Basic cardiovascular anatomy and physiology

The cardiovascular system at the most basic level comprises of the heart, vasculature and blood. Circulating volume within this system – blood – deliver nutrients and oxygen while also removing waste products such as carbon dioxide.

Cardiac anatomy

Blood flow through the heart and lungs

 

Follow the arrows in the diagram below to see the blood flow through the heart

  • 1. De-oxygenated blood returns to the heart

    Venous blood returns from around the body via the vena cava, the vena cava delivers this blood into the right atrium.

  • 2. De-oxygenated blood flows from right atria to right ventricle

    During diastole blood passively flows from the atrium to the ventricles. From the right atrium the blood passes through the tricuspid valves into the right ventricle.

  • 3. De-oxygenated blood flows from right ventricle into pulmonary artery

    During systole the increased pressure from ventricular contraction causes the pulmonary valve to open and blood is ejected into the pulmonary arteries, blood then passes through the lungs to undergo gas exchange.

  • 4. Oxygenated blood returns from lungs into left atrium

    Oxygenated blood returns from the lungs via the four pulmonary veins into the left atrium

  • 5. Oxygenated blood flows from left atrium to left ventricle

    During diastole this blood flows passively from the left atrium to left ventricle through the mitral valve.

  • 6. Oxygenate blood flows from left ventricle into the aorta and out of the heart

    Left ventricular contraction causes the aortic valve to open and blood is ejected into the aorta, from there it travels through arterial circulation to the end organs and tissues

Cardiac Output:

Cardiac output (CO) is defined as the volume of blood pumped from the heart every minute. This can be calculated by knowing the stroke volume (SV), which is the volume of blood ejected from the left ventricle every beat, and the heart rate (HR).

CO = SV x HR

The stroke volume can be affected by several factors:

  • Preload

    Preload reflects the amount of blood returning to the right side of the heart. It is the filling pressure. Pre-load affects heart contractility and cardiac output/ stroke volume. Frank Starling’s law of the heart demonstrated that an increase in ventricular filling promotes an increase in pressure/ force of contraction, up to a certain point. Once completely full, the heart contractility will not increase further.

  • Contractility

    This is the force with which the myocardium contracts, and can be affected by changes in pre-load and afterload.

    Contractility can be reduced by diseases of the heart or cardiac disorders. Hypoxia and hypercapnia can cause myocardial dysfunction.

    Contractility can be increased by endogenous catecholamines released during times of stress response (fight or flight). Some drugs can cause positive inotropy such as adrenaline, digoxin.

  • Afterload

    This is the resistance against which the heart has to pump to eject blood from the heart during systole. High afterload will result in decreased cardiac output from lower stroke volumes.

    To create enough pressure in the arterial system it requires some resistance, this is called systemic vascular resistance.

  • Blood pressure

    Blood pressure measures the peak pressure exerted against the arterial wall during contraction of the heart (systolic BP) and relaxation of the heart (diastolic BP).