Physiology: Internal Environment and Homeostasis

Internal Environment

• Physiology is the study of the normal functioning of a living organism and its component parts.
• Key principles of physiology include homeostasis and the regulation of the internal environment.
• Homeostasis - the ability to keep the human internal environment relatively stable (a dynamic steady state).
• The human body is an open system and exchanges heat and materials with the outside environment.
• To maintain homeostasis, the body must maintain mass balance.
• The body’s internal and external environments are separated by the cell membrane.
• Most cells are completely surrounded by the body’s internal environment, composed of the extracellular fluid (ECF).
• The extracellular fluid exchanges material with the outside world and the fluid inside cells, or intracellular fluid (ICF).

Levels of Organization: Body Compartments

• The general functions of the cell membrane include:
  • Physical isolation
  • Regulation of exchange with the environment
  • Communication between the cell and its environment
  • Structural support

Body Fluid Compartments

• Cations and anions are not distributed equally between the body compartments, creating a state of electrical disequilibrium.

Water Movement

• Water moves freely between the cells and the extracellular fluid.
• Osmosis is the flow of water across a semipermeable membrane from a solution with low solute concentration to a solution with high solute concentration.
• Osmolarity (mOsM/L) is the number of particles per liter of solution.
• Tonicity describes a solution and how that solution would affect cell volume if a cell were placed in the solution and allowed to come to equilibrium.
• Diffusion is the passive movement of molecules down a chemical gradient (Fick’s law of diffusion).

Transport Processes

• The cell membrane are selectively permeable.
• Substances can be transported across the cell membrane by diffusion, using membrane proteins, or active transport.

Blood

• Total blood volume (in a 70kg man) is 7%; or 5 Liters.
• Blood is a connective tissue composed of plasma, red blood cells, white blood cells, and platelets.
• Plasma is the liquid component of blood and contains water, proteins, electrolytes, nutrients, and waste products.
• Red blood cells (erythrocytes) are responsible for transporting oxygen to the tissues.
• White blood cells (leukocytes) are responsible for fighting infection.
• Platelets are small fragments of cells and are responsible for blood clotting.

Red Blood Cells

• Red blood cells live for 120 days and are made in the bone marrow.
• Hemoglobin is a protein found in red blood cells that binds to oxygen.
• Globin proteins isoforms include alpha, beta, gamma, and delta.
• Adult hemoglobin (HbA – 2⍺+2β; HbA2/2,5%/ -2⍺+2𝜹)
• Hemoglobin A1C is used to measure blood sugar levels over a period of time.

Hemoglobin Function

• Hemoglobin binds to oxygen and releases it to tissues as needed.
• The Bohr effect states that the affinity of hemoglobin for oxygen increases with the pH.
• The Haldane effect states that the affinity of hemoglobin for carbon dioxide is higher when it is bound to oxygen.

Hemoglobin Abnormalities

• Polycythemia vera is a stem cell dysfunction that leads to an increase in red blood cells.
• Relative polycythemia is caused by dehydration and leads to a decrease in plasma volume.

Platelets and Coagulation

• Platelets are small fragments of cells and their typical lifespan is 10 days.
• Hemostasis is the process of keeping blood within a damaged blood vessel.
• Hemostasis involves vasoconstriction, temporary blockage of a break by a platelet plug, and coagulation.

Coagulation

• Platelets adhere to collagen with the help of integrins.
• Platelets release serotonin, ADP, and platelet-activating factor (PAF) which initiate a positive feedback loop that activates more platelets.
• Platelet membrane phospholipids are converted to thromboxane A2.

Coagulation Pathways

• There are two main pathways for coagulation:
  • The intrinsic pathway begins when damage to the tissue exposes collagen.
  • The extrinsic pathway starts when damaged tissues expose tissue factor, also called tissue thromboplastin (III).

Clot Dissolution

• Clots are temporarily fixed and are broken down by the enzyme plasmin (plasminogen→plasmin by thrombin and tPA /tissue plasminogen activator/).

Anticoagulants

• Anticoagulants include:
  • Heparin and antithrombin III produced by endothelial cells.
  • Protein C produced by endothelial cells.
  • Streptokinase and tPA (drugs).
  • Aspirin (blocks the COX enzymes that promote synthesis of the platelet activator thromboxane A2).
  • Coumarin anticoagulants such as warfarin (blocks vitamin K→a cofactor in the synthesis of factors II, VII, IX, X)

Hemophilia

• Hemophilia is a bleeding disorder caused by a deficiency in one of the clotting factors.
  • Hemophilia A is caused by a deficiency in factor VIII.
  • Hemophilia B is caused by a deficiency in factor IX.

Acid-Base Balance

• The pH of human blood is maintained within a narrow range of 7.35 to 7.45.
• This balance is achieved through the action of buffers, the lungs, and the kidneys.
• Buffers are molecules that moderate changes in pH by combining with or releasing H+.
• The primary buffer system in the blood is the bicarbonate buffer system.
• The respiratory system also plays a role in regulating acid-base balance.
• The kidneys control the excretion of hydrogen ions and the reabsorption of bicarbonate ions.

Acid-Base Balance Disturbances

• Acid-base disturbances can be caused by a variety of conditions including respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.
• Respiratory acidosis is caused by a decrease in ventilation, which leads to an increase in carbon dioxide.
• Respiratory alkalosis is caused by an increase in ventilation, which leads to a decrease in carbon dioxide.
• Metabolic acidosis is caused by a decrease in bicarbonate ions, which can be caused by a number of conditions such as diabetes mellitus.
• Metabolic alkalosis is caused by an increase in bicarbonate ions, which can be caused by conditions such as vomiting.

Test your knowledge on the principles of physiology, focusing on the internal environment and homeostasis. Explore how the human body maintains balance between its internal and external environments through various mechanisms. This quiz covers key concepts like mass balance and cell membrane functions.