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Questions and Answers

What is the primary function of metarterioles in the circulatory system?

  • To facilitate gas exchange directly within tissues.
  • To maintain blood pressure to vital organs through blood flow shunting. (correct)
  • To reduce blood flow resistance in the arteries.
  • To increase the speed of blood flow through veins.
  • Which type of capillary is characterized by a tube of endothelial cells with intercellular clefts?

  • Sinusoids
  • Continuous capillaries (correct)
  • Fenestrated capillaries
  • Collagenous capillaries
  • Which method is considered the most important for capillary exchange?

  • Diffusion (correct)
  • Osmosis
  • Transcytosis
  • Filtration
  • What term describes the network of capillaries that originated from a single metarteriole?

    <p>Capillary bed</p> Signup and view all the answers

    What is a key feature of the blood-brain barrier that affects capillary diffusion?

    <p>Tight junctions that limit diffusion</p> Signup and view all the answers

    What is the primary function of smooth muscle cells in the tunica media?

    <p>To regulate the diameter of the lumen through contraction and relaxation</p> Signup and view all the answers

    Which layer of the blood vessel is in direct contact with the blood?

    <p>Tunica interna (intima)</p> Signup and view all the answers

    What structural feature distinguishes elastic arteries from muscular arteries?

    <p>Presence of elastic lamellae</p> Signup and view all the answers

    How do arterioles contribute to capillary flow control?

    <p>By changing their diameter through smooth muscle contraction</p> Signup and view all the answers

    What term is used to describe the union of two or more branches supplying the same body region?

    <p>Anastomosis</p> Signup and view all the answers

    What is the main role of the tunica externa in blood vessels?

    <p>To anchor blood vessels to surrounding tissues</p> Signup and view all the answers

    Which structure controls the flow of blood at the pre-capillary junction?

    <p>Sphincters</p> Signup and view all the answers

    What characteristic of arterioles allows them to effectively regulate vascular resistance?

    <p>High smooth muscle content</p> Signup and view all the answers

    Which layer of the blood vessel is in direct contact with the blood?

    <p>Tunica interna</p> Signup and view all the answers

    What is the primary function of arterioles in the vascular system?

    <p>To control blood flow to capillaries</p> Signup and view all the answers

    Which characteristic of capillaries allows for efficient substance exchange?

    <p>Very thin walls</p> Signup and view all the answers

    Which statement best describes venules?

    <p>They unite to form larger veins</p> Signup and view all the answers

    Which of the following factors does NOT influence vascular resistance?

    <p>Heart rate</p> Signup and view all the answers

    What is primarily responsible for the control of blood pressure in the vascular system?

    <p>Both neurological and endocrine processes</p> Signup and view all the answers

    Which structure acts as the smallest branch of arteries and plays a crucial role in regulating blood flow?

    <p>Arterioles</p> Signup and view all the answers

    The vascular system is best defined as what type of system?

    <p>A closed delivery system beginning and ending at the heart</p> Signup and view all the answers

    Study Notes

    Cardiovascular System - Blood Vessels and Haemodynamics

    • The cardiovascular system is a closed delivery system that begins and ends at the heart.

    • Arteries carry blood away from the heart.

    • Arterioles are smaller arteries.

    • Capillaries are branches of arterioles and have very thin walls for substance exchange.

    • Venules collect blood from capillaries.

    • 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

    • Blood hydrostatic pressure (BHP): pressure against walls, varying from 35-16 mmHg across the capillary bed.

    • Interstitial fluid hydrostatic pressure (IFHP): near zero mmHg.

    • Interstitial fluid osmotic pressure (IFOP): small (0.1-5 mmHg).

    • Blood colloid osmotic pressure (BCOP): high (26 mmHg) due to large plasma proteins.

    • Net Filtration Pressure (NFP): the driving force for bulk flow.

    • NFP calculation and results at arterial and venous ends of capillaries, emphasizing filtration at arterial end and reabsorption at venous end.

    • Starling's Law: nearly 85% of filtered fluid is reabsorbed, while 3 liters enter lymphatic system.

    • 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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