Comprehensive Circulatory System Notes: Pulmonary and Systemic Circulation, Vessels, and Portal System

Pulmonary Circulation

  • Blood returning to the heart from the systemic circuit enters the right atrium via the superior and inferior venae cavae and the coronary sinus (drains the heart muscle).
    • Blood in the right atrium is relatively low in O$2$ and high in CO$2$ (carbon dioxide from tissues).
  • From the right atrium, blood moves into the right ventricle and is pumped to the lungs for gas exchange via the pulmonary circuit.
  • The single vessel leaving the right ventricle is the pulmonary trunk.
    • At its base, the pulmonary trunk has a pulmonary semilunar valve that prevents backflow during ventricular diastole.
  • The pulmonary trunk rapidly bifurcates into the left and right pulmonary arteries, which branch repeatedly within the lungs to form smaller arteries and arterioles leading to pulmonary capillaries.
  • Pulmonary capillaries surround alveoli where gas exchange occurs (O$2$ uptake, CO$2$ release).
  • Oxygenated blood returns from the lungs via the four pulmonary veins (two on the left, two on the right) to the left atrium, completing the pulmonary circuit.
  • Table 20.4 outlines major arteries and veins of the pulmonary circuit:
    • Pulmonary trunk: single vessel exiting the right ventricle; divides into right and left pulmonary arteries.
    • Pulmonary arteries: deliver blood to pulmonary capillaries.
    • Pulmonary veins: return blood from the lungs to the left atrium.
  • Note on terminology: refer to the vessel exiting the right ventricle as the pulmonary trunk, not as a pulmonary artery, to avoid confusion.

Systemic Circulation: The Aorta and Major Branches

  • After returning to the left atrium via the four pulmonary veins, blood moves into the left ventricle and is pumped into the aorta, the main systemic artery.
  • The aorta distributes oxygenated blood to virtually all tissues via its major branches (Figure 20.24).
  • The aorta has three main regions:
    • Ascending aorta
    • Aortic arch
    • Descending aorta (thoracic aorta superior to the diaphragm; abdominal aorta below the diaphragm)
  • The aortic valve (aortic semilunar valve) prevents backflow into the left ventricle.
  • After exiting the heart, the ascending aorta rises ~5 cm before forming the aortic arch; the arch ends at the intervertebral disk between T4 and T5.
  • The aorta then descends, continuing as the thoracic aorta above the diaphragm and the abdominal aorta below it, terminating at the level of the intervertebral disk between L4 and the bifurcation into the common iliac arteries.
  • The abdominal aorta bifurcates into the two common iliac arteries at L4.
  • Terms to know:
    • Aortic hiatus: opening through which the descending aorta passes the diaphragm.
    • Thoracic aorta vs abdominal aorta: regions above vs below the diaphragm.
  • Aortic regions and major branches are summarized in Tables 20.5–20.7:
    • Ascending aorta
    • Aortic arch
    • Descending aorta (thoracic and abdominal portions)
  • Coronary circulation (the first branches of the ascending aorta):
    • Left coronary artery arises from the left posterior aortic sinus.
    • Right coronary artery arises from the anterior aortic sinus.
    • Coronary arteries encircle the heart and branch to supply cardiac tissue; this vascular ring is critical for myocardial perfusion.
  • Aortic arch branches (three major branches):
    • Brachiocephalic (trunk) artery: on the right side; divides into the right subclavian artery and the right common carotid artery.
    • Left common carotid artery: arises independently from the aortic arch and supplies the left side of the head and neck.
    • Left subclavian artery: arises independently from the aortic arch and supplies the left upper limb and thorax.
  • Elastic arteries: the arch branches are classified as elastic arteries due to their high elastin content, helping to dampen the pulsatile output of the heart.
  • Subclavian arteries give rise to three major branches:
    • Internal thoracic (mammary) artery: supplies thymus, pericardium, and anterior chest wall.
    • Vertebral artery: travels through cervical vertebrae, passes through foramen magnum to supply brain and spinal cord; part of the posterior circulation and contributes to the arterial circle (circle of Willis).
    • Thyrocervical trunk: supplies thyroid, neck region, and upper back/shoulder.
  • The common carotid arteries divide into:
    • Internal carotid artery: supplies brain; forms part of the arterial circle with vertebral arteries and contributes to cerebral circulation; gives rise to carotid sinus (baroreceptors) and carotid body (chemoreceptors).
    • External carotid artery: supplies many structures of the face, jaw, neck, esophagus, and larynx (including lingual, facial, occipital, maxillary, superficial temporal branches).
  • Internal carotid and vertebral arteries are the primary suppliers of brain blood; brain receives ~ extapproximately0.20≈20% of total blood flow at any momentext{approximately } 0.20 \approx 20\% \text{ of total blood flow at any moment}
    • This high flow to the brain requires constant perfusion; interruption can cause TIAs (transient ischemic attacks) or CVAs (cerebrovascular accidents).
  • The arterial circle (circle of Willis) is an anastomotic ring at the base of the brain formed by branches of the internal carotid and vertebral arteries; it ensures continual brain perfusion via possible collateral routes.
  • Brain arteries and branches (examples):
    • Anterior cerebral artery (frontal lobe)
    • Middle cerebral artery (temporal and parietal lobes; common site of CVAs)
    • Ophthalmic artery (eyes)
    • Anterior communicating artery (anastomosis between left and right anterior cerebral arteries; forms anterior part of the circle)
    • Posterior communicating arteries (join with posterior cerebral arteries to complete the circle)
    • Basilar artery (formed by vertebral arteries; supplies brainstem and cerebellum; divides into posterior cerebral arteries)
  • Thoracic aorta visceral branches include:
    • Bronchial arteries (to lungs and visceral pleura)
    • Pericardial arteries (to pericardium)
    • Esophageal arteries (to esophagus)
    • Mediastinal arteries (to mediastinum)
  • Thoracic aorta parietal (somatic) branches include:
    • Intercostal arteries (to thoracic wall and vertebral column)
    • Superior phrenic arteries (to superior surface of the diaphragm)
  • Abdominal aorta branches (major ones):
    • Celiac trunk: left gastric, splenic, common hepatic (which gives hepatic proper, right gastric, cystic arteries)
    • Superior mesenteric artery: small intestine, pancreas, much of large intestine
    • Inferior mesenteric artery: distal large intestine and rectum
    • Inferior phrenic arteries (diaphragm)
    • Adrenal (suprarenal) arteries
    • Renal arteries (to kidneys)
    • Gonadal arteries (ovarian or testicular; ovarian arteries supply uterus and uterine tube via uterine anastomosis; testicular arteries travel to testes and form part of spermatic cord)
    • Lumbar arteries (4 pairs; sometimes a 5th pair from median sacral artery)
    • Median sacral artery: continuation into the sacrum
    • Common iliac arteries: bifurcate into internal and external iliac arteries; external iliac becomes femoral artery in the thigh; internal iliac supplies pelvic organs
  • Abdominal aorta divisions and summaries are listed in Tables 20.7 and 20.8.

