Lecture 1

Hemodynamics

• Purpose of the cardiovascular system:

  • To deliver oxygen and nutrients to cells.

  • To remove waste products, such as carbon dioxide, from cells.

  • It is a rapid system that provides a steep concentration gradient within the vicinity of every cell.

  • This is vital for multicellular organisms because diffusion alone is too slow.

• Hemodynamics:

  • The study of blood flow.

  • Relates Ohm’s Law to fluid flow.

  • Examines the relationship between blood flow, blood pressure, and resistance to blood flow.

Ohm's Law Applied to Blood Flow

• F = \frac{\Delta P}{R}

  • F = flow

  • \Delta P = pressure difference between two fixed points (P1 and P2)

  • R = resistance to flow

• Blood flow is directly related to the pressure difference between two points and inversely proportional to resistance.

• Resistance is the friction that impedes flow, indicating how difficult it is for blood to move between two points at any given pressure.

• Blood always flows from a region of higher pressure to a region of lower pressure.

• The pressure difference between two points is the driving force for blood flow.

• For flow to occur, the pressure difference must overcome resistance to flow (\Delta P > R).

• The body can alter blood flow by changing resistance, mainly through the resistance of arterioles.

• Blood flow is determined by the pressure difference between two points, not absolute pressures.

• Hydrostatic pressure:

  • The pressure exerted by the volume of blood within the circulatory system on the walls of blood vessels.

  • Often referred to as 'pressure'.

  • It varies throughout the cardiovascular system.

• Hemodynamics is determined by the pressure difference between two relevant points, not absolute pressure at any point.

  • If there is no pressure difference, the flow will be 0 mL/min.

  • The pressure difference must be greater than the sum of all resistances to create flow.

Factors Determining Resistance to Blood Flow

• Resistance cannot be directly measured but can be calculated using the formula F = \frac{\Delta P}{R}.

• Factors include:

  • Viscosity of the blood.

  • Length of the blood vessel.

  • Diameter of the blood vessel.

• Viscosity:

  • The friction between molecules of a flowing fluid.

  • Blood contains molecules and formed elements (red blood cells, white blood cells, and platelets).

  • Interactions between these components produce friction, contributing to blood flow resistance.

  • Hematocrit (number of red blood cells) affects viscosity.

• Blood vessel length and diameter affect the contact between blood and vessel walls.

  • Friction develops between moving blood and stationary vessel walls.

  • Greater contact leads to greater friction and resistance.

  • Longer vessels produce more friction than shorter ones.

  • Vessel diameter can change via constriction or dilation.

    • Constriction reduces diameter.

    • Dilation increases diameter.

• Blood flows in concentric layers.

  • Smaller diameter vessels:

    • More blood in contact with the vessel wall.

    • Fewer concentric layers.

    • Generates more friction.

  • Larger diameter vessels:

    • Some blood contacts the vessel walls, but many layers move without contact.

    • Less friction compared to smaller vessels.

Poiseuille’s Equation

• Defines the relationship between viscosity, vessel radius, and vessel length.

• R = \frac{8Lη}{πr^4}

  • R = resistance to blood flow

  • η = blood viscosity

  • L = vessel length

  • r = vessel radius

• Resistance is directly proportional to vessel length and blood viscosity, and inversely proportional to the radius to the fourth power.

• Vessel diameter has the greatest effect on resistance because r is raised to the fourth power.

• Small changes in vessel diameter lead to large changes in resistance.

• The body can alter vessel diameter by constricting and relaxing vascular smooth muscle, changing resistance.

Cardiovascular System Functions and Components

Functions

• Deliver oxygen and nutrients; remove waste products of metabolism.

• Fast chemical signalling via circulating hormones or neurotransmitters.

• Thermoregulation

• Mediation of inflammatory and host defense responses against invading microorganisms.

Components

• Heart

• Blood vessels

• Blood

Types of Vessels

• Arteries

• Arterioles

  • Small branching vessels with high resistance

• Capillaries

  • Transport blood between small arteries and veins

  • Exchange materials between blood and body cells

• Venules

• Veins

• All arteries carry blood away from the heart.

• All veins carry blood back to the heart.

• Closed circulatory system allows for greater pressures to be generated when the heart contracts

Anatomy of the Heart

• Four chambers: two atria and two ventricles.

Atria

• Thin-walled chambers

• Low pressure chambers

• Receive blood returning to the heart.

Ventricles

• Thick-walled chambers (thicker than atria)

• Responsible for the forward propulsion of blood when they contract.

• Apex: the lowest superficial surface of the heart

• Base: the upper surface of the heart where blood vessels attach

Septa

• Muscular walls dividing the left and right sides.

  • Interatrial septum: separates left and right atria.

  • Interventricular septum: separates left and right ventricles.

• Allows the heart to function as a dual pump.

  • Left side: pumps highly oxygenated blood to the systemic circuit (body).

  • Right side: pumps poorly oxygenated blood to the pulmonary circuit (lungs).

Blood Flow

• The circulatory system is divided into two serial circuits:

  • Pulmonary circulation

  • Systemic circulation

Pulmonary Circuit

• Carries blood to and from the gas exchange surfaces of the lungs.

• Blood entering the lungs is poorly oxygenated.

• Oxygen diffuses from the lung tissues to the blood.

• Blood leaving the lungs is highly oxygenated.

Systemic Circuit

• Transports blood to and from the rest of the body.

• Blood entering the body tissues is highly oxygenated.

• Oxygen diffuses from the blood to the interstitial fluid surrounding the tissue cells.

• Blood leaving the tissues is poorly oxygenated.

• "Serial" means in sequence.

Path of Blood Flow

• Systemic circuit moves blood to and from body tissues.

• The left side of the heart receives blood from the pulmonary circulation and pumps it to the systemic circulation.

• The right side of the heart receives blood from the systemic circulation and pumps it to the pulmonary circulation.

• Blood moves from the pulmonary circuit to the heart, then to the systemic circuit before returning to the heart.

• It moves in series.

• Arteries: carry blood away from the heart.

  • Most carry highly oxygenated blood except the pulmonary trunk and pulmonary arteries, which carry poorly oxygenated blood to the lungs.

• Veins: carry blood to the heart.

  • Most carry poorly oxygenated blood back to the heart except pulmonary venules and pulmonary veins, which carry highly oxygenated blood back to the left atrium from the lungs.

Series vs. Parallel Flow

• Series blood flow is found in the cardiovascular system (pulmonary and systemic circuits).

• Parallel flow to most organs.

  • Each organ is supplied by a different artery, so its blood flow can be independently regulated.

• Exception: the liver receives blood flow in parallel and in series.

Distribution of Blood Flow

• The cardiovascular system can increase or decrease the rate of blood flow.

• It can alter the distribution of blood flow, depending on the body’s needs, by increasing blood flow to areas that need more blood and decreasing blood flow to areas that do not need as much blood at that time.

The Pericardium

• A fibrous sac surrounding the heart and the roots of the great blood vessels.

Functions of the Pericardium

• Stabilizes the heart in the thoracic cavity.

• Provides protection to the heart by physically surrounding it.

• Reduces friction as the heart beats by secreting pericardial fluid.

• Limits overfilling of the heart chambers.

Pericardial Structure

• Three-layered sac:

  • Fibrous pericardium

    • Outer layer.

    • Provides protection and stabilization in the thoracic cavity by attaching to structures in the chest.

    • Holds the heart in place.

    • Limited distensibility prevents sudden, rapid overfilling.

  • Parietal pericardium

    • Part of the serous pericardium.

    • Lies underneath the fibrous pericardium and is attached to it.

  • Visceral pericardium

    • Part of the serous pericardium.

    • Innermost layer.

    • Also called the epicardium when in contact with the heart muscle.

  • Pericardial cavity

    • Separates the parietal pericardium from the visceral pericardium.

    • Both layers secrete fluid to decrease friction during heartbeats.

    • Serous layer: composed of cells that secrete a fluid.

Cardiac Tamponade and Pericarditis

• Pericarditis: inflammation of the pericardium.

  • Caused by viruses, bacteria, fungi, trauma, or malignancy.

  • Leads to fluid accumulation in the pericardial cavity.

• Cardiac tamponade: compression of heart chambers due to excessive accumulation of pericardial fluid.

  • Limits heart's movement and prevents chambers from filling with adequate blood (decreased ventricular filling).

The Heart Wall and Cardiac Muscle Cells

Ventricular Walls

• The muscular wall of the left ventricle is thicker than the right ventricle.

  • The increased thickness allows the left ventricle to generate higher pressures as it contracts compared to the right ventricle.

  • Allows it to pump blood around the entire systemic circulatory system.

• The right ventricle only needs to pump blood to the lungs and does not need to develop pressures as great as those developed by the left ventricle.

The Heart Wall Layers

• Epicardium

  • Also called visceral pericardium.

  • Layer immediately outside the heart muscle.

  • Covers the outer surface of the heart.

  • Connective tissue attaches it to the myocardium.

  • Functions as a protective layer.

• Myocardium

  • The muscular wall of the heart.

  • Lies underneath the epicardium.

  • Contains muscle cells or myocytes, nerves, and blood vessels.

• Endocardium

  • Innermost layer of the heart wall.

  • Lines heart cavities and heart valves.

  • Thin layer of endothelium continuous with the endothelium of attached blood vessels.

• The entire circulatory system (heart chambers, heart valves, and blood vessels) is lined by endothelium which forms an interface between the blood and the heart chamber or blood vessel wall, providing a smooth surface for blood to flow over.

• Three layers (endocardium, myocardium, and epicardium) are found in both atria and both ventricles.

  • The layers do show variation between the different chambers of the heart.

  • The ventricles have a thicker myocardium than the atria; the left ventricle has a thicker myocardium than the right ventricle.

Cardiac Muscle Cells (Myocytes)

• Branched or Y-shaped cells.

• Joined longitudinally or end-to-end to adjacent myocytes allowing for greater connectivity in the heart.

• Striated or stripped appearance (actin and myosin).

• A single, centrally located nucleus.

• Rich in mitochondria (provide ATP for muscle cell contraction).

• Adjacent cells are held together by intercalated disks.

• The membranes of two different myocytes are closely opposed and very intertwined at their region of attachment.

Intercalated Disks

• Two types of specialized intercellular junctions:

  • Desmosomes

    • Adhering junctions that hold cells together in tissues subject to considerable mechanical stress or stretching.

    • Mechanically couple one heart cell to another.

    • Proteins involved: cadherins, plaques, intermediate filaments.

    • Cadherins from one cell attach to cadherins from another cell.

  • Gap junctions

    • Communicating junctions.

    • Electrically couple heart cells, allowing ions to move between cells.

    • Important for the spread of action potentials.

    • Proteins involved: connexons.

Arrangement of the Heart Muscles

• Muscle fibers are arranged spirally around the heart chambers.

• Important for emptying blood into arteries when ventricles contract.