Lecture 11: Cardiovascular System: Vasculature and Regulation Lecture Dynamics and Regulation Study Guide

Overview of Cardiovascular Dynamics and Vasculature Regulation

The study of the cardiovascular system involves understanding the complex interrelationships between three primary factors: flow, resistance, and pressure. Control mechanisms that regulate blood pressure and blood flow are critical for the proper functioning of the entire circulatory system. These dynamics ensure that blood is delivered to tissues according to their metabolic demands while maintaining a stable internal environment. Dr. Shaun D. Cain of Eastern Oregon University presents these concepts in the context of Human Anatomy and Physiology III (Biol 233).

iClicker Question: Mechanisms of Vasodilation

During the lecture, an iClicker question was posed to test student understanding of vessel mechanics: The arteries can undergo vasoconstriction and vasodilation. Vasodilation is the result of what force? The options provided included: muscle contraction of the circular muscles around the vessel, muscle contraction of longitudinal muscles along the vessel, increased pressure of the blood itself, the elastic properties of the vessel, or none of the above. The correct understanding is that while contraction of circular smooth muscle causes vasoconstriction, vasodilation occurs when those muscles relax and the increased pressure of the blood itself pushes the vessel walls outward.

Laminar and Turbulent Flow in Vessels

Blood flow through vessels can be characterized as either laminar or turbulent. Laminar flow is defined as streamlined flow that occurs when the interior of a blood vessel is smooth and of equal diameter along its length. In laminar flow, the blood moves in layers; the outermost layer moves the slowest due to friction against the vessel wall, while the center moves the fastest.

Conversely, turbulent flow is interrupted flow that occurs when the rate of flow exceeds a critical velocity or when fluid passes a constriction, a sharp turn, or a rough surface. Turbulent flow is partially responsible for heart sounds. However, sounds due to turbulence are not normal in arteries and usually indicate a constriction; such turbulence increases the probability of thrombosis, or the formation of blood clots.

Measurement of Blood Pressure

Blood pressure is a measure of the force exerted by blood against the vessel wall, and it is the primary force that moves blood through the vasculature. It can be measured directly by inserting a cannula into a blood vessel or indirectly using the auscultatory method. The auscultatory method involves a sphygmomanometer and a stethoscope to listen for Korotkoff sounds. These sounds are produced by turbulent flow in the arteries as pressure is released from the blood pressure cuff. The pressure recorded during the very first sound is the systolic pressure, while the pressure level where the sound disappears is the diastolic pressure.

Mathematical Principles of Blood Flow

Blood flow is expressed as the volume of blood passing a specific point per unit of time. For example, the average cardiac output at rest is $5\,L/min$, meaning the flow through the aorta is also $5\,L/min$. The governing equation for flow is:

$Flow = \frac{P_1 - P_2}{R}$

In this equation, $P_1$ and $P_2$ represent pressures at two distinct points in the vessel, and $R$ represents the resistance to flow. Flow is directly proportional to the pressure difference and inversely proportional to resistance. Resistance itself is determined by the following formula:

$Resistance = \frac{128 \times v \times l}{D^4}$

Here, $v$ is the viscosity of the blood, $l$ is the length of the vessel, and $D$ is the diameter of the vessel.

Poiseuille’s Law and Physical Adaptations

Poiseuille’s Law states that flow decreases when resistance increases and vice versa. Because resistance is inversely proportional to the fourth power of the diameter ($D^4$), even small changes in vessel diameter result in significant changes in flow. The combined formula is expressed as:

$Flow = \frac{\pi \times (P_1 - P_2) \times D^4}{128 \times v \times l}$

During exercise, the heart beats with greater force to increase pressure in the aorta. Simultaneously, capillaries leading to skeletal muscle increase in diameter, which decreases resistance and increases flow. Consequently, the flow in the aorta can increase from the resting $5\,L/min$ to five times that amount ($25\,L/min$).

Viscosity and Blood Composition

Viscosity is the measure of a liquid's resistance to flow. Resistance is proportionate to viscosity; therefore, as viscosity increases, the pressure required to maintain flow also increases. Blood viscosity is influenced largely by the hematocrit, which is the percentage of total blood volume composed of red blood cells (RBCs). Conditions such as dehydration or the uncontrolled production of RBCs lead to increased viscosity, which significantly increases the workload on the heart to pump the thicker blood.

Laplace’s Law and Critical Closing Pressure

Critical closing pressure is defined as the pressure at which a blood vessel collapses and blood flow stops. Mechanical stress on the vessel wall is governed by Laplace’s Law, which states that the force acting on the blood vessel wall is proportional to the diameter of the vessel multiplied by the blood pressure:

$F = D \times P$

As the diameter of a vessel increases, the force acting on the wall also increases. This principle explains why a weakened part of a vessel wall might bulge out, forming an aneurysm.

