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Cerebrovascular System

Image : “Cerebrovascular System” by BruceBlaus. License: CC BY-SA 4.0

Coronary Circulation

The blood vessels that supply the cardiac muscles constitute the coronary circulation. The arteries supplying oxygenated blood to the cardiac muscles are the right coronary artery and the left coronary artery.

The veins that carry deoxygenated blood away from the heart are called the cardiac veins. These include the great cardiac vein, the anterior cardiac vein, the small cardiac vein, the posterior cardiac vein, and the middle cardiac vein.

The coronary arteries are autoregulated. When they are fully patent, they adjust the supply of blood according to myocardial demand. These, however, are the end arteries, the only source of oxygen and nutrients to the heart muscles.

The arterial supply of the heart (anterior view)

Image: “The arterial supply of the heart (anterior view).” by Patrick J. Lynch, medical illustrator derivative work: Fred the Oyster (talk) adaption and further labeling: Mikael Häggström – Coronary.pdf. License: CC BY-SA 3.0

Two coronary arteries arise from the root of the aorta, just before it exits the left ventricle.

The aorta has three sinuses:

  • The right coronary artery (RCA) emerges from the anterior aortic sinus
  • The left coronary artery (LCA) emerges from the posterior aortic sinus
  • No artery emerges from the right posterior aortic sinus

However, in some people, the two arteries may emerge as one trunk, or there is an extra third branch, known as the posterior coronary artery. The arteries that remain on the surface of the heart and follow the sulci are called epicardial coronary arteries, as shown in the figures.

The left coronary artery supplies blood to the left atrium, left ventricle, and interventricular septum. It gives off two main branches as follows:

  • The left circumflex artery (LCX)
  • The anterior interventricular artery or left anterior descending artery (LAD)

The left circumflex artery follows the coronary sulcus pathway on the left side of the heart. It then fuses with the branches of the right coronary artery.

coronary vessels posterior view

Image: “Coronary circulation viewed from posterior.” by staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. – Own work. License: CC BY 3.0

The anterior interventricular artery, also known as the left anterior descending artery, follows the route of the interventricular sulcus, where it gives off several branches that anastomose with the posterior interventricular artery.

The right coronary artery supplies blood to the right atrium, right ventricle, the conductive system of the heart, and a portion of the left ventricle. It gives off the following branches:

  • Marginal branches
  • Posterior interventricular artery or posterior descending artery

The anastomosis occurring in the coronary circulation is referred to as an anatomical anastomosis. These arteries do not supply blood efficiently if one of them is blocked by an atheroma. The three anastomoses are:

  • Branches of the anterior interventricular artery anastomose with the posterior interventricular artery
  • The circumflex artery anastomoses with the right coronary artery in the atrioventricular groove
  • The septal branches of both right and left coronary artery anastomose in the interventricular septum

The great cardiac vein receives tributaries from the left atrium and both ventricles. It begins at the apex and ascends along the anterior longitudinal sulcus.

The middle cardiac vein also begins at the apex of the heart and ascends in the posterior longitudinal sulcus.

The anterior cardiac vein drains blood from the right ventricle and opens into the right atrium.

The small cardiac vein drains blood from the posterior portion of the right atrium and ventricle.

All the cardiac veins except the anterior cardiac vein drain into coronary sinus as shown in the figures.

When the coronary arteries develop an atheroma in them, it leads to the decreased blood supply to the cardiac muscles. In such cases, when the oxygen demand exceeds the supply, it results in left-sided chest pain called angina. If the coronary artery is completely blocked, the area of the heart supplied by that particular artery undergoes infarction, often referred to as myocardial infarction or heart attack in layperson language. These lesions can be visualized in echocardiography or deduced from electrocardiography findings.

Coronary Blood Flow During Contraction and Relaxation

“Coronary Blood Flow During Contraction and Relaxation” Image created by Lecturio

The effect of ventricular contraction of Coronary Blood Flow is that the contracting myocytes collapse vessels. Arterial blood is forced backward toward aorta. During contraction, there is reduced blood flow, but during relaxation, there is engorgement, increasing coronary blood flow.

Cerebral Circulation

Cerebral circulation refers to the vessels supplying the brain. The brain receives approximately 750 ml of blood each minute, which is 15% of the cardiac output.

The arterial supply of the brain is divided into two categories:

  • The anterior circulation from the internal carotid artery system
  • The posterior cerebral circulation from the vertebral-basilar circulatory system

The anterior cerebral circulation supplies blood to the anterior portion of the brain. It comprises of the following arteries:

Circle of Willis

Image: “Schematic representation of the circle of Willis, arteries of the brain, and brain stem.” by Rhcastilhos – Gray519.png, License: Public Domain

The right and left internal carotid arteries are the branches of the common carotid artery. Each internal carotid artery gives off an anterior cerebral artery branch and continues as the middle cerebral artery as shown in the figure.

The anterior cerebral artery supplies blood to the frontal lobe and the superior medial portion of the parietal lobe. The anterior cerebral artery syndrome is a result of blockage of the anterior cerebral artery. The symptoms include behavioral changes as well as motor and sensory weakness of the lower leg and foot on the contralateral side.

The anterior communicating artery connects the two anterior cerebral arteries as shown in the diagram.

The middle cerebral artery supplies a major portion of the hemisphere except for the part of the cerebrum supplied by the anterior cerebral artery. Deep branches also supply the basal ganglia. Middle cerebral artery syndrome occurs as a result of occlusion of the middle cerebral artery. It results in contralateral loss of motor and sensory sensations of the face and arm. Damage to the dominant hemisphere results in aphasia since the middle cerebral artery also supplies Broca’s and Wernicke’s areas.

The posterior cerebral circulation supplies the occipital lobes of the brain, the medulla, and the brain stem. The arteries forming the posterior cerebral circulation are as shown below:

The vertebral arteries, arising from the subclavian artery, fuse to form the basilar artery in the cranium. It has two posterior inferior cerebellar artery branches that supply the cerebellum. Occlusion of the posterior inferior cerebellar artery results in lateral medullary syndrome.

The basilar artery supplies the midbrain and the cerebellum. It has the following branches:

  • The posterior cerebral artery supplies the occipital lobe
  • The superior cerebellar artery supplies the superior half of the cerebellum and a portion of the midbrain
  • The anterior inferior cerebellar artery supplies the cerebellum; occlusion of this artery causes lateral pontine syndrome 
  • The pontine arteries, which are small branches arising from the main trunk of the basilar artery, supply the pons and the adjacent areas of the brain

The posterior communicating artery connects the posterior cerebral artery to the middle cerebral artery and the internal carotid artery.

Venous drainage of the brain

The venous drainage of the brain consists of a superficial system of dural sinuses lying between the periosteum and the meningeal layer of the dura mater. The deep system consists of traditional veins inside the deep structures of the brain.

the dural sinuses of the brain

Image: “The dural sinuses of the brain.” by OpenStax – License: CC BY 4.0

The dural venous sinuses are valve-less vessels which ultimately drain into the internal jugular vein. The inferior sagittal sinus and the great cerebral vein unite to form the straight sinus.

The superior sagittal sinus, the straight sinus, the transverse sinus, and the occipital sinus converge at the confluence of sinuses as shown in the figure to the right.

The sigmoid sinus is a continuation of the transverse sinus and drains into the internal jugular vein.

The cavernous sinus is found on either side of the sella turcica. It drains the blood from the ophthalmic veins, as shown in the figure below. The blood then drains into the internal jugular vein via the superior and inferior petrosal sinuses.

the cavernous sinus

Image: “The cavernous sinus.” by Henry Vandyke Carter, Henry Gray (1918) Anatomy of the Human Body. Gray’s Anatomy, Plate 572. License: Public Domain

The veins of the brain lie within the brain structures. They are further categorized into a superficial and deep system.

The superficial venous system consists of:

  • The superficial cerebral vein drains into the superior sagittal sinus
  • The superficial middle cerebral vein drains into the cavernous sinus or the transverse sinus after draining the lateral surface of the cerebral hemispheres
  • The superior anastomotic vein connects the superficial middle cerebral vein to the cavernous sinus
  • The inferior anastomotic vein connects the superficial middle cerebral vein to the transverse sinus
  • The inferior cerebral vein drains into the cavernous and transverse sinuses after receiving blood from the inferior surface of the cerebral hemispheres

The deep venous system consists of:

  • Subependymal veins receive blood from the medullary veins and drain into the dural sinuses
  • The medullary veins drain into the subependymal veins
  • The great cerebral vein, also known as the vein of Galen, receives blood from the cerebrum and drains into the straight sinus

Cutaneous Circulation

cutaneous circulation

Image: “The cutaneous circulation.” by Henry Vandyke Carter, Henry Gray (1918) Anatomy of the Human Body. Gray’s Anatomy, Plate 942. License: Public Domain

The needs of the skin are minimal; as such, the blood supply is mainly via arteriovenous anastomosis as opposed to supplying by arterioles. The epidermis of the skin does not contain any arterioles or arteries to supply blood. Nutrients and waste products travel through simple diffusion from the underlying skin layers. The dermis consists of two regions – the papillary region and the reticular region.

The papillary region is the superficial layer composed of areolar connective tissues. As the name suggests, it consists of papillary projections into the epidermis that contain a network of blood capillaries.

The reticular region lies deeper in the papillary region and is composed of dense connective tissues made up of collagen, elastin, and reticular fibers. This layer is comparatively thicker than the papillary layer.

The network of capillaries in the papillary region is derived from the arteries of the reticular region. Similarly, these arteries drain into veins present in the deeper layer. The arteries are called rete arteriosum and the veins are called rete venosum. The skin’s blood supply is shown in the figure below.

The skin is a thermoregulatory organ, so its blood supply depends largely upon the internal body temperature and the external environment. For instance, if the external temperature is low, the brain sends sympathetic signals to the skin vessels. This causes vasoconstriction of the arterioles to prevent blood flow to the superficial layers of skin and to preserve heat loss.

Similarly, if the external temperature is high, the arterioles of the skin are dilated. This is to allow more blood flow to the superficial layers of the skin and allow the loss of heat via the skin.

Non-glabrous systemic responses

Cutaneous Circulation

“Cutaneous Circulation” Image created by Lecturio

Sympathetic vasoconstrictor

  • Tonically active but can be increased
  • Norepinephrine (α1- & α2-AR)
  • Adrenergic co-released neuropeptide Y

Active vasodilation

  • Not solely acetylcholine (ACh) mediated
  • Might be cholinergic co-released
  • Nitric oxide (NO) is involved but is not the mediator
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