Arteries Serving the Upper Limbs

  • The subclavian artery becomes the axillary artery as it enters the axillary region; the axillary artery mainly continues as the brachial artery in the upper limb.
  • The brachial artery supplies the brachial region and branches into:
    • Deep brachial arteries (posterior arm supplied)
    • Ulnar collateral arteries (toward the elbow region)
  • At the elbow, the brachial artery bifurcates into:
    • Radial artery (parallels radius)
    • Ulnar artery (parallels ulna)
  • In the wrist, these arteries form the superficial and deep palmar arches, giving rise to digital arteries for the fingers.
  • The flow pattern for the upper limb arteries is summarized in Table 20.9 and Figure 20.31–20.32.
  • The subclavian-axillary-brachial continuum is the main arterial supply to the arm; there are numerous named branches feeding the chest, shoulder, back, and CNS via the vertebral and internal thoracic arteries.

Arteries Serving the Lower Limbs

  • The external iliac artery becomes the femoral artery as it enters the thigh (femoral region).
  • Branches of the femoral artery include:
    • Lateral deep femoral artery (profunda femoris)
    • Genicular arteries (knee region)
  • As the femoral artery passes posterior to the knee, it becomes the popliteal artery, which branches into:
    • Anterior tibial artery (becomes dorsalis pedis in the foot)
    • Posterior tibial artery (gives rise to fibular/peroneal artery)
  • The anterior tibial artery supplies the anterior leg; dorsalis pedis supplies the dorsal foot; posterior tibial supplies the posterior leg and plantar surface; the fibular (peroneal) artery supplies lateral compartments and contributes to plantar arches.
  • The posterior and anterior tibial arteries form the dorsal and plantar arches of the foot, with the medial and lateral plantar arteries forming the plantar arch; dorsal arch is formed by the dorsalis pedis and the plantar arteries.
  • Major systemic arteries of the lower limb are summarized in Table 20.10 and Figures 20.33–20.34.

