Biology - Human Physiology (Systems: Circulatory, Respiratory, and Digestive)

Mechanical vs. Chemical Digestion: Digestion involves the breakdown of food. Chemical digestion is mediated by enzymes such as Amylase (converting starch to maltose), Protease (proteins to amino acids), and Lipase (lipids to fatty acids and glycerol). The reaction for glucose oxidation during respiration is $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$.

Absorption and Surface Area: The small intestine contains villi and microvilli to increase the surface area ($A$) for nutrient absorption. The rate of diffusion is influenced by Fick's Law, where $Rate \propto \frac{Surface\ Area \times Concentration\ Gradient}{Diffusion\ Distance}$.

Circulatory System Structure: The heart functions as a double pump. The systemic circuit carries oxygenated blood to the body, while the pulmonary circuit carries deoxygenated blood to the lungs. Blood pressure is highest in the Aorta and lowest in the Vena Cava, following the gradient $\Delta P$.

Gas Exchange in Alveoli: Gas exchange occurs via simple diffusion. $O_2$ moves from the alveoli (high $P_{O_2}$) to the capillaries (low $P_{O_2}$), while $CO_2$ moves in the opposite direction following its own partial pressure gradient.

Hemoglobin and Oxygen Transport: Oxygen binds to hemoglobin in red blood cells to form oxyhemoglobin: $Hb + 4O_2 \rightleftharpoons Hb(O_2)_4$. The affinity of hemoglobin for oxygen changes based on the $pH$ and $CO_2$ concentration (the Bohr Shift).

Breathing Mechanics: Ventilation is driven by volume and pressure changes. According to Boyle's Law, $P \propto \frac{1}{V}$. When the diaphragm contracts, thoracic volume ($V$) increases, causing pressure ($P$) to decrease below atmospheric pressure ($P_{atm}$), forcing air into the lungs.