HESI A2 Elsevier (HESI): HESI Admission Assessment - Set 4 - Part 3

Correct Answer: Diaphragm

The diaphragm is the primary muscle of inspiration during quiet breathing. When it contracts, it flattens and increases thoracic volume, lowering intrathoracic pressure so that air flows down a pressure gradient into the lungs. Option a is incorrect because The external intercostal muscles assist with inspiration by elevating the ribs and expanding the thoracic cage. They are accessory muscles of breathing and contribute to chest expansion but are not the primary inspiratory muscle. Option b is incorrect because The internal intercostal muscles depress the ribs during forced expiration, helping to push air out of the lungs. They are not the primary muscle responsible for normal quiet inspiration in healthy adults. Option c is incorrect because The abdominal muscles contract during forced expiration to push the diaphragm upward and reduce thoracic volume. They support coughing and heavy exhalation but are not the main inspiratory muscle in quiet breathing.

Correct Answer: Alveoli

Alveoli are tiny air sacs surrounded by pulmonary capillaries. Their thin walls and large surface area allow oxygen and carbon dioxide to diffuse between air and blood, completing pulmonary gas exchange. Option a is incorrect because The trachea is a conducting airway lined with ciliated pseudostratified epithelium. It transports air to and from the lungs and traps debris in mucus but does not participate in gas exchange. Option b is incorrect because The bronchi branch from the trachea and conduct air into the lungs through smaller bronchioles. They are part of the conducting zone of the airway, not the respiratory zone where gas exchange occurs. Option d is incorrect because The larynx is the voice box that connects the pharynx to the trachea and houses the vocal cords. It plays a role in airway protection and phonation but not in gas exchange.

Correct Answer: Gas exchange between air and blood

Gas exchange between air and blood is a function of the respiratory system, occurring across the thin walls of the alveoli. It is not performed by the digestive tract or its accessory organs. Option b is incorrect because Enzymatic breakdown of food is a key function of the digestive tract. Enzymes from the salivary glands, stomach, pancreas, and small intestine digest carbohydrates, proteins, and fats into absorbable molecules in the lumen. Option c is incorrect because Absorption of nutrients into the blood is a major function of the digestive tract. Most digested carbohydrates, amino acids, vitamins, and minerals enter capillaries through the lining of the small intestine each day. Option d is incorrect because Elimination of unabsorbed waste is a function of the digestive tract. The large intestine compacts indigestible material and forms feces that are stored in the rectum and eliminated through the anus during defecation.

Correct Answer: Small intestine

The small intestine, with its plicae, villi, and microvilli, has an enormous surface area for absorption. Most carbohydrates, proteins, fats, vitamins, and minerals are absorbed here into the blood and lymphatic vessels. Option a is incorrect because The stomach mixes food with hydrochloric acid and pepsin and absorbs only small amounts of substances such as alcohol, water, and some medications. It is not the primary site of nutrient absorption in the body. Option c is incorrect because The esophagus is a muscular tube that transports a swallowed bolus from the pharynx to the stomach by peristalsis. It does not absorb nutrients and serves only as a conduit for food and liquids. Option d is incorrect because The large intestine absorbs water, electrolytes, and some vitamins produced by resident bacteria. The bulk of nutrient absorption from food occurs upstream in the small intestine before material reaches the colon.

Correct Answer: Large intestine

Bacteria living in the large intestine synthesize vitamin K, which is then absorbed across the colonic wall into the blood. Vitamin K is essential for the production of several clotting factors in the liver. Option a is incorrect because The liver uses vitamin K to produce several clotting factors and stores fat-soluble vitamins. It does not synthesize vitamin K itself, however, and serves mainly as a site of action and storage. Option b is incorrect because The lungs are the primary site of gas exchange between air and blood. They have no role in producing vitamins for use elsewhere in the body and do not synthesize or absorb vitamin K. Option c is incorrect because The gallbladder stores and concentrates bile produced by the liver and releases it into the small intestine to emulsify fats. It does not synthesize vitamin K, although bile aids absorption of fat-soluble vitamins.

Correct Answer: Nephron

The nephron is the basic functional unit of the kidney, with about one million in each kidney. Each nephron filters blood at the glomerulus, reabsorbs needed substances, secretes wastes, and produces urine. Option a is incorrect because A renal pyramid is a triangular region of the renal medulla containing collecting ducts and parts of nephrons. It is a structural region of the kidney rather than the basic functional unit itself. Option b is incorrect because The renal pelvis is a funnel-shaped structure that collects urine from the calyces and channels it into the ureter. It transports urine but does not filter blood or form urine through tubular processing. Option d is incorrect because The glomerulus is a tuft of capillaries within a nephron where filtration of blood plasma begins. It is part of a nephron rather than a complete functional unit that performs all steps of urine formation.

Correct Answer: Glomerular filtration

Glomerular filtration moves water and small solutes from blood in the glomerular capillaries into the capsular space under hydrostatic pressure. This filtrate is the starting fluid for urine formation in each nephron of the kidney. Option b is incorrect because Tubular reabsorption returns water, glucose, ions, and other useful substances from the tubule back into peritubular blood. It refines the filtrate after filtration but does not initiate the process of urine formation. Option c is incorrect because Tubular secretion moves additional wastes, drugs, and ions from peritubular blood into the tubule for elimination. It supplements filtration in clearing substances from the blood but is not the first step of urine formation. Option d is incorrect because Micturition is the process of urination, in which urine is expelled from the bladder through the urethra. It occurs after urine has already been produced by the nephrons and stored in the urinary bladder.

Correct Answer: Ureter

The ureter is a muscular tube that carries urine from the renal pelvis of each kidney to the urinary bladder. Smooth muscle in its wall produces peristaltic waves that move urine downward toward the bladder. Option a is incorrect because The urethra carries urine from the urinary bladder to the outside of the body during urination. It is the final passageway for urine, not the connection between the kidney and the urinary bladder. Option c is incorrect because The renal artery carries oxygenated blood from the abdominal aorta into the kidney to be filtered. It supplies blood for filtration and metabolism but does not transport urine away from the kidney. Option d is incorrect because The collecting duct is part of the nephron system within the kidney itself. It receives fluid from many distal tubules and drains into the renal pelvis, not directly into the urinary bladder.

Correct Answer: Testes, sperm production

The testes contain the seminiferous tubules where spermatogenesis takes place under hormonal control. They are the male gonads responsible for producing sperm and the steroid hormone testosterone in interstitial Leydig cells. Option a is incorrect because The vas deferens transports sperm from the epididymis to the urethra during ejaculation. It is a conducting tube within the spermatic cord and does not produce sperm, which are formed within the testes themselves. Option b is incorrect because The prostate gland secretes a milky alkaline fluid that nourishes and protects sperm and contributes to semen volume. It does not store sperm; storage occurs primarily in the epididymis next to each testis. Option c is incorrect because Seminiferous tubules are the coiled structures within the testes where spermatogenesis occurs. They produce sperm rather than transport urine, which travels separately through the urethra during the act of urination.

Correct Answer: Luteinizing hormone

A sharp surge of luteinizing hormone from the anterior pituitary at midcycle triggers ovulation. It causes the mature dominant follicle to rupture, releasing the secondary oocyte from the ovary into the fallopian tube. Option a is incorrect because Estrogen levels rise during the follicular phase as a dominant follicle develops and contribute to the luteinizing hormone surge through positive feedback. Estrogen alone, however, does not directly trigger the release of an oocyte. Option b is incorrect because Progesterone is secreted mainly by the corpus luteum after ovulation has already occurred. It maintains the uterine lining for possible implantation but is not the hormone whose midcycle surge causes ovulation each cycle. Option d is incorrect because Follicle-stimulating hormone promotes development of ovarian follicles during the early follicular phase. Its levels rise modestly at midcycle but it does not, on its own, trigger ovulation in the same way as luteinizing hormone.