Veins: General Principles and Patterns

  • Systemic veins return blood to the right atrium; venous blood is typically low in O$_2$.
  • In neck and limbs, there are often two levels: superficial and deep veins. Deep veins generally mirror the arterial pattern; superficial veins often lack direct arterial counterparts but contribute to temperature regulation (thermoregulation).
  • Veins accompany arteries in many regions (complementary pattern), but there is considerable variability in venous anatomy.
  • The same voyage analogy used for arteries also helps with veins, but veins present numerous anastomoses and branches.
  • In the thorax, most venous blood drains into the azygos system and then to the superior vena cava (SVC); however, coronary venous blood drains directly into the coronary sinus and then into the right atrium.
  • The flow of systemic venous return is described in Figures 20.35–20.42 and Tables 20.11–20.16.

Veins of the Head and Neck

  • Internal jugular veins drain blood from the brain and superficial facial veins; they run with the internal carotid arteries and merge with subclavian veins to form the brachiocephalic veins.
  • External jugular veins drain superficial head/neck regions and drain into the subclavian veins.
  • Major head/neck veins include:
    • Internal jugular vein
    • Temporal vein
    • Maxillary vein
    • External jugular vein
  • Venous drainage of the brain involves intracranial sinuses (see Table 20.13 and Figure 20.37), including:
    • Superior sagittal sinus
    • Inferior sagittal sinus
    • Straight sinus
    • Cavernous sinus
    • Petrosal sinuses
    • Occipital sinus
    • Transverse sinuses
    • Sigmoid sinuses
  • These sinuses ultimately drain into the internal jugular vein.

Veins of the Thorax and Abdominal Regions (Systemic Veins)

  • Major thoracic veins: SVC, brachiocephalic veins, vertebral veins, internal thoracic (mammary) veins, intercostal veins, esophageal veins, bronchial veins, azygos vein, hemiazygos vein, and superior intercostal veins.
  • The azygos system drains much of the thoracic wall and mediastinal region and ultimately drains into the SVC.
  • The azygos and hemiazygos systems provide collateral routes to the SVC if primary pathways are obstructed.
  • Veins draining the head/neck flow into internal and external jugular pathways that join the subclavian veins to form the brachiocephalic veins.
  • Table 20.11 summarizes thoracic veins; Table 20.12 summarizes head/neck veins; Table 20.13 covers brain venous drainage.

Veins Draining the Upper Limbs

  • Venous drainage of the upper limb follows a pattern that mirrors the arterial supply:
    • Digital veins drain into palmar venous arches.
    • Radial and ulnar veins join to form the brachial vein in the arm.
    • Median antebrachial vein parallels the ulnar vein, draining into the basilic vein.
    • Basilic vein drains into the axillary vein; cephalic vein drains superficial lateral aspects and drains into the axillary vein via the subclavian region.
    • Median cubital vein (in the antecubital fossa) connects the cephalic and basilic veins and is a common site for blood draws.
  • The axillary vein continues as the subclavian vein, which joins the internal jugular to form the brachiocephalic vein.
  • Table 20.14 summarizes upper-limb veins; Figure 20.39 provides a flow chart of the venous flow into the SVC.

Veins Draining the Lower Limbs

  • Major pathways: plantar and dorsal venous arches in the foot feed into the anterior and posterior tibial veins, fibular veins, and deep venous networks.
  • The popliteal vein is formed behind the knee from the fusion of the fibular, anterior tibial, and posterior tibial veins and becomes the femoral vein as it travels upward.
  • The great saphenous vein (longest superficial vein) drains the medial leg and thigh and drains into the femoral vein; the small saphenous drain into the popliteal vein.
  • Deep veins (deep femoral, femoral circumflex, etc.) drain deep thigh regions and feed into the femoral vein; the femoral vein becomes the external iliac vein as it moves into the pelvic area.
  • Internal iliac veins drain pelvic organs and walls; external iliac veins continue as the femoral and then external iliac pathways.
  • The inferior vena cava (IVC) receives blood from hepatic, renal, gonadal, lumbar, and other abdominal veins and returns blood to the right atrium.
  • Table 20.16 lists major veins of the lower limbs; Figure 20.42 summarizes venous flow in the lower limb.

