Cardiovascular System – Blood Vessels & Circulation (BIOL 2252 • Ch 20)

Overview of Blood Vessels

• Blood vessels form a dynamic delivery system that begins and ends at the heart.

• They work closely with the lymphatic system to circulate body fluids.

• There are three main types of blood vessels:

  • Arteries: Carry blood away from the heart, typically towards capillaries.

    • Most carry oxygen-rich blood, except for the pulmonary arteries (to lungs) and fetal umbilical arteries.

  • Capillaries: Microscopic, very thin-walled vessels where blood directly touches tissue cells. This is the main site for exchanging gases, nutrients, and waste products.

  • Veins: Carry blood back toward the heart.

    • Most carry oxygen-poor blood, except for the pulmonary veins (from lungs) and fetal umbilical veins.

Structure of Blood Vessel Walls

• All blood vessels (except capillaries) have a central open space called a lumen where blood flows, surrounded by several layers called tunics.

• Tunica intima (innermost layer):

  • Made of a smooth lining (endothelium) similar to the heart's inner lining.

  • Its slick surface reduces friction, allowing blood to flow easily.

• Tunica media (middle layer):

  • Contains circular smooth muscle and elastic fibers.

  • It also has sympathetic nerves that control muscle contraction.

  • Vasoconstriction occurs when the muscle contracts, narrowing the lumen.

  • Vasodilation occurs when the muscle relaxes, widening the lumen.

  • This layer is crucial for maintaining blood pressure (BP) and regulating blood flow.

• Tunica externa (adventitia) (outermost layer):

  • Made of loose connective tissue with elastic and collagen fibers.

  • It anchors the vessel to surrounding structures and protects it.

  • Large vessels in this layer contain tiny blood vessels called vasa vasorum ("vessels to vessels") that nourish the walls of the large vessels themselves.

Comparison of Vessel Types

• Arteries vs. Veins (called "companion vessels" because they often run together):

  • Arteries: Have a thicker tunica media and a narrower lumen (central opening). They contain more elastic and collagen fibers, making them strong and resilient, so they maintain their shape even when empty.

  • Veins: Have a thicker tunica externa and a larger lumen. They have less elastic tissue, so they tend to collapse when empty. Veins also have valves (folds of the tunica intima with elastic/collagen fibers) that prevent blood from flowing backward, especially in the limbs where gravity can be an issue.

• Capillaries: Consist only of the tunica intima (endothelium plus a basement membrane). This thinness allows for the fastest and most efficient exchange of substances between blood and tissues.

• **Key Blood Pressure Ranges:

  • Arterial BP: From about 100\,\text{mm Hg} as it leaves the heart to about 40\,\text{mm Hg} at the capillary end.

  • Venous BP: From about 20\,\text{mm Hg} to nearly 0\,\text{mm Hg} as it returns to the heart.

Arteries: Types & Characteristics

• As arteries branch further away from the heart, their lumen diameter and amount of elastic fibers generally decrease, while their relative amount of smooth muscle increases.

1. Elastic (Conducting) Arteries:

   • These are the largest arteries, like the aorta and its major branches.

   • They have a large lumen and abundant elastin in all three tunics.

   • They stretch and recoil with each heartbeat, helping to smoothly propel blood onward.

   • They are not very active in vasoconstriction.

2. Muscular (Distributing) Arteries:

   • Ranging from about 1\,\text{cm} down to 0.3\,\text{mm} in diameter.

   • They have the thickest tunica media layer and distinct internal & external elastic laminae (sheets of elastic tissue).

   • These arteries are very active in vasoconstriction and vasodilation, which controls blood distribution to different organs.

   • Most named arteries (like the brachial artery in your arm, or coronary arteries supplying the heart) are muscular arteries.

3. Arterioles (Resistance Arteries):

   • Smallest arteries, with diameters from about 0.3\,\text{mm} down to 10\,\mu\text{m}.

   • Larger arterioles have all three tunics, while smaller ones may just have a single layer of smooth muscle and endothelium.

   • They exhibit a baseline level of contraction called vasomotor tone, which is regulated by the vasomotor center in the brain (medulla). Arterioles are crucial for controlling systemic blood pressure.

Clinical Conditions – Arteries

• Atherosclerosis:

  • Involves the formation of atheroma (plaque) within the tunica intima, causing it to thicken and the lumen to narrow.

