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Cell membranes are primarily composed of phospholipids and proteins. The lipid bilayer consists of phospholipids with a hydrophilic glycerol backbone and two hydrophobic fatty acid tails.
Lipid-soluble substances, such as O2, CO2, and steroid hormones, can cross cell membranes by dissolving in the hydrophobic lipid bilayer due to their nonpolar nature.
Water-soluble substances, like Na+, Cl−, glucose, and H2O, cannot dissolve in the lipid of the membrane. Instead, they may cross through water-filled channels or pores, or be transported by specific carrier proteins.
Proteins in cell membranes serve various functions, including acting as receptors, channels, and transporters, facilitating communication and transport across the membrane.
The lipid bilayer structure is crucial for maintaining the integrity of the cell, providing a barrier that separates the internal environment from the external environment, and allowing selective permeability.
Key physiology topics for USMLE Step 1 include cell physiology, autonomic physiology, cardiovascular physiology, respiratory physiology, renal and acid-base physiology, gastrointestinal physiology, and endocrine physiology.
The Frank-Starling law states that the strength of the heart's contraction is directly proportional to the degree of stretch of the cardiac muscle fibers; increased venous return leads to increased stroke volume.
Baroreceptors detect changes in blood pressure and send signals to the central nervous system to adjust heart rate and vascular resistance, thereby regulating arterial pressure.
The hemoglobin–O2 dissociation curve illustrates how hemoglobin's affinity for oxygen changes with varying levels of oxygen and carbon dioxide, pH, and temperature, affecting oxygen delivery to tissues.
Hypoxemia is a low level of oxygen in the blood, which can be caused by respiratory diseases, high altitudes, or hypoventilation. Hypoxia refers to insufficient oxygen at the tissue level and can result from hypoxemia or other factors affecting oxygen utilization.
The cardiac cycle consists of phases including atrial systole, ventricular systole, and diastole, during which the heart chambers contract and relax to pump blood through the circulatory system.
The renin-angiotensin II-aldosterone system regulates blood pressure and fluid balance by promoting vasoconstriction and increasing sodium and water reabsorption in the kidneys.
During exercise, the body increases heart rate, stroke volume, and cardiac output to meet the heightened oxygen and nutrient demands of active muscles, while also redistributing blood flow to essential organs.
Key equations include Fick's principle for cardiac output, the ideal gas law for respiratory physiology, and the Henderson-Hasselbalch equation for acid-base balance.
The ventilation-perfusion (V/Q) ratio is crucial for efficient gas exchange; an optimal V/Q ratio ensures that the amount of air reaching the alveoli matches the blood flow in the pulmonary capillaries.
Changes in posture can affect venous return and blood pressure; for example, standing up quickly can lead to orthostatic hypotension due to a temporary decrease in venous return and cardiac output.
The autonomic nervous system regulates involuntary bodily functions, including heart rate, digestion, respiratory rate, and the fight-or-flight response, through its sympathetic and parasympathetic divisions.
Excitation-contraction coupling is the process by which an electrical signal (action potential) leads to muscle contraction, involving the release of calcium ions from the sarcoplasmic reticulum and interaction with contractile proteins.
Normal blood values include a hemoglobin level of 12-16 g/dL for women and 14-18 g/dL for men, a white blood cell count of 4,500-11,000 cells/mcL, and a platelet count of 150,000-450,000 cells/mcL.
The kidneys maintain acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate from urine, thus regulating blood pH and maintaining homeostasis.
Compliance curves illustrate the relationship between lung volume and pressure, indicating how easily the lungs can expand; decreased compliance can indicate restrictive lung diseases.