During quiet inspiration, the diaphragm contracts and moves downward, and the external intercostal muscles expand the thorax. Intrapleural pressure becomes more negative, the lungs expand, and alveolar pressure falls slightly below atmospheric pressure, drawing air into the lungs. During quiet expiration, these muscles relax and elastic recoil reduces lung volume; alveolar pressure rises slightly above atmospheric pressure, so air flows out. Thus A does not describe inspiration.

Alveoli are air sacs that function as the location for gas exchange in the lungs. The pharynx, larynx, and bronchioles are the route for air passage. The epiglottis is a flap of tissue that prevents food from entering the respiratory tract.

In this question, CO₂ travels from capillaries out through the nasal cavity via the lungs. Capillaries → anterior or posterior vena cava → right atrium → right ventricle → pulmonary artery → pulmonary capillaries → alveoli → bronchioles → bronchi → trachea → larynx → pharynx → out through the nasal cavity. It’s a long journey!!

An artery is a large vessel with a thick muscular wall to resist the high pressure. The thinnest and smallest vessels are the capillaries, which function to exchange gas, nutrients, and waste with body cells. Venules connect capillaries and veins. Vessels that transport blood into the heart are called veins. Veins have low blood pressure, so there are valves to prevent back flow.

Cholecystokinin (CCK), released by the small intestine, stimulates pancreatic enzyme secretion and gallbladder contraction, which releases stored bile into the duodenum. Bile is produced by the liver, not by the gallbladder. Enterogastrone is a historical term for intestinal hormonal signals that inhibit gastric secretion and motility. Gastrin stimulates gastric acid secretion. Glucagon and insulin are pancreatic hormones that regulate blood glucose: glucagon increases blood glucose, whereas insulin decreases it.

B is the exception. In this standard comparison, the immediate fight-or-flight response is mediated by sympathetic activity, including adrenal medullary epinephrine and norepinephrine release. ACTH stimulates adrenal cortical cortisol secretion, which supports fuel availability and vascular responsiveness during prolonged stress. These pathways overlap in time; cortisol is not confined exclusively to long-term events. Aldosterone is regulated mainly by angiotensin II and blood K⁺, rather than being the principal ACTH-mediated stress hormone.

Insects have hemolymph fluid, in which blood and interstitial fluid are the same. Fish have two chambers and one circuit in a closed circulatory system. Frogs use lungs and skin to exchange O₂ and CO₂ and have three chambers and two circuits. Birds and mammals have the most well developed circulatory system, with four chambers and two circuits.

A–D are correct, so E is the answer. Gas exchange occurs by diffusion across moist respiratory surfaces. In human lungs, O₂ diffuses from alveoli into pulmonary capillary blood and CO₂ diffuses in the opposite direction down their partial-pressure gradients. In birds, airflow through the main gas-exchanging parabronchi remains largely unidirectional during both inhalation and exhalation. Air sacs act as ventilatory bellows; they are not the principal sites of gas exchange. The large total cross-sectional area of systemic capillaries accounts for their low blood-flow velocity.

A and D are correct, so E is the answer. The epiglottis helps prevent food from entering the airway. Salivary amylase begins starch digestion, producing maltose and short carbohydrate fragments rather than completing digestion to monosaccharides. Pepsin begins protein digestion in the stomach; pancreatic amylase digests carbohydrates in the small intestine. Pancreatic lipase acts mainly on triglycerides to yield monoglycerides and free fatty acids, rather than converting all triglycerides directly into glycerol and fatty acids.

Steroid hormones (such as androgens, estrogens, progesterone, cortisol, and aldosterone) can bind to receptors in the nucleus through plasma membranes. Most other hormones, like antidiuretic hormones, are peptide hormones.

External respiration is the exchange of O₂ and CO₂ between the alveoli and pulmonary capillary blood. Internal respiration refers to gas exchange between systemic capillaries and body tissues. Cellular respiration is the metabolic process by which cells produce ATP. Gas transport by the blood connects external and internal respiration. D is therefore correct.

Most long-chain fatty acids and monoglycerides enter intestinal epithelial cells, where they are reassembled into triglycerides and packaged into chylomicrons. Chylomicrons enter lacteals and travel through the lymphatic system before reaching the bloodstream. Short- and medium-chain fatty acids can enter the portal blood more directly. Glucose and amino acids enter intestinal blood capillaries and travel to the liver via the hepatic portal circulation. Parietal cells and chief cells are found in the stomach.