Vascular Compliance and Systemic Physiology

Vascular compliance is the tendency for blood vessel volume to increase as blood pressure increases. It is calculated as:

$Compliance = \frac{\text{increase in volume}}{\text{increase in pressure}}$

The more easily the vessel wall stretches, the greater its compliance. The venous system has a large compliance, which is approximately $24$ times greater than that of the arterial system. This allows veins to act as a blood reservoir for the body.

In terms of systemic circulation, the total cross-sectional area increases as the diameter of individual vessels decreases. For instance, the single aorta has a cross-sectional area of $5\,cm^2$, while the millions of capillaries together have a total cross-sectional area of $2500\,cm^2$. This results in a decrease in the velocity of blood flow in the capillaries, similar to a stream flowing rapidly through a narrow gorge but slowing down as it reaches a broad plane.

Pressure Gradients and Resistance Distribution

Blood pressure averages $100\,mm\,Hg$ in the aorta and drops to $0\,mm\,Hg$ by the time it reaches the right atrium. The greatest drop in pressure occurs in the arterioles, which are the primary regulators of blood flow through specific tissues. Capillaries and veins do not experience large fluctuations in pressure. Muscular arteries and arterioles can constrict or dilate in response to autonomic and hormonal stimulation. Muscular arteries regulate flow into a region of the body, while arterioles regulate flow into specific tissues.

Pulse Pressure

Pulse pressure is the numerical difference between systolic and diastolic pressures. It increases when stroke volume increases or when vascular compliance decreases. Compliance tends to decrease with age due to conditions like arteriosclerosis, which causes pressure to rise. Pulse pressure is used to determine heart rate and rhythmicity. The most frequent site for measuring pulse rate is the radial artery in the carpus, known as the radial pulse.

Capillary Exchange and Fluid Dynamics

Capillary exchange is the movement of substances into and out of capillaries, with diffusion being the most important mechanism. Lipid-soluble substances, such as $O_2$, $CO_2$, steroid hormones, and fatty acids, cross the capillary walls by diffusing directly through the plasma membrane. Water-soluble substances, such as glucose and amino acids, diffuse through intercellular spaces or fenestrations.

Fluid movement is affected by blood pressure, capillary permeability, and osmosis. Most fluid moves out of the capillaries at the arterial end; however, only about $9/10$ of that volume returns to the capillaries at the venous end. The remaining $1/10$ that stays in the tissues is collected by the lymphatic system and eventually returned to venous circulation.

Net Filtration Pressure (NFP)

Net Filtration Pressure (NFP) is the force responsible for moving fluid across capillary walls. It is determined by the balance of hydrostatic and osmotic pressures. Hydrostatic pressure is the physical pressure of blood in vessels or fluid in interstitial spaces. Osmotic pressure relates to the movement of solutes through a membrane in the presence of non-diffusible solutes like large proteins. Because these proteins do not pass freely through capillary walls, the difference in protein concentration between blood and interstitial fluid drives osmosis.

NFP is calculated using the following equations:

$Net\,hydrostatic\,pressure = CHP - IFP$ (Where $CHP$ is capillary hydrostatic pressure and $IFP$ is interstitial fluid hydrostatic pressure)

$Net\,osmotic\,pressure = BCOP - ICOP$ (Where $BCOP$ is blood colloid osmotic pressure and $ICOP$ is interstitial fluid colloid osmotic pressure)

$NFP = (CHP - IFP) - (BCOP - ICOP)$

At the arterial end, the net hydrostatic pressure outward ($33\,mm\,Hg$) is greater than the net osmotic pressure inward ($20\,mm\,Hg$). At the venous end, the net osmotic pressure inward ($20\,mm\,Hg$) is greater than the net hydrostatic pressure outward ($13\,mm\,Hg$).

Venous Function and Control of Blood Flow

Venous return to the heart increases with increases in blood volume, venous tone, and arteriole dilation. Venous tone refers to the continual state of partial contraction of veins resulting from sympathetic stimulation. Gravity also affects blood pressure; in a standing position, hydrostatic pressure increases blood pressure below the heart and decreases it above the heart. Muscular movement helps improve venous return against gravity.

In most tissues, blood flow is controlled locally to match metabolic needs. Factors like increased $CO_2$, lactate, and potassium, or a lack of nutrients, act as vasodilators that relax metarterioles and precapillary sphincters. This increases blood flow to serve working tissues. Periodic contraction and relaxation of these sphincters is called vasomotion. Long-term local control can lead to increased capillary density in regions with consistently high metabolic rates.

Nervous and Hormonal Regulation

The nervous system provides minute-to-minute regulation and can shunt blood from one area to another by increasing resistance. The sympathetic division is the most important for this, innervating all vessels except capillaries, precapillary sphincters, and most metarterioles. The vasomotor center is located in the lower pons and upper medulla oblongata. Its excitatory part is tonically active, causing vasomotor tone through norepinephrine.

Sympathetic stimulation also triggers the adrenal medulla to release epinephrine and norepinephrine into the blood. This typically causes vasoconstriction via $\alpha\text{-adrenergic}$ receptors, except in skeletal muscle, where it causes vasodilation via $\beta\text{-adrenergic}$ receptors.