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Questions and Answers
What is the primary function of metarterioles in the circulatory system?
What is the primary function of metarterioles in the circulatory system?
Which type of capillary is characterized by a tube of endothelial cells with intercellular clefts?
Which type of capillary is characterized by a tube of endothelial cells with intercellular clefts?
Which method is considered the most important for capillary exchange?
Which method is considered the most important for capillary exchange?
What term describes the network of capillaries that originated from a single metarteriole?
What term describes the network of capillaries that originated from a single metarteriole?
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What is a key feature of the blood-brain barrier that affects capillary diffusion?
What is a key feature of the blood-brain barrier that affects capillary diffusion?
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What is the primary function of smooth muscle cells in the tunica media?
What is the primary function of smooth muscle cells in the tunica media?
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Which layer of the blood vessel is in direct contact with the blood?
Which layer of the blood vessel is in direct contact with the blood?
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What structural feature distinguishes elastic arteries from muscular arteries?
What structural feature distinguishes elastic arteries from muscular arteries?
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How do arterioles contribute to capillary flow control?
How do arterioles contribute to capillary flow control?
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What term is used to describe the union of two or more branches supplying the same body region?
What term is used to describe the union of two or more branches supplying the same body region?
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What is the main role of the tunica externa in blood vessels?
What is the main role of the tunica externa in blood vessels?
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Which structure controls the flow of blood at the pre-capillary junction?
Which structure controls the flow of blood at the pre-capillary junction?
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What characteristic of arterioles allows them to effectively regulate vascular resistance?
What characteristic of arterioles allows them to effectively regulate vascular resistance?
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Which layer of the blood vessel is in direct contact with the blood?
Which layer of the blood vessel is in direct contact with the blood?
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What is the primary function of arterioles in the vascular system?
What is the primary function of arterioles in the vascular system?
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Which characteristic of capillaries allows for efficient substance exchange?
Which characteristic of capillaries allows for efficient substance exchange?
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Which statement best describes venules?
Which statement best describes venules?
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Which of the following factors does NOT influence vascular resistance?
Which of the following factors does NOT influence vascular resistance?
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What is primarily responsible for the control of blood pressure in the vascular system?
What is primarily responsible for the control of blood pressure in the vascular system?
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Which structure acts as the smallest branch of arteries and plays a crucial role in regulating blood flow?
Which structure acts as the smallest branch of arteries and plays a crucial role in regulating blood flow?
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The vascular system is best defined as what type of system?
The vascular system is best defined as what type of system?
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Study Notes
Cardiovascular System - Blood Vessels and Haemodynamics
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The cardiovascular system is a closed delivery system that begins and ends at the heart.
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Arteries carry blood away from the heart.
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Arterioles are smaller arteries.
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Capillaries are branches of arterioles and have very thin walls for substance exchange.
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Venules collect blood from capillaries.
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Veins carry blood towards the heart.
Outcomes
- Describe the anatomy of arteries, veins, and capillaries and factors affecting them.
- Describe the process of exchange in capillaries.
- Understand the flow mechanics of arterioles, capillaries, and venules.
- Explain physiological processes controlling blood pressure (neurological and endocrine).
Overview of the Vascular System
- Cardiovascular system delivers blood to tissues.
Overview of Systemic Circulation
- Arteries carry oxygenated blood away from the heart.
- Arterioles are smaller arteries.
- Capillaries are branches of arterioles in which exchange occurs.
- Venules collect blood from capillaries.
- Veins carry deoxygenated blood towards the heart.
Anatomy of Blood Vessels
- Arteries are large and elastic, carrying blood away from the heart.
- Arterioles are smaller, branching arteries.
- Capillaries are thin-walled branches of arterioles, facilitating exchange.
- Venules collect blood from capillaries.
- Veins carry blood back to the heart.
Basic Structure
- Tunica interna (intima) is the innermost layer, coming in contact with blood.
- Tunica media is the middle layer with muscular tissue.
- Tunica externa (adventitia) is the outer layer with connective tissue.
3 Layers of Blood Vessels
- Tunica interna (intima): innermost layer, direct contact with blood flowing through the lumen (opening). It includes endothelium, basement membrane, and internal elastic lamina.
- Tunica media: a thick layer of muscle and connective tissue, composed of smooth muscle cells and external elastic lamina. Smooth muscle cells regulate the vessel diameter (vasoconstriction and vasodilation).
- Tunica externa (adventitia): outermost layer, composed of elastic and collagen fibres, containing nerves and vasa vasorum (vessels of vessels). It anchors the vessel to surrounding tissues.
Histology Section of Blood Vessels
- Muscular arteries and large veins are shown with labeled components: adventitia, media, artery lumen, vein lumen, nerve, and adipose tissue.
Arteries (3 layers)
- Thicker tunica media compared to veins (more muscle).
- High compliance vasoregulation: elastic aorta and arteries stretch during ventricular contraction and recoil during ventricular relaxation, allowing for efficient blood flow.
Elastic Arteries (Conducting Arteries)
- Largest diameter with thin walls.
- Thick tunica media with elastic fibres (lamellae).
- Help push blood onward.
- Accommodate blood volume ejected by ventricles.
- Store elastic energy, converting it to kinetic energy to propel blood.
Muscular Arteries (Distributing Arteries)
- Medium size with more smooth muscle cells to regulate blood flow.
- Branch to distribute blood to various organs.
- Tunica externa thick with elastic fibres allowing for changes in diameter (but cannot recoil).
- Anastomoses: unions of blood vessels providing alternate routes for blood flow.
Arterioles (Resistance Vessels)
- Control blood flow to capillary beds.
- Thin tunica interna.
- Metarterioles: ends of arterioles that lead to capillaries.
- Pre-capillary sphincters: regulate blood flow within the capillary bed.
- Vasomotion: blood flow shunting through metarterioles to maintain blood pressure to vital organs.
Capillaries (Exchange Vessels)
- Smallest vessels (5-10µm) forming networks.
- Connect arterial and venous blood flow.
- Thin walls (no tunica media and externa): only endothelial cells and basement membrane for exchanges.
- Extensive network surrounding body cells for material exchange.
- Microcirculation: blood flow from metarterioles to venules via capillary bed or thoroughfare channel.
Types of Capillaries
- Continuous: tube of endothelial cells with intercellular clefts (gaps).
- Fenestrated: fenestrations (pores) for larger substances.
- Sinusoids: wider with large fenestrations and clefts, facilitating larger material exchange.
Capillary Exchange
- Diffusion: most important method, substances moving down concentration gradients through clefts, fenestrations or endothelial cells.
- Transcytosis: transport of larger, lipid-insoluble molecules in vesicles.
- Bulk flow: filtration and reabsorption (passive) driven by pressure gradients crucial for regulating blood and interstitial fluid volumes.
Capillary Exchange Pressures
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Blood hydrostatic pressure (BHP): pressure against walls, varying from 35-16 mmHg across the capillary bed.
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Interstitial fluid hydrostatic pressure (IFHP): near zero mmHg.
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Interstitial fluid osmotic pressure (IFOP): small (0.1-5 mmHg).
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Blood colloid osmotic pressure (BCOP): high (26 mmHg) due to large plasma proteins.
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Net Filtration Pressure (NFP): the driving force for bulk flow.
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NFP calculation and results at arterial and venous ends of capillaries, emphasizing filtration at arterial end and reabsorption at venous end.
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Starling's Law: nearly 85% of filtered fluid is reabsorbed, while 3 liters enter lymphatic system.
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Edema: filtration exceeding reabsorption, causing interstitial fluid overflow.
Venules
- Thin walls, collecting capillary blood.
- Postcapillary venules are porous, allowing exchange.
- Venules become thicker (50-200µm) with more smooth muscle.
- Elastic walls act as reservoirs.
Veins
- Thin walls (0.5-3cm).
- Tunica externa thickest with collagen and elastic fibres.
- Lack internal and external elastic laminae.
- Larger lumen for blood distension (preventing high pressure).
- More numerous than arteries.
- Superficial veins = subcutaneous layer, deep veins = between skeletal muscles.
Veins and Valves
- Capacitance vessels due to large volumes accommodated.
- Up to 65% of blood volume stored.
- Low blood pressure, small risk of bursting, structural adaptations.
- Venous valves (from tunica intima), preventing backflow.
- Large lumen = lower resistance.
Venous Return
- Mechanism for blood flow back to the heart..
