CARDIO-VASCULARSYSTEM:

BLOODVESSELS

VESSEL WALLS

Blood vessel walls:-tunica intima (inner layer)

-endothelium -tunica media (middle layer)

-smooth muscle-under control of sympathetic NS -helps maintain blood pressure

-tunica adventitia (outer layer)-connective tissue

-protection-has nerve fibers, lymphatics

ARTERIAL SYSTEM

ELASTIC ARTERIES - aorta and its branches- large thick walled - has elastin in all tunics, especially tunica media- has "pressure smoothing effect"- stretches to accomodate large fluctuations in blood volume- keeps BP relatively continuous (vs stop and go with each

beat)MUSCULAR ARTERIES - most of the arteries

- thickest media - more smooth muscle- active in vasoconstriction

ARTERIOLES- 0.3-10nm diameter of lumen- blood flow to capillaries is determined by diameter of

arterioles

CAPILLARIESCAPILLARIES- smallest 1mm long x 8-10nm diameter- tunica intima only (endothelium)- exchange of gases, nutrients with interstitial fluidTYPES OF CAPILLARIES

Continuous - one endothelial cell wraps all around ends joined by tight junctions

(brain)Fenestrated - have windows

very permeable to fluids vs solutesSinusoidal - modified, very leaky

CAPILLARY BEDShave sphincters so that capillary bed can be bypassed metarterioles for bypass

VENOUS SYSTEM

VENULES- just endothelium + fibroblasts - very leaky for easy exchange of fluid and WBC- larger venules all three tunics

VEINS- three tunics walls much thinner- not much smooth muscle or elastin- thickest layer is tunica adventitia- most of blood supply at any one time is in the veins- valves prevent backflow (folds of tunica intima)

HEMODYNAMICS

• Ejection of blood from the heart into the arterial system permits a volume containing nutrients and oxygen to flow to the periphery.

• There are factors that impede this flow and others that facilitate it. This factors influence the arterial or driving pressure as blood flows toward the tissues.

BLOOD FLOWIn considering flow through the cardiovascular system some complex factors must be considered:

• There are two pumps situated in series with different characteristics.

• The pumps produce pulsatile, not constant, flow. • Pump output (cardiac output) must be altered

constantly to meet changing demands for peripheral blood flow.

• Distribution of flow is via a viscoelastic system. • Structure of the distribution vessels is constantly

changing. • Output of the two pumps must be balanced to maintain

volume distribution in the system. • The fluid being pumped is not homogeneous (it is non-

Newtonian).

VOLUME FLOW, VELOCITY AND CROSS SECTIONAL AREA

• The volume flow throughout the cardiovascular system of an adult approximates 5.0 L/min at rest.

• The total cross-sectional area increases, the linear velocity of flow decreases:

V = Q / A, where: V = linear velocity (cm/sec), Q = volume flow (cm3/min), A = area (cm2).

• Because Q is a constant: V = 1/A, or velocity is inversely proportional to the the cross-sectional area.

VELOCITY OF FLOW

• As the vascular cross-sectional area increases (for example, moving from small arteries to capillaries) the velocity of flow falls, exchange of substrates and waste takes place in the capillaries, and velocity increases again as blood returns toward the heart.

• The velocity of flow is lowest in the capillaries, highest in the aorta (120 cm/sec during systole).

EFFECT OF RESISTANCETO BLOOD FLOW

Fluid moves through a tube in response to a pressure gradient. There are frictional forces within the fluid and between the fluid molecules and the vessel wall that tend to oppose the flow. As fluid moves through a segment of tube, there is a fall in pressure due to the loss of energy attributable to these frictional forces.

EFFECT OF RESISTANCE TO BLOOD FLOW

Increased tube length or decreased tube radius also increases the frictional forces or resistance that must be overcome to maintain flow.Thus the Resistance to flow (R) is determined by: a) tube geometry b) nature of the fluid

(viscosity)

Resistance in the CVS

• Because the length of vessels (L) and the viscosity of blood (V) in the cardiovascular system are relatively constant, the resistance (R) depends primarily on the radius of the vessels.

