During inspiration, the diaphragm and external intercostal muscles contract. Thoracic volume increases, alveolar pressure falls below atmospheric pressure, and air enters the lungs. During quiet expiration, these muscles relax and elastic recoil raises alveolar pressure above atmospheric pressure.

Alveoli are air sacs where gas exchange with pulmonary capillary blood occurs. The pharynx, larynx, and terminal bronchioles conduct air. Respiratory bronchioles mark the start of the respiratory zone and have alveoli in their walls. The epiglottis helps prevent food from entering the respiratory tract.

Carbon dioxide travels from tissue capillaries through systemic veins, the superior vena cava, the right atrium, the right ventricle, and the pulmonary arteries to pulmonary capillaries. It then diffuses into alveoli and exits through the airways. This route does not pass through the left atrium.

Arteries carry blood away from the heart and have muscular walls suited to relatively high pressure. Capillaries are the smallest vessels and permit exchange with tissues. Venules collect blood from capillaries and drain into veins. Veins return blood to the heart at lower pressure; many have valves that prevent backflow.

Cholecystokinin (CCK), released by the small intestine, stimulates gallbladder contraction to release stored bile and stimulates pancreatic enzyme secretion. The liver produces bile. Gastrin promotes gastric acid secretion; insulin and glucagon regulate blood glucose. Enterogastrone is a historical term for intestinal hormonal signals that inhibit gastric activity.

ACTH stimulates cortisol secretion from the adrenal cortex. Cortisol supports longer-lasting stress responses, including maintenance of blood glucose. The cortex also produces aldosterone, regulated mainly by angiotensin II and blood K⁺. Rapid epinephrine and norepinephrine release comes from the adrenal medulla.

Statements I, III, and IV are correct. Insects have an open circulatory system with hemolymph. Fish have a closed circulatory system with two main heart chambers and one circuit. Adult frogs have three main heart chambers and use lungs and skin for gas exchange. Birds and mammals have four-chambered hearts.

Gas exchange occurs by diffusion across moist respiratory surfaces. Systemic capillaries have a large combined cross-sectional area, so average blood velocity is low. In birds, air sacs maintain unidirectional airflow through the gas-exchanging parabronchi during both inhalation and exhalation. Statements A–D are correct.

A and D are true. The epiglottis helps keep swallowed material out of the airway. Salivary amylase begins starch digestion, producing maltose and shorter carbohydrates rather than directly producing monosaccharides. Pepsin digests proteins in the stomach. Pancreatic lipase acts in the small intestine, producing mainly monoglycerides and free fatty acids.

Androgens, estrogens, aldosterone, and glucocorticoids are steroid hormones. Their classical receptors are intracellular, in the cytoplasm or nucleus, and regulate gene expression. Antidiuretic hormone (ADH) is a peptide hormone that acts through cell-surface receptors.

External respiration is gas exchange between alveoli and pulmonary capillaries. Internal respiration is gas exchange between systemic capillaries and tissues. Cellular respiration uses oxygen in cells to support ATP production.

Enterocytes reassemble absorbed long-chain fatty acids and monoglycerides into triglycerides and package them into chylomicrons. Chylomicrons enter lacteals, the lymphatic capillaries in intestinal villi, before reaching the blood. Glycerol and shorter fatty acids can enter portal blood directly.

ADH promotes water reabsorption from collecting ducts, concentrating the tubular fluid. The inner medullary collecting duct also becomes more permeable to urea, allowing urea recycling that helps maintain the medullary osmotic gradient. Urea concentration does not simply increase at every preceding nephron segment.

Albumin is the main contributor to plasma colloid osmotic pressure and also helps buffer pH. Fibrinogen is involved in clot formation, and immunoglobulins function as antibodies. Myoglobin binds oxygen in muscle cells.

Vitamin A is a fat-soluble vitamin needed for vision. Retinal, a form of vitamin A, is part of the light-sensitive pigment in photoreceptors. Vitamin A deficiency can impair dark adaptation and cause night blindness.

All five statements are true. The exocrine pancreas releases bicarbonate and digestive enzymes or enzyme precursors into ducts. Trypsinogen is activated to trypsin in the small intestine. Endocrine pancreatic cells release hormones such as insulin and glucagon into the blood.