E is correct. During antidiuresis, water reabsorption can concentrate urea in collecting-duct fluid. Urea concentration does not simply increase at every successive nephron segment: it is influenced by water reabsorption, urea reabsorption and secretion, and medullary urea recycling. The flow pathway is Bowman’s space → proximal tubule → descending limb → ascending limb → distal tubule → collecting duct → renal pelvis → ureter → urinary bladder → urethra.

Plasma proteins are albumin, fibrinogen, and immunoglobulins. Albumin regulates osmotic balance or blood pressure and pH buffering. Fibrinogen is one component for blood clotting, and immunoglobulins are antibodies. Thrombin is an enzyme, which converts fibrinogen to fibrin in blood clotting. Myoglobin is a protein with a monomer shape, which binds O₂ and is stored in muscles.

Vitamin A is a group of fat-soluble retinoids, including retinol and its derivatives. Retinal, a vitamin A derivative, combines with opsin to form visual pigments in retinal photoreceptors. Vitamin A deficiency can impair vision and cause night blindness; vitamin A and retinal are not interchangeable names for a single compound.

All five statements are correct, so E is the answer. The exocrine pancreas secretes bicarbonate ions, amylase, lipase, and trypsinogen. The pancreas secretes trypsinogen, an inactive precursor that is activated to trypsin in the small intestine. Its endocrine cells secrete hormones including glucagon and insulin.

Aldosterone, secreted by the adrenal cortex, promotes Na⁺ reabsorption and K⁺ secretion in the distal nephron. Water can follow osmotically when water permeability and other conditions permit; ADH is a major regulator of collecting-duct water permeability. Aldosterone secretion is regulated mainly by the renin–angiotensin–aldosterone system and blood K⁺ concentration. ACTH primarily stimulates cortisol secretion and has only a minor, transient influence on aldosterone. TSH stimulates thyroid secretion of T₃ and T₄; growth hormone promotes growth directly and through IGF-1.

The complete airflow pathway is: nasal cavity → pharynx (nasopharynx → oropharynx → laryngopharynx) → larynx → trachea → primary bronchi → secondary bronchi → tertiary bronchi → smaller bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli.

A omits the larynx and bronchioles, among other intermediate structures. B and D incorrectly include the esophagus, which carries food rather than air to the lungs. C omits the nasal cavity and larynx and incorrectly places bronchioles before bronchi. None of A–D shows the complete sequence, so E (None of the above) is correct.

I, II, and IV are correct. Reduced atmospheric pressure lowers inspired PO₂ and alveolar PO₂, impairing oxygen loading and reducing arterial oxygen content. This can reduce tissue oxygen delivery and cause breathlessness and fatigue. Slow circulation is not the primary cause; ventilation and cardiac output can increase as compensatory responses. CO₂ changes must not be treated as identical to oxygen changes: hyperventilation at altitude often lowers arterial CO₂.

A and C are correct, so E is the answer. Mature human red blood cells lack nuclei and mitochondria and transport respiratory gases. Plasma cells differentiated from B lymphocytes secrete antibodies. Basophils release histamine and other mediators in inflammatory and allergic responses. Monocytes can differentiate into macrophages.

Increased metabolism rates increase body temperatures and partial blood pressure of CO₂ and decrease blood pH level. Therefore, the O2 binding affinity of hemoglobin is decreased.

D is false. Under normal CO₂-driven respiratory control, a rise in arterial CO₂ stimulates ventilation, whereas a fall reduces this stimulus. Central chemoreceptors respond mainly to changes in cerebrospinal-fluid pH caused by CO₂; peripheral chemoreceptors also respond to blood gases and pH. Atrioventricular valves prevent backflow into the atria during ventricular contraction, and semilunar valves prevent backflow into the ventricles during ventricular relaxation. Much of the CO₂ transported from tissues to the lungs is carried as bicarbonate.