  • Risk factors include high cholesterol (hypercholesterolemia), smoking, and high blood pressure (hypertension). It's more common in males than females.

  • Treatments can include angioplasty (widening the vessel) or coronary bypass surgery.

• Aneurysm:

  • A localized weakening of an arterial wall, leading to a bulge.

  • There's a significant risk of rupture, which can cause massive, life-threatening bleeding.

  • Commonly occurs in the aorta and cerebral arteries in the brain. Incidence increases with age due to loss of elastin in vessel walls.

Capillaries: Structure & Types

• Capillaries are tiny, about 1\,\text{mm} long, with a diameter of 8–10\,\mu\text{m}, just wide enough for red blood cells to pass through in a single file line (called a rouleau).

• Their primary function is to exchange gases (like oxygen and carbon dioxide), nutrients, wastes, and hormones between the blood and tissue cells.

• There are three structural types of capillaries, differing in their permeability:

1. Continuous Capillaries:

   • Have a complete endothelial lining with tight junctions between cells.

   • They have small gaps called intercellular clefts that allow small solutes (like glucose) to pass through.

   • Found in most tissues, including muscle, skin, the central nervous system (CNS), and lungs.

2. Fenestrated Capillaries:

   • Have endothelial cells with pores (fenestrations) about 100\,\text{nm} in size.

   • These pores increase permeability, allowing larger molecules like plasma proteins to pass through.

   • Located in areas where significant fluid transport occurs, such as intestinal villi (for absorption), ciliary processes of the eye, kidneys (for filtration), and endocrine glands (for hormone release).

3. Sinusoids (Discontinuous Capillaries):

   • Have an incomplete endothelial lining and basement membrane, with large openings.

   • These large gaps allow formed elements of blood (like blood cells) and very large proteins to easily pass in and out.

   • Found in organs like red bone marrow (where blood cells are made), spleen (for blood cleansing), liver (for processing blood), and some endocrine glands.

Capillary Beds & Perfusion

• A capillary bed is a network of capillaries where exchange occurs. Blood flows from an arteriole into a metarteriole, which then leads to a thoroughfare channel, and finally to a post-capillary venule.

• True capillaries, where the actual exchange happens, branch off the metarteriole.

• Blood flow into true capillaries is controlled by precapillary sphincters, which are rings of smooth muscle.

  • When sphincters are relaxed, blood flows through the true capillaries, allowing perfusion (blood entering the capillary bed).

  • When sphincters are contracted, blood bypasses the true capillaries and flows directly through the thoroughfare channel.

• Perfusion is the volume of blood entering a capillary bed per unit of time. Not all capillary beds are perfused simultaneously; the body directs blood flow where it's most needed.

Capillary Exchange Mechanisms

• Substances move between blood and tissues through capillaries using several mechanisms:

  • Diffusion: Movement of substances down their concentration gradients (from high to low concentration).

    • Lipid-soluble substances (like oxygen, O2, and carbon dioxide, CO2) can cross directly through the capillary cell membranes.

    • Water-soluble substances (like glucose, ions) pass through the small intercellular clefts or fenestrations.

  • Vesicular Transport (Transcytosis): Large molecules (like certain hormones or fatty acids) are transported across the endothelial cells within tiny sacs called pinocytotic vesicles.

Bulk Flow (Starling's Law of the Capillaries)

• This refers to the mass movement of fluid into or out of the capillary, driven by pressure differences. This overall process is known as Starling's Law of the Capillaries and explains fluid exchange at the capillary level.

• Filtration: Fluid moves out of the blood vessel at the arterial end of the capillary.

• Reabsorption: Fluid moves back into the blood vessel at the venous end of the capillary.

• These movements are governed by two main opposing forces:

  • Hydrostatic Pressure (HP): A "pushing" force.

    • Blood Hydrostatic Pressure (HP_b): The pressure exerted by blood within the capillaries. It's higher at the arterial end (35\,\text{mm Hg}) and lower at the venous end (17\,\text{mm Hg}).

    • Interstitial Fluid Hydrostatic Pressure (HP_{if}): The pressure of fluid outside the capillaries in the tissue spaces. It's usually very low, approximately 0\,\text{mm Hg}, because the lymphatic system continuously drains excess fluid.

  • Colloid Osmotic Pressure (COP): A "pulling" force, mainly due to the presence of large proteins that cannot easily cross the capillary walls.

