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Cardiovascular system Summary
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Physiology of the cardiovascular system
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Summary of the cardiovascular system lecturesThe main function of the cardio-vascular system is the transport of useful substances (O2, nutrients)
from their sources to the cells, and waste products (CO2, H2O, urea, creatinine) from cells to lungs
and kidneys. It also transports hormones, clotting factors, immune factors around the body and is
involved in thermoregulation by distribution of warm core blood. Blood consist of plasma: water
with all soluble elements, red blood cells (erythrocytes) specialised to carry O2, and white blood
cells that are responsible for the immune response.
The main cardiovascular system is one long continuous system of tubes, with two pumps (fused into
one heart) providing pressure to keep blood flowing and capillary beds where exchange of molecules
takes place.
On its way around the complete circuit a blood cell, coming from the vena cava enters the right
atrium and moves through the tricuspid valve into the right ventricle during the refill phase
(diastole). When the ventricle contracts in the squeeze-out phase (systole) and the pressure
increases rapidly, this shuts the tricuspid valve close and opens the pulmonary (semi-lunar) valve,
ejecting the blood via the pulmonary artery into the pulmonary circulation. The pulmonary artery
splits many times, eventually into pulmonary arterioles that feed pulmonary capillaries that
exchange gasses by diffusion down their concentration gradients. Oxygen from air into the blood,
carbon dioxide from the blood into the air. Onwards moving through tiny venules joining into
smaller veins into the large pulmonary veins that empty into the left atrium. When the left ventricle
relaxes in diastole it is filled by blood flowing through the bicuspid or mitral valve. After the atrium
squeezes the last blood into the ventricle, the left ventricle contracts, increasing the pressure, which
closes the bicuspid valve and opens the aortic (semilunar) valve to the aorta, the start of the
systemic circulation. From there our blood cell is forced down the arteries splitting into arterioles
that allow flow into capillaries, where gasses, nutrients and waste products are exchanged down
their diffusion gradients (e.g. O2 from blood into tissue, CO2 from tissue into blood). Leaving the
capillary, blood flows through venules and then veins back into the vena cava: Full circle.
The interior of all vessels is lined by a single layer of flat, pancake-like endothelium cells, making
diffusing of molecules easy, where it is the only layer like in capillaries. In addition to the
endothelium, arteries and arterioles have smooth muscles that can be contracted by activation of
nerves of the autonomic nervous system (ANS), causing vaso-constriction, or, in absence of ANS
nerve activity: vaso-dilation.
The ventricles pump like a bellows pump with an one-way inlet vale and an outlet valve. The
myocardium (heart muscle) contracts spontaneously at a low rate, but can contract at a higher heart
rate (HR) in response to an electrical signal starting in the sino-atrial (SA) node in the top of the right
atrium. The SA node acts as a pace maker: it is rhythmically active and triggers the contraction of
both atria. When electrical activity reaches the atrio-ventricular (AV) node the activity does not
spread to the top of the ventricles, but goes quickly through the bundle of His and then spreads
through the Purkinje fibres to reach every part of the ventricles that then contract in systole. Then
the myocardium relaxes in wait for the next electrical signal and contraction triggered by the SA
node. This sinus rhythm rate or HR is controlled by the ANS that can increase or decrease it.
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