Calcitonin inhibits osteoclast activity and can lower blood Ca²⁺. PTH acts on osteoblast-lineage cells to increase signals such as RANKL that promote osteoclast-mediated bone resorption; it does not primarily stimulate osteoclasts directly. PTH also increases renal calcium conservation and promotes calcitriol formation. In humans, calcitonin has a relatively limited role in normal calcium homeostasis compared with PTH and vitamin D. Thus A is correct.

Minerals are inorganic nutrients that are regulated by the excretion of urine or sweat. Important minerals in the body are sodium, potassium, calcium, phosphorus, magnesium, chlorine, and sulfur.

Vitamins are not inorganic nutrients but organic compounds; so, vitamins are not minerals.

Vitamin K is required for the liver to produce several functional clotting factors; it does not directly convert prothrombin to thrombin. Prothrombin is activated by the prothrombinase complex, including factor Xa, factor Va, phospholipid surfaces, and Ca²⁺. Platelets provide surfaces and help form the initial plug. Thrombin converts fibrinogen into fibrin, which forms the clot framework. K⁺ is not a standard component of this clotting pathway, so E is the answer.

All are true. The liver has multiple functions, such as bile production, destruction of red blood cells, glycogen synthesis and breakdown, synthesis of plasma proteins, conversion of ammonia into urea, and detoxification.

Filtration at the glomerulus produces filtrate in Bowman’s space. The pathway is: Bowman’s capsule → proximal tubule → descending limb of the loop of Henle → ascending limb of the loop of Henle → distal tubule → connecting tubule → collecting duct → papillary duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra → outside the body.

A omits several intermediate structures. B reverses the limbs of the loop of Henle; C places the distal tubule before the loop of Henle; D places Bowman’s capsule before the glomerulus and omits intermediate structures. None of A–D gives the complete pathway, so E (None of the above) is correct.

The anterior pituitary releases GH, TSH, ACTH, LH, FSH, and prolactin. ADH and oxytocin are synthesized in hypothalamic neurons, transported along their axons, and stored and released from the posterior pituitary. Thus both A and B are exceptions, making E correct.

Animals use chemical substances to communicate with each other by odors outside the body. These substances are called pheromones. Pheromones function in courtship, marking territory, and classifying social order between members of the same species.

The amount of blood pumped from the ventricle is called stroke volume.

Platelets are fragments derived from megakaryocytes and are principally involved in hemostasis and clotting, so D is correct. Neutrophils and monocytes/macrophages are major phagocytes. Eosinophils have limited phagocytic ability but are particularly associated with responses to parasites and allergic inflammation. The question therefore distinguishes principal roles rather than claiming that all listed cells are equally phagocytic.

Most clean and oxygenated blood is contained in the pulmonary vein because this blood is from the lungs right after oxygenation, while most deoxygenated blood is contained in the anterior and posterior vena cava. The capillaries have the lowest velocity of blood.

D is correct. Bile is produced by the liver and stored and concentrated in the gallbladder. Bile salts are not enzymes: they emulsify fats and form mixed micelles that help lipid digestion products and fat-soluble vitamins reach the intestinal absorptive surface. They therefore support absorption as well as emulsification.

The lower esophageal sphincter (LES), assisted by the crural diaphragm, forms an antireflux barrier at the gastroesophageal junction. Impaired barrier function can allow acidic gastric contents to reflux and injure the esophageal lining. The upper esophageal sphincter controls passage between the pharynx and esophagus, and the pyloric sphincter controls passage from the stomach into the small intestine. Peristalsis moves contents along the gastrointestinal tract. Chief cells secrete pepsinogen, the inactive precursor of pepsin.

Water, glucose, and small solutes are filtrated from glomerulus to Bowman’s capsule. Salt and water are secreted or reabsorbed between distal tubule and peritubular capillary.

Glomerular filtration is a passive process driven by hydrostatic pressure across the filtration barrier. Glucose and other small solutes are filtered with water into Bowman’s space. Water reabsorption in the descending limb of the loop of Henle occurs passively by osmosis. Glomerular filtration is pressure-driven bulk flow rather than active transport or simple diffusion, so D is uniquely correct.

One of the metabolic disorders of the thyroid gland is hypothyroidism, which is caused by insufficient secretion of thyroid hormone. Symptoms of hypothyroidism include a low rate of metabolism with cold intolerance, lethargy, and weight gain. The features of hyperthyroidism are the opposite of hypothyroidism. Graves’ disease, weight loss, poor emotional control, and high blood pressure are symptoms of hyperthyroidism.

