LF 129 14: Blood Circulation

Introduction to the Cardiovascular System

Circulation Overview

• The cardiovascular system is a dual circulation system in series:
  • Systemic circulation: Left ventricle (LV) to Right atrium (RA).
  • Pulmonary circulation: Right atrium (RA) to Left atrium (LA) to Right ventricle (RV).
• Functions:
  • Oxygenation of blood.
  • Excretion of $CO_2$.
  • Supply of $O_2$ and nutrients.
  • Removal of $CO_2$ and metabolic waste products.

Vessels and Pressure

• Capillaries: Exchange vessels.
• Different vessels accommodate different pressures:
  • High arterial pressure: ~$100$ mmHg.
  • Low venous pressure: $0 – 8$ mmHg.
• Types of arteries:
  • Elastic arteries: Smooth pulsatile flow.
  • Muscular arteries: Control resistance to flow.
• Veins: Capacitance vessels holding a reservoir of blood.
• Pump: RA -> LV.

Blood Vessel Structure

• Common theme:
  • Lumen.
  • Tunica intima (interna):
    • Endothelium.
    • Supporting connective tissue.
    • Release of paracrine signals.
  • Tunica media:
    • Elastic tissue.
    • Smooth muscle.
  • Tunica adventitia (externa):
    • Principally collagen.

Artery and Vein Specializations

• Blood vessels require specializations to cope with different pressures and functions.
• Artery:
  • External elastic membrane.
  • Smooth muscle.
  • Internal elastic membrane.
  • Lumen.
  • Endothelium.
• Vein
  • Valve.

Elastic Arteries

• Elastic arteries absorb high pressure.
• Smooth pulsatile flow.
• Store energy.
• Lumen
• Tunica media
• Elastic fibres in laminae (black)
• Smooth muscle (red)

Elastic Arteries - Windkessel Effect

• The energy stored in the wall of elastic arteries during systole is released in diastole, maintaining the blood flow at this time and smoothing it.

Rigid Arteries - Hypothetical Scenario

• If the great arteries were completely rigid:
  • Arterial pressure:
    • Ventricular pressure rises to a maximum during systole – the systolic pressure.
    • It falls to a low level during diastole.
    • So would arterial pressure if arteries were rigid.
  • If flow follows pressure it would be intermittent.
• Dicrotic notch.

Arterial Compliance and the Windkessel Effect

• Arteries are sufficiently compliant that their volume increases with pressure.
• As blood is pumped into the aorta and major arteries, they stretch.
• Thus in systole, more blood flows in than out.
• The walls of the aorta and elastic arteries recoil in diastole, maintaining blood flow. This is known as the Windkessel effect.

Elastic Arteries - Pressure and Flow

• Elastic arteries convert intermittent pressure into pulsatile flow.
• Aortic pressure rises to a maximum during systole – the systolic pressure.
• It falls to a minimum during diastole – the diastolic pressure.
• Flow follows the pressure, but never reaches zero - It is pulsatile rather than intermittent.

Pulmonary and Systemic Circulation

• Pulmonary and systemic circulation follow the same pattern at different pressure.

Muscular Arteries and Flow Control

• Muscular arteries control flow.

Blood Flow Factors

• Blood flow depends on blood vessel radius.
• Resistance will be determined by 3 factors:
  • Length of blood vessels
    • Longer blood vessels would provide greater resistance
    • The length of each vessel remains constant
  • Viscosity of blood
    • Blood with a lot of solute would provide more resistance
    • Solutes such as hemocrit, albumin, etc do not change much under normal circumstances
  • Radius of blood vessels

Small Arterioles

• Small arteries and arterioles are the site of controlled resistance to flow

Blood Flow Under Pressure

• Blood flows under pressure; the flow will be related to radius by:

$Q = (P<em>1 – P</em>2)/R$

• Where:
  • $Q$ = flow
  • $(P<em>1-P</em>2)$ = pressure difference between the two ends
  • $R$ is the resistance of the vessel
• This resistance will rise if the vessel is narrower; the flow will be reduced; and the pressure drop will be increased: $P<em>1 ↓P</em>2 ↓Q$

Laminar Blood Flow

• Blood flow is normally laminar.
• The flow is considered essentially as layers of fluid that move over each other.
• Flow is fastest at the centre and slowest – essentially stationary – at the outside.

