Internal respiration is the gas exchange between blood and body tissues, and it occurs mainly in the systemic capillaries. Here, oxygen diffuses from the blood into tissue cells while carbon dioxide diffuses from the tissues into the blood to be carried away. Capillaries are the right site because their walls are extremely thin and they form a dense network right next to every cell, keeping the diffusion distance short and providing lots of surface area for exchange. This is distinct from external respiration, which happens in the lungs at the alveolar level, where gases move between air and blood. The trachea and bronchioles merely conduct air and do not participate in gas exchange.

Endocrine signaling works through hormones released into the bloodstream to reach distant targets. Because hormones travel through the circulatory system, their onset is slower, since it takes time for hormones to circulate and encounter receptors. Once they act, their effects can be long-lasting because hormones can persist in the blood for extended periods and trigger intracellular cascades that keep producing effects even after the initial signal is gone. In contrast, nervous system signaling uses fast electrical impulses and neurotransmitters released at synapses directly onto adjacent cells. This yields rapid, precise responses that are typically brief, ending quickly once neurotransmitters are cleared or reabsorbed. So, compared with nervous signaling, endocrine signaling is generally slower to start but longer-lasting. The endocrine system is also a key regulator of homeostasis, so the idea that it isn’t involved in homeostasis isn’t correct.

The olfactory sense corresponds to the ability to smell. In the nasal cavity, odor molecules bind to specialized receptors on olfactory neurons, triggering signals that travel via the olfactory nerve to the brain. This pathway leads to regions such as the olfactory bulb and areas involved in smell processing and emotion. This is distinct from vision, which uses the eyes to detect light; hearing, which uses the ears to detect sound; and taste, which uses taste buds on the tongue to detect dissolved chemicals. So the olfactory sense is responsible for smelling.

Mechanical digestion is the physical breakdown of food into smaller pieces to increase surface area for enzymes. This starts in the mouth, where teeth chew and mix food with saliva. In the stomach, muscular contractions churn and mix the contents, further breaking them into a semi-liquid form. In the small intestine, rhythmic segmentation helps mix and move the contents, aiding further breakdown and preparation for chemical digestion and absorption. So the set that includes mouth, stomach, and small intestine matches all the sites where these mechanical processes actively occur. The other options pair organs that aren’t primarily involved in mechanical breakdown—esophagus mainly moves everything along, the large intestine focuses on water absorption, and liver/pancreas/gallbladder are mainly secretory, not mechanical, functions.

Osteoblasts are the cells that build bone and drive its remodeling. They synthesize and secrete osteoid, the organic matrix rich in collagen, and then promote mineralization with calcium phosphate to form hard bone. In the remodeling process, osteoclasts resorb old bone, and osteoblasts come in to lay down new bone, restoring structure and strength. While osteocytes help regulate remodeling in response to mechanical stress, and cartilage or tendon are not bone-forming cells, osteoblasts are the key players in both forming and renewing bone.