Aldosterone, produced by the adrenal cortex, increases Na⁺ reabsorption and K⁺ secretion in the distal nephron. Its main regulators are angiotensin II and blood K⁺. ADH primarily increases collecting-duct water permeability. ACTH primarily stimulates cortisol secretion.

Air passage from outside the body into the lungs:

Nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli

Statements I, II, and IV are correct. Lower atmospheric pressure reduces inspired and alveolar oxygen partial pressure, which can reduce arterial oxygenation and the oxygen reserve available during exertion. Blood circulation does not necessarily slow; heart rate and ventilation often increase during the initial response. Reduced oxygen availability should not be generalized to impaired carbon dioxide removal.

A and C are correct. Mature human red blood cells lack nuclei and mitochondria. Plasma cells are differentiated B lymphocytes that secrete antibodies. Monocytes can become macrophages, while basophils release histamine during inflammatory and allergic responses.

These local changes reduce hemoglobin’s affinity for O₂, promoting oxygen unloading to active tissues. Increased CO₂ and lower pH contribute to the Bohr effect.

Atrioventricular valves prevent backflow into the atria during ventricular contraction. Semilunar valves prevent backflow from arteries into the ventricles. An increase in arterial CO₂ normally stimulates ventilation, largely through CO₂-related changes in cerebrospinal-fluid pH. Much of the carbon dioxide in blood is transported as bicarbonate.

Calcitonin inhibits osteoclast-mediated bone resorption and can favor calcium retention in bone. PTH raises blood calcium and can promote bone resorption indirectly through signaling by osteoblast-lineage cells. Calcitonin has a relatively minor role in normal adult human calcium regulation.

Minerals are inorganic nutrients, such as calcium, sodium, potassium, magnesium, and iron. They support functions including fluid balance, nerve signaling, and enzyme activity. Deficiency or excess can impair normal function.

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

Platelets form a plug and support coagulation reactions. Ca²⁺ is a coagulation cofactor, and vitamin K is needed to produce several functional clotting proteins. Thrombin converts soluble fibrinogen into fibrin, which stabilizes the clot. K⁺ is not a standard coagulation cofactor.

All listed processes are liver functions: glycogen synthesis, synthesis of many plasma proteins, conversion of ammonia into urea, and detoxification. Therefore, none of A–D is an exception.

Urea is filtered from glomerular capillaries into Bowman’s capsule. Tubular fluid then passes through the proximal tubule, descending and ascending limbs of the loop of Henle, distal tubule, and collecting duct. Urine drains through the renal pelvis, ureter, bladder, and urethra. Choice A lists structures in the correct relative order.

Oxytocin and ADH are synthesized in the hypothalamus and released from the posterior pituitary. GH and ACTH are released from the anterior pituitary, along with TSH, FSH, LH, and prolactin. Thus, A and B are the exceptions.

Pheromones are chemicals released into the environment that influence other members of the same species. They can help coordinate courtship, territorial signaling, alarm responses, or social behavior.

Stroke volume is the volume ejected by one ventricle per heartbeat. Cardiac output is the volume pumped by one ventricle per minute and equals stroke volume multiplied by heart rate. Ventricular systole is contraction; ventricular diastole is relaxation.

Neutrophils and monocytes are important phagocytic leukocytes. Eosinophils also have some phagocytic capacity, although their major roles include parasite defense and allergic responses. Platelets are cell fragments involved in hemostasis, not leukocytes.

Pulmonary veins return oxygenated blood from the lungs to the left atrium. Average blood velocity is lowest in systemic capillaries because their combined cross-sectional area is large. The superior and inferior venae cavae return deoxygenated systemic blood to the right atrium.

Bile is produced by the liver and stored and concentrated in the gallbladder. Bile salts are not enzymes. They emulsify fats and help form micelles, improving lipid digestion and absorption in the small intestine.

The lower esophageal sphincter helps prevent acidic stomach contents from refluxing into the esophagus. The upper esophageal sphincter regulates entry from the pharynx, and the pyloric sphincter regulates passage from the stomach to the small intestine. Chief cells secrete pepsinogen, the inactive precursor of pepsin.

