The blood vessels which supply the cardiac muscles constitute the coronary circulation.
The arteries supplying oxygenated blood to the cardiac muscles are known as the coronary arteries. There are two main coronary arteries: right coronary artery and left coronary artery.
The veins which 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 i.e., when they are fully patent, they adjust the supply of blood according to myocardial demand. These, however, are the end arteries which means they are the only source of oxygen and nutrients to the heart muscles.
Two coronary arteries arise from the root of aorta, just before it exits the left ventricle. The arteries which 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:
- The left circumflex artery
- The anterior interventricular artery or left anterior descending artery
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.
The anterior interventricular artery, also known as the left anterior descending artery, follows the route of interventricular sulcus where it gives off several branches which anastomose with the posterior interventricular artery.
The right coronary artery supplies blood to the right atrium, right ventricle, conductive system of the heart and some area of the left ventricle. It gives off the following branches:
- The marginal branches
- The posterior interventricular artery or posterior descending artery.
The anastomosis occurring in the coronary circulation is referred to as anatomical anastomosis as these arteries do not supply blood efficiently if one of them is blocked by an atheroma. The three anastomoses occurring 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 decreased blood supply to the cardiac muscles. In such cases when the oxygen demand exceeds, 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 lay man language.
Cerebral circulation comprises of 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 and the posterior cerebral circulation, both of which are outlined below.
The anterior cerebral circulation supplies blood to the anterior portion of the brain. It comprises of the following arteries:
- 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, 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 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, as Broca’s and Wernicke’s areas are also supplied by the middle cerebral artery.
- The vertebral arteries arising from the subclavian artery fuse to form the basilar artery in the cranium. It gives off two posterior inferior cerebellar artery branches which 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 gives off 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. Lateral pontine syndrome occurs as a result of occlusion of the anterior inferior cerebellar artery.
- The pontine arteries are small branches arising from the main trunk of the basilar artery. These arteries 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 venous sinuses are valve-less vessels which ultimately drain into the internal jugular vein. The straight sinus is formed by the inferior sagittal sinus and the great cerebral vein.
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 beside.
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 veins of the brain lie within the brain structures. They are further categorized into superficial and deep system.
The superficial venous system consists of:
- The superficial cerebral vein, which drains into the superior sagittal sinus.
- The superficial middle cerebral vein, which drains into the cavernous sinus or the transverse sinus after draining the lateral surface of the cerebral hemispheres. Superior anastomotic vein connects the superficial middle cerebral vein to the cavernous sinus while the inferior anastomotic vein connects the superficial middle cerebral vein to the transverse sinus.
- 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, which, after receiving blood from the medullary veins, drain into the dural sinuses.
- The medullary veins, which drain into the subependymal veins.
- The great cerebral vein, also known as the vein of Galen, which receives blood from the cerebrum and drains into the straight sinus.
The epidermis of the skin does not contain any blood vessels. 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 and as the name suggests consists of papillary projections into the epidermis. The projections contain a network of blood capillaries. It is composed of areolar connective tissues.
The reticular region lies deeper to the papillary region and is composed of dense connective tissues made up of collagen, elastic 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 blood supply of the skin is shown in the figure.
As skin is a thermoregulatory organ, the blood supply to the skin 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 loss of heat via skin.