrect because Venules are small veins that drain capillary beds. They merge into larger veins and carry blood toward the heart, not away from it, so they do not match the description in the question. Option d is incorrect because Capillaries are tiny vessels that connect arterioles to venules. They are the site of exchange between blood and tissues, where oxygen, nutrients, and wastes diffuse, rather than a primary direction of blood flow.

Correct Answer: Erythrocytes

Erythrocytes, or red blood cells, are filled with hemoglobin that binds oxygen in the lungs and releases it in tissues. They are the main oxygen carriers in the blood and also help transport carbon dioxide. Option a is incorrect because Platelets are small cell fragments derived from megakaryocytes that participate in blood clotting. They form plugs at sites of vessel injury and release clotting factors but do not bind or transport oxygen. Option b is incorrect because Neutrophils are white blood cells that phagocytize bacteria and other pathogens during the early stages of infection. They are an important part of the innate immune response and do not transport oxygen. Option d is incorrect because Lymphocytes are white blood cells involved in adaptive immunity. B cells produce antibodies and T cells destroy infected or abnormal cells, but lymphocytes do not transport oxygen through the circulatory system.

Correct Answer: Vasoconstriction of arterioles

Vasoconstriction of arterioles increases systemic vascular resistance throughout the body. With cardiac output and blood volume held constant, the increased resistance raises arterial pressure, which is why vasoconstrictors are used to treat shock. Option b is incorrect because Decreased cardiac output reduces the volume of blood ejected by the heart per minute. Lower cardiac output, with other factors held constant, tends to lower arterial blood pressure rather than raise it. Option c is incorrect because Decreased blood volume, such as from hemorrhage or dehydration, reduces venous return and stroke volume. The result is a lower cardiac output and a lower arterial blood pressure, not an increase in pressure. Option d is incorrect because Vasodilation of arterioles decreases systemic vascular resistance throughout the circulation. The reduced resistance allows blood to flow more easily through arterioles and capillaries, which lowers arterial blood pressure rather than raising it.