PY1.1-7 | General Physiology — Summary & Reflection

REFLECT

Try these thought experiments to consolidate your understanding:

1. The IV fluid challenge: Your patient is dehydrated and hypotensive. You have three options: 0.9% normal saline, 5% dextrose, and pure water. Think through WHERE each distributes and why only one of them effectively raises blood pressure. Can you explain this to a colleague in 30 seconds?

1. The ORS connection: You're in a rural health centre during a cholera outbreak. No IV fluids are available. Explain to the patient's family why a mixture of salt, sugar, and water (ORS) works — using the concept of SGLT1 cotransport. Why does sugar help absorb salt?

1. The membrane walk-through: Imagine you are a glucose molecule approaching a muscle cell membrane after a meal. Trace your journey: Which transporter lets you in? What drives you across? What hormone had to act first? What happens differently in a diabetic patient?

1. The pH detective: A diabetic patient is breathing deeply and rapidly (Kussmaul breathing). Her blood pH is 7.1. Walk through the three lines of defence: Which buffer is being overwhelmed? What is the respiratory system trying to do? What will the kidneys do over the next 24 hours?

1. Cross-subject integration: Take a single concept — the Na⁺/K⁺ pump. Where does Anatomy describe the cells that contain it? What does Biochemistry say about the ATP it uses? What does Physiology say about what the pump achieves? Can you see how the three subjects describe different aspects of the same reality?

KEY TAKEAWAYS

Key takeaways — your study checklist:

1. Cell structure (PY1.1): 37 trillion cells share a common blueprint — membrane, cytoplasm, nucleus. Key organelles: mitochondria (ATP), RER (secretory proteins), SER (lipid/steroid synthesis), Golgi (modification/sorting), lysosomes (digestion), peroxisomes (oxidation). The cell membrane follows the fluid mosaic model — phospholipid bilayer + cholesterol + integral/peripheral proteins + glycocalyx.

1. Body fluids (PY1.2): Total body water = 60% of body weight. ICF = 2/3 (K⁺ dominant), ECF = 1/3 (Na⁺ dominant). ECF = plasma + interstitial + transcellular. Normal saline stays in ECF; 5% dextrose distributes across all water.

1. Membrane transport (PY1.3): Passive (no ATP): simple diffusion, osmosis, facilitated diffusion. Active (ATP): Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in), Ca²⁺-ATPase, H⁺/K⁺-ATPase. Secondary active: cotransport (SGLT1 — basis of ORS) and countertransport (Na⁺/H⁺ exchanger).

1. Homeostasis (PY1.4): Sensor → control centre → effector. Negative feedback (most common, opposes stimulus — e.g., insulin and glucose). Positive feedback (amplifies stimulus — e.g., oxytocin in labour, clotting cascade).

1. Blood composition (PY1.5): Plasma (55%) + formed elements (45% = haematocrit). Plasma proteins: albumin (oncotic pressure), globulins (immunity), fibrinogen (clotting). Formed elements: RBCs, WBCs (5 types), platelets.

1. pH regulation (PY1.6): Normal pH 7.35–7.45. Three defences: chemical buffers (seconds) — bicarbonate most important; respiratory compensation (minutes) — blow off CO₂; renal compensation (hours-days) — excrete H⁺, reabsorb HCO₃⁻. Henderson-Hasselbalch: pH = 6.1 + log([HCO₃⁻]/0.03 × pCO₂).

1. Bone & calcium (PY1.7): 99% calcium in bone. Three hormones: PTH (raises Ca²⁺ — bone resorption, renal reabsorption, vitamin D activation), calcitonin (lowers Ca²⁺), vitamin D (intestinal absorption). Osteoblasts build, osteoclasts crush, osteocytes communicate.