Cardiac Output and Stroke Volume

Cardiac Output (CO)

• Stroke volume is the volume of blood ejected per beat, measured in milliliters (mL).
• Heart rate is the number of beats per minute.
• Cardiac output is the volume of blood flowing through the circulation in one minute.
• CO is determined by heart rate (HR) multiplied by stroke volume (SV).
• CO = HR x SV

Regulation of Heart Rate

• Regulation of heart rate regulation involves the medulla, vagus nerve, spinal cord and sympathetic trunk.
• It involves the SA node, AV node, and cardiac accelerator nerve.

Factors Affecting Stroke Volume

• Preload, afterload, and contractility affect stroke volume.
• Preload (Frank-Starling law) refers to the stretch of heart muscle cells (cardiomyocytes) at the end of ventricular filling during diastole.
• Contractility describes the heart's ability to eject blood (stroke volume) given specific afterload and preload conditions.
• Afterload is the work the heart must perform to pump blood to the rest of the body.

Blood Flow, Resistance, and Pressure

• Blood encounters resistance as it flows through blood vessels.
• Total Peripheral Resistance (TPR) is the sum of all resistance throughout the circulation.
• A pressure gradient is needed for blood to overcome resistance.
• The Hagen-Poiseuille equation describes the relationship between blood flow, resistance, and pressure gradient.

Determinants of Blood Flow

• The difference in pressure between the start and end of a vessel is the pressure gradient
• Blood flow is directly proportional to the pressure difference
• Resistance is opposition of blood flow through a vessel.
• Blood flow is inversely proportional to resistance.
• Blood flow (F) = Pressure gradient (Pi-Pf) / Resistance (R).

Resistance to Blood Flow

• Blood viscosity is directly proportional to resistance.
• Total blood vessel length is directly proportional to resistance.
• Blood vessel radius is inversely proportional to resistance.
• Resistance (R) = 8ηL / πr4
• Resistance is approximately equal to 1/r^4

Pressures within the Systemic Circulation

• Graph illustrates pressure throughout circulation, from aorta to vena cavae.
• It displays systolic, diastolic, and mean pressure.

Vessel Structure Review

• Elastic arteries withstand high pressure.
• Elastic arteries exhibit the Windkessel effect.
• Muscular arteries distribute blood and dampen pulsatility.
• Arterioles regulate blood flow to capillaries.

Arterioles and Their Function

• Arterioles are the primary site of vascular resistance.
• Arterioles regulate TPR through vasoconstriction and vasodilation.
• Arterioles regulate blood flow to tissues, based on metabolic needs, ensuring oxygen and nutrient supply.
• Arterioles control blood pressure by adjusting diameter and play a key role in short-term regulation.
• They respond to neural, hormonal, and local chemical signals for dynamic blood flow.

Systemic Control of Arteriolar Diameter

• Local factors causing vasodilation include decreased O2, increased CO2, adenosine, increased H+, increased K+, prostaglandins, and nitric oxide.
• Decreased sympathetic tone and atrial natriuretic peptide also promote vasodilation.
• Factors causing vasoconstriction include increased sympathetic tone, noradrenaline, angiotensin II, antidiuretic hormone, stretch, and endothelins.
• Intrinsic controls (autoregulation) distribute blood flow to individual organs and tissues.
• Extrinsic controls maintain mean arterial pressure and redistribute blood flow during exercise/thermoregulation.

Capillaries

• Composed of a layer of endothelial cells facilitate easy diffusion.
• The pressure is relatively low.
• Capillaries serve as exchange vessels for oxygen and nutrients, and excretion of waste products.
• Oxygen, carbon dioxide, nutrients, and metabolic wastes move between blood and interstitial fluid via diffusion.
• Diffusion occurs down concentration gradients, from high-concentration to low-concentration areas.
• Lipid-soluble substances (gases) diffuse through the lipid bilayer of plasma membranes.
• Small, water-soluble solutes pass through intercellular clefts/fenestrations, substances can also cross due to a pressure gradient.

Fluid Filtration Across Capillaries

• Hydrostatic pressure is the exerted pressure against the vessel wall.
• Osmotic pressure is from the concentration of colloidal proteins in the blood.
• Fluid filtration depends on Starling forces, capillary and osmotic pressures.
• Net Filtration Pressure (NFP) is net pressure moving out - net pressure inwards .
• NFP = (BHP + IFOP) – (BCOP + IFHP)

Venous Return

• Veins act as conduits and capacitance vessels.
• At rest, 64% of blood in the systemic veins and venules.
• Venous return represents the rate of blood flow back to the heart.

Factors Affecting Venous Return

• Venous return is supported by postural changes, the muscular pump, respiratory pump, and venoconstriction.
• The muscular pump involves contraction of skeletal muscles.
• The respiratory pump involves thoracic pressure shifts during inspiration.

Blood Pressure

• The systemic circulation can be compared to a network of parallel vessels
• As blood flows through these vessels, resistance is encountered
• The cumulative resistance across all vessels (TPR) is systemic vascular resistance (SVR)
• The mean arterial pressure (MAP) is determined by cardiac output (CO) and total peripheral resistance (TPR)
• MAP = CO x TPR
• Determined by factors influencing cardiac output and TPR.
• Homeostasis is controlled by mechanisms affecting heart rate, stroke volume, vessel radius, blood viscosity, and blood volume.
• Systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial pressure (MAP).
• Normal blood pressure and MAP is influenced by blood volume, salt/water balance, and sympathetic activity.

Autonomic Innervation

• Baroreceptors detect pressure changes in the aortic arch and carotid sinus
• The cardio centre in the medulla receives information relayed be the baroreceptors
• Sympathetic and parasympathetic activity adjusts appropriately through it.
• SA and AV nodes innervated by both Sympathetic nervous system (SNS) and Parasympathetic nervous system (PNS).
• Ventricles and blood vessels are only innervated by SNS.

Short-Term Control of Blood Pressure

• The baroreceptor reflex controls blood pressure short term.
• Stimuli (changes in blood pressure) are identified y the baroreceptors.
• Impulses from baroreceptors activate or inhibit centres in the brain.
• Sympathetic impulses change heart to decrease of increase contractility and CO .
• Vasomotor fibres trigger vasodilation or vasoconstriction.

Sympathetic Nervous System

• The SNS is responsible for immediate changes in flow (e.g. during exercise).
• SNS fibres release noradrenaline (NA).
• In the heart NA acts on β1-receptors to increase HR and SV.
• In blood vessels NA acts on:
• α1-receptors to cause vasoconstriction
• β2-receptors to cause vasodilation

Long Term Control of Blood Pressure

• Long term blood pressure is controlled through direct and indirect renal mechanisms
• Direct renal mechanisms use filtration in the kidneys affecting urine formation and blood volume.
• Indirect renal mechanisms influence the renin-angiotensin-aldosterone.