The chordae tendineae of the AV valves are anchored to the ______ of the ventricles.

A. Baroreceptors: These sensory nerve endings are located in the carotid sinuses and aortic arch to monitor systemic blood pressure. They detect mechanical stretch in the vessel walls to regulate cardiovascular reflexes. They do not directly sense the plasma osmolarity changes that trigger thirst.

B. Proprioceptors: These sensors are located in muscles, tendons, and joints to provide information regarding body position and movement. they allow the brain to track the spatial orientation of limbs. They are not involved in the monitoring of fluid balance or hypothalamic thirst regulation.

C. Nociceptors: These are specialized sensory receptors that respond to potentially damaging stimuli by sending signals to the spinal cord and brain. They are responsible for the perception of pain. They do not have a functional role in governing fluid intake or osmotic pressure.

D. Osmoreceptors: Located in the anterior hypothalamus, these specialized neurons detect changes in the osmotic pressure of the extracellular fluid. When plasma osmolarity increases, these cells shrink, triggering the sensation of thirst and the release of antidiuretic hormone. They are the primary regulators of fluid homeostasis.

E. Mechanoreceptors: These receptors respond to physical deformation such as touch, pressure, and vibration. While found throughout the skin and internal organs, they do not specifically monitor the chemical or osmotic status of the blood. They provide tactile and structural feedback rather than metabolic regulation.

A. 6: Hemoglobin lacks the structural capacity to bind 6 oxygen molecules simultaneously. The protein is a tetramer, meaning it consists of only 4 polypeptide subunits. Each subunit is limited to the binding of a single heme group and its associated iron ion.

B. 2: This number represents only half of the potential carrying capacity of a fully saturated hemoglobin molecule. In the high partial pressure of oxygen found in pulmonary capillaries, hemoglobin typically binds more than two molecules. This would reflect a low oxygen saturation level of 50%.

C. 3: Binding 3 molecules would result in 75% oxygen saturation, which occurs as blood unloads oxygen to resting tissues. However, it does not represent the maximum theoretical or physiological limit of the transport protein. The molecular structure allows for one additional binding site to be filled.

D. 4: Each hemoglobin molecule is a tetramer composed of 4 globin chains, each containing a central heme group with a ferrous iron atom. Each iron atom can reversibly bind 1 molecule of O2. Therefore, a single hemoglobin molecule can carry a maximum of 4 oxygen molecules.

E. 5: Human hemoglobin does not possess a fifth binding site or heme group to accommodate an extra oxygen molecule. The quaternary structure is strictly limited to 4 subunits. Any value above 4 is biologically and chemically impossible for a standard adult hemoglobin A molecule.