Question 5 Fluid Dynamics and Tissue Perfusion
Blood Flow and Tissue Perfusion
Blood flow and tissue perfusion perform additional functions beyond nutrient delivery and waste removal.
Gas Exchange in the Lungs
Tissue perfusion to the lungs facilitates gas exchange, specifically:
Oxygen Loading: Blood absorbs oxygen from the lungs.
Carbon Dioxide Unloading: Blood releases carbon dioxide into the lungs.
Nutrient Absorption in the Digestive Tract
Tissue perfusion to the digestive system allows for:
Breakdown of nutrients into smaller building blocks.
Absorption of these building blocks into the bloodstream, enabling transport to various organ systems and tissues.
Kidney Functions
The kidneys are vital for:
Blood Filtration: Removal of waste products from the bloodstream.
Urine Formation: Waste is excreted from the body.
Metabolite Processing: Removal of excess electrolytes and maintenance of acid-base balance.
Tissue perfusion to the kidneys is crucial for effective waste removal and urine formation.
Regulation of Blood Flow to Organs
Maintaining appropriate blood flow to organs and tissues is essential for their proper function.
Blood flow is regulated through:
Extrinsic Control: Regulation from outside the organ system (nervous and endocrine systems).
Intrinsic Control: Local regulation from within the organ (autoregulation).
Extrinsic Control of Blood Flow
This involves the nervous system and hormonal influences.
Autonomic Nervous System: Acts on smooth muscle within arterioles:
Vasodilation: Increases blood flow to capillary beds when arterioles dilate.
Vasoconstriction: Reduces blood flow when arterioles constrict.
Cardiac Output Distribution: Cardiac output must be distributed to organs based on their metabolic needs.
Intrinsic Control of Blood Flow (Autoregulation)
Autoregulation allows arterioles to respond to metabolic demands by altering diameter.
Influential Factors:
Increased metabolic activity = Vasodilation (increased blood flow).
Decreased metabolic activity = Vasoconstriction (decreased blood flow).
Resistance and Flow Relationship:
Vasodilate: Increases flow by decreasing resistance.
Vasoconstrict: Decreases flow by increasing resistance.
Blood Flow Changes During Exercise
During exercise, cardiac output increases significantly:
At Rest: Skeletal muscle receives about 20% of cardiac output.
During Exercise: Skeletal muscle can receive over 70%.
Increased Oxygen Demand: Skeletal muscle arterioles vasodilate, allowing more blood flow due to higher metabolic activity.
Decreased Flow to Other Organs: Reduced blood flow to kidneys and digestive tract during exercise.
Mean Arterial Pressure (MAP)
Definition: MAP is crucial and should be maintained within narrow parameters.
Relation to Cardiac Output:
Increasing blood flow to active tissues decreases resistance at those sites while balancing decreased perfusion to less active tissues.
Blood Flow Distribution During Exercise vs. Rest
Blood volume distribution:
At Rest:
Brain: ~750 mL
Skeletal Muscle: ~1,200 mL
During Strenuous Exercise:
Total Blood Flow: ~17,500 mL/min
Skeletal Muscle: ~12,500 mL
Heart: Increased from 250 mL to 750 mL
Skin: Increased from 500 mL to 1,900 mL
Kidneys: Decreased from 1,100 mL to 600 mL
Abdominal Organs: Decreased from 1,400 mL to 600 mL
Brain Blood Flow: Remains constant despite exercise or rest conditions.
Autoregulation Mechanisms
Reactive Hyperemia: Arterioles respond to changes in tissue metabolism.
Metabolic Controls: Changes due to tissue activity:
Increased metabolism = Decreased oxygen, increased CO2, increased protons (lower pH), increased potassium.
Myogenic Controls: Relate to stretch of arterial walls:
Increased blood pressure = Increased stretch = Vasoconstriction
Decreased blood pressure = Decreased stretch = Vasodilation
Key Points on Metabolic and Myogenic Controls
Metabolic controls signal arterioles for increased blood flow according to metabolic activity.
Myogenic controls help maintain a relatively constant blood flow and pressure in the capillary system to prevent damage.
Neurohormonal Extrinsic Control
Neural Controls:
Sympathetic division activation leads to vasoconstriction.
Hormones like angiotensin II and the sympathetic neurotransmitters epinephrine and norepinephrine affect arterial diameter.
Hormonal Control:
Atrial natriuretic peptide (ANP) has vasodilatory effects, helping regulate blood pressure.
Objective: Maintain mean arterial pressure and blood flow based on metabolic needs.
Capillary Exchange Mechanism
Blood flow slows in capillaries due to high cumulative cross-sectional area, facilitating material exchange.
Capillary Structure:
Thin walls with endothelial cells, only one cell layer thick, allowing exchange.
Methods of Exchange in Capillaries
Diffusion:
Molecules move from areas of higher concentration to lower concentration without energy input.
Movement through Clefts:
Water-soluble substances pass through clefts between endothelial cells.
Fenestrations:
Larger openings allow for movement of bigger water-soluble solutes.
Active Transport:
Large substances like proteins are moved across via vesicles.
Bulk Flow in Capillaries
Definition: Bulk flow is the movement of fluid out of, and back into, capillary beds.
Hydrostatic Pressure:
A pushing force that promotes fluid movement out of the capillaries.
Colloid Osmotic Pressure:
A pulling force that draws fluid back into capillaries due to plasma proteins remaining in the capillary.
Summary of Capillary Dynamics
Higher pressure at the arterial end leads to fluid being pushed out.
Fluid returns at the venous end due to higher colloidal osmotic pressure.
About 3 liters of fluid, if not returned, accumulate, necessitating removal via the lymphatic system.
Lymphatic System Functionality
Role in Fluid Balance: Collects excess fluid from tissues and returns it to the cardiovascular system.
Immune Response: Lymphatic vessels carry leukocytes through lymph nodes to monitor for pathogens.
The intrinsic cardiac conduction system is responsible for initiating and conducting electrical impulses that stimulate the heart to contract, thereby maintaining the heart's rhythm. The main components of the intrinsic conduction system, in order, starting at the sinoatrial (SA) node, are as follows:
Sinoatrial (SA) Node:
Located in the right atrium, the SA node serves as the primary pacemaker of the heart, generating impulses that initiate the heartbeat.
Atrioventricular (AV) Node:
Located at the junction between the atria and ventricles, the AV node receives impulses from the SA node and briefly delays them before sending them to the ventricles. This delay allows the atria to contract before the ventricles.
Bundle of His:
Also known as the atrioventricular bundle, this structure conducts the impulses from the AV node down to the ventricles. It splits into right and left bundle branches.
Right and Left Bundle Branches:
These branches carry the electrical impulses through the interventricular septum to the respective ventricles, ensuring simultaneous contraction of both ventricles.
Purkinje Fibers:
These fibers spread throughout the ventricles, distributing the electrical impulse that triggers ventricular contraction.
An example of an irregularity in the intrinsic conduction system is heart block. In heart block, the electrical signals are partially or completely blocked as they travel through the AV node. This can lead to various consequences, such as:
Bradycardia: A slower than normal heart rate, which could cause fatigue and fainting.
Syncope: Sudden loss of consciousness due to inadequate blood flow to the brain.
Potential need for a pacemaker: In severe cases, where the heart cannot maintain an appropriate rhythm on its own, surgical intervention may be required to implant a device that regulates heartbeats.