The Hepatic Portal System

  • The liver acts as a processing plant for absorbed nutrients and wastes; many digestive-system-derived materials are directed to the liver via the hepatic portal system before entering systemic circulation.
  • Portal system basics: portal systems begin and end in capillaries; blood travels from digestive organs to the liver via the hepatic portal vein and then drains into hepatic sinusoids.
  • Components of the hepatic portal system:
    • Hepatic portal vein: formed at L2 by the confluence of the superior mesenteric and splenic veins; receives inferior mesenteric vein and other tributaries.
    • Superior mesenteric vein drains small intestine, most of the large intestine, and stomach.
    • Splenic vein drains spleen, pancreas, portions of the stomach, and tributaries of the portal system.
    • Inferior mesenteric vein drains distal large intestine, including descending colon, sigmoid colon, and rectum.
    • The portal vein also receives gastric veins and cystic veins from the gall bladder.
  • Blood from the portal system reaches the liver for processing; after hepatic processing, blood exits via hepatic veins into the IVC.
  • The liver thus receives dual blood supply: systemic blood via the hepatic artery and portal blood via the hepatic portal vein.
  • Figure 20.43 illustrates the hepatic portal system and its flow into the liver.

Key Concepts and Terms

  • Anastomosis: connection between two previously branched blood vessels, common in veins and some arteries (e.g., brain arteries).
  • Circle of Willis: arterial circle at the base of the brain formed by internal carotid and vertebral arteries; provides collateral routes to maintain cerebral perfusion.
  • Arterial trunk: a large artery that gives rise to several smaller arteries (e.g., celiac trunk).
  • Elastic arteries: arteries like those arising from the aortic arch; high elastin content aiding pressure dampening.
  • Pulmonary circuit vs systemic circuit: pulmonary sends blood to lungs for gas exchange; systemic sends oxygenated blood to the rest of the body.
  • TIAs and CVAs: transient ischemic attacks and cerebrovascular accidents; timing and duration of blood flow interruption impact brain tissue.
  • Hepatic portal system: a portal circulation between digestive organs and liver; allows liver to process absorbed nutrients and wastes before systemic delivery.

Quick Reference: Notable Levels and Branch Points (selected)

  • Aorta and its major regions: ext{Ascending aorta}
    ightarrow ext{Aortic arch}
    ightarrow ext{Descending (thoracic) aorta}
    ightarrow ext{Abdominal aorta}
    ightarrow ext{Common iliac arteries at } L_4
  • Aortic arch branches: ext{Brachiochephalic trunk}
    ightarrow ext{Right subclavian and right common carotid}, ext{Left common carotid}, ext{Left subclavian}
  • Thoracic vertebral landmarks: T5exttoT12extforthethoracicaortaregionT5 ext{ to } T12 ext{ for the thoracic aorta region}
  • Diaphragm landmarks: aortichiatus;belowthediaphragmtheaortaistheabdominalaortaaortic hiatus; below the diaphragm the aorta is the abdominal aorta
  • Brain blood flow share: ≈20% of cardiac output to the brain at any moment\approx 20\%\text{ of cardiac output to the brain at any moment}
  • Spinal and pelvic levels: L4 − level for iliac bifurcation,L2 for hepatic portal vein formationL4\,-\,\text{level for iliac bifurcation}, L2\text{ for hepatic portal vein formation}

Practical Implications and Clinical Relevance

  • Understanding vessel naming changes by location (e.g., left subclavian becoming axillary and then brachial) helps in locating vessels during anatomy or surgical procedures.
  • Recognizing vessel anastomoses, particularly in brain circulation (circle of Willis) and thoracic/abdominal venous networks, explains how collateral flow protects tissue during obstruction.
  • The hepatic portal system highlights the liver’s role in processing nutrients and wastes before systemic distribution, with clinical relevance to liver function and metabolic diseases.
  • Knowledge of veins and venous flow patterns is essential for understanding procedures like venipuncture, assessment of venous return, and interpretation of hemodynamic changes in disease states.

Summary Connections to Foundational Principles

  • Flow follows pressure gradients from high-pressure arteries to low-pressure veins, with valves preventing backflow where present (e.g., aortic semilunar valve, pulmonary semilunar valve).
  • Oxygen transport and gas exchange drive the separation of pulmonary and systemic circuits; lungs oxygenate blood which then circulates to tissues with varying oxygen demands.
  • The nervous and endocrine systems rely on stable perfusion to the brain and heart, with reflexive control via baroreceptors and chemoreceptors located in the aortic and carotid sinuses.
  • The principle of parallel vs series circulation is illustrated by the circle of Willis and hepatic portal system, where multiple tributaries feed into shared networks that feed different organs, maintaining systemic balance.