• There is a small pressure drop as blood moves from vessels near the heart toward the periphery, until blood reaches vessels the size of arterioles.

RESISTANCE IN SERIESAND IN PARALLEL

• Various types of vessels are arranged in series with one another. Thus, the aorta, femoral artery, and arterioles of skeletal muscle in the leg are in series.

• Each category of vessel, such as all arterioles and all capillaries, are arranged in parallel with one another.

VISCOSITY AND RESISTANCE

• According to Poiseuille's equation, increased viscosity increases resistance and tends to impede the flow of blood.

• The blood has a viscosity that can change depending on several factors, which include the following:

- Hematocrit (Hct)

- Velocity of Flow

BLOOD FLOW AND RESISTANCE

PERIPHERAL RESISTANCE (PR)- opposition to flow, determined by:

• blood viscosity - increased viscosity = increased resistance

• blood vessel length -• longer = more resistance• blood vessel diameter

resistance varies inversely

with the radius  

BLOOD PRESSURE

BLOOD PRESSURE - force per unit area exerted on the blood vessel wall by the blood (measured in mmHg)

SYSTEMIC BLOOD PRESSURE:– fluid driven by a pump - pressure highest

closest to pump– heart beat initiates blood flow - pressure is

caused by resistance

BLOOD PRESSURE• Arterial blood pressure- systolic BP - peak arterial pressure 120

mmHg (after semilunar valve closes, pressure drops as blood flows down pressure gradient)

- diastolic BP - lowest pressure at aorta 70-80mm Hg (peripheral resistance dissipates most of kinetic energy in vessels)

- mean BP - average arterial pressure

• Capillary pressure - 20-40mm Hg • Venous pressure - 20mmHg

CAPILLARY FUNCTION

Fluid and solute exchange between blood and the interstitium occurs almost exclusively in capillaries, so it is at this level that the cardiovascular system fulfils its primary function: the support of cellular metabolism.

ORGANIZATION OF THE MICROCIRCULATION

Arterioles branch into smaller metarterioles, each of which supplies a number of capillary vessels. The opening to each capillary is surrounded by circular smooth muscle forming a precapillary sphincter. These sphincters contract and relax spontaneously, and this activity leads to continuous changes in blood flow through a single capillary. Mean flow and pressure within an entire capillary bed remain fairly constant. Shunt vessels can also be opened, diverting blood away from adjacent capillaries.

CAPILLARY STRUCTURE

The capillary wall contains no smooth muscle and consists of a single layer of endothelial cells surrounded by a basement membrane. There are potential spaces (intercellular clefts) between adjacent cells. In some tissues these gaps are effectively closed, reducing the permeability of the capillary wall to plasma solutes. This is seen in the brain (blood-brain barrier). In the kidney, there are holes, or fenestrations, through the endothelial cells themselves. Fenestrated capillaries offer much less resistance to fluid exchange across the capillary wall than normal.

CAPILLARY EXCHANGE MECHANISMS - DIFFUSION

The direction and rate of diffusion for any molecule or ion depends on:1. Transcapillary concentration gradients provide the driving force for diffusion. Glucose and O2 are more concentrated in plasma than in the interstitium, and they diffuse out of the capillary, while waste products like CO2, which tend to accumulate around metabolizing cells, diffuse in.2. Capillary permeability dictates the rate of diffusion under any given concentration conditions. Lipid-soluble substances can rapidly diffuse across the endothelial cells themselves (e.g. O2 and CO2). Polar substances, such as ions and glucose, are lipid insoluble but they also cross the capillary wall through the intercellular clefts (large polar molecules, like the plasma proteins, cannot easily escape from the capillary).

