Arteries vs Veins: Tunica Layers & Differences

• Arteries and veins have three layers: tunica intima, tunica media, and tunica adventitia

Tunica Intima

• The innermost layer of arteries and veins
• It consists of a thin layer of endothelial cells to promote smooth blood flow
• It directly contacts the blood

Tunica Media

• The middle layer of arteries and veins
• It is primarily made up of smooth muscle cells and elastic fibers
• It allows for vessel constriction and dilation to regulate blood pressure

Tunica Adventitia

• Outermost layer of arteries and veins
• Composed of connective tissue that anchors blood vessels to surrounding tissues
• Contains nerves and tiny blood vessels

Key Differences Between Arteries and Veins

• Arteries generally have a thicker tunica media with more smooth muscle cells compared to veins
• The thicker tunica media enables arteries to withstand higher blood pressure
• Veins tend to have a larger lumen (inner space) than arteries, due to their thinner walls

Hydrostatic Pressure

• The force a fluid applies against the walls of its container, acting as a pushing force
• Mechanism: Pressure exerted by the fluid within a blood vessel due to the volume of blood pushing against the vessel walls
• Higher hydrostatic pressure causes fluid to move out of the blood vessel and into the surrounding tissues
• The pressure is highest at the arterial end of a capillary where blood pressure is highest and pushes fluid out

Blood Osmotic Pressure

• Caused by the concentration of solutes (like proteins) in the blood
• It creates a pulling force that draws fluid back into the blood vessels
• Mechanism: Driven by the concentration of proteins (like albumin) in the blood plasma, draws water towards a higher protein concentration
• It pulls fluid back in at the venous end
• It helps to maintain proper fluid balance within the circulatory system
• It pulls fluid back into the blood vessel from the surrounding tissues

Capillary Exchange

• At the arterial end of a capillary, high hydrostatic pressure forces fluid out of the vessel
• At the venous end, lower hydrostatic pressure allows osmotic pressure to draw fluid back in

Starling's Forces

• Describes the dynamic balance between hydrostatic and osmotic pressure

• Govern the movement of fluid across the capillary wall

• High blood pressure or plasma protein levels can increase hydrostatic pressure, causing fluid leakage from capillaries

Blood Pressure Determinants

• Systolic blood pressure is impacted by the elasticity of large arteries

• Arteries can become stiffer with age, also leading to higher systolic pressure

• Diastolic blood pressure is influenced by the resistance in small arterioles, which control blood flow to capillaries

• The normal blood pressure range is around 120/80

Peripheral Resistance

• The resistance blood faces while moving through the body's blood vessels, especially in the smaller arterioles
• It works against blood flow and is important for keeping blood pressure stable
• Peripheral resistance goes up when blood vessels narrow
• It goes down when blood vessels widen

Pulse Pressure

• The difference between systolic and diastolic blood pressure
• It indicates how much force the heart is using with each beat
• Influenced by how flexible arteries are
• Stiffer arteries lead to larger pulse pressure, possibly indicating heart problems, especially with age
• Other factors include the amount of blood the heart pumps, heart rate, and the resistance in blood vessels

Hydrostatic Pressure Changes

• Pressure decreases as blood travels from the arterial side to the venous side of blood vessels and capillaries
• Pressure is highest at the start of a capillary and decreases as blood flows through
• The drop in pressure happens mainly because fluid leaves the capillary and goes into nearby tissues
• Capillary exchange where nutrients and wastes are traded between blood and tissues

Anastomosis

• Is a link between two blood vessels that lets blood move between them
• This creates backup routes if one vessel gets blocked, helping to keep blood flowing to a certain area
• Provides collateral circulation, ensuring blood supply even if one major vessel is blocked
• Can be found naturally in the body or created surgically in bypass procedures
• Important in organs like the heart, where consistent blood flow is crucial

Metarterioles

• They are tiny vessels that connect arterioles to venules, letting blood skip capillary beds when necessary
• They mainly control blood flow to certain tissues based on the body's needs
• They connect arterioles to venules, acting as a shortcut to bypass capillary beds
• Regulation is by precapillary sphincters, which control blood flow into the capillary network
• It plays a role in temperature regulation by shunting blood away from the skin surface when necessary

Precapillary Sphincters

• They are small muscle valves located at the start of capillary beds
• They open and close to regulate blood flow into the capillaries based on the needs of nearby tissues
• Influenced by oxygen and carbon dioxide levels
• Tissues needing more oxygen cause sphincters to relax and open, letting blood in
• Enough oxygen causes sphincters to tighten and close

Precapillary Sphincter Control

• They regulate blood flow into capillary beds, ensuring that tissues receive the appropriate oxygen and nutrients based on their current needs
• When surrounding tissues have low oxygen or high carbon dioxide levels, chemicals trigger relaxation of the precapillary sphincters, allowing blood flow into the capillary bed

Cardioacceleratory Centers

• Located in the brainstem to increase heart rate via the sympathetic nervous system

