Cardiovascular System — revision sheet

The cardiovascular system is a closed circuit made of a four-chambered muscular pump, about 100 000 km of blood vessels and roughly 5 L of blood. It delivers oxygen, nutrients and hormones to every tissue, removes carbon dioxide and wastes, distributes heat and keeps the internal environment stable. Two circulations run in series: the low-pressure pulmonary circuit and the high-pressure systemic circuit.

Resting cardiac output: ≈5 L/min (HR ≈70/min × SV ≈70 mL)Resting heart rate: 60–100 beats/minEjection fraction (normal): ≈55–70%Heart mass: ≈250–300 g (female), ≈300–350 g (male)Blood volume: ≈70 mL/kg (≈5 L in a 70 kg adult)Haematocrit: ≈40–50% (men), ≈36–46% (women)Arterial blood pH: 7.35–7.45Optimal blood pressure: <120/80 mmHg; MAP ≈93 mmHgPulmonary artery pressure: ≈25/10 mmHg (mean ≈15; PH if >20)Blood volume in systemic veins: ≈64% (arteries ≈13%, arterioles + capillaries ≈7%)Number of capillaries: ≈10 billion; ≈500–700 m² surfaceResting coronary blood flow: ≈225–250 mL/min (4–5% of CO)

Heart — overview & coverings

Heart Organ: The heart is a hollow, four-chambered muscular pump about the size of a clenched fist. It lies inside the pericardium in the middle mediastinum and drives blood through the pulmonary and systemic circulations.
Pericardium Membrane: The pericardium is a fibroserous sac that encloses the heart and the roots of the great vessels. It has a tough outer fibrous layer and an inner double-layered serous membrane, with a thin film of lubricating fluid between the serous layers.
Fibrous skeleton of the heart Organ part: The fibrous skeleton is a framework of dense collagen in the plane of the atrioventricular groove. Four fibrous rings joined by fibrous trigones anchor the valves and myocardium and electrically insulate the atria from the ventricles.

Heart — wall layers

Epicardium Membrane: The epicardium is the outermost layer of the heart wall and the visceral layer of the serous pericardium. It consists of mesothelium over connective tissue and fat, and it carries the main coronary vessels and nerves.
Myocardium Tissue: The myocardium is the thick middle layer of the heart wall. It is made of cardiac muscle cells that contract together to pump blood, and it is thickest in the left ventricle.
Endocardium Tissue: The endocardium is the thin, smooth inner lining of the heart chambers and valves. It is made of endothelium on a layer of connective tissue and is continuous with the tunica intima of the great vessels.

Heart — chambers & septa

Right atrium Organ part: The right atrium receives deoxygenated blood from the SVC, IVC and coronary sinus and passes it through the tricuspid valve to the right ventricle. It contains the sinoatrial and atrioventricular nodes.
Right ventricle Organ part: The right ventricle is a crescent-shaped chamber that forms most of the anterior (sternocostal) surface of the heart. It receives blood through the tricuspid valve and pumps it at low pressure through the pulmonary valve into the pulmonary trunk.
Left atrium Organ part: The left atrium is the most posterior chamber of the heart and forms most of its base. It receives oxygenated blood from four pulmonary veins and passes it through the mitral valve to the left ventricle.
Left ventricle Organ part: The left ventricle is the thick-walled main pumping chamber. It receives blood through the mitral valve, ejects it at high pressure through the aortic valve into the systemic circulation, and forms the apex of the heart.
Interatrial septum Organ part: The interatrial septum is the thin wall between the right and left atria. On its right side is the fossa ovalis, the remnant of the fetal foramen ovale.
Interventricular septum Organ part: The interventricular septum is the thick, mostly muscular wall between the two ventricles. It bulges into the right ventricle and carries the bundle of His and the bundle branches.
Papillary muscles Organ part: Papillary muscles are cone-shaped projections of ventricular muscle whose tips anchor the chordae tendineae. They contract early in systole so that the AV valve cusps do not turn inside out into the atria.
Chordae tendineae Organ part: The chordae tendineae are fine fibrous cords that connect the papillary muscles to the cusps of the tricuspid and mitral valves. Like the lines of a parachute, they stop the cusps blowing back into the atria during systole.
Moderator band (septomarginal trabecula) Organ part: The moderator band is a muscular bridge across the right ventricle, running from the interventricular septum to the base of the anterior papillary muscle. It carries part of the right bundle branch.