Under the stated simplified classical model, II, IV, and V are correct, so D is the answer. Capillary hydrostatic pressure favors outward filtration; plasma oncotic pressure, produced mainly by plasma proteins, favors inward fluid movement. This assumed pressure comparison is a teaching model, not a universal description of capillaries. The revised Starling principle emphasizes the endothelial glycocalyx and the oncotic gradient across it. In many tissues, sustained venous-end reabsorption is limited, and lymphatic drainage returns most net filtered fluid to the circulation.

D is the exception: chief cells secrete inactive pepsinogen, not active pepsin. Gastric HCl creates the acidic conditions that initiate pepsinogen activation; the resulting pepsin then activates additional pepsinogen autocatalytically. Parietal cells secrete HCl. Mucus protects the lining, the pyloric sphincter regulates gastric emptying, and the stomach stores and mechanically mixes food, begins digestion, and absorbs small amounts of some substances.

Secretin is released primarily by S cells in the duodenum in response to acidic chyme. It stimulates pancreatic bicarbonate secretion, helping neutralize acid in the small intestine, and inhibits gastric acid secretion. Thus A, B, C, and E describe secretin correctly. D is the sole exception: its primary secretion site is the duodenum, not the stomach.

E is false. The large intestine secretes mucus for lubrication and protection, but it does not secrete substantial amounts of digestive enzymes for nutrient breakdown. It absorbs water, electrolytes, and some products of intestinal microbial activity, including some vitamin K, and stores feces. Most digestion and nutrient absorption occur in the small intestine, where epithelial microvilli increase absorptive surface area. The pancreas, liver, and gallbladder support digestion there.

D is correct. Tubular secretion transfers selected substances from peritubular blood or the interstitium into tubular fluid and contributes to ion balance and acid–base regulation. Many secretory processes use active transport, including secretion of organic ions; secretion should not be defined as universally active. Passive urea secretion into thin limbs also contributes to medullary urea recycling. Reabsorption moves substances from tubular fluid back toward the blood, the opposite direction to secretion.

Endocrine glands, called ductless glands, secrete their products or hormones into the bloodstream. Exocrine glands secrete their products into ducts. Giantism is related to Growth Hormone (GH) in the anterior pituitary gland.

All are correct.

E is false. Most vitamin requirements must be met through diet, but there are synthesis exceptions that vary by species and conditions. Human skin can produce vitamin D with adequate UVB exposure, niacin can be formed from tryptophan, and intestinal microorganisms can produce some vitamins such as vitamin K. Many animals synthesize vitamin C, whereas humans cannot. These exceptions do not mean that animals can synthesize every vitamin in sufficient amounts.

The cardiovascular system supplies oxygen and nutrients to body tissues and takes out wastes from the body. Heart, blood pressure, blood, and blood vessels are all necessary to transport these materials.

Pepsin digests proteins into smaller peptides, so D is correct. Chief cells secrete pepsinogen, which is activated in the acidic gastric environment to form pepsin. The stomach also carries out some fat digestion through gastric lipase, so a general question about substances digested in the stomach could include fat. This question specifically asks about pepsin. Further digestion of these nutrient classes occurs in the small intestine.

Water, salts, glucose, and amino acids are normally filtered and then reabsorbed along the nephron. Glycogen is a large glucose polymer and is not a typical small solute normally filtered at the glomerulus and reabsorbed by renal tubules, so E is the exception. Most glucose and amino-acid reabsorption occurs in the proximal tubule. Water is reabsorbed from the descending limb of the loop of Henle, whereas the ascending limb reabsorbs NaCl and is relatively impermeable to water.

Alcohol can transiently inhibit ADH (vasopressin) release, reducing renal water reabsorption and increasing urinary water loss, so B is correct. Caffeine can have a modest, dose-dependent diuretic or natriuretic effect, influenced by habitual intake; it should not be described as universally suppressing ADH or inevitably causing dehydration. Aldosterone promotes distal-nephron Na⁺ reabsorption and K⁺ secretion. PTH raises blood Ca²⁺, while calcitonin can lower it and has a relatively limited role in normal human calcium homeostasis.