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Chapter 2—Pharmacodynamics
Pharmacology Chapter 2 - Pharmacology Metabolic Kinetics, including transmembrane transport of drug molecules, the process of drugs in the body, important parameters of pharmacokinetics, design and optimization of drug doses, etc.
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Chapter 2—Pharmacodynamics
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Chapter 2—Pharmacodynamics
Transmembrane transport of drug molecules
Transmembrane mode
Passive fortune
Concept: The process of drug diffusing from the high concentration side like the low concentration side along the concentration gradient.
Features
Concentration gradient
No vector required
No energy consumption
No saturation and competition suppression
Classification
Filtered
concept
It refers to the transmembrane transport of water-soluble polar or non-polar drug molecules with hydrostatic or osmotic pressure along with the body fluid passing through the aqueous channels of the cell membrane, also known as water-soluble diffusion.
Simple diffusion
concept
It refers to the lipid layer in which the lipid soluble drug is dissolved in the cell membrane and passes through the cell membrane at a concentration gradient, also known as lipid soluble diffusion.
Diffusion speed
It mainly depends on the oil and water distribution coefficient of the drug (fat-soluble) and the difference in drug concentration on both sides of the membrane.
The greater the difference in fat solubility and concentration, the faster the diffusion will be.
Carrier Transport
Features
Be selective
Need carrier protein
Saturation and competitive
Structural and site specific
Classification
Actively transfer
Concept: Drugs are transported from low concentration side to high concentration side by means of carrier or enzymatic systems.
Features
Need carrier protein
To consume energy
Saturation and competitive inhibition
Inverse concentration gradient
Easy diffusion
Concept: The process of drug diffusion from high concentration to low concentration side with the help of cell membrane carriers
Features
Need carrier protein
No energy consumption
Saturation and competitive inhibition
Concentration gradient
Membrane Transport
concept
Large molecular substances are transported through the movement of the membrane
Classification
Endorf
Cell squirt
Factors affecting drug permeability of cell membranes
Dissociation of drugs and pH of body fluids
Molecular (non-dissociation) drugs are hydrophobic and lipophilic, and are easy to pass through the cell membrane; ionic drugs are extremely polar and do not easily pass through the cell membrane lipid layer. This phenomenon is called ion barrier.
The degree of drug dissociation depends on the body fluid pH and drug dissociation constant (Ka).
Weak acidic drugs have less dissociation and more absorption in acidic environments; otherwise.
Weakly alkaline drugs have more dissociation and less absorption in acidic environments; otherwise.
Weak acidic drugs are absorbed from the stomach; weak alkaline drugs are absorbed from the intestine.
The concentration of weakly acidic drugs in extracellular fluid is higher than that intracellular fluid. The concentration of weakly alkaline drugs in the intracellular fluid is higher than that in the extracellular fluid.
Poor drug concentration and cell membrane permeability, area and thickness
Blood flow
The amount and function of cell membrane transporter
The process of drugs in the body
absorb
Concept: refers to the process in which the drug enters the blood circulation at its own site.
way
Oral administration (absorption in the digestive tract)
Drugs➡ Gastrointestinal mucosa➡ Portal vein➡ Liver➡ Body circulation
The small intestine is the main absorption site when taking the drug orally
Influencing factors: First-level effect (first-pass elimination): refers to the phenomenon that the drugs absorbed by the gastrointestinal tract are partially metabolized by the intestinal wall and liver before reaching the blood circulation throughout the body, thereby reducing the amount of effective drugs entering the blood circulation throughout the body.
To avoid the primary effect, sublingual and lower rectal administration is usually used.
Sublingual ➡ Superior vena cava ➡ Systemic circulation eg: nitroglycerin (angina pectoris)
Lower rectal ➡ body circulation
Injection (parental absorption)
Intravenous injection
There is no absorption process
Intramuscular injection
Subcutaneous injection
Arterial injection
Intraperitoneal injection of IP.