- Skeletal muscle pump and respiratory pump.
Skeletal Muscle Pump
- Muscle contractions propel blood superiorly while proximal valves open and distal valves close. - Muscle relaxation further facilitates blood movement towards the heart by opening proximal valves and closing distal valves.
Respiratory Pump
- Pressure changes in thoracic and abdominal cavities during breathing.
- Inhalation: reduced thoracic pressure and increased abdominal pressure help propel venous blood.
- Exhalation: increased thoracic pressure and reduced abdominal pressure assist.
Hemodynamics and Blood Flow
- Blood flow through tissues quantified in mL/min, representing cardiac output (CO = heart rate x stroke volume).
- Blood flow distribution dependent on blood pressure.
Blood Pressure
- Blood flows from high to low pressure.
- Systolic blood pressure (highest): during ventricular contraction.
- Diastolic blood pressure (lowest): during ventricular relaxation.
- Pressure progressively decreases along blood vessel pathways; arteries > arterioles > capillaries > venules > veins.
Mean Arterial Pressure (MAP)
- Mean of pressure across cardiac cycle, vital to organ perfusion.
- Calculated as MAP = SBP + 2DBP / 3 (where SBP is systolic blood pressure and DBP is diastolic blood pressure.)
- Blood pressure depends on total blood volume, cardiac output, and vascular resistance.
Vascular Resistance
- Opposition to blood flow due to friction.
- Depends on vessel lumen size, viscosity, and total length.
Velocity of Blood Flow
- Velocity inversely related to cross-sectional area—slowest in capillaries, fastest where venules unite.
Systemic Blood Circulation
- Relationship between blood flow velocity and total cross-sectional area of blood vessels throughout the body.
Control of Blood Pressure and Blood Flow
- Interconnected negative feedback systems regulate blood pressure and blood flow (heart rate, stroke volume, systemic vascular resistance, blood volume).
Cardiovascular (CV) Center
- Regulates heart rate and stroke volume.
- Neural (baroreceptors and chemoreceptors), hormonal, and local feedback mechanisms for BP control.
Sensory Receptors (Inputs to CV Center)
- Baroreceptors: pressure receptors in large arteries; detect pressure and stretch changes.
- Chemoreceptors: detect chemical changes (O2, CO2, H+).
- Proprioceptors: detect joint and muscle movements.
Neural Regulation of Blood Pressure (BP) (Baroreceptors)
- Carotid sinus reflex: baroreceptors in the carotid arteries regulate blood pressure changes.
- Aortic reflex: baroreceptors in the aorta control changes in systemic blood pressure.
Control of Blood Pressure (BP) (Responses to BP Changes)
- BP falls: less baroreceptor stimulation → less parasympathetic/more sympathetic stimulation → increased HR, CO, and vasoconstriction → increased BP.
- BP rises: more baroreceptor stimulation → more parasympathetic/less sympathetic stimulation → decreased HR, CO, and vasodilation → decreased BP.
Chemoreceptors
- Detect chemical changes (O2, CO2, H+) in blood and influence the CV centre.
- Changes in blood chemistry trigger sympathetic stimulation to increase heart rate and vasoconstriction for higher blood pressure.
Hormonal Regulation of BP
- Renin-angiotensin-aldosterone system (RAAS): Reduced kidney blood flow triggers RAAS, causing vasoconstriction and aldosterone release leading to increased sodium and water reabsorption and thereby increased blood volume leading to increased blood pressure.
- Adrenal medulla releases epinephrine and norepinephrine in response to sympathetic stimulation, increasing CO, HR, vasoconstriction (arterioles) and venous return.
- Antidiuretic hormone (ADH) : produced by hypothalamus, controls water reabsorption from kidneys, increasing blood volume and thus blood pressure.
- Atrial natriuretic peptide (ANP): released in atria (of heart) in response to increased blood volume promoting vasodilation and decreasing sodium/water reabsorption thereby causing decreased blood volume and blood pressure.
Autoregulation
- Capacity of tissues to automatically adjust blood flow, based on needs.
- Vasodilation and vasoconstriction occur in response to local factors such as physical or chemical changes.
Resources
- Visible Body (A&P/3D atlas).
- Gray's Anatomy (for Students)
- Marieb (Human Anatomy & Physiology)
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