    • Blood Colloid Osmotic Pressure (COP_b): The "pull" exerted by proteins in the blood plasma, typically about 26\,\text{mm Hg}. This tends to draw fluid into the capillaries.

    • Interstitial Fluid Colloid Osmotic Pressure (COP_{if}): The "pull" exerted by proteins in the interstitial fluid. It's very low, about 1\,\text{mm Hg}, as very few proteins are normally present in the interstitial fluid.

• Net Filtration Pressure (NFP): Determines the net movement of fluid. NFP = (HPb - HP{if}) - (COPb - COP{if})

  • At the arterial end: NFP \approx (35 - 0) - (26 - 1) = 35 - 25 = +10\,\text{mm Hg}. A positive NFP means net filtration (fluid moves out).

  • At the venous end: NFP \approx (17 - 0) - (26 - 1) = 17 - 25 = -8\,\text{mm Hg}. A negative NFP means net reabsorption (fluid moves in).

• On average, about 20\,\text{L} of fluid exits the capillaries daily, and about 17\,\text{L} is reabsorbed. The remaining 3\text{ to }4\,\text{L} of excess fluid in the interstitial space is returned to the blood circulation by the lymphatic system.

Lymphatic System Connection

• The lymphatic system picks up this excess interstitial fluid (now called lymph) and returns it to the venous circulation, specifically into the subclavian veins near the heart.

• This system is vital for maintaining fluid volume balance and for immunity. (Will elaborate on this in a dedicated Lymphatic System section).

Local Blood Flow Regulation

• Blood flow to specific tissues is carefully controlled by:

  • Vascularity (Angiogenesis/Regression): Tissues with high metabolic rates (like the heart, brain, and active skeletal muscles) have a dense network of capillaries (high vascularity), formed through angiogenesis (new vessel growth). If a tissue's metabolic needs decrease, some vessels might regress.

  • Myogenic Response: The smooth muscle in blood vessel walls automatically responds to changes in stretch. If blood pressure stretches the vessel, it constricts to reduce flow. If pressure drops and stretch decreases, it dilates. This helps keep blood flow to a tissue constant despite systemic blood pressure changes.

  • Local Vasoactive Chemicals: Chemicals produced by tissues or blood cells can cause local changes in vessel diameter.

    • Vasodilators (widen vessels, increase flow): Low oxygen (O2), high carbon dioxide (CO2), high hydrogen ions (H^+), high potassium ions (K^+), lactate (from anaerobic metabolism), nitric oxide (NO), histamine, bradykinin, atrial natriuretic peptide (ANP), and epinephrine (adrenaline) acting on beta-2 receptors (\beta_2).

    • Vasoconstrictors (narrow vessels, decrease flow): High oxygen (O2), endothelin, thromboxane, leukotrienes, Angiotensin II (Ang II), antidiuretic hormone (ADH), norepinephrine (NE) acting on alpha-1 receptors (\alpha1), and cold temperatures.

  • Reactive Hyperemia: A temporary increase in blood flow to a tissue that occurs after a period of inadequate blood supply (occlusion) is removed.

Hemodynamics: Flow, Pressure, Resistance

• Blood flow (F): The volume of blood moving through a vessel or organ in a given time (e.g., in mL per minute, mL min⁻¹). For the entire systemic circulation, total blood flow equals cardiac output (CO).

• Blood Pressure (BP): The force exerted by blood against the inner walls of blood vessels, measured in millimeters of mercury (mm Hg). Blood flows from an area of higher pressure to an area of lower pressure, creating a pressure gradient that drives flow.

• Resistance (R) (also called Total Peripheral Resistance for the entire systemic circulation): The opposition to blood flow, primarily caused by friction between blood and vessel walls. The main sources of resistance are:

  1. Blood Viscosity: The "thickness" or stickiness of blood. Higher hematocrit (more red blood cells) increases viscosity and thus increases resistance.

  2. Vessel Length: Longer blood vessels offer more resistance. For example, obese individuals have longer vessel networks, which increases resistance.

  3. Vessel Radius (Diameter): This is by far the most influential factor. Even small changes in vessel radius have a dramatic effect on resistance, as described by Poiseuille's relationship: F \propto r^4 (Flow is proportional to the fourth power of the vessel radius).

     • This means if the radius is halved, the resistance increases 16 times (2^4), and flow decreases significantly.

• Relationship between Flow, Pressure, and Resistance: F = \dfrac{\Delta P}{R}

  • This formula states that blood flow (F) is directly proportional to the pressure gradient (\Delta P) and inversely proportional to resistance (R).

  • Therefore, an increase in the pressure gradient (larger difference between high and low pressure) or a decrease in resistance will both lead to increased blood flow.

  • Also, Blood Pressure (BP) is directly related to Cardiac Output (CO) and Total Peripheral Resistance:
    BP \approx CO \times R

Blood Pressure Profiles

• Arterial BP (pulsatile, meaning it fluctuates with each heartbeat):

  • Systolic Blood Pressure (SBP): The peak pressure in the arteries during ventricular contraction (systole), typically around 120\,\text{mm Hg} .

  • Diastolic Blood Pressure (DBP): The lowest pressure in the arteries during ventricular relaxation (diastole), typically around 80\,\text{mm Hg}.

  • Pulse Pressure (PP): The difference between systolic and diastolic pressure. It reflects the force of the heart's contraction.
    PP = SBP - DBP (e.g., 120\,\text{mm Hg} - 80\,\text{mm Hg} = 40\,\text{mm Hg}).

  • Mean Arterial Pressure (MAP): The average pressure driving blood through the arteries over the entire cardiac cycle. This is the most important pressure value because it indicates the adequacy of tissue perfusion (blood flow to tissues).

    • Formula A: MAP = DBP + \dfrac{1}{3}(SBP-DBP)

    • Formula B: MAP = \dfrac{SBP + 2\,DBP}{3}

    • A normal MAP is approximately 93\,\text{mm Hg}. A MAP below 60\,\text{mm Hg} indicates inadequate tissue perfusion and can be life-threatening.

• Capillary BP: Starts at approximately 40\,\text{mm Hg} at the arterial end and drops to less than 20\,\text{mm Hg} at the venous end. This low pressure prevents capillary rupture but is still high enough for fluid and nutrient exchange.

• Venous BP: Very low, ranging from 20\,\text{mm Hg} down to 0\,\text{mm Hg} as it reaches the heart. It is non-pulsatile.

Venous Return Aids

• Despite low pressure, blood returns efficiently to the heart due to:

  • Large Lumen, Low Resistance: Veins have large diameters, offering minimal resistance to flow.

  • Valves: Prevent backflow, especially against gravity in limbs.

  • Skeletal Muscle Pump: Contraction of skeletal muscles surrounding veins "milks" blood toward the heart, as valves prevent backflow.

  • Respiratory Pump: During inspiration (breathing in), pressure in the chest cavity decreases, and abdominal pressure increases, pushing blood from the abdomen towards the chest. During expiration (breathing out), pressures reverse, aiding flow towards the heart.

  • Sympathetic Venoconstriction: Smooth muscle in venous walls contracts under sympathetic nervous system control, narrowing veins and pushing blood towards the heart.

Regulation of Blood Pressure

Blood pressure is tightly regulated to ensure adequate blood flow to all tissues. This involves short-term and long-term mechanisms.

Short-Term Regulation: Neural Control

• Primarily controlled by the cardiovascular center located in the medulla oblongata of the brainstem. It oversees:

  • Cardiac Centers:

    • Cardioacceleratory center: Increases heart rate (HR) and the force of heart contractions (contractility).

    • Cardioinhibitory center: Decreases heart rate.

  • Vasomotor Center: Controls the diameter of blood vessels, mainly arterioles, by maintaining sympathetic vasomotor tone (a baseline level of constriction).

• Baroreceptors: Pressure-sensitive receptors located in the carotid sinuses (in the neck, supplying the brain) and the aortic arch (main artery leaving the heart).

  • When BP increases, baroreceptors stretch more, sending signals that decrease sympathetic nervous system activity and increase parasympathetic activity. This leads to a decrease in heart rate, vasodilation (widening of vessels), and ultimately a decrease in BP.

  • When BP decreases, the reverse happens, leading to increased HR, vasoconstriction, and an increase in BP.

• Chemoreceptors: Respond to changes in blood chemistry, located in the carotid and aortic bodies (near baroreceptors).

  • They are sensitive to increased carbon dioxide (CO2), decreased pH (more acidic blood), or significantly decreased oxygen (O2).

  • When activated, they signal the cardiovascular center to increase cardiac output (CO) and cause widespread vasoconstriction, which raises BP to improve blood flow and oxygen delivery.

• Higher Brain Centers: The hypothalamus (involved in stress, temperature regulation) and limbic system (emotions) can also influence blood pressure responses, often by modifying the cardiovascular center's activity (e.g., during fright or exercise).

Short-Term Regulation: Hormonal Control

• Various hormones rapidly affect blood pressure:

  • Epinephrine (Adrenaline) and Norepinephrine (NE) (from adrenal medulla):

    • Act on beta-1 (\beta_1) receptors in the heart to increase cardiac output.

    • Act on alpha-1 (\alpha1) receptors in most blood vessels to cause vasoconstriction, raising resistance and BP. (Note: Epinephrine can also cause vasodilation via \beta2 receptors in specific areas like skeletal muscle).

  • Angiotensin II: A very potent vasoconstrictor, directly increasing peripheral resistance and BP.

  • Antidiuretic Hormone (ADH or Vasopressin): Primarily promotes water reabsorption by the kidneys, increasing blood volume. At very high concentrations, it also causes significant vasoconstriction, hence its alternative name "vasopressin."

  • Aldosterone: Promotes sodium (Na^+) and water reabsorption in the kidneys, leading to an increase in blood volume and thus BP.

  • Atrial Natriuretic Peptide (ANP) (from heart atria): Promotes vasodilation and increases the excretion of sodium and water by the kidneys, leading to a decrease in blood volume and BP. It acts antagonistically to ADH and aldosterone.

Long-Term Regulation: Renal Control (Kidneys)

• The kidneys play a crucial role in long-term blood pressure control by regulating blood volume.

• Direct Renal Mechanism:

  • When BP increases, the kidneys filter more blood (increased glomerular filtration), leading to an increase in urine output. This reduces blood volume, which in turn lowers BP.

  • When BP decreases, the kidneys filter less, leading to decreased urine output, thus conserving blood volume and increasing BP.

• Indirect Renal Mechanism (Renin-Angiotensin-Aldosterone System, RAAS):

  1. When blood pressure is low (or sympathetic stimulation occurs), the kidneys release an enzyme called renin.

  2. Renin converts a protein from the liver, angiotensinogen, into Angiotensin I (Ang I).

  3. As Ang I passes through the lung capillaries, an enzyme called Angiotensin-Converting Enzyme (ACE) converts Ang I into the active hormone Angiotensin II (Ang II).

  4. Angiotensin II has several powerful effects that raise blood pressure:

     • It is a strong vasoconstrictor, directly increasing total peripheral resistance and BP.

     • It stimulates the thirst center in the hypothalamus, encouraging water intake and increasing blood volume.

     • It stimulates the release of ADH (from the pituitary gland) and aldosterone (from the adrenal cortex), both of which lead to increased water and sodium retention by the kidneys, further increasing blood volume and BP.

Venous System: Structure & Reservoir Function

• Veins are often called capacitance vessels because they are very distensible and can hold a large volume of blood. At rest, the venous system (veins and venules) contains about 65% of the body's total blood volume.

• **Blood Distribution at Rest (Approximate):

  • Systemic veins/venules: \approx 55\%

  • Pulmonary circuit (lungs): \approx 18\%

  • Heart: \approx 12\%

  • Systemic arteries/capillaries: \approx 15\%

Clinical Correlations – Veins & BP Disorders

• Deep Vein Thrombosis (DVT): Formation of a blood clot (thrombus) in a deep vein, usually in the calf or thigh. Symptoms include calf pain and swelling. There is a serious risk of the clot breaking off and traveling to the lungs, causing a pulmonary embolism, which can be life-threatening.

• Hypertension (High Blood Pressure): Chronically elevated blood pressure, generally defined as Systolic BP greater than 140\,\text{mm Hg} and/or Diastolic BP greater than 90\,\text{mm Hg}. It's often called the "silent killer" because it typically has no early symptoms.

  • Consequences: Long-term hypertension significantly increases the risk of atherosclerosis (hardening of arteries), heart failure, kidney disease, and stroke.

• Hypotension (Low Blood Pressure): Blood pressure that is too low, generally Systolic BP less than 90\,\text{mm Hg} and/or Diastolic BP less than 60\,\text{mm Hg}.

  • Orthostatic Hypotension: A common type where a sudden drop in BP occurs when standing up from a sitting or lying position, causing dizziness or lightheadedness, as blood pools in the lower limbs.

• Circulatory Shock: A critical condition resulting from inadequate blood flow (perfusion) to meet the body's tissue needs. This leads to cell damage and organ failure. Several types include:

  • Hypovolemic Shock: Caused by severe blood or fluid loss (e.g., hemorrhage, severe dehydration).

  • Vascular Shock: Due to extreme vasodilation (widespread vessel widening) leading to a massive drop in total peripheral resistance and BP, even if blood volume is normal (e.g., anaphylactic shock, septic shock).

  • Cardiogenic Shock: Occurs when the heart is unable to pump enough blood to meet the body's needs (e.g., after a severe heart attack).

Redistribution of Blood Flow (Exercise vs Rest)

• During rest, Cardiac Output (CO) is approximately 5250\,\text{mL min}^{-1}.

• During strenuous exercise, CO can increase significantly, reaching about 17,500\,\text{mL min}^{-1} or even more.

• The distribution of blood flow changes dramatically:

  • At rest, skeletal muscles receive about 20\% of CO, but during intense exercise, this can increase to over 70\% of CO. This is achieved through:

    • Intrinsic vasodilation within the skeletal muscles themselves (due to local factors like low O2, high CO2).

    • Extrinsic vasoconstriction in areas less vital during exercise (like the digestive tract and kidneys) directed by the sympathetic nervous system, which shunts blood away from these areas towards the muscles.

• Despite widespread vasodilation in active muscles, Mean Arterial Pressure (MAP) is generally maintained or even increases during exercise due to the significant increase in cardiac output and persistent sympathetic tone in other vascular beds.

Special Circulations (Circulatory Routes)

The body has several specialized circulatory routes to meet the unique needs of different organs:

• Systemic Circulation: The vast network of vessels that carries oxygenated blood from the left side of the heart to all body tissues and returns deoxygenated blood to the right side of the heart. It operates at high pressure.

• Pulmonary Circulation: A short, low-pressure loop that carries deoxygenated blood from the right side of the heart to the lungs for oxygenation and returns oxygenated blood to the left side of the heart. Pulmonary arteries carry deoxygenated blood, and pulmonary veins carry oxygenated blood (opposite of systemic patterns).

• Coronary Circulation: A dedicated system of arteries (coronary arteries) that branch off the aorta to supply the heart muscle (myocardium) itself with oxygen and nutrients. Veins then drain the deoxygenated blood from the heart muscle.

• Cerebral Circulation (Brain): Maintains a remarkably constant blood flow of approximately 750\,\text{mL min}^{-1}, regardless of major changes in systemic BP. It is tightly autoregulated by local chemical factors, primarily the brain's response to pH and carbon dioxide (CO_2) levels, to ensure a stable supply of oxygen and glucose.

• Hepatic Portal System: A unique venous system that collects nutrient-rich (and potentially toxin-laden) blood from the digestive organs (stomach, intestines, pancreas, spleen) and delivers it to the liver via the hepatic portal vein. The liver then processes these absorbed nutrients and detoxifies harmful substances before the blood returns to the general circulation via hepatic veins, which drain into the inferior vena cava (IVC). This is an example of a portal system where blood passes through two capillary beds in series (one in the GI tract, one in the liver).

• Fetal Circulation: Adapted for life in the womb, bypassing the non-functional lungs and liver. Key features include:

  • Foramen Ovale: An opening between the right and left atria that allows most blood to bypass the pulmonary circulation. It typically closes shortly after birth, becoming the fossa ovalis.

  • Ductus Arteriosus: A vessel connecting the pulmonary trunk directly to the aorta, shunting blood away from the lungs into the systemic circulation. It closes after birth, becoming the ligamentum arteriosum.

  • Ductus Venosus: A shunt that allows a significant portion of oxygenated blood from the umbilical vein to bypass the fetal liver and go directly into the inferior vena cava (IVC). It closes after birth.

  • Umbilical Arteries: Carry deoxygenated blood and waste products from the fetus to the placenta.

  • Umbilical Vein: Carries oxygenated blood and nutrients from the placenta to the fetus.

Vascular Pathways Highlights

• Arterial System Flow: Aorta → Elastic arteries → Muscular arteries → Arterioles → Capillaries.

• Many major superficial and deep veins are paired. Veins in the limbs often have both superficial versions (like the great saphenous vein or cephalic vein) and deeper counterparts.

