pH homeostasis relies on three coordinated defenses: chemical buffering with bicarbonate, respiratory control of carbon dioxide, and renal handling of bicarbonate and hydrogen ions. The bicarbonate buffer system acts quickly: bicarbonate ions neutralize added hydrogen ions to form carbonic acid, which then rapidly dissociates into water and CO2. This dampens sharp pH changes right away. The lungs then regulate CO2 levels through breathing. When acid loads or CO2 rise, ventilation increases to blow off more CO2, shifting the balance toward less H+ and a higher pH; when CO2 accumulates, ventilation decreases, CO2 builds up, and H+ increases, lowering pH. Over the longer term, the kidneys fine-tune pH by reabsorbing filtered bicarbonate, generating new bicarbonate, and excreting hydrogen ions in the urine. This renal adjustment replenishes bicarbonate and removes acid, sustaining pH near normal despite ongoing metabolic or respiratory demands. Together, these systems explain why pH remains tightly controlled: buffering provides immediate protection, respiration adjusts gas-derived influences, and the kidneys provide durable correction. The other statements miss essential parts: buffering and renal contributions are real and important, not just lung control; diet does not set blood pH in a way that fixes it, and the bicarbonate system does not acidify the blood but helps prevent acidification.

Protection in the skin is built by a stratified squamous epithelium. The multiple layers provide resilience to friction and wear, while the flat, thin cells in the outer layers form a tight surface. In the epidermis, these cells become keratin-filled (keratinized), creating a waterproof, tough barrier that guards against dehydration, mechanical damage, and pathogens. Other epithelial types are suited to different jobs: simple squamous is thin and good for diffusion, simple cuboidal handles secretion and absorption, and pseudostratified ciliated columnar is involved in movement and absorption in the respiratory tract. So for the outer skin’s protective role, stratified squamous epithelium is the best fit.

Understanding how blood moves in the pulmonary circuit helps here: blood that needs to be oxygenated travels from the heart to the lungs and back. The vessel that moves deoxygenated blood away from the heart toward the lungs is the pulmonary artery. After the right ventricle pumps into the pulmonary trunk, it splits into the pulmonary arteries that carry that blood to the lungs for gas exchange. In contrast, arteries normally carry blood away from the heart, but the pulmonary arteries are the exception by delivering deoxygenated blood to the lungs. Oxygenated blood then returns to the heart through the pulmonary veins. For completeness: the aorta carries oxygenated blood to the body, the inferior vena cava brings deoxygenated blood from the body back to the heart, and the pulmonary veins bring oxygenated blood from the lungs back to the heart.

The stomach stays highly acidic mainly to activate digestive enzymes and help kill ingested pathogens. The stomach secretes strong acid (HCl) using parietal cells, which lowers the pH to about 1–3. This acidic environment converts inactive pepsinogen into active pepsin, an enzyme that begins digesting proteins. It also helps reduce microbial load, acting as a barrier to many pathogens. A protective mucus layer with bicarbonate keeps the stomach lining from being damaged by this acidity. When chyme moves into the duodenum, that acidity would harm the intestinal lining and impair enzyme function, so the duodenum neutralizes it. The big players are bicarbonate-rich pancreatic juice and bicarbonate that comes with bile. Pancreatic ducts release bicarbonate-containing fluid in response to secretin, which is triggered by the acidic chyme. Bile contributes additional buffering, and the duodenal mucosa—especially Brunner’s glands—secretes bicarbonate-rich mucus to provide a protective and buffering layer. This buffering raises the pH toward neutral, creating an optimal environment (around pH 6–7) for pancreatic enzymes and brush-border enzymes to work efficiently. So, the stomach’s acidity serves digestion and protection, while the duodenum neutralizes that acid with pancreatic and biliary bicarbonate and bicarbonate-rich mucus to protect tissue and enable further digestion.