Heart — valves

Tricuspid valve Organ part: The tricuspid valve is the right atrioventricular valve. Its anterior, posterior and septal cusps guard the opening between the right atrium and right ventricle.
Pulmonary valve Organ part: The pulmonary valve is the semilunar valve where the right ventricular outflow tract joins the pulmonary trunk. Its three cusps (anterior, right and left) stop blood flowing back into the right ventricle.
Mitral valve Organ part: The mitral valve is the left atrioventricular valve. Its anterior (aortic) and posterior cusps control flow from the left atrium into the left ventricle. It is named for its resemblance to a bishop's mitre.
Aortic valve Organ part: The aortic valve is the semilunar valve between the left ventricle and the ascending aorta. Its three cusps (right, left and posterior or non-coronary) lie in front of the aortic sinuses, from which the coronary arteries arise.

Heart — coronary circulation

Right coronary artery Artery: The right coronary artery arises from the right aortic sinus and runs in the right atrioventricular groove to the crux of the heart. It supplies the right atrium, most of the right ventricle, the SA node in about 60% of people and the AV node in about 80–90%. In right-dominant hearts it also supplies the inferior wall of the left ventricle through the posterior interventricular artery.
Left coronary artery Artery: The left coronary artery arises from the left aortic sinus as a short left main stem. It passes between the pulmonary trunk and the left auricle and then divides into the LAD and circumflex arteries, which together supply most of the left ventricle, the left atrium and the anterior two-thirds of the septum.
Left anterior descending artery Artery: The left anterior descending (LAD) artery is the anterior interventricular branch of the left coronary artery. It runs down the anterior interventricular groove to the apex and often around it, supplying the anterior left ventricular wall, the apex and the anterior two-thirds of the septum. It is the coronary artery most often occluded.
Circumflex artery Artery: The circumflex artery is the branch of the left coronary artery that follows the left atrioventricular groove around the left border to the back of the heart. It supplies the lateral and posterolateral left ventricular wall and the left atrium.
Posterior interventricular artery Artery: The posterior interventricular artery runs in the posterior interventricular groove from the crux toward the apex. It supplies the inferior left ventricular wall and the posterior third of the septum, and the artery that gives rise to it defines coronary dominance.
Coronary sinus Vein: The coronary sinus is the main venous channel of the heart. It is a wide vein about 2–5 cm long in the posterior part of the left atrioventricular groove that collects most of the heart's venous blood and empties it into the right atrium.
Great cardiac vein Vein: The great cardiac vein is the main tributary of the coronary sinus. It drains the territory of the left coronary artery, running up the anterior interventricular groove beside the LAD and then around the left atrioventricular groove beside the circumflex.
Middle cardiac vein Vein: The middle cardiac vein runs up the posterior interventricular groove beside the posterior interventricular artery and drains the inferior walls of both ventricles into the coronary sinus, close to its opening.
Small cardiac vein Vein: The small cardiac vein runs in the right atrioventricular groove alongside the right coronary artery and drains part of the right atrium and right ventricle into the end of the coronary sinus.

Heart — conduction system

Sinoatrial node Organ part: The sinoatrial (SA) node is the heart's natural pacemaker. It is a crescent of specialised muscle cells where the superior vena cava meets the right atrium, and it depolarises spontaneously 60–100 times a minute at rest.
Internodal pathways and Bachmann bundle Organ part: The internodal pathways are routes of faster conduction through the atrial walls. The anterior, middle and posterior internodal tracts carry the impulse from the SA node to the AV node, and Bachmann bundle carries it across to the left atrium.
Atrioventricular node Organ part: The atrioventricular (AV) node is a small mass of slowly conducting tissue in the floor of the right atrium, inside the triangle of Koch. It is the only normal electrical gateway to the ventricles, and it delays the impulse by about 0.1 s so that the atria can finish emptying.
Bundle of His Organ part: The bundle of His is a bundle of conducting fibres that continues from the AV node, passes through the central fibrous body and runs along the membranous septum before dividing into the right and left bundle branches. It is the only normal electrical bridge across the fibrous skeleton.
Right bundle branch Organ part: The right bundle branch is a thin, cord-like branch of the bundle of His. It runs down the right side of the interventricular septum and through the moderator band to the anterior papillary muscle and the Purkinje network of the right ventricle.
Left bundle branch Organ part: The left bundle branch is a broad, sheet-like branch of the bundle of His. It runs down the left side of the septum and divides into an anterior (superior) fascicle and a posterior (inferior) fascicle, with additional septal fibres, to activate the left ventricle.
Purkinje fibres Tissue: Purkinje fibres form a network of large, fast-conducting specialised muscle cells beneath the ventricular endocardium. They deliver the impulse almost simultaneously to the whole ventricular myocardium.

