Human Body

1. Direction of Blood Flow: Arteries carry blood away from the heart to various body organs; Veins carry blood toward the heart from the tissues.
   2. Vessel Wall Structure: Arteries have thick, highly muscular, and elastic walls with a narrow lumen to withstand high pumping pressure; Veins have thin, less muscular walls with a wide lumen under low pressure.
   3. Internal Valves: Arteries have no internal valves (except at base of aorta); Veins possess one-way semilunar valves throughout their length to prevent backflow of blood.
   (Exceptions: Pulmonary Artery carries deoxygenated blood; Pulmonary Vein carries oxygenated blood).

• Definition: Double Circulation is the cardiovascular mechanism wherein blood passes through the human heart twice in one complete circuit around the body, completely separating oxygenated from deoxygenated blood.
• 1. Pulmonary Circulation (Heart $\to$ Lungs $\to$ Heart):

$$\text{Right Ventricle} \xrightarrow{\text{Pulmonary Artery}} \text{Lungs (Oxygenated)} \xrightarrow{\text{Pulmonary Veins}} \text{Left Atrium}$$

• 2. Systemic Circulation (Heart $\to$ Body $\to$ Heart):

$$\text{Left Ventricle} \xrightarrow{\text{Aorta}} \text{Body Tissues (Deoxygenated)} \xrightarrow{\text{Vena Cava}} \text{Right Atrium}$$

• Advantage: Guarantees that oxygen-rich blood is delivered to metabolically active tissues at high pressure without mixing with deoxygenated blood.

• The Right Ventricle only pumps deoxygenated blood a short distance to the nearby lungs through the low-resistance pulmonary circuit.
• In contrast, the Left Ventricle must pump oxygenated blood through the colossal systemic circuit to the entire rest of the body—from the crown of the head to the tips of the toes—against high peripheral resistance and gravity.
• Consequently, the left ventricle requires an extraordinarily thick, powerful myocardium (three times thicker than the right) to generate the high systolic pressure ($~120\text{ mm Hg}$) needed.

• Definition: Bile is an alkaline, yellowish-green digestive fluid that contains bile salts (sodium glycocholate and taurocholate) but contains NO digestive enzymes.
• Production & Storage: Synthesized continuously by the Liver and stored and concentrated in the Gallbladder.
• Roles in Digestion:
1. Neutralization: Its alkaline nature neutralizes the acidic chyme coming from the stomach, creating the necessary alkaline $pH$ ($~7.8-8.5$) for pancreatic enzymes to function in the duodenum.
2. Emulsification of Fats: Bile salts break down large, insoluble fat globules into millions of microscopic droplets, dramatically increasing surface area for lipase enzyme to hydrolyze fats into fatty acids and glycerol.

1. Immense Surface Area: Millions of tiny, finger-like projections (villi and microvilli) line the inner mucosa of the ileum, multiplying the effective absorptive surface area by over $600\text{ times}$ (surface area of a tennis court!).
   2. Ultra-Thin Epithelium: The walls of villi consist of a single-cell-thick layer of epithelium, minimizing the diffusion distance for absorbed nutrients.
   3. Rich Capillary Network: Each villus contains a dense network of blood capillaries to absorb glucose, amino acids, vitamins, and minerals directly into the bloodstream.
   4. Central Lacteal: Each villus houses a central lymph vessel (lacteal) dedicated to absorbing digested fatty acids and glycerol.

1. Diaphragm Contraction: The dome-shaped muscular diaphragm contracts and flattens downward toward the abdominal cavity.
   2. Rib Movement: The external intercostal muscles contract, pulling the rib cage and sternum upward and outward.
   3. Volume Increase: These two coordinated muscular movements dramatically increase the internal volume of the thoracic (chest) cavity.
   4. Pressure Drop: According to Boyle's Law, the expansion decreases air pressure inside the lungs below outside atmospheric pressure.
   5. Air Inflow: Fresh atmospheric air rushes through the nasal cavity and trachea to inflate the lungs.

• Definition: Alveoli are microscopic, balloon-like air sacs located at the terminus of respiratory bronchioles in the lungs where pulmonary gas exchange takes place.
• Two Adaptations:
1. Extremely Thin Membrane: Alveolar walls are composed of a single layer of squamous epithelial cells ($< 1\text{ micrometer}$ thick), surrounded by a dense web of blood capillaries for rapid diffusion.
2. Moist Inner Lining: The inner surface of each alveolus is coated with a thin film of moisture, allowing oxygen gas to dissolve rapidly before diffusing across the respiratory membrane into blood hemoglobin.

• (a) Hydrochloric Acid ($HCl$) in Stomach:
1. Creates a strongly acidic medium ($pH \approx 1.5 - 2$) essential to convert inactive pepsinogen into active pepsin enzyme for protein digestion.
2. Kills potentially harmful microorganisms, bacteria, and pathogens swallowed along with food.
• (b) Salivary Amylase (Ptyalin) in Mouth:
Initiates the chemical breakdown of complex cooked carbohydrates, hydrolyzing insoluble starch into the soluble disaccharide maltose ($pH \approx 6.8$).