• Venous anastomoses (interconnections between veins) are very common, providing alternative pathways for venous drainage.

• Arterial anastomoses (interconnections between arteries) are less common but important in some areas (e.g., Circle of Willis in the brain, epigastric arteries) as they provide collateral flow, ensuring blood supply even if one vessel is blocked.

• A Portal System refers to a circulatory route where blood flows through two capillary beds in series before returning to the heart. Examples include the hepatic portal system and the hypothalamo-hypophyseal portal system (connecting the hypothalamus to the pituitary gland).

Key Formulas & Relationships (LaTeX)

• Cardiac Output (CO): The volume of blood pumped by the heart per minute.
  CO = SV \times HR (Stroke Volume x Heart Rate)

• Blood Flow (F): Directly proportional to the pressure difference and inversely proportional to resistance.
  F = \dfrac{\Delta P}{R}

• Mean Arterial Pressure (MAP): The average pressure in the arteries.
  MAP = DBP + \dfrac{1}{3}(SBP-DBP)

• Pulse Pressure (PP): The difference between systolic and diastolic pressure.
  Pulse\;Pressure = SBP - DBP

• Resistance (R): Strongly influenced by vessel radius.
  Resistance \propto \dfrac{1}{r^{4}} (Inversely proportional to the fourth power of the radius)

• Net Filtration Pressure (NFP): Determines fluid movement across capillaries (Starling's Law).
  NFP = (HPb - HP{if}) - (COPb - COP{if})

The Lymphatic System & Lymphoid Organs

The lymphatic system is a vital part of both the circulatory and immune systems, performing several key functions:

1. Fluid Recovery: It collects excess interstitial fluid (which originates from blood plasma that leaks out of capillaries) and returns it to the bloodstream. This maintains blood volume and helps prevent edema (swelling). About 3 to 4 liters of fluid are returned this way daily.

2. Immunity: It houses and transports immune cells, playing a central role in the body's defense against pathogens (bacteria, viruses), abnormal cells (like cancer cells), and foreign substances.

3. Lipid Absorption: In the small intestine, specialized lymphatic capillaries (lacteals) absorb dietary fats and fat-soluble vitamins that are too large to directly enter blood capillaries.

Origin of Lymph & Fluid Balance

• Origin of Lymph: Lymph is essentially interstitial fluid that has entered lymphatic vessels. Interstitial fluid, in turn, originates from blood plasma that filters out of capillaries due to hydrostatic pressure, becoming tissue fluid surrounding cells.

• Relationship to Tissue Fluid Balance: This continuous leakage of fluid from capillaries means that if not returned, tissues would swell, and blood volume would drop. The lymphatic system acts as an "overflow" returning this excess fluid, maintaining proper fluid balance between the blood and tissues.

• Pathway of Lymph Return:

  1. Interstitial fluid enters lymphatic capillaries.

  2. Flows into progressively larger collecting lymphatic vessels.

  3. Passes through lymph nodes (where it's cleaned and immune cells monitor it).

  4. Merges into larger lymphatic trunks.

  5. Collects into one of two large lymphatic ducts (right lymphatic duct or thoracic duct).

  6. The ducts empty into the subclavian veins in the neck, returning lymph to the systemic blood circulation.

• Factors Affecting Lymph Flow:

  • Skeletal Muscle Pump: Contractions of skeletal muscles compress lymphatic vessels, pushing lymph forward (similar to venous return).

  • Respiratory Pump: Pressure changes in the thoracic and abdominal cavities during breathing help draw lymph towards the thorax.

  • Smooth Muscle Contraction: Walls of larger lymphatic vessels contain smooth muscle that rhythmically contracts, aiding in lymph propulsion.

  • Valves: Lymphatic vessels also have valves to prevent backflow of lymph.

  • Arterial Pulsations: Pulsations of nearby arteries can also compress lymphatic vessels.

Structural Features of Lymphatic Capillaries

• Blind-Ended: Unlike blood capillaries that form a loop, lymphatic capillaries are blind-ended (closed at one end), forming mini-valves.

• Highly Permeable: They are remarkably permeable to interstitial fluid, proteins, cells, and even larger debris. This high permeability is due to:

  • Endothelial Cells: Overlap loosely, forming flap-like mini-valves that open inward when interstitial fluid pressure is high and close when lymphatic pressure is high, preventing backflow.

  • Lack of Basement Membrane: They typically lack a continuous basement membrane, further enhancing permeability.

  • Collagen Filaments: Anchor the endothelial cells to surrounding connective tissue, pulling the mini-valves open when interstitial fluid volume increases.

• Formation of Lymph: When interstitial fluid pressure is higher than inside the lymphatic capillary, the mini-valves open, allowing fluid and anything dissolved or suspended in it (including large proteins and cells) to easily enter the lymphatic capillary, forming lymph.

Lymphoid Cells

The lymphatic system is rich in immune cells, often referred to as lymphoid cells:

1. Lymphocytes: The main cells of the adaptive immune system.

   • T lymphocytes (T cells): Directly attack virus-infected cells and tumor cells, and regulate other immune cells.

   • B lymphocytes (B cells): Produce antibodies, which "mark" pathogens for destruction by other immune cells.

2. Macrophages: Phagocytize (engulf) foreign substances and activate T cells. They are derived from monocytes.

3. Dendritic Cells: Spiny-looking cells that capture antigens (foreign substances) and bring them to lymph nodes to activate T cells. They act as antigen-presenting cells.

4. Reticular Cells: Produce a network of reticular fibers (stroma) that support other cell types in lymphoid organs, forming the "scaffolding" of these tissues.

Lymphoid Organs: Structure and Function

Lymphoid organs are categorized as primary (where lymphocytes mature) or secondary (where mature lymphocytes encounter antigens).

1. Lymph Nodes:

   • Structure: Small, bean-shaped organs clustered along lymphatic vessels. Each node has a dense fibrous capsule, an outer cortex (containing B cells, T cells after maturation, and dendritic cells), and an inner medulla (containing macrophages and lymphocytes).

   • Functions:

     • Filter Lymph: Act as "filters" through which lymph must pass before returning to the blood. Macrophages within the nodes remove debris, microorganisms, and abnormal cells.

     • Immune Surveillance: Provide a site where lymphocytes (T and B cells) can be activated and mount an immune response against antigens presented by dendritic cells, thus preventing spread of infection or cancer.

2. Spleen:

   • Structure: The largest lymphoid organ, located in the upper left abdomen. It contains two main functional areas:

     • White Pulp: Rich in lymphocytes and located around central arteries, involved in immune functions.

     • Red Pulp: Contains many red blood cells and macrophages, involved in disposing of old blood cells and pathogens.

   • Functions:

     • Immune Surveillance and Response: Cleanses blood of aged cells, platelets, and debris; houses macrophages; and is a site for lymphocyte proliferation (multiplication) and immune surveillance.

     • Blood Cleansing: Removes old and damaged red blood cells and platelets from circulation.

     • Stores Platelets and Monocytes: Serves as a blood reservoir for platelets and monocytes.

     • Fetal Red Blood Cell Production: In the fetus, the spleen is a site of red blood cell formation.

3. Thymus:

   • Structure: A bilobed organ located in the mediastinum, behind the sternum. It is most active during childhood and gradually atrophies (shrinks) after puberty.

   • Function:

     • T-Lymphocyte Maturation: The primary function of the thymus is to provide an environment for the maturation of T lymphocytes. Immature T cells migrate from the bone marrow to the thymus, where they learn to distinguish between the body's own cells and foreign invaders, making them "immunocompetent."

Other Lymphoid Tissues (MALT)

These are diffuse collections of lymphoid cells and reticular fibers found in various organs, guarding potential entry points for pathogens.

• Mucosa-Associated Lymphoid Tissue (MALT): Collections of lymphoid tissue found in mucous membranes throughout the body. These strategically positioned tissues protect the body from pathogens entering via respiratory, digestive, and urogenital tracts. Examples include:

  • Tonsils: Ring of lymphoid tissue around the pharynx (throat) that trap and remove pathogens entering the throat through food or air.

  • Peyer's Patches: Clusters of lymphoid follicles located in the wall of the small intestine (ileum), which prevent bacteria from passing through the intestinal wall and into the bloodstream.

  • Appendix: A tubular offshoot of the large intestine, also containing lymphoid tissue, thought to serve a similar immune function as Peyer's patches.

  • Diffuse Lymphoid Tissue: Loosely organized collections of lymphoid cells in almost every organ, particularly abundant in the lamina propria of mucous membranes (e.g., digestive tract, respiratory tract).