Cardiovascular System Notes

Heart Function

The heart is a double pump where the atria contract simultaneously while the ventricles are relaxing. The contraction phase is systole, and the relaxation phase is diastole. The first heart sound (LUB) is due to the closing of the AV valves during ventricular systole, and the second heart sound (dub) is due to the closing of the semilunar valves during ventricular diastole.

Cardiac Cycle

The cardiac cycle begins with atrial systole (P wave on EKG) followed by isovolumetric contraction (QRS complex). The ventricles begin to contract, slamming the AV valves shut, and increasing pressure. The ejection phase starts when the semilunar valves open, and blood is ejected. Isovolumetric relaxation (T wave) occurs as the ventricles relax, causing the semilunar valves to shut. Finally, the filling stage occurs when the atria fill and the mitral valve opens. Most of the ventricular filling is passive.

Cardiac Output (CO)

Cardiac Output = Heart Rate (HR) \times Stroke Volume (SV)

Factors that affect CO include heart rate and stroke volume. The sympathetic nervous system increases heart rate (positive chronotropic effects) by increasing SA node activity. The parasympathetic nervous system decreases heart rate (negative chronotropic effects) by increasing the hyperpolarization of the SA node.

Stroke Volume

The Frank-Starling Law states that as the amount of blood entering the heart during diastole increases, the contractility of the heart increases, and more blood is pumped from the heart. The amount of blood in the heart at the end of diastole is called the end-diastolic volume (EDV), also known as preload. Total peripheral resistance (TPR), also known as afterload, is another factor determining stroke volume. Norepinephrine and epinephrine can increase the contractility of the heart, which is called positive inotropy.

Vessel Basics

Arteries lead away from the heart and branch into arterioles, which lead into capillaries. Capillaries drain into venules, which lead to the veins leading back to the heart. The tunica externa is the outermost layer, the tunica media is primarily composed of smooth muscle tissue, and the tunica intima is the innermost layer.

Arteries

Large arteries have lots of elastic fibers for elastic recoil during systole and diastole. Small arteries and arterioles have less elastic tissue and are more muscular, contributing the most to TPR.

Capillaries

Capillaries are the site of gas and nutrient exchange. Blood flow through capillaries is controlled by precapillary sphincters and the constriction/dilation of arterioles. Continuous capillaries have no large gaps, while fenestrated capillaries have large pores.

Veins

Veins are low-pressure vessels with high compliance. Venous valves, the skeletal muscle pump, and the respiratory pump help return blood to the heart. Veins act as blood reservoirs, storing about 70% of the blood at rest.

Blood Flow

Flow = \frac{Change in Pressure}{Resistance}

Resistance = \frac{length \times viscosity}{radius^4}

Changing the radius of blood vessels is the primary way to control blood flow. Intrinsic control (myogenic and metabolic) and extrinsic control (sympathetic and parasympathetic nervous systems) regulate resistance and blood flow.

Regulation of Blood Pressure (BP)

BP = CO \times TPR

Long-term regulation of blood pressure is done through changes in blood volume, regulated by the Renin-Angiotensin system and ADH. Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II in the lungs by ACE. Angiotensin II causes vasoconstriction and the release of aldosterone, which increases Na+ and water retention.

Antidiuretic Hormone (ADH)

ADH causes vasoconstriction and water retention by the kidneys. ADH is released in response to high osmolarity.

Baroreceptor Reflex

The baroreceptor reflex is a rapid response to rapid drops in blood pressure. Pressure sensors in the aortic arch and carotid bodies sense the decrease and initiate the reflex.

Cardiovascular Shock

Cardiovascular shock is a condition of empty blood vessels and inadequate blood flow. Hypovolemic shock is due to a lack of blood volume, while vascular shock is due to systemic vasodilation.