CARDIO-VASCULARSYSTEM:

BLOOD FLOWIN SPECIAL AREAS (part - II)

SPECIAL CIRCULAIONS

CEREBRAL CIRCULATION

CEREBRAL CIRCULATION

• The carotid arteries are the major vessels supplying the brain in humans. The vertebral vessels supplement the carotid flow.

• The brain relies on a continuous blood flow for adequate function.

• Interruption of blood flow for only 5-10 seconds causes a loss of consciousness.

• Circulatory arrest for only 3-4 minutes results in irreversible brain damage.

NORMAL CEREBRAL BLOOD FLOW

Represents approximately 15% of the resting cardiac output (750 ml/min), or about 50-55 ml blood/100 g tissue/min. CORONARY FLOW IN DIFFERENT

PHASES OF CARDIAC CYCLE

CONTROL OF CEREBRAL BLOOD FLOW

The brain, is surrounded by a rigid skull, but vascular diameter can be altered in response to changing blood flow needs by a narrowing of the venous bed, which compensates for vasodilation on the arterial side.

- Cerebral autoregulation is very effective in controlling blood flow over a range of arterial pressures.

- CO2 concentration is the major controller. Moderate increases or decreases in CO2 levels may double or halve normal cerebral blood flow, respectively.

CONTROL OF CEREBRAL BLOOD FLOW

Neural control of cerebral blood flow appears to be limited to the larger vessels and those located within the pia mater. Both sympathetic and parasympathetic neurons supply the cerebral vessels, and the role of these neurons remains unknown.

CBF & CSF pressure

CSF is an ultrafiltrate of plasma that circulates freely throughout the cerebral ventricles and central canal of the spinal cord. It is formed and reabsorbed at the rate of about 500 ml/day. An acute increase in CSF pressure may elicit vascular responses (Cushing's reflex, or the CNS ischemic reflex).

CUSHING REFLEX

• Cushing's reflex is a systemic vasoconstriction in response to an increase in cerebrospinal fluid (CSF) pressure.

• The marked systemic hypertension helps maintain adequate blood flow to the brain in the presence of a high intracranial pressure.

• The reflex is initiated by the medullary centers, which activate when they become hypoxic as a result of limited blood flow.

CONDITIONS AFFECTINGTHE FLOW

• Mental hard work or sleep does not produce a change in cerebral blood flow. But this does not mean that there is actually no change.

• The active part becomes more vascular and readjustment of brain blood flow occurs in other parts of the brain, so that the total flow does not change.– Ex-1: repeated exposure to light in animals, causes a

rise in blood flow in the visual cortex– Ex-2: when hand movements are (mentally)

conceived by a man, the frontal regions of brain show a marked rise in blood supply

CBF - IN SUMMARY

• Cerebral blood flow is determined by factors within the brain itself.

• Flow remains constant over a wide range of varying conditions, but local changes in blood flow are linked to local changes in metabolic activity.

• Nerves play little, if any, role and under normal conditions, metabolic autoregulation predominates.

• The cerebral vasculature is very sensitive to changes in blood CO2 levels, but responds only slightly to changes in O2 levels.

SKELETAL MUSCLE

In an average adult man, 40%-50% of the total body weight is made up of skeletal muscle.

RESTING SKELETAL MUSCLE

• Resting skeletal muscle consumes about 20% of the resting O2 consumption and receives about 20% of the cardiac output.

• Blood flow in resting skeletal muscle varies from 1.5-6 ml/100 g tissue/min.

• Muscles composed primarily of red fibers receive a larger blood flow than those comprised mainly of white fibers.

EXERCISING SKELETAL MUSCLE

• Exercising skeletal muscle receives approximately 80 ml blood/100 g tissue/min (a 15- to 20-fold increase over the resting state) and extracts approximately 80% of the O2 from the arterial blood.

• The release of metabolic products from the muscle tissue during exercise results in a large increase in blood flow. These breakdown products include increased levels of CO2, H+, K+, lactate, adenosine, and decreased O2 levels.

MODERATE EXERCISE

• Even with the marked vasodilation that occurs during moderate exercise, blood flow is inadequate to maintain adequate O2 delivery to the cells.

• The anaerobic threshold is defined as the level of exercise that produces a significant rise in the level of lactic acid in the bloodstream.

ANAEROBIC THRESHOLD

The anaerobic threshold occurs at an O2 consumption that is approximately 60% of the maximal consumption for any individual.

