BMS100 Physiology Concepts V PDF

Physiology Concepts V Physicochemical Laws in Physiology Dr. Vargo BMS 100 Week 6 Today’s Overview Mass Balance in Physiology Renal Clearance Elimination of Gases from the Blood and Tissues Boyle’s Law and the Respiratory System Laplace’s Law Mass Balance is Simple Assume an aqueous system with a ce...

Physiology Concepts V Physicochemical Laws in Physiology Dr. Vargo BMS 100 Week 6 Today’s Overview Mass Balance in Physiology Renal Clearance Elimination of Gases from the Blood and Tissues Boyle’s Law and the Respiratory System Laplace’s Law Mass Balance is Simple Assume an aqueous system with a certain amount (mass) of substance Z dissolved in it Also assume that this system is at a steady state – the total amount of Z in the system doesn’t change therefore If you add Z to the system, then the same amount of Z has to leave the system to maintain steady state (keep Z constant in the system) ▪ If there’s an imbalance, then a new steady state will be reached Mass Balance – a Model Theoretical System: ▪ A fluid reservoir that has a constant volume ▪ A mechanism by which a dissolved substance is added at a constant rate Substance Z ▪ A filtering system that removes Z at the same rate that it’s added Mass Balance Fundamentals The model as illustrated is at steady state ▪ The concentration of Z in the reservoir isn’t changing because there’s a balance between Z added and Z removed What happens if you decrease the amount of Z removed? ▪ Does Z accumulate “infinitely” in the reservoir? ▪ This is a question with great physiological relevance: Renal filtration changes Medications that are eliminated by the kidney Changes in ventilation Changes in oxygen supply to tissues Mass Balance and Clearance Clearance = the volume of a fluid that has been completely “cleared” of a substance Example – substance Z was removed from the pipe by the filtering system ▪ Although there’s still some Z in the pipe, it’s at a lower concentration ▪ Imagine separating the fluid in the pipe into 2 volumes: The volume that has the same concentration of Z – [Z] – as the original reservoir The volume that has no Z ([Z] = 0 in that volume) ▪ This is the volume that has been cleared Mass Balance and Clearance The math is pretty simple: CLZ = clearance of Z ▪ This is a flow rate – volume/time [Zr] = concentration of Z in the reservoir Qb = flow of fluid into the “bladder” ▪ Again – volume/time [Zb] = concentration of Z in the bladder CLZ × [Zr] = Qb × [Zb] Qb × [Zb] CLZ = [Zr] Clearance in Physiology So, what happens if you damage your clearance mechanism and clearance drops? ▪ In the kidneys, this can be expressed as changing the rate of fluid filtration Glomerular filtration rate ▪ In the lungs, this can be expressed as changing the rate of ventilation Ventilation is our clearance mechanism for carbon dioxide To the whiteboard… Try calculating yourself: Our regular approximate alveolar ventilation is just over 4 L/min ▪ This is our clearance mechanism – CLZ ▪ For the sake of simplification, let’s say it’s 4.4 L/min The concentration of CO2 in the alveoli is about 40 mm Hg ▪ [Zb] The concentration of carbon dioxide in the pulmonary arterial blood is about 45 mm Hg ([Zr]) and the cardiac output to the lung is about 5 L/min (QB) Let’s see what happens to the arterial CO2 concentration if we cut ventilation in half → 2.2 L/min NOTE – this is an oversimplification – it’s not quite what happens with CO2 elimination and the numbers aren’t quite right but it’s not terribly inaccurate and it’s much less unpleasant than the actual calculation Hypoventilation and [CO2art] We weren’t that far off… Boron and Boulpaep’s Medical Physiology, 3rd ed. Fig. 31-4 Boyle’s Law Boyle’s Law is our missing link in understanding the mechanics of ventilation ▪ ONLY applies to gases – fluids are pretty much incompressible (they don’t change volume when you apply pressure) Simply: ▪ If you increase the pressure in a container with a gas in it, the volume of the gas will decrease ▪ For a gas, volume and pressure are inversely related Boyle’s Law P1V1 = P2V2 Case 1 Anatomy & Physiolgy, 2nd ed. Fig. 22.15, p. 962 Case 2 Boyle’s Law and Ventilation - review Ventilation = the process by which we move atmospheric air into and out of the alveoli ▪ uses pressure gradients, Poiseuille’s law can be applied ▪ NOT diffusion Anatomy & Physiolgy, 2nd ed. Fig. 22.16, p. 963 Boyle’s Law and the Lung – to consider: When gas moves into the lung during inspiration, how does the pressure in the alveoli compare to atmospheric pressure? ▪ How about during expiration? How do we change the pressure in the alveoli in each case? Boyle’s Law and the Lung – what do you recall? What are the major muscles that contract during: ▪ Expiration? ▪ Inspiration? How does the pleural fluid help us transmit force from the muscles that change the volume of the thoracic cavity? Laplace’s Law There are many circular, (roughly) spherical, or cylindrical objects in our body ▪ Blood vessels and airways in the respiratory tree ▪ Intestines ▪ The heart These objects all contain gas or fluid, which can exert pressure against the walls… ▪ What sort of stress does this pressure put on those walls (wall tension)? ▪ Is there anything that “increases” the tension on the walls, even when the pressure is constant? Laplace’s Law: Tension (Te) = the tension that is placed on the walls of a cylindrical/spherical structure Thickness (t) = the thickness of the wall Pressure (P) = the pressure inside of the structure Radius (r) = the radius of the structure 𝑷 ×𝒓 𝑻𝒆 = 𝒕 Laplace’s Law: Tension (Te) increases when: ▪ Radius? ( or ) ▪ Pressure? ( or ) ▪ Thickness? ( or ) 𝑷 ×𝒓 𝑻𝒆 = 𝒕 Small Law, Large Relevance Aneurysm = a pathological increase in the diameter of a blood vessel ▪ In many organs, this is due to high blood pressure or atherosclerosis ▪ What impact does an increase in the diameter have on the tension/stress across the wall of this unhealthy vessel? https://commons.wikimedia.org/wiki/File:Abd ominal_Aortic_Aneurysm_Location.png Small Law, Large Relevance Dilated cardiomyopathy – the diameter of the heart increases ▪ Genetic, uncommon infections, toxins, idiopathic As the radius of the ventricle increases, the tension increases ▪ It doesn’t explode… but it has to use way, way more energy to counteract wall tension when it contracts → heart failure https://commons.wikimedia.org/wiki/File:Blau sen_0165_Cardiomyopathy_Dilated.png “Answer” – pulmonary question Ventilation X [CO2art] = Blood flow X [CO2alv] 2.2 L/min X [CO2art] = 5 L/min X 40 mm Hg [CO2art] = 5 L/min X 40 mm Hg 2.2 L/min [CO2art] = 91 mm Hg (!!!) [CO2art] wouldn’t get quite this high – a number of other parameters would correct for the rising carbon dioxide levels --> many inaccuracies ▪ However, hypoventilation does result in the development of a new steady state and a higher arterial carbon dioxide