Great vessels

Aorta Artery: The aorta is the largest artery in the body, an elastic artery about 2.5–3.5 cm wide at its root. It carries oxygenated blood from the left ventricle and, through its branches, supplies every systemic tissue. It is divided into the ascending aorta, the aortic arch and the descending (thoracic and abdominal) aorta.
Pulmonary trunk Artery: The pulmonary trunk is a short, wide artery that leaves the right ventricle carrying deoxygenated blood toward the lungs. Under the aortic arch it divides into the right and left pulmonary arteries.
Pulmonary arteries (right and left) Artery: The right and left pulmonary arteries are the terminal branches of the pulmonary trunk. They carry deoxygenated blood into each lung and branch alongside the bronchial tree down to the alveolar capillaries.
Pulmonary veins Vein: There are usually four pulmonary veins, right and left superior and inferior. They are valveless and return oxygenated blood from the lungs to the left atrium.
Superior vena cava Vein: The superior vena cava is a large, valveless vein about 7 cm long. It returns blood from the head, neck, upper limbs and chest wall to the right atrium.
Inferior vena cava Vein: The inferior vena cava is the largest vein in the body. It returns blood from the abdomen, pelvis and lower limbs to the right atrium, beginning at L5 and passing through the diaphragm at T8.

Arteries — head & neck

Brachiocephalic trunk Artery: The brachiocephalic trunk is the first and largest branch of the aortic arch. It ascends behind the manubrium and divides behind the right sternoclavicular joint into the right common carotid and right subclavian arteries.
Common carotid artery Artery: The common carotid artery is the main artery of the neck on each side. It ascends in the carotid sheath and divides at the upper border of the thyroid cartilage (about C3–C4) into the internal and external carotid arteries.
Carotid sinus Receptor: The carotid sinus is a widening at the origin of the internal carotid artery. Its wall contains stretch-sensitive baroreceptors that monitor arterial pressure and signal through the glossopharyngeal nerve.
Internal carotid artery Artery: The internal carotid artery is the terminal branch of the common carotid that supplies the front of the brain and the eye. It has no branches in the neck and ends by dividing into the anterior and middle cerebral arteries.
External carotid artery Artery: The external carotid artery is the terminal branch of the common carotid that supplies the face, scalp, neck, jaws, tongue and most of the dura. It has eight named branches and ends inside the parotid gland as the maxillary and superficial temporal arteries.
Middle meningeal artery Artery: The middle meningeal artery is the largest meningeal artery. It branches from the maxillary artery, enters the skull through the foramen spinosum and runs in grooves on the inner surface of the skull. Rupture of its anterior branch at the pterion causes an extradural (epidural) haematoma.
Vertebral artery Artery: The vertebral artery is the first branch of the subclavian artery. It ascends through the transverse foramina of C6 to C1, enters the skull through the foramen magnum and joins its partner to form the basilar artery. Together, the two vertebral arteries and the basilar artery make up the posterior circulation of the brain.
Basilar artery Artery: The basilar artery is formed where the two vertebral arteries join at the junction of the pons and medulla. It runs up the front of the pons and divides at the level of the midbrain into the two posterior cerebral arteries.
Cerebral arterial circle (circle of Willis) Vessel: The circle of Willis is a ring of linked arteries at the base of the brain. It joins the internal carotid (anterior) and vertebrobasilar (posterior) systems and surrounds the optic chiasm and the pituitary stalk.
Anterior cerebral artery Artery: The anterior cerebral artery is a terminal branch of the internal carotid. It runs forward and then back along the medial surface of the hemisphere above the corpus callosum, supplying the medial frontal and parietal lobes, including the leg and foot areas of the motor and sensory cortex.
Middle cerebral artery Artery: The middle cerebral artery is the largest terminal branch of the internal carotid. It runs laterally in the lateral sulcus and supplies most of the lateral surface of the hemisphere, including the face and arm areas of the motor and sensory cortex and the language areas. Its lenticulostriate branches supply the basal ganglia and internal capsule.
Posterior cerebral artery Artery: The posterior cerebral artery is a terminal branch of the basilar artery. It curves around the midbrain to supply the occipital lobe, including the visual cortex, and the inferomedial temporal lobe. Through perforating branches it also supplies the thalamus and midbrain.

Arteries — thorax & upper limb

Subclavian artery Artery: The subclavian artery is the main artery at the root of the upper limb. It arches over the dome of the pleura and the first rib, passing behind anterior scalene, and becomes the axillary artery at the lateral border of the first rib.
Internal thoracic artery Artery: The internal thoracic artery is a branch of the first part of the subclavian artery. It runs down about 1 cm from the edge of the sternum, behind the costal cartilages, supplying the anterior chest wall, breast and pericardium. It is the preferred graft for coronary artery bypass.
Intercostal arteries Artery: The intercostal arteries run in each intercostal space and supply the chest wall. The posterior intercostal arteries come mainly from the thoracic aorta and the anterior intercostal arteries from the internal thoracic and musculophrenic arteries. The two sets join (anastomose) in each space.
Axillary artery Artery: The axillary artery continues the subclavian artery from the lateral border of the first rib to the inferior border of teres major. Pectoralis minor divides it into three parts, and it runs with the cords of the brachial plexus inside the axillary sheath.
Brachial artery Artery: The brachial artery is the main artery of the arm. It continues the axillary artery at the inferior border of teres major and divides in the cubital fossa, at the level of the neck of the radius, into the radial and ulnar arteries.
Radial artery Artery: The radial artery is the smaller, lateral terminal branch of the brachial artery. It runs down the lateral side of the forearm to the wrist, where the radial pulse is felt, then crosses the anatomical snuffbox and forms the deep palmar arch.
Ulnar artery Artery: The ulnar artery is the larger, medial terminal branch of the brachial artery. It descends deep in the forearm with the ulnar nerve, crosses the wrist superficial to the flexor retinaculum in Guyon's canal and forms most of the superficial palmar arch.
Palmar arterial arches Artery: The palmar arches are two arterial arcades in the palm. The superficial arch comes mainly from the ulnar artery and the deep arch mainly from the radial artery. Because the two are connected, the fingers have a double blood supply.

Arteries — abdomen & pelvis

Coeliac trunk Artery: The coeliac trunk is the first unpaired branch from the front of the abdominal aorta, at T12. It is a short trunk, about 1–2 cm long, that divides into the left gastric, splenic and common hepatic arteries. These supply the foregut, the liver, the gallbladder, part of the pancreas and the spleen.
Left gastric artery Artery: The left gastric artery is the smallest branch of the coeliac trunk. It rises to the junction of the oesophagus and stomach, then runs along the lesser curvature of the stomach, supplying the lower oesophagus and the lesser curvature.
Splenic artery Artery: The splenic artery is the largest branch of the coeliac trunk. It follows a winding course to the left along the upper border of the pancreas to the hilum of the spleen, supplying the spleen, the body and tail of the pancreas and the greater curvature of the stomach.
Common hepatic artery Artery: The common hepatic artery is the branch of the coeliac trunk that runs to the right along the upper border of the pancreas. It divides into the gastroduodenal artery and the proper hepatic artery, which supplies the liver and gallbladder.
Superior mesenteric artery Artery: The superior mesenteric artery (SMA) is the second unpaired branch from the front of the abdominal aorta, arising at L1 about 1 cm below the coeliac trunk. It runs through the root of the mesentery and supplies the midgut, from the second part of the duodenum to the distal transverse colon.
Inferior mesenteric artery Artery: The inferior mesenteric artery (IMA) is the third and smallest unpaired branch from the front of the abdominal aorta, at L3. It supplies the hindgut, from the distal third of the transverse colon to the upper anal canal.
Renal artery Artery: The renal arteries are paired branches from the sides of the abdominal aorta at about L1–L2, just below the SMA. Together they carry about 20–25% of the cardiac output to the kidneys.
Gonadal arteries (testicular / ovarian) Artery: The gonadal arteries are long, slender paired branches from the front of the abdominal aorta at about L2. They run down behind the peritoneum to the testes, through the spermatic cord, or to the ovaries, through the suspensory ligament of the ovary.
Common iliac artery Artery: The common iliac arteries are the terminal branches of the abdominal aorta, formed where it divides at L4. Each runs about 4 cm downward and outward and divides in front of the sacroiliac joint (L5/S1) into the internal and external iliac arteries.
Internal iliac artery Artery: The internal iliac artery is the main artery of the pelvis. It arises from the common iliac artery at the pelvic brim and divides into anterior and posterior divisions, which supply the pelvic organs, the perineum, the buttock and the medial thigh.
External iliac artery Artery: The external iliac artery is the larger terminal branch of the common iliac artery. It runs along the medial border of psoas on the pelvic brim and passes under the inguinal ligament at the mid-inguinal point, where it becomes the femoral artery.

Arteries — lower limb

Femoral artery Artery: The femoral artery is the main artery of the thigh. It continues the external iliac artery under the inguinal ligament, runs through the femoral triangle and the adductor canal, and becomes the popliteal artery at the adductor hiatus.
Deep femoral artery Artery: The deep femoral artery (profunda femoris) is the largest branch of the femoral artery. It supplies the thigh muscles and, through the medial circumflex femoral artery, the head of the femur.
Popliteal artery Artery: The popliteal artery continues the femoral artery from the adductor hiatus. It runs through the popliteal fossa as its deepest structure and divides at the lower border of popliteus into the anterior and posterior tibial arteries.
Anterior tibial artery Artery: The anterior tibial artery is the smaller terminal branch of the popliteal artery. It passes forward above the interosseous membrane into the anterior compartment of the leg and becomes the dorsalis pedis artery at the ankle.
Posterior tibial artery Artery: The posterior tibial artery is the larger terminal branch of the popliteal artery. It runs down the deep posterior compartment of the leg with the tibial nerve, passes behind the medial malleolus and divides into the medial and lateral plantar arteries.
Fibular artery Artery: The fibular (peroneal) artery is the largest branch of the posterior tibial artery. It runs down close to the fibula in the deep posterior compartment of the leg and supplies the lateral leg, the fibula and the heel.
Dorsalis pedis artery Artery: The dorsalis pedis artery continues the anterior tibial artery onto the dorsum of the foot at the ankle joint. Its pulse is felt just lateral to the tendon of extensor hallucis longus.

Veins — head, neck & thorax

Dural venous sinuses Vein: The dural venous sinuses are endothelium-lined, valveless channels between the two layers of the dura mater. They drain blood from the brain to the internal jugular veins, and through the arachnoid granulations they also return cerebrospinal fluid to the blood.
Internal jugular vein Vein: The internal jugular vein is the main vein draining the brain, face and neck. It begins at the jugular foramen, continuing the sigmoid sinus, runs down in the carotid sheath and joins the subclavian vein behind the sternal end of the clavicle to form the brachiocephalic vein.
External jugular vein Vein: The external jugular vein is a superficial neck vein. It forms near the angle of the mandible, crosses sternocleidomastoid obliquely beneath platysma and drains into the subclavian vein.
Subclavian vein Vein: The subclavian vein continues the axillary vein from the lateral border of the first rib. It crosses the first rib in front of anterior scalene and joins the internal jugular vein behind the sternoclavicular joint to form the brachiocephalic vein.
Brachiocephalic vein Vein: Each brachiocephalic vein is formed behind the sternal end of the clavicle by the union of the internal jugular and subclavian veins. The two brachiocephalic veins join behind the right first costal cartilage to form the superior vena cava.
Azygos vein Vein: The azygos vein is the main vein of the posterior thoracic wall. It ascends on the right side of the thoracic vertebral bodies and arches over the root of the right lung at T4 to drain into the SVC. It is an important bypass route between the IVC and the SVC.

Veins — upper limb

Axillary vein Vein: The axillary vein forms at the inferior border of teres major, where the basilic vein joins the brachial veins. It ascends on the medial side of the axillary artery and becomes the subclavian vein at the lateral border of the first rib.
Cephalic vein Vein: The cephalic vein is the superficial vein on the lateral (thumb) side of the upper limb. It runs from the dorsal venous network of the hand up through the deltopectoral groove and drains into the axillary vein.
Basilic vein Vein: The basilic vein is the superficial vein on the medial (little-finger) side of the upper limb. It pierces the deep fascia in the middle of the arm and joins the brachial veins to form the axillary vein.
Median cubital vein Vein: The median cubital vein is a superficial vein in front of the elbow that links the cephalic vein to the basilic vein. It is the usual site for venepuncture.

Veins — abdomen & portal system

Hepatic veins Vein: The hepatic veins are the right, middle and left veins that drain blood from the liver into the inferior vena cava just below the diaphragm.
Hepatic portal vein Vein: The hepatic portal vein carries nutrient-rich, deoxygenated blood from the gut, spleen, pancreas and gallbladder to the liver. It forms behind the neck of the pancreas, where the superior mesenteric vein and the splenic vein join.
Splenic vein Vein: The splenic vein drains the spleen. It runs to the right behind the body of the pancreas and joins the superior mesenteric vein behind the pancreatic neck to form the portal vein.
Superior mesenteric vein Vein: The superior mesenteric vein drains the midgut. It runs in the root of the mesentery to the right of the superior mesenteric artery and joins the splenic vein behind the neck of the pancreas to form the portal vein.
Inferior mesenteric vein Vein: The inferior mesenteric vein drains the hindgut. It begins as the superior rectal vein, ascends behind the peritoneum to the left of the duodenojejunal flexure and usually joins the splenic vein behind the pancreas.
Portosystemic (portocaval) anastomoses Vessel: Portosystemic anastomoses are sites where tributaries of the hepatic portal system connect with systemic veins. Normally they carry very little blood. In portal hypertension they dilate into varices that carry blood around the liver.
Renal vein Vein: The renal veins drain the kidneys into the inferior vena cava. The left renal vein is about three times longer than the right. It crosses in front of the aorta, just below the origin of the superior mesenteric artery.
Gonadal veins (testicular / ovarian) Vein: The gonadal veins drain the testes or ovaries. The right gonadal vein drains into the IVC, while the left drains into the left renal vein. This asymmetry explains why varicoceles are much more common on the left.
Common iliac vein Vein: Each common iliac vein forms in front of the sacroiliac joint, where the external and internal iliac veins join. The two common iliac veins unite at L5, just right of the midline, to form the inferior vena cava.

Veins — lower limb

Femoral vein Vein: The femoral vein is the main deep vein of the thigh. It continues the popliteal vein from the adductor hiatus, runs with the femoral artery through the adductor canal and femoral triangle, and becomes the external iliac vein behind the inguinal ligament.
Popliteal vein Vein: The popliteal vein is the deep vein behind the knee. It forms at the lower border of popliteus from the veins accompanying the tibial arteries, receives the small saphenous vein, and becomes the femoral vein at the adductor hiatus.
Great saphenous vein Vein: The great saphenous vein is the longest vein in the body. It runs from in front of the medial malleolus up the medial side of the leg and thigh and drains into the femoral vein at the saphenofemoral junction in the groin.
Small saphenous vein Vein: The small saphenous vein is the superficial vein of the back of the calf. It begins behind the lateral malleolus, runs up the midline of the calf and drains into the popliteal vein behind the knee.

Blood vessels — structure & microcirculation

Venous valves Organ part: Venous valves are pairs of half-moon-shaped cusps formed from folds of the inner lining (tunica intima) of a vein. They let blood flow only toward the heart. They are most numerous in the veins of the limbs, where they work with the skeletal muscle pump.
Arteries (general structure) Vessel: Arteries carry blood away from the heart under high pressure. Their walls are thick, with a prominent tunica media. They are classed as elastic (conducting) or muscular (distributing) arteries and branch eventually into arterioles.
Veins (general structure) Vessel: Veins are thin-walled, distensible vessels that return blood to the heart at low pressure. They hold about two-thirds of the blood volume, which makes them the circulation's reservoir (capacitance vessels). In the limbs they contain valves.
Tunica intima Tissue: The tunica intima is the innermost layer of a blood vessel wall. It consists of endothelium on a basement membrane and a thin layer of connective tissue beneath it. In arteries it is bounded on the outside by an internal elastic lamina.
Tunica media Tissue: The tunica media is the middle layer of the vessel wall. It is made mainly of smooth muscle cells arranged circularly (in a helix), with elastic and collagen fibres. It is thickest in arteries, where it controls vessel diameter and stores elastic energy.
Tunica externa (adventitia) Tissue: The tunica externa (adventitia) is the outermost layer of the vessel wall. It is made mainly of type I collagen and elastic fibres with fibroblasts. It anchors the vessel and carries the vessel's own blood vessels (vasa vasorum) and nerves (nervi vasorum).
Vascular endothelium Tissue: The vascular endothelium is the single layer of flattened endothelial cells that lines every blood vessel and the heart. It behaves like an organ in its own right, actively controlling vascular tone, permeability, clotting, inflammation and the growth of new vessels.
Arterioles Vessel: Arterioles are the smallest arteries, about 10–100 µm in diameter, with one to three layers of smooth muscle. They are the main resistance vessels: they set total peripheral resistance and control flow into each capillary bed.
Capillaries Capillary bed: Capillaries are the smallest blood vessels, about 5–10 µm wide and about 1 mm long. Their walls are a single layer of endothelium, which lets gases, nutrients, wastes and fluid pass between blood and tissue. There are about 10 billion of them, with a total surface area of about 500–700 m².
Venules Vessel: Venules are small veins, about 10–100 µm across, that collect blood from capillaries. The smallest of them, the postcapillary venules, are a key site of fluid exchange and the main place where leukocytes leave the blood during inflammation.
Microcirculation Capillary bed: The microcirculation is the working unit of the smallest vessels in an organ: terminal arterioles, metarterioles, capillaries, venules and arteriovenous anastomoses. Here blood flow is matched to local metabolic need, and exchange with the tissues takes place.
Capillary exchange (Starling forces) Physiological process: Fluid moves across capillary walls according to the balance between hydrostatic pressure and colloid osmotic (oncotic) pressure. Fluid is filtered out at the arteriolar end and partly reabsorbed at the venular end, and the lymphatics return the rest.

Blood

Blood Tissue: Blood is a fluid connective tissue made of plasma carrying erythrocytes, leukocytes and platelets. It transports gases, nutrients, wastes, hormones and heat, buffers pH, defends against infection and seals injured vessels.
Plasma Fluid: Plasma is the straw-coloured liquid part of blood, making up about 55% of its volume. It is about 92% water and contains proteins, electrolytes, nutrients, gases, hormones and wastes. Serum is plasma that has lost its clotting factors.
Erythrocyte (red blood cell) Cell: Erythrocytes (red blood cells) are biconcave cells about 7.5 µm across, without a nucleus and packed with haemoglobin. They carry oxygen from the lungs to the tissues, help return CO₂ to the lungs and circulate for about 120 days.
Haemoglobin Molecule: Haemoglobin is the iron-containing protein that fills red blood cells. It is a tetramer of four globin chains, each carrying a haem group with ferrous iron (Fe²⁺). The four haems bind oxygen cooperatively.
Platelets (thrombocytes) Cell: Platelets are small cell fragments without a nucleus, about 2–4 µm across, shed from megakaryocytes in the bone marrow. They stick to damaged vessel walls, clump together into a plug and provide the surface on which the clotting cascade runs.
Haemostasis Physiological process: Haemostasis is the rapid, local arrest of bleeding from an injured vessel. It works in three overlapping steps: vascular spasm, formation of a platelet plug (primary haemostasis) and formation of a fibrin clot by coagulation (secondary haemostasis). Controlled fibrinolysis then removes the clot.
Coagulation cascade Physiological process: The coagulation cascade is a chain of enzyme reactions in which each plasma clotting factor activates the next, amplifying the signal at each step. It is triggered mainly by tissue factor and produces a burst of thrombin that converts soluble fibrinogen into a cross-linked fibrin mesh.
Blood groups (ABO and Rh) Molecule: Blood groups are inherited antigens on the surface of red blood cells. The most important are the ABO carbohydrate antigens and the Rh(D) protein. They determine which blood can be safely transfused and the risk of haemolytic disease of the fetus and newborn.

Cardiovascular physiology

Cardiac cycle Physiological process: The cardiac cycle is the sequence of electrical and mechanical events in a single heartbeat: atrial systole, ventricular systole (isovolumetric contraction and ejection) and ventricular diastole (isovolumetric relaxation and filling). At 75 beats/min it lasts about 0.8 s.
Heart sounds and murmurs Physiological process: Heart sounds are vibrations heard through a stethoscope, produced mainly by valve closure. S1 comes from closure of the AV valves and S2 from closure of the semilunar valves. S3 and S4 are extra sounds that may be normal or pathological. Murmurs are produced by turbulent blood flow.
Cardiac output Physiological process: Cardiac output is the volume of blood each ventricle pumps per minute. It equals heart rate multiplied by stroke volume, about 70 beats/min × 70 mL ≈ 5 L/min at rest, and can rise four- to sevenfold during exercise.
Frank–Starling mechanism Physiological process: The Frank–Starling mechanism is the heart's built-in ability to pump out whatever blood returns to it. When more blood fills the ventricle (higher preload), the muscle fibres are stretched further and contract more strongly, so stroke volume rises.
Cardiac action potentials Physiological process: The heart has two types of action potential. Atrial and ventricular muscle cells and Purkinje fibres have fast-response potentials, with a rapid Na⁺-driven upstroke and a long Ca²⁺-driven plateau. SA and AV nodal cells have slow-response potentials, with spontaneous depolarisation between beats and a Ca²⁺-driven upstroke.
Electrocardiogram (ECG) Physiological process: The electrocardiogram (ECG) records, from electrodes on the skin, the combined electrical activity of the heart over time. Its waves (P, QRS, T) and intervals (PR, QRS, QT) trace the sequence of atrial and ventricular depolarisation and repolarisation.
Arterial blood pressure Physiological process: Arterial blood pressure is the pressure blood exerts on the arterial walls. It rises to a systolic peak and falls to a diastolic trough with each beat. Its mean value, mean arterial pressure (MAP ≈ cardiac output × total peripheral resistance), drives the perfusion of every organ.
Baroreceptor reflex Physiological process: The baroreceptor reflex is a fast negative-feedback loop that buffers moment-to-moment changes in arterial pressure. Stretch receptors in the carotid sinus and aortic arch signal to the medulla, which adjusts heart rate, contractility and vascular tone within seconds.
Renin–angiotensin–aldosterone system (RAAS) Physiological process: The renin–angiotensin–aldosterone system (RAAS) is a hormonal cascade that defends blood pressure and extracellular fluid volume. Renin from the kidney converts angiotensinogen to angiotensin I. ACE then converts this to angiotensin II, which constricts arterioles and drives sodium and water retention through aldosterone, ADH and thirst.
Haemodynamics (flow, pressure and resistance) Physiological process: Haemodynamics is the physics of blood flow. Flow equals the pressure gradient divided by resistance, resistance depends very steeply on vessel radius (Poiseuille's law), and the speed of blood varies inversely with the total cross-sectional area of the vessels.