CUTANEOUS CIRCULATION

• Cutaneous arterioles form a dense network just under the dermis layer of the skin. These arterioles give rise to metarterioles, which subdivide into capillary loops.

• The capillary loops provide a large surface area for heat exchange.

• The venules form an extensive subpapillary venous plexus.

• Arteriovenous anastomoses are located in the distal parts of the extremities and the nose, lips, and ears. These vessels are wide, low-resistance connections that serve as shunts and allow blood to bypass the superficial capillary loops.

CUTANEOUS CAPILLARIES

INNERVATION OF THE CUTANEOUS VESSELS

Innervation of the cutaneous vessels occurs through the sympathetic adrenergic system, which provides a tonic vasoconstrictor effect.

INNERVATION OF THE CUTANEOUS VESSELS

• The cutaneous arterioles and metarterioles have both alpha- and beta-receptors. The arteriovenous anastomoses have only alpha-receptors.

• The venous plexus has separate neural connections and can undergo marked venoconstriction, which minimizes the amount of blood in the skin.

FUNCTIONS OF THE CUTANEOUS VESSELS• The metabolic rate of the skin is relatively

small so that a minimal amount of blood flow to the skin can supply the nutritive functions.

• The major function of the cutaneous blood flow is to aid in the regulation of body temperature.

• Heat conservation is accomplished by markedly diminishing the rate of blood flow to the skin.

• Body heat is preserved because less heat is dissipated to the environment.

HEAT DISSIPATION

• When body temperature rises as a result of increased metabolic activity, the added heat load must be eliminated.

• The increased blood temperature stimulates centers in the hypothalamus that inhibit the normal tonic vasoconstrictor outflow to the cutaneus vessels and increase blood flow to the skin.

HEAT DISSIPATION

• The increased flow carries heat to the surface of the body, where it is dissipated by radiation, evaporation, and conduction to the environment.

• If the environmental temperature is higher than the body temperature, heat can only be dissipated by means of evaporation of sweat.

CLINICAL NOTE: THE TRIPLE

RESPONSE (WHITE LINE) • If the skin is stroked lightly by a smooth end

of a stick or glass rod, along the line described by the stroke a WHITE LINE develops in a few seconds, which lasts for about 5 minutes.

• The WHITE LINE develops due to contraction of the subpapillary venous plexus which contracts as a direct response to the stroke and is not mediated by any nerve or chemical.

CLINICAL NOTE:RED REACTION

If the stroke is made very firmly, the characteristic triple response develops, particularly in a sensitive subject. The features of the triple response are :

• The RED REACTION (also called RED LINE) appears where the stroke is made. This is due to capillary engorgement which in turn is due relaxation of the precapillary sphincter, in the area.

• The line appears within about 10 seconds and become well developed in about 1 minute. The cause of the relaxation of the precapillary sphincter is the release of histamine from the damaged tissue (histamine is a relaxant of vascular smooth muscles).

• According to some authors, the red line is due to dilatation of the small venules.

PULMONARY BLOOD FLOW

Pulmonary blood flow is controlled locally by the alveolar gas tensions in a given region.

• Pulmonary artery pressure ~ 25/10 mmHg

• Right atrial pressure ~0-3 mmHg

• Left atrial pressure normally about 8-10 mmHg

PATTERN OF LOCAL CONTROL • If the partial pressure of O2 in

the alveoli falls, or the partial pressure of CO2 rises, this stimulates pulmonary vasoconstriction.

• The resulting increase in resistance reduces flow to that region, diverting blood to areas of the lung with more normal gas pressures.

• This pattern of local control of blood flow is the reverse of that seen in the systemic circulation, where low tissue O2 and high tissue CO2 are associated with vasodilatation.

CLINICAL NOTE: COR PULMONALE

If pulmonary vasoconstriction is widespread, there may be an increase in pulmonary arterial pressure because of increased total pulmonary resistance (which is normally only about one-third of systemic resistance). This type of pulmonary hypertension can lead to hypertrophy and then failure of the right ventricle. Right ventricular hypertrophy secondary to lung disease is called «cor pulmonale».

SPLANCHNIC BLOOD FLOW

The splanchnic circulation comprises the vasculature of the entire gastrointestinal tract as well as that of the liver, pancreas and spleen, and receives about 25% of the resting cardiac output. Blood flow through this system is greatly reduced by reflex activation of adrenergic sympathetic vasoconstrictor nerves in response to hypotension or during exercise. This tends to raise peