Unit Summary: Lymphoid, Respiratory, and Digestive Systems

Functions: Drains excess interstitial fluid, transports dietary lipids (via lacteals in the small intestine), and carries out adaptive immune responses.
Components: Lymph, lymphatic vessels, lymphoid tissues (reticular connective tissue with lymphocytes), and red bone marrow.
Circulation Path: Blood capillaries $\rightarrow$ Interstitial spaces $\rightarrow$ Lymphatic capillaries $\rightarrow$ Lymphatic vessels $\rightarrow$ Lymph nodes $\rightarrow$ Lymphatic trunks $\rightarrow$ Ducts (Thoracic and Right Lymphatic) $\rightarrow$ Junction of internal jugular and subclavian veins.

Innate (Nonspecific): Present at birth; acts against all microbes. Includes physical barriers (skin, mucus), antimicrobial substances (interferons, complement), natural killer (NK) cells, phagocytes, inflammation, and fever.
Adaptive (Specific): Recognition of specific antigens. Involves B cells (antibody-mediated) and T cells (cell-mediated: helper $T$ and cytotoxic $T$ ). Distinctive for specificity and memory.

Primary: Sites of maturation ( $Red bone marrow$ for B cells; $Thymus$ for T cells).
Secondary: Sites of immune response ( $Lymph nodes$ , $Spleen$ , $Lymphoid nodules/MALT$ ).

Respiration Steps:
1. Pulmonary ventilation (breathing).
2. External respiration (gas exchange at lungs).
3. Internal respiration (gas exchange at tissues).
Boyle’s Law: Pressure is inversely proportional to volume ( $P \propto \frac{1}{V}$ ). Inhalation occurs when lung volume increases and alveolar pressure drops below atmospheric pressure ( $760\,mmHg$ ).
Muscles: Diaphragm (main muscle) and external intercostals elevate ribs for inhalation. Exhalation is typically passive (elastic recoil).

Dalton’s Law: Total pressure is the sum of partial pressures ( $P_{total} = \sum P_x$ ).
Oxygen Transport: $98.5\%$ bound to hemoglobin ( $Hb$ ) as oxyhemoglobin ( $Hb-O_2$ ). Affinity is decreased (unloading enhanced) by low pH, high $CO_2$ , high temperature, and high $BPG$ .
Carbon Dioxide Transport: $7%\% dissolved in plasma,$ 23\% $as carbaminohemoglobin ($ Hb-CO_2 $), and$ 70\% $as bicarbonate ions ($ HCO_3^-).
Chemical Reaction: CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-

Respiratory Center: Medullary respiratory center (DRG for quiet rhythm; VRG for forceful breathing) and Pontine group.
Regulation: Central chemoreceptors (medulla) respond to H^+ $or$ PCO_2 $in CSF; Peripheral chemoreceptors (aorta/carotid) respond to$ PO_2 $,$ PCO_2 $, and$ H^+$$ in blood.

Processes: Ingestion, secretion, motility (mixing/propulsion), digestion (mechanical/chemical), absorption, and defecation.
Wall Layers: Mucosa (epithelium, lamina propria, muscularis mucosae), Submucosa, Muscular layer (circular/longitudinal), and Serosa/Adventitia.
Neural Innervation: Enteric Nervous System (ENS) containing the Myenteric plexus (motility) and Submucosal plexus (secretions).

Stomach: Secretes gastric juice (HCl from parietal cells; pepsinogen from chief cells). HCl kills microbes and activates pepsin (protein digestion).
Pancreas: Produces pancreatic juice containing sodium bicarbonate (buffers acid) and enzymes (amylase, trypsin, lipase, nucleases).
Liver/Gallbladder: Liver produces bile for emulsification of lipids; gallbladder stores and concentrates it.
Small Intestine: Major site of digestion and absorption. Features circular folds, villi, and microvilli to increase surface area.
Large Intestine: Absorbs water, ions, and vitamins; forms and expels feces via the defecation reflex.

Cephalic: Smell/sight/taste prepares mouth and stomach.
Gastric: Stimulated by stomach distension and gastrin; promotes motility and secretion.
Intestinal: Triggered by chyme entering the duodenum; involves hormones Secretin (buffers) and Cholecystokinin (CCK; enzyme release/bile ejection).