Cardiovascular Regulation, Hemodynamics, Circulation Circuits, and Aging Adaptations

Reflex Control of Cardiovascular Function

• The cardiovascular centers in the medulla oblongata constantly monitor arterial blood through reflex pathways to maintain homeostatic blood pressure and tissue perfusion.
• Baroreceptor Reflexes:
  • Receptors consist of stretch receptors located in the elastic walls of target vessels and heart chambers:
  1. Carotid Sinuses: Located at the base of the internal carotid arteries; maintain adequate blood flow to the brain.
  2. Aortic Sinuses: Located in the walls of the ascending aorta; monitor blood pressure at the start of the systemic circuit.
  3. Right Atrium: Monitors blood pressure at the end of the systemic circuit.
  • Aortic Reflex: Triggered by blood pressure changes in the ascending aorta to adjust blood pressure and blood flow throughout the systemic circuit.
  • Baroreceptor Responses to Blood Pressure Elevation:
  • Increased stretch stimulates baroreceptors in carotid and aortic sinuses.
  • Signals to medulla oblongata activate the cardioinhibitory center while inhibiting the cardioacceleratory center and vasomotor center.
  • Effector response: Decreased cardiac output and peripheral vasodilation reduce blood pressure back to normal homeostasis.
  • Baroreceptor Responses to Blood Pressure Fall:
  • Decreased stretch inhibits baroreceptors.
  • Cardiovascular center in medulla oblongata stimulates the vasomotor center and cardioacceleratory center while inhibiting the cardioinhibitory center.
  • Effector response: Increased cardiac output and peripheral vasoconstriction elevate blood pressure back to normal homeostasis.
  • Atrial Baroreceptors & Bainbridge Reflex:
  • Atrial baroreceptors monitor pressure at the systemic circuit terminus.
  • The Bainbridge reflex (atrial reflex) responds specifically to stretching of the right atrial wall (e.g., during increased venous return) by adjusting cardiac rate.

• Chemoreceptor Reflexes:
  • Receptors consist of sensory neurons located in the carotid bodies, aortic bodies, and the ventrolateral surfaces of the medulla oblongata.
  • Respond to chemical changes in blood and cerebrospinal fluid (CSF), specifically:
  • Increased $\text{CO}_2$ levels (hypercapnia).
  • Decreased $pH$ (acidosis).
  • Decreased $\text{O}_2$ levels (hypoxia).
  • Chemoreceptor Reflex Response:
  • Stimulation of receptors sends sensory impulses to the medulla oblongata.
  • Respiratory centers are stimulated, leading to an increased respiratory rate.
  • Cardiovascular centers stimulate the cardioacceleratory center, inhibit the cardioinhibitory center, and stimulate the vasomotor center.
  • Result: Vasoconstriction in peripheral extremities and increased cardiac output elevate blood pressure and overall blood flow, promoting increased $pH$ and $\text{O}_2$ levels with reduced $\text{CO}_2$ levels.

Hormonal Regulation of Blood Pressure and Volume

• Antidiuretic Hormone (ADH):
  • Released by the neurohypophysis (posterior lobe of the pituitary gland).
  • Triggers for release: Low blood volume, high plasma osmotic concentration (hypertonic condition), and circulating angiotensin II.
  • Cardiovascular actions: Elevates blood pressure through peripheral vasoconstriction and reduces water loss at the kidneys to preserve plasma volume.
• Angiotensin II:
  • Activation: Released into circulation in response to a drop in renal blood pressure (via renal release of renin).
  • Systemic effects:
  • Stimulates aldosterone secretion by the adrenal cortex (promoting renal $\text{Na}^+$ and water retention).
  • Stimulates ADH secretion by the posterior pituitary.
  • Stimulates thirst centers in the hypothalamus.
  • Increases cardiac output and potent peripheral vasoconstriction, driving blood pressure elevation.
• Erythropoietin (EPO):
  • Released by the kidneys in response to low blood pressure or low $\text{O}_2$ concentration in the blood.
  • Actions: Stimulates peripheral vasoconstriction and accelerates red blood cell maturation and production (erythropoiesis) in red bone marrow, increasing blood volume and oxygen-carrying capacity.
• Natriuretic Peptides:
  • Atrial Natriuretic Peptide (ANP): Produced by cardiac muscle cells in the wall of the right atrium.
  • Brain Natriuretic Peptide (BNP): Produced by ventricular muscle cells.
  • Trigger: Excessive diastolic stretching of cardiac muscle fibers due to elevated blood volume and pressure.
  • Biological actions (lower blood pressure and volume):
  • Increases $\text{Na}^+$ excretion in urine (natriuresis).
  • Increases water excretion in urine (diuresis).
  • Reduces thirst sensations.
  • Inhibits release of ADH, aldosterone, epinephrine, and norepinephrine.
  • Causes peripheral vasodilation.