• Found in the medulla oblongata of the brainstem

• Stimulated by signals from the nervous system indicating a need for increased heart rate (like during exercise)

• Sends signals via the sympathetic nervous system to increase heart rate and cardiac contractility

Baroreceptors

• Located in the walls of blood vessels to detect changes in blood pressure

• They send signals to the cardiovascular center to adjust heart rate accordingly, maintaining blood pressure homeostasis

• Found primarily in the carotid sinus (at the bifurcation of the carotid artery) and aortic arch Sensitive to stretch in the arterial walls, which is related to blood pressure

• When blood pressure increases, baroreceptors fire more rapidly, sending signals to the cardiovascular center to slow down the heart rate

Continuous Capillaries

• Consist of tight junctions between endothelial cells, forming a continuous lining with minimal gaps, allowing only small molecules to pass through
• Found in muscle tissue, nervous tissue, and fat tissue
• Facilitates the exchange of small molecules like gases (oxygen and carbon dioxide) through diffusion due to its tight junctions

Fenestrated Capillaries

• Contain small pores or "fenestrae" (holes) in the endothelial cells, allowing for rapid exchange of larger molecules

• Found in the kidneys, small intestine, endocrine glands, and choroid plexus of the brain

• Enables efficient absorption of nutrients in the intestines and filtration of waste products in the kidneys

Sinusoidal Capillaries

• Have large gaps between endothelial cells, creating wide open spaces within the capillary lumen

• Found in the liver, spleen, and bone marrow

• Allows for the movement of large molecules like proteins and blood cells between the blood and tissue due to its large open spaces

Fetal Structures

• Foramen ovale: A shunt that moves blood from the right atrium to the left atrium of the heart
• Ductus arteriosus: A shunt that moves blood from the pulmonary artery to the aorta
• Ductus venosus: A shunt that moves oxygenated blood through the liver to the inferior vena cava
• Umbilical arteries: A pair of vessels that carry fetal blood to the placenta
• Umbilical vein: A vessel that carries oxygenated blood from the placenta to the fetus

Adult Structures

• Fossa ovalis: The adult structure that replaces the foramen ovale
• Ligamentum teres: The adult structure that replaces the intra-abdominal part of the umbilical vein
• Ligamentum venosum: The adult structure that replaces the ductus venosus
• Medial umbilical ligaments: The adult structure that replaces the umbilical arteries
• Superior vesicular artery: The adult structure that supplies the bladder
• Ligamentum arteriosum: The fibrous remnant that replaces the ductus arteriosus

Tetralogy of Fallot

• Heart defect with four heart problems: ventricular septal defect, overriding aorta, pulmonary stenosis, and right ventricular hypertrophy
• Causes cyanotic, or blue, skin on babies due to a lack of oxygen
• Pulmonary valve stenosis: The valve between the heart and lungs becomes narrow, making it harder for blood to flow from the heart to the lungs
• Ventricular septal defect: A gap in the lower heart chambers affecting blood flow in the heart and lungs
• Blood lacking oxygen from the lower right chamber mixes with oxygen-rich blood from the lower left chamber
• The aorta is misplaced and positioned right above the hole in the heart
• Right ventricular hypertrophy: The right lower chamber of the heart thickens due to working too hard, leading to a weak heart and heart failure

Hormones that Increase Blood Pressure

• Epinephrine (adrenaline): Released during stress, causing rapid heart rate and vasoconstriction
• Norepinephrine (noradrenaline): Released during stress, responsible for vasoconstriction
• Vasopressin (antidiuretic hormone): Promotes water reabsorption in the kidneys, increasing blood volume and pressure

Sympathetic and Parasympathetic Nervous Systems

• Sympathetic activation triggers the release of epinephrine and norepinephrine
• Parasympathetic system activation counteracts the effects of the sympathetic system by promoting vasodilation and slowing heart rate

Cardiac vs Skeletal Muscle

• The main difference lies in the long plateau phase in cardiac muscle, which is caused by calcium ions (Ca2+) entering through L-type calcium channels
• Skeletal muscle action potentials do not have this plateau phase, resulting in shorter cardiac action potentials

Electrolyte and Plateau Phase

• The key electrolyte responsible for the plateau phase in a cardiac action potential is calcium (Ca2+)

Stroke Volume

• Stroke volume is the amount of blood ejected from each ventricle

Cardiac Cycle Phases

• End Diastolic Volume (EDV): The volume of blood in the ventricle is greatest at diastole end
• End Systolic Volume (ESV): The volume of blood in the ventricle is smallest at systole end
• Atrial contraction causes a rise in pressure
• The cardiac cycle lasts 0.8 seconds, with atrial contraction occurring in the first 0.1 seconds
• The remaining 0.4 seconds is a period of total heart relaxation (quiescent period)

Explore the three layers of arteries and veins: tunica intima, media, and adventitia. Learn about their composition and function in blood flow and pressure regulation. Discover the key structural differences between arteries and veins, particularly in their tunica media and lumen size.