Respiratory inhalation medication
/
Local drug delivery
The purpose is to produce local effects on the skin, eyes, throat and vagina.
Sublingual medication
Avoid first-level effect
Absorption rate: inhalation>sublingual>rectal>muscle>subcutaneous>oral>skin
distributed
Concept: refers to the process of circulating from the blood to various organs and tissues of the body after the drug is absorbed. )
The degree and speed of drug distribution in various tissues in the body mainly depends on the blood flow of tissues and organs and the binding ability of the drug to plasma proteins and tissue cells.
Factors
Blood flow of tissues and organs
Binding rate of plasma proteins
Conjugated drugs cannot be transported across the membrane and are a temporary form of storage of drugs in the blood.
Drug binding to plasma proteins: specificity; reversibility; saturation; competition inhibition.
Histocellular binding
The strong affinity of drugs with certain tissues is an important reason for the selectivity of drug sites of action.
Physical fluid pH and drug dissociation
Intrabody barrier
Blood-brain barrier
Including blood and brain tissue, blood and cerebrospinal fluid, and. Three barriers: cerebrospinal fluid and brain tissue.
Only drugs with high fat solubility and low binding rate to plasma proteins can passively spread through the blood-brain barrier.
Placenta barrier
The barrier between the placental villus and the uterine sinus is called the placental barrier.
It has no obvious difference in its permeability to drugs and general capillaries.
Blood Eye Barrier
The barrier between blood and the retina, water atrium, and vitreous body is called the blood eye barrier.
Fat-soluble drugs and water-soluble drugs with molecular weight less than 100 Da are easy to pass.
Metabolism (biotransformation)
It refers to a series of chemical reactions in the body after the drug is absorbed through enzymes or other actions, which leads to the transformation of the chemical structure of the drug, also known as biotransformation.
Phase of metabolizing drug
Phase I reaction produces metabolites with increased polarity through oxidation, reduction and hydrolysis.
‖Phase reaction is a combination that produces a combination with high polarity and high water solubleness and is excreted through the urine.
Drug metabolizing enzymes
Liver-cytochrome P450 monooxygenase system, referred to as CYP
CYP has low selectivity, low variability in action, and is susceptible to influence by a variety of factors.
Factors that affect drug metabolism
Genetic factors
Induction and inhibition of drug metabolizing enzymes
Enzyme inhibitors - drugs that reduce the activity of drug metabolizing enzymes and slow down drug metabolism.
Enzyme inducers - drugs that can increase the activity of drug metabolic enzymes and accelerate drug metabolism.
Changes in liver blood flow
Liver blood flow
Other factors
excretion
Concept: The process of drug being excreted from the body through different pathways in the form of a prototype or metabolite.
Pathway: One, two internal organs and four fluids (gastrointestinal tract-lung/kidney-salvae/lotion/stomach juice/sweat)
Biliary circulation, enterohepatic circulation
Some drugs are converted into water-soluble metabolites with strong polarity through the liver, which are secreted into the bile and enter the intestinal cavity through the bile duct and common bile duct, and then excreted with feces. The drug part that is discharged into the intestinal cavity through bile can be further passed through small intestinal epithelial cells. It is absorbed into the blood circulation through the liver. This circulation between the liver, bile, and small intestine is called the intestinal liver circulation.
Enterohepatic circulation can prolong the plasma half-life and maintenance time of the drug.
Volatile drugs and inhaled anesthetics can be excreted from the body through the lungs.
Intraventricular model
Drug elimination kinetics
Blood concentration of the drug-time relationship
After one dose, blood drug concentration can be measured at different times. The curve of the relationship between hemorrhagic drug concentration and time (drug-time curve).
Pharmaceutical elimination kinetics type
equation
First-level elimination dynamics
concept
The drugs in the body are eliminated in a constant proportion, and the amount of elimination per unit time is proportional to the concentration of plasma drugs.