FLUID FILTRATION AND ABSORPTION

• Pressure gradients across the capillary wall lead to bulk flow of fluid, in which water and the small ions and molecules dissolved in it are driven across the capillary wall. The following factors are important in determining the resulting rates of capillary filtration or absorption.

• The hydrostatic pressure gradient is the difference between capillary pressure and interstitial fluid pressure. The hydrostatic gradient acts out of the capillary, favouring filtration.

FLUID FILTRATION AND ABSORPTION

• The osmotic pressure gradient is generated because the capillary wall acts as a semipermeable membrane with respect to plasma proteins, i.e. water can cross the capillary but proteins cannot. Protein concentration, which determines the osmotic effect, is higher in plasma than in interstitial fluid, so the osmotic gradient favours absorption of fluid into the capillary. The osmotic pressure generated by the plasma proteins is called the oncotic pressure of plasma.

• The net filtration pressure gradient determines the direction of fluid transfer at any given point along the capillary. As blood travel from the arteriolar to the venular end, capillary pressure falls from about 30 mmHg to about 10 mmHg. This decline represents the pressure gradient necessary to drive blood flow through the capillary resistance.

FLUID FILTRATION AND ABSORPTION

• Interstitial fluid pressure is subatmospheric in many tissues (about -2 mmHg), i.e. it actually increases the hydrostatic gradient across the capillary wall.

• Therefore, the hydrostatic gradient acting out of the capillary, falls from 32 mmHg at the arteriolar end to about 12 mmHg at the venular end.

• The colloid osmotic gradient is constant at about 20 mmHg along the length of the capillary, since the concentrations of protein in the plasma and interstitium do not change.

• This gradient reflects the difference between the colloid osmotic pressure of plasma (25 mmHg) and that of the interstitial fluid (5 mmHg).

Pressure relations in the capillaries

PA = blood pressure at the arteriole end of the capillary.

PV = blood pressure at the venule end of the capillary.(the horizontal line represents the osmotic pressure of the blood)

• When the blood pressure is greater than the osmotic pressure, filtration of interstitial fluid occurs (downward-pointing arrows).

• When the blood pressure is less than the osmotic pressure, reabsorption of interstitial fluid occurs (up arrows).

Pressure relations in the capillaries

a) The normal situation. Filtration and absorption are balance.

b) Result of dilating the arterioles. PA increases and the tissue space becomes engorged with interstitial fluid.

c) Result of constricting the arterioles. PA decreases and interstitial fluid is withdrawn from the tissue space.

d) Result of a lowered concentration of protein in the blood (such as occurs during prolonged malnutrition). Because of the reduced osmotic pressure (lower horizontal line), fluid accumulates in the tissue spaces resulting in edema.

CLINICAL NOTE: OEDEMA

OEDEMA is swelling caused by an accumulation of interstitial fluid and is a common finding in a range of clinical conditions. It is best understood in terms of the factors which control transcapillary fluid exchange

CLINICAL NOTE: OEDEMA

The main causes of OEDEMA are:1. Increases in the hydrostatic gradient increase the rate of capillary filtration. This occurs whenever capillary pressure increases, e.g. because of a rise in venous pressure. This may result from prolonged standing, heart failure or venous obstruction by a clot or tumour. Reductions in interstitial fluid pressure also increase filtration; this can occur during prolonged air flights since cabin pressure is less than atmospheric pressure at sea level. Swelling of the feet often results.

CLINICAL NOTE: OEDEMA

2. Decreases in the osmotic gradient reduce absorption, increasing net filtration. This commonly results from a low plasma protein concentration, e.g. because of liver failure, renal disease. If the capillary permeability to protein rises, this also reduces the effective osmotic gradient and this helps account for the oedema seen in inflammatory responses to infection or trauma.3. Lymphatic obstruction prevents both fluid and protein from being cleared from the tissues. The resulting swelling is referred to as lymphoedema.