Osmoregulation, Excretion, and the Immune System

Part I: Osmoregulation and Excretion

Nitrogenous Wastes

Animals must eliminate nitrogenous waste — primarily from protein and nucleic acid metabolism — but the form varies with environment:

This represents an evolutionary tradeoff: ammonia is energetically cheap to produce but requires abundant water to dilute its toxicity — ideal for aquatic organisms. Urea can be concentrated, saving water. Uric acid is the most energetically expensive but the most water-efficient — essential for animals that develop in shelled eggs (birds, reptiles) where water is severely limited and waste must be stored without poisoning the embryo.

The Mammalian Kidney and Nephron

The functional unit of the kidney is the nephron (~1 million per human kidney). Each nephron performs four processes:

1. Filtration: Blood enters the glomerulus — a tuft of capillaries within Bowman's capsule. Blood pressure forces water, ions, glucose, amino acids, and nitrogenous wastes through the filtration membrane into Bowman's space, forming filtrate. Blood cells and large proteins are retained. The glomerular filtration rate (GFR) is ~125 mL/min in humans — the entire plasma volume is filtered ~60 times per day.

1. Reabsorption: As filtrate flows through the proximal tubule, loop of Henle, distal tubule, and collecting duct, most water, all glucose and amino acids, and many ions are reabsorbed back into peritubular capillaries. This is highly selective — useful substances are reclaimed; wastes remain.

1. Secretion: Additional wastes, toxins, excess ions (H⁺, K⁺), and drugs are actively transported from peritubular capillaries INTO the tubule lumen — a second chance to eliminate substances that escaped filtration.

1. Excretion: The final product — urine — exits via the collecting duct, ureter, bladder, and urethra.

The loop of Henle creates an osmotic gradient in the kidney medulla via countercurrent multiplication: the descending limb is permeable to water but not NaCl; the ascending limb actively transports NaCl out but is impermeable to water. This establishes an increasingly concentrated medullary interstitium (from ~300 mOsm at the cortex to ~1200 mOsm at the inner medulla). The collecting duct passes through this gradient; when ADH is present, water is reabsorbed, producing concentrated urine.

Hormonal Regulation

• ADH (antidiuretic hormone, vasopressin): Released from posterior pituitary when blood osmolarity rises or blood pressure falls. ADH increases water permeability of collecting ducts (via aquaporin-2 insertion), promoting water reabsorption → concentrated urine. Alcohol inhibits ADH release → increased urine output (diuresis).
• Aldosterone: Released from adrenal cortex in response to low blood pressure/volume (via renin-angiotensin-aldosterone system). Increases Na⁺ reabsorption (and K⁺ secretion) in distal tubule and collecting duct. Water follows osmotically.
• Atrial natriuretic peptide (ANP): Released from heart atria when stretched by increased blood volume. Opposes aldosterone — promotes Na⁺ and water excretion.

Part II: The Immune System

Innate Immunity

Innate immunity is the rapid, nonspecific first line of defense present from birth:

• Barriers: Skin (physical + acidic + antimicrobial peptides), mucous membranes, lysozyme (in tears/saliva), stomach acid
• Phagocytes: Neutrophils and macrophages engulf and destroy pathogens
• Natural killer (NK) cells: Destroy virus-infected and tumor cells
• Inflammation: Histamine release from mast cells → vasodilation, increased capillary permeability → redness, heat, swelling, recruitment of immune cells
• Complement system: ~30 plasma proteins that, when activated, lyse pathogens, enhance phagocytosis (opsonization), and promote inflammation
• Interferons: Proteins produced by virus-infected cells that "warn" neighboring cells and activate immune defenses

Adaptive Immunity

Adaptive immunity is specific, slower to develop on first exposure, and features immunological memory. It has two branches:

Humoral Immunity (B Cells and Antibodies)

• B lymphocytes mature in bone marrow and produce antibodies (immunoglobulins).
• Each B cell carries a unique receptor specific to a particular antigen.
• Upon encountering its antigen (and usually with T helper cell assistance), a B cell proliferates and differentiates into:
  • Plasma cells: Antibody factories — secrete ~2,000 antibody molecules/second
  • Memory B cells: Long-lived; enable rapid response upon re-exposure

Antibody functions: Neutralization (block pathogen binding), opsonization (tag for phagocytosis), complement activation, agglutination (clumping).

Cell-Mediated Immunity (T Cells)

• T lymphocytes mature in the thymus.
• Helper T cells (CD4+): The central coordinators. Activated by antigen-presenting cells (dendritic cells, macrophages) displaying antigen on MHC class II molecules. Secrete cytokines that activate B cells, cytotoxic T cells, and macrophages. HIV targets and destroys CD4+ T cells, crippling the immune system.
• Cytotoxic T cells (CD8+): Kill infected or cancerous cells by recognizing antigen displayed on MHC class I molecules (found on nearly all nucleated cells) and releasing perforin (pore-forming) and granzymes (induce apoptosis).

Primary vs Secondary Immune Response

The secondary response is faster, stronger, and more sustained because of memory cells generated during the primary response. This is the basis of vaccination.

Active vs passive immunity:

• Active: Individual's own immune system produces antibodies and memory cells (infection or vaccination)
• Passive: Antibodies transferred from another individual (mother to fetus via placenta/breast milk; antivenom; immunoglobulin therapy). Provides immediate protection but no memory — temporary.