Fetal circulation

Fetal circulation Physiological process: Before birth, the placenta, not the lungs, oxygenates the blood. Three shunts (the ductus venosus, foramen ovale and ductus arteriosus) send the best-oxygenated blood to the heart and brain and bypass the liver and the fluid-filled lungs.
Foramen ovale (and fossa ovalis) Organ part: The foramen ovale is an oval opening in the fetal interatrial septum, guarded by a flap of septum primum. It lets oxygen-rich blood from the IVC pass from the right atrium to the left atrium. After birth it closes, leaving the fossa ovalis.
Ductus arteriosus Artery: The ductus arteriosus is a fetal artery joining the pulmonary trunk to the underside of the aortic arch, just beyond the left subclavian artery. It diverts most right ventricular output away from the lungs. After birth it closes and becomes the ligamentum arteriosum.
Ductus venosus Vein: The ductus venosus is a fetal vein inside the liver. It carries part of the oxygen-rich umbilical venous blood straight to the IVC, bypassing the liver sinusoids. After birth it closes and becomes the ligamentum venosum.
Umbilical arteries and vein Vessel: The umbilical cord contains three vessels. The two umbilical arteries carry deoxygenated fetal blood from the internal iliac arteries to the placenta. The single umbilical vein returns oxygenated blood to the fetus.

Mnemonics

Branches of the aortic arch: ABC'S: the Aortic arch gives the Brachiocephalic trunk, the left Common carotid and the left Subclavian artery.
Branches of the external carotid artery: "Some Anatomists Like Freaking Out Poor Medical Students": superior thyroid, ascending pharyngeal, lingual, facial, occipital, posterior auricular, maxillary, superficial temporal.
Branches of the subclavian artery: VIT C & D: vertebral, internal thoracic, thyrocervical trunk, costocervical trunk, dorsal scapular.
Abdominal aorta: vertebral levels: "12 – 1 – 1/2 – 2 – 3 – 4": coeliac trunk T12, superior mesenteric artery L1, renal arteries L1–L2, gonadal arteries L2, inferior mesenteric artery L3, aortic bifurcation L4.
Tributaries of the IVC: "I Like To Rise So High": Iliac (common iliac) veins, Lumbar veins, Testicular/ovarian vein (right), Renal veins, Suprarenal vein (right), Hepatic veins.
Auscultation areas: "All Physicians Take Money". Moving from the upper right sternal border to the apex: Aortic (2nd right intercostal space), Pulmonary (2nd left intercostal space), Tricuspid (4th–5th left intercostal space at the sternal edge), Mitral (apex, 5th left intercostal space, midclavicular line).
Tetralogy of Fallot: PROV: Pulmonary (right ventricular outflow) stenosis, Right ventricular hypertrophy, Overriding aorta, Ventricular septal defect.
Right shift of the O₂–Hb dissociation curve: "CADET, face Right". CO₂, Acid, 2,3-DPG (BPG), Exercise and Temperature all shift the curve to the right, which lowers affinity and makes O₂ unloading easier.

From the Corpus anatomy atlas — for education only.