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# Cardiovascular System ## Blood Vessels ### Arteries * Carry blood **away** from the heart * Thick, strong walls * Elastic: can stretch and recoil * Three layers: * **Endothelium**: inner layer, epithelial cells * **Smooth muscle**: middle layer, thickest layer. Allows for v...

# Cardiovascular System ## Blood Vessels ### Arteries * Carry blood **away** from the heart * Thick, strong walls * Elastic: can stretch and recoil * Three layers: * **Endothelium**: inner layer, epithelial cells * **Smooth muscle**: middle layer, thickest layer. Allows for vasoconstriction and vasodilation * **Connective tissue**: outer layer * **Arterioles**: small arteries that deliver blood to capillaries. * Have all three layers, but are much thinner than arteries * Control blood flow from arteries to capillaries via vasodilation and vasoconstriction ### Capillaries * Microscopic vessels that connect arterioles and venules * Thin walls: endothelium + basement membrane * Allow for exchange of substances between blood and interstitial fluid ### Veins * Carry blood **towards** the heart * Thinner walls than arteries, less smooth muscle and connective tissue * Not elastic * Three layers: same as arteries, but thinner * Contain **valves**: prevent backflow of blood * **Venules**: small veins that receive blood from capillaries ## Blood Flow ### Factors Affecting Blood Flow * **Blood pressure**: the force exerted by blood against the walls of blood vessels. * Blood flows from areas of high pressure to areas of low pressure * **Resistance**: opposition to blood flow * Increased resistance = decreased blood flow * Factors that affect resistance: * Blood viscosity: thickness of blood (increased viscosity = increased resistance) * Blood vessel length: longer vessel = increased resistance * Blood vessel diameter: smaller diameter = increased resistance ### Regulation of Blood Pressure * **Nervous system**: * **Baroreceptors**: pressure sensors in aorta and carotid arteries * Detect changes in blood pressure and send signals to the cardiovascular center in the brainstem * Cardiovascular center regulates heart rate, contractility, and blood vessel diameter * **Chemoreceptors**: detect changes in blood pH, $O_2$, and $CO_2$ levels in blood * Send signals to the cardiovascular center, which adjusts heart rate and blood vessel diameter * **Hormones**: * **Epinephrine and norepinephrine**: increase heart rate and contractility, vasoconstriction * **Antidiuretic hormone (ADH)**: causes kidneys to reabsorb water, increasing blood volume and pressure, vasoconstriction * **Atrial natriuretic peptide (ANP)**: causes kidneys to excrete sodium and water, decreasing blood volume and pressure, vasodilation * **Angiotensin II**: vasoconstriction, stimulates release of ADH and aldosterone ### Circulatory Routes * **Systemic circulation**: carries blood between the heart and the rest of the body * Left ventricle $\rightarrow$ aorta $\rightarrow$ arteries $\rightarrow$ arterioles $\rightarrow$ capillaries $\rightarrow$ venules $\rightarrow$ veins $\rightarrow$ superior/inferior vena cava $\rightarrow$ right atrium * **Pulmonary circulation**: carries blood between the heart and the lungs * Right ventricle $\rightarrow$ pulmonary artery $\rightarrow$ arterioles $\rightarrow$ capillaries in lungs $\rightarrow$ venules $\rightarrow$ pulmonary veins $\rightarrow$ left atrium * **Hepatic portal circulation**: carries blood from the digestive organs to the liver * Nutrient-rich blood from digestive organs flows to the hepatic portal vein, which delivers blood to the liver for processing before returning to the heart * **Fetal circulation**: exists only in the fetus * Nutrients and oxygen are supplied by the mother's blood via the placenta * Blood bypasses the fetal lungs via the foramen ovale and ductus arteriosus