Features
1. Elimination of drugs is carried out in a constant ratio;
2. The speed of drug elimination is related to Co;
3. The half-life is constant and has nothing to do with Co;
4. Curve: The normal coordinate is a concave curve, and the vertical coordinate is a straight line when it is logarithmic.
Zero-order elimination dynamics
concept
The drug is eliminated in the body at a constant rate.
Features
1. In most cases, the amount of medicine in the body is too large, exceeding the body's maximum elimination ability;
2. The elimination of drugs is eliminated at a constant dose;
3. When the blood drug concentration drops below the body's elimination ability, it will be converted to first-order kinetics;
4. Elimination speed has nothing to do with Co;
5. The half-life is not constant and changes with Co;
6. Curve: When the normal coordinate is a straight line, the vertical coordinate is logarithmic, it is a convex curve.
Hybrid elimination dynamics
Important parameters of pharmacokinetics
Peak concentration and peak time
Peak concentration
The highest point of the time-time curve during extravascular administration is called the plasma peak concentration, Cmax
Peak time
The time when the peak concentration is reached is called the peak time, Tmax
Area under the curve
The area covered by the drug-time curve is called the area under the curve, AUC
Its size reflects the relative amount of drug absorption into the blood circulation
Bioavailability (F)
It refers to the relative amount and speed of the drug being absorbed into the systemic blood circulation after being administered through extravascular routes.
Classification
Relative bioavailability
Determine the absorption degree of different administration routes of the same drug.
Absolute bioavailability
Determine the preparations for good or bad
Apparent distribution volume (Vd)
It refers to the volume of body fluids required to distribute the drug in the body according to the plasma drug concentration when the drug distribution in the plasma is in equilibrium.
Eliminate rate constant
It is the fraction of eliminating the drug within a unit of time.
Eliminate half-life
concept
The half-life of drug elimination is the time it takes to drop plasma drug concentration by half. Reflects the speed of elimination of drugs in the body.
significance
1. The basis for the classification of similar drugs into long-acting, medium-acting and short-acting;
2. The basis for the interval between continuous medications;
3. Influenced by liver and kidney functions;
4. Estimate the amount of drug.
If the drug is given at a fixed dose, fixed interval time, or constant speed intravenous infusion, the steady-state blood drug concentration will basically reach after 4 to 5 half-life.
Steady state blood drug concentration: dosage = elimination amount
When taking the medicine at one time, after 4 to 5 half-life, the medicine is basically eliminated in the body.
Clearance rate
concept
It is the body's elimination organ to eliminate the plasma volume of drugs within a unit of time, that is, how much volume of drugs contained in the plasma is clearly understood by the body.
significance
Reflects liver and kidney function.
Drug dosage design and optimization
Steady state plasma concentration
concept
The total amount of drugs eliminated according to the first-order kinetic law gradually increases with continuous administration until the amount of drugs eliminated from the body is equal to the amount of drugs entering the body, thus achieving equilibrium. The plasma drug concentration at this time is called for steady-state plasma concentration, Css.
Related to dosage interval and dose; related to bioavailability and elimination rate.
Increasing the frequency of dosage or increasing the dosage cannot achieve steady-state plasma concentration in advance, but can only change the total amount of drugs in the body (i.e. increasing the steady-state concentration level) or the difference between peak concentration and trough concentration.
Loading dosage
concept
It refers to the first dose increase, and then the maintenance dose is given, so that the steady-state blood drug concentration (i.e. the target concentration set for the patient in advance) is generated in advance.
reason
When administering the drug at a maintenance dose, it usually takes 4 to 5 half-life to reach the steady-state blood drug concentration. If the dose is increased or the dosage is shortened, the steady-state cannot be reached in advance, and the drug concentration can only be increased. Therefore, load-dose dosage method can be adopted.
If oral intermittent administration is administered once every half-life, the loading dose can be doubled by the first dose; when continuous intravenous infusion, the loading dose can be 1.44 times the first half-life intravenous drip.