Glomerular filtration moves water and small solutes from glomerular blood into Bowman’s capsule. Reabsorption returns needed substances from tubular fluid to blood; secretion moves substances from blood into tubular fluid. The proximal tubule normally reabsorbs filtered glucose, and the loop of Henle reabsorbs water or salt in its different limbs. The distal tubule is not the main filtration site.

Glucose enters Bowman’s capsule with water by pressure-driven filtration, a passive bulk-flow process. Water leaves the descending limb by osmosis, also passive. This differs from active or secondary active solute transport during tubular reabsorption.

A is correct. Hypothyroidism involves insufficient thyroid hormone and may cause reduced metabolic rate, cold intolerance, lethargy, and weight gain. Graves disease commonly causes hyperthyroidism, with excess thyroid hormone and often weight loss and emotional symptoms. High blood pressure alone does not distinguish the conditions: hypothyroidism can also cause diastolic hypertension. Option E describes excess thyroid hormone, which indicates hyperthyroidism.

Statements II, IV, and V are correct. Plasma proteins, especially albumin, generate an osmotic force favoring water retention in the bloodstream and opposing outward hydrostatic filtration. The balance varies across tissues and conditions; universal steady reabsorption at venous capillary ends should not be assumed. Lymphatic vessels return much of the filtered fluid to the blood.

Chief cells secrete inactive pepsinogen. Acidic conditions initiate its conversion to pepsin, and pepsin can activate additional pepsinogen. Parietal cells secrete HCl. The stomach stores and mechanically mixes food, begins protein digestion, and absorbs limited amounts of some substances.

Secretin is a peptide hormone released mainly by duodenal S cells in response to acidic chyme. It stimulates bicarbonate-rich pancreatic secretion and helps inhibit gastric acid secretion. Its main secretion site is the small intestine, not the stomach, so D is the exception.

Most enzymatic digestion and nutrient absorption occur in the small intestine. Microvilli expand the absorptive surface. The large intestine absorbs water and ions, secretes protective mucus, and houses bacteria that ferment remaining material and produce some vitamins. It does not secrete pancreatic enzymes.

Tubular secretion moves substances from peritubular blood into tubular fluid. Secretion of H⁺, K⁺, drugs, and other substances helps regulate electrolyte balance and blood pH and remove wastes. Many secretory processes use active transport; mechanisms vary by substance. Reabsorption moves substances in the opposite direction.

Endocrine glands release hormones into interstitial fluid and then blood; they do not use ducts. Exocrine glands release products into ducts. Excess GH before growth-plate closure can cause gigantism. Classical steroid hormone receptors regulate gene transcription after binding their hormones.

All pairs are correct. Prolactin stimulates milk production, and oxytocin promotes uterine contractions and milk ejection. GH supports body growth. FSH supports spermatogenesis in the testes and follicle development in the ovaries.

Vitamins are organic compounds needed in small amounts and are classified as water-soluble or fat-soluble. Deficiencies can impair normal function. Humans can synthesize some vitamins, such as vitamin D under suitable conditions, but cannot make every vitamin in sufficient amounts; dietary sources remain necessary.

All listed items are directly associated with the cardiovascular system. The heart pumps blood through vessels, and blood pressure helps drive flow. Blood delivers oxygen and nutrients and carries metabolic wastes to organs that remove them.

Pepsin digests proteins into smaller peptides in the stomach. Chief cells secrete its inactive precursor, pepsinogen. Fat digestion can also begin in the stomach through lipases, so protein is not the only nutrient that can undergo gastric digestion.

Water, salts, glucose, and amino acids enter the filtrate, and the kidneys reabsorb needed amounts. Most filtered glucose and amino acids are normally reabsorbed in the proximal tubule. Glycogen is a large intracellular storage polysaccharide, not a normal filtered solute.

ADH, also called vasopressin, increases collecting-duct water permeability and water reabsorption. Alcohol can suppress its release, promoting water loss in urine. Caffeine’s renal effects involve other mechanisms and should not be described simply as the same ADH-suppression pathway.