Poiseuille's Law

• It was shown by Poiseuille that, for laminar flow:

$R = [8 \eta l] / [\pi r^4]$

• Where:
  • $R$ is resistance
  • $\eta$ is viscosity
  • $l$ is length
  • $r$ is radius
  • $\pi$ is the mathematical constant (approximately 3.14).
• Thus a reduction in radius by ½ gives an increase in resistance of 16 fold.

Turbulent Flow

• Flow becomes turbulent if velocity is high.
• The layers (laminae) of laminar flow break up and flow becomes disordered.
• In these circumstances, the resistance to flow is raised.
• Turbulent flow tends to lead to endothelial damage and hence to arterial disease.

Capillaries

• Capillaries are the site of exchange.

Microcirculation Control

• Sphincters control access to the microcirculation.
• Constrict/dilate arterioles.
• Open/close precapillary sphincters.

Capillary Structure and Function

• Capillaries are thin walled, essentially a single layer of endothelial cells (on a basement membrane).
• Their structure minimises resistance to diffusion into and from the interstitium.
• Capillaries are designed to optimise diffusion.

Diffusion Optimization

• Exchange of blood gases and metabolites.
• Generation of an equilibrium between plasma interstitial fluid.

Gas and Nutrient Exchange

• Metabolites and respiratory gases cross capillary walls by diffusion.
  $J = P \cdot A \cdot (C<em>i - C</em>o)$
• Where:
  • $P$ is the permeability coefficient
  • $A$ is the area of exchange
  • $(C<em>i-C</em>o)$ is the concentration gradient.

Permeability Factors

• Permeability is determined in part by the nature of the molecule itself.
• Lipid soluble molecules, which include $O<em>2$ and $CO</em>2$, diffuse easily through capillary cell membranes.
• Hydrophilic molecules travel through pores, via a paracellular route.
• Molecules >60kd are not transferred and many plasma proteins are retained in the circulation – important in the equilibrium between plasma and the e.c.f.

Fluid Equilibrium

• Capillaries are the site of fluid equilibrium.
• The extracellular fluid volume has two compartments:
  • 10. 5 litres interstitial fluid
  • 3 litres plasma
• There is an equilibrium between these two compartments, so far as their volumes are concerned.
• Capillaries are the site at which this equilibrium is determined.

Starling Forces

• Capillary beds as the site at which the equilibrium between plasma and interstitial fluid is established.
• Governed by ‘Starling forces’.
• Loss of fluid from the plasma, owing to hydrostatic pressure.
• Reabsorption of fluid into plasma, owing to colloid osmotic pressure or oncotic pressure.

Hydrostatic Pressure

• Equilibrium between plasma & interstitial fluid is determined by hydrostatic pressure.

Osmotic Pressure

• This is counteracted by osmotic pressure of plasma proteins.
• Large molecules (>60kd) do not cross capillary walls.
• Their osmotic pressure draws fluid back into capillaries.
• Colloid osmotic pressure or oncotic pressure.

Filtration and Re-absorption

• Filtration/reabsorption depends on the difference between hydrostatic and oncotic pressure.
• Filtration pressure = hydrostatic pressure – oncotic pressure.
• The Starling hypothesis.

Lymphatic System and Fluid Balance

• Balance is not be perfect, but it’s necessary for lymphatic sampling.
• Any excess of fluid is taken up into lymphatics and returned to the circulation.
• Larger lymphatics have valves and contract rhythmically.
• Samples the blood for foreign particles.

Lymphatic System Association

• Capillary beds are associated with the lymphatic system.

Veins as Capacitance Vessels

• Veins are capacitance vessels providing a reservoir of blood.

Blood Volume

• The pulmonary circuit does not hold any excess blood volume.
• Blood volume is approximately 5 litres: ~3.3 litres reservoir that can be recruited in time of need.
• Pulmonary circuits do not hold any fluid reserve.

Muscle Pump

• Muscle pump enhances venous return.

Muscle Pump and Orthostatic Intolerance

• Muscle pump enhances venous return
  • Orthostatic intolerance – Postural hypotension
  • Exercise
    • Venous return
    • Stroke volume
    • Cardiac output

Respiratory Pump

• Respiratory pump enhances venous return.
• Diaphragmatic movement.
• Change in PIT.
• Blood pools in lungs due to drop in pulmonary vascular pressure.
• Determine by venous and RA pressure gradient.

Lecture Summary

• The structure and function of blood vessels in the systemic circulation
• The role of elastic arteries in smoothing pulsatile flow
• The role of resistance vessels in regulating blood flow
• The role of capillaries in exchange and in the equilibrium between plasma and interstitial fluid
• The role of veins as capacitance vessels