Overview
Physiology takes cell and molecular mechanisms and asks what they do in a whole body. Most questions are about regulation: a variable changes, a sensor detects it, and an effector responds. Be able to trace that loop for blood glucose, blood pressure, blood pH, and body water.
Human physiology is the focus here. Comparative anatomy across animal groups is in Animal Biology.
Core concepts
7.1Neurons and signaling
Resting potential (about −70 mV) comes mainly from K⁺ leak channels and the Na⁺/K⁺ pump. An action potential starts when voltage-gated Na⁺ channels open (depolarization), followed by Na⁺ channel inactivation and K⁺ channel opening (repolarization). Myelin allows saltatory conduction.
At chemical synapses, Ca²⁺ entry triggers neurotransmitter release. Whether the postsynaptic effect is excitatory or inhibitory depends on the receptor, not only on the transmitter.
Key terms: resting potential · voltage-gated channel · refractory period · EPSP / IPSP · saltatory conduction
7.2Endocrine regulation
Most hormone systems use negative feedback. In the hypothalamus–pituitary–target axis, releasing hormones trigger tropic hormones, which trigger the target hormone, and the target hormone inhibits the earlier steps. Knowing the axis lets you tell a primary (gland) defect from a secondary (pituitary) defect by the pattern of hormone levels.
Insulin lowers blood glucose, and glucagon raises it. Antidiuretic hormone (ADH) increases water reabsorption through aquaporins in the collecting duct.
Key terms: negative feedback · tropic hormone · insulin / glucagon · ADH · aldosterone
7.3Circulation and gas exchange
Cardiac output = heart rate × stroke volume. Blood pressure depends on cardiac output and peripheral resistance. Flow is highest where resistance is lowest, and arteriole diameter is the main control point.
Hemoglobin's oxygen dissociation curve is sigmoidal because binding is cooperative. Lower pH, higher CO₂, higher temperature, and 2,3-BPG shift the curve right, so hemoglobin releases more O₂ to active tissues (the Bohr effect). Fetal hemoglobin has higher O₂ affinity than adult hemoglobin.
Key terms: cardiac output · Bohr effect · cooperative binding · partial pressure · surfactant
7.4The kidney
Filtration at the glomerulus, then reabsorption and secretion along the nephron. The loop of Henle's countercurrent multiplier builds a concentration gradient in the medulla. ADH makes the collecting duct permeable to water, so urine becomes concentrated.
Clearance and glucose threshold questions come up often. Glucose appears in urine when the filtered load exceeds the transport maximum.
Key terms: glomerular filtration · countercurrent multiplier · transport maximum · ADH · renin–angiotensin
7.5The immune system
Innate defenses (barriers, phagocytes, complement, inflammation) act fast and are not specific. Adaptive immunity uses B cells (antibodies) and T cells. Helper T cells recognize antigens on MHC class II, and cytotoxic T cells recognize antigens on MHC class I.
Memory cells explain why a second exposure produces a faster and larger response. This is the basis of vaccination.
Key terms: MHC I / II · clonal selection · antibody classes · memory cells · complement
Practice
Review the concepts above, then complete the practice set. Missed a question? Read the explanation and try a similar problem.
Common mistakes
| Mistake | Instead |
|---|---|
| Thinking the Na⁺/K⁺ pump directly causes the action potential. | Voltage-gated channels cause the action potential. The pump maintains the gradients over the longer term. |
| Reading a right shift of the O₂ curve as "hemoglobin holds more oxygen." | A right shift means lower affinity, so more O₂ is released at a given partial pressure. |
| Assuming high hormone levels always mean the gland making them is overactive. | Check the upstream tropic hormone. High TSH with low T₄ points to a thyroid (primary) defect. |
Recommended resources
- OpenStax Anatomy and Physiology 2e Textbook (free)
- OpenStax Biology 2e, Chapters 33–43 Textbook (free)
- Khan Academy: Human biology Videos
Further reading
- Campbell Biology, unit on animal form and function.
- Silverthorn, Human Physiology: An Integrated Approach.