Cardiovascular Adaptation and Hemodynamic Responses

• Cerebral Blood Flow Priority:
  • Blood supply to the brain is held at top physiological priority due to high neuronal oxygen demand.
  • When peripheral blood vessels constrict in response to systemic stressors, cerebral blood vessels dilate via autoregulation to maintain stable blood flow.
• Cerebrovascular Accident (CVA / Stroke):
  • Occurs when blood supply to a portion of the brain is interrupted due to vascular blockage (thrombus/embolus) or arterial rupture.
  • The middle cerebral artery is the most common site for stroke occurrence.
• Hemodynamic Changes During Exercise:
  • Blood distribution changes dramatically between resting conditions, light exercise, and strenuous exercise:
  • Skeletal Muscles: Rest = $1200\,\text{mL/min}$; Light Exercise = $4500\,\text{mL/min}$; Strenuous Exercise = $12,500\,\text{mL/min}$.
  • Heart: Rest = $250\,\text{mL/min}$; Light Exercise = $350\,\text{mL/min}$; Strenuous Exercise = $750\,\text{mL/min}$.
  • Brain: Rest = $750\,\text{mL/min}$; Light Exercise = $750\,\text{mL/min}$; Strenuous Exercise = $750\,\text{mL/min}$ (maintained constant).
  • Skin: Rest = $500\,\text{mL/min}$; Light Exercise = $1500\,\text{mL/min}$; Strenuous Exercise = $1900\,\text{mL/min}$.
  • Kidney: Rest = $1100\,\text{mL/min}$; Light Exercise = $900\,\text{mL/min}$; Strenuous Exercise = $600\,\text{mL/min}$.
  • Abdominal Viscera: Rest = $1400\,\text{mL/min}$; Light Exercise = $1100\,\text{mL/min}$; Strenuous Exercise = $600\,\text{mL/min}$.
  • Miscellaneous: Rest = $600\,\text{mL/min}$; Light Exercise = $400\,\text{mL/min}$; Strenuous Exercise = $400\,\text{mL/min}$.
  • Total Cardiac Output: Rest = $5800\,\text{mL/min}$; Light Exercise = $9500\,\text{mL/min}$; Strenuous Exercise = $17,500\,\text{mL/min}$.
• Cardiovascular Responses to Blood Loss (Hemorrhage):
  • Initial disruption: Acute blood loss lowers blood pressure and volume.
  • Short-Term Neural Mechanisms:
  • Baroreceptor and chemoreceptor stimulation, combined with hypothalamic input (pain, stress, anxiety, fear), causes general sympathetic activation.
  • Secretion of Epinephrine (E) and Norepinephrine (NE) from the adrenal medulla.
  • Increases cardiac output, promotes peripheral vasoconstriction, and mobilizes venous blood reservoirs (venoconstriction).
  • Long-Term Endocrine Mechanisms:
  • Renal hypoperfusion triggers release of EPO (increasing erythropoiesis and red blood cell count).
  • Renin activation leads to release of ADH, Angiotensin II, and Aldosterone, promoting fluid intake (thirst) and water retention at the kidneys.
  • Combined effect restored normal blood pressure and blood volume homeostasis.

The Pulmonary Circuit

• Path of Blood Flow:
  • Deoxygenated blood returns from the systemic circuit into the right atrium, passes through the tricuspid valve into the right ventricle, and is pumped across the pulmonary semilunar valve into the pulmonary trunk.
  • Pulmonary Arteries:
  • The pulmonary trunk divides into the left pulmonary artery and right pulmonary artery.
  • Pulmonary arteries divide into smaller pulmonary arterioles.
  • Pulmonary arterioles supply dense capillary networks that encircle individual alveoli (air sacs).
  • Functional significance of progressive vessel branching: Maximizes surface area for efficient gas exchange across the respiratory membrane, removing $\text{CO}_2$ and adding $\text{O}_2$ to blood.
  • Pulmonary Veins:
  • Alveolar capillary networks drain into pulmonary venules.
  • Venules merge to form four pulmonary veins (two left, two right).
  • The four pulmonary veins empty oxygenated blood directly into the left atrium, where it is routed to the left ventricle for systemic circulation.

The Systemic Circuit – Arterial Architecture

• The Aorta:
  • Ascending Aorta: Rises from the left ventricle; gives off coronary arteries.
  • Aortic Arch: Curves posteriorly and to the left over the heart.
  • Descending Aorta: Extends inferiorly through the thoracic cavity (thoracic aorta) and abdominal cavity (abdominal aorta).

• Branches of the Aortic Arch:
  1. Brachiocephalic Trunk (Innominate Artery): Branches only on the right side into:
  • Right Subclavian Artery.
  • Right Common Carotid Artery.
  1. Left Common Carotid Artery.
  2. Left Subclavian Artery.
• Subclavian Arteries & Upper Limb Branching:
  • Supply blood to the arms, chest wall, shoulders, back, and central nervous system.
  • Primary subclavian branches:
  1. Internal Thoracic Artery: Supplies skin/muscles of chest and abdomen, mammary glands, and pericardium.
  2. Vertebral Artery: Ascends via cervical transverse foramina to supply the brain.
  3. Thyrocervical Trunk: Supplies neck tissues, thyroid gland, shoulders, and upper back.
  • Transition in upper limb: Subclavian artery exits thoracic cavity \n\rightarrow\n Axillary Artery (in shoulder region) \n\rightarrow\n Brachial Artery (in arm).
  • Division of brachial artery: At the coronoid fossa of the humerus, the brachial artery divides into:
  • Radial Artery: Supplies the lateral side of the forearm (thumb side).
  • Ulnar Artery: Supplies the medial side of the forearm (pinky side).
  • Hand supply: Radial and ulnar arteries interconnect at the wrist via anastomoses forming the Superficial Palmar Arch and Deep Palmar Arch, which give rise to Digital Arteries.
• Head and Neck Arteries:
  • Each Common Carotid Artery bifurcates at the neck into:
  • External Carotid Artery: Supplies external structures of the neck, lower jaw, face, and cranium. Branches include superficial temporal, maxillary, occipital, facial, and lingual arteries.
  • Internal Carotid Artery: Enters cranium via the carotid canal to supply the brain. Major branches include the ophthalmic artery, anterior cerebral artery, and middle cerebral artery.

• Cerebral Blood Supply & Circle of Willis:
  • Left and right vertebral arteries enter the skull through the foramen magnum and fuse along the ventral surface of the pons to form the single Basilar Artery.
  • Basilar artery branches into posterior cerebral arteries, which connect to posterior communicating arteries.
  • Cerebral Arterial Circle (Circle of Willis): Ring-shaped vascular anastomosis surrounding the pituitary gland that interconnects the internal carotid arteries and basilar artery systems, maintaining collateral cerebral blood flow.

• Abdominal Aorta Branches:
  • Unpaired Visceral Branches:
  1. Celiac Trunk: Divides into three branches:
     • Left Gastric Artery: Supplies stomach and esophagus.
     • Splenic Artery: Supplies spleen, stomach, and pancreas.
     • Common Hepatic Artery: Supplies liver, gallbladder, and stomach.
  2. Superior Mesenteric Artery: Supplies pancreas, duodenum, small intestine, and proximal colon.
  3. Inferior Mesenteric Artery: Supplies distal colon and rectum.
  • Paired Branches of Abdominal Aorta:
  • Inferior Phrenic Arteries (diaphragm).
  • Adrenal (Suprarenal) Arteries (adrenal glands).
  • Renal Arteries (kidneys).
  • Gonadal Arteries (testicular or ovarian arteries).
  • Lumbar Arteries (vertebrae and spinal cord/body wall).

• Arteries of the Pelvis and Lower Limbs:
  • The terminal segment of the abdominal aorta splits into the right and left common iliac arteries (and median sacral artery).
  • Common iliac arteries branch into:
  • Internal Iliac Artery: Supplies pelvic muscles, skin, urinary and reproductive organs, perineum, gluteal region, and medial thigh.
  • External Iliac Artery: Crosses inferior to the inguinal ligament to become the Femoral Artery.
  • Femoral Artery Branching:
  • Gives off the Deep Femoral Artery (supplies deep thigh structures).
  • Passes through the adductor canal and emerges posterior to the knee as the Popliteal Artery.
  • Division of Popliteal Artery:
  • Anterior Tibial Artery: Passes anteriorly between tibia and fibula; continues onto the dorsum of the foot as the Dorsalis Pedis Artery.
  • Posterior Tibial Artery: Gives off the Fibular (Peroneal) Artery; divides in the sole of the foot into Medial Plantar Artery and Lateral Plantar Artery.
  • Foot supply: Dorsalis pedis and plantar arteries interconnect to form the Dorsal Arch and Plantar Arch, supplying digital arteries to toes.

The Systemic Circuit – Venous Architecture

• General Systemic Venous Features:
  • Complementary arteries and veins run side-by-side.
  • Peripheral neck and limb regions possess two sets of veins: deep veins (accompanying major arteries) and superficial veins (located subcutaneously; crucial for thermoregulation).

• Superior Vena Cava (SVC) Drainage System:
  • Receives blood from head, neck, chest, shoulders, and upper limbs.
  • Venous Drainage of Head and Neck:
  • Superficial head structures drain via temporal, maxillary, and facial veins.
  • Temporal and maxillary veins form the External Jugular Vein.
  • Facial vein drains into the Internal Jugular Vein.
  • Cranial venous sinuses (superior sagittal, straight, transverse, sigmoid sinuses) collect brain blood and drain into the Internal Jugular Vein.

• Upper Limb Venous Drainage:
  • Superficial Veins:
    • Digital veins drain into palmar venous arches.
    • Cephalic Vein: Ascends along lateral side of arm.
    • Basilic Vein: Ascends along medial side of arm.
    • Median Antebrachial Vein: Drains forearm.
    • Median Cubital Vein: Antecubital vein interconnecting cephalic and basilic veins (common site for venipuncture).
  • Deep Veins:
    • Deep palmar arches drain into paired radial and ulnar veins.
    • Radial and ulnar veins fuse above elbow to form the Brachial Vein.
  • Fusion pathways: Brachial vein merges with basilic vein to become the Axillary Vein. Cephalic vein joins axillary vein to form the Subclavian Vein.
  • Subclavian vein merges with internal and external jugular veins to form the Brachiocephalic Vein, which enters the SVC.
• Thoracic Drainage: Intercostal veins, azygos vein (right side), hemi-azygos vein, and accessory hemi-azygos vein (left side) drain thoracic wall structures into the SVC.

• Inferior Vena Cava (IVC) Drainage System:
  • Receives blood from all structures inferior to the diaphragm.
  • Lower Limb Venous Drainage:
  • Superficial Veins:
    • Dorsal Venous Arch: Collects blood from foot surface and digital veins.
    • Great Saphenous Vein: Longest vein in body; ascends along medial leg and thigh to empty into the femoral vein.
    • Small Saphenous Vein: Ascends along posterior calf to empty into the popliteal vein.
  • Deep Veins:
    • Plantar and dorsal venous arches drain into anterior tibial, posterior tibial, and fibular veins.
    • Merge at knee to form Popliteal Vein \n\rightarrow\n Femoral Vein (receives deep femoral, femoral circumflex, and great saphenous veins) \n\rightarrow\n External Iliac Vein.
  • Pelvis drainage: External iliac vein and internal iliac vein join to form the Common Iliac Vein. Right and left common iliac veins merge to form the Inferior Vena Cava.
• The Hepatic Portal System:
  • Directs nutrient-rich blood from capillary beds of digestive organs (stomach, intestines, pancreas, spleen) directly into a second capillary bed within the liver (liver sinusoids) for metabolic processing prior to entering systemic circulation.
  • Major tributaries forming the Hepatic Portal Vein:
  • Superior Mesenteric Vein.
  • Inferior Mesenteric Vein.
  • Splenic Vein.
  • Cystic Vein and Gastric Veins.
  • Distinguishing distinction: The Hepatic Portal Vein enters the liver carrying unprocessed blood; Hepatic Veins exit the liver and empty processed blood directly into the Inferior Vena Cava.

Fetal, Neonatal, and Pathological Circulation

• Fetal Circulation Adaptations:
  • Fetal lungs are collapsed and non-functional during gestation; nutrient and gas exchange occurs entirely across placenta via the umbilical cord.
  • Umbilical Cord Vessels:
  • One Umbilical Vein: Carries oxygenated, nutrient-rich blood from placenta toward fetal heart.
  • Two Umbilical Arteries: Carry deoxygenated, waste-laden blood from internal iliac arteries back to placenta.
  • Fetal Shunts:
  1. Ductus Venosus: Connects umbilical vein directly to inferior vena cava, bypassing non-functional fetal liver.
  2. Foramen Ovale: Interatrial opening covered by a valve flap; allows blood to flow directly from right atrium to left atrium, bypassing pulmonary circuit.
  3. Ductus Arteriosus: Short muscular vessel connecting pulmonary trunk to aortic arch, shunting blood away from lungs.

• Cardiovascular Changes at Birth (Neonatal Transition):
  • Newborn takes first breath and lungs expand with air.
  • Expansion of pulmonary blood vessels reduces vascular resistance, drastically increasing pulmonary blood flow.
  • Increased venous return to left atrium elevates left atrial pressure above right atrial pressure, forcing valve flap of foramen ovale closed (forming the fossa ovalis).
  • High blood oxygen levels cause smooth muscle constriction of ductus arteriosus, isolating pulmonary trunk from aorta (forming the fibrous ligamentum arteriosum).
• Congenital Heart Anomalies:
  • Patent Foramen Ovale:
  • Foramen ovale remains open after delivery.
  • Driven by high systemic pressures, blood recirculates from left atrium to right atrium ("left-to-right shunt").
  • Arterial oxygen content remains normal, but left ventricular workload increases markedly.
  • Patent Ductus Arteriosus:
  • Ductus arteriosus fails to constrict post-delivery.
  • Elevation in pulmonary circuit pressure forces blood from pulmonary trunk into systemic circulation ("right-to-left shunt").
  • Deoxygenated blood enters systemic circulation, producing cyanosis ("blue baby").

Effects of Aging on the Cardiovascular System

• Age-Related Changes in the Heart:
  1. Reduction in maximum cardiac output.
  2. Changes in nodal and conducting cells (pacemaker degeneration).
  3. Reduced elasticity of the cardiac fibrous skeleton.
  4. Progressive coronary atherosclerosis.
  5. Replacement of damaged cardiac muscle cells by non-contractile fibrous scar tissue.
• Age-Related Changes in Blood Vessels:
  1. Arteries become less elastic (arteriosclerosis); pressure surges can lead to aneurysm formation.
  2. Calcium deposits accumulate on vessel walls, increasing risk of stroke (CVA) or myocardial infarction.
  3. Lipid accumulation in tunica media combined with calcium salts forms atherosclerotic plaques.
  4. Thrombi (blood clots) form at sites of atherosclerotic plaques, posing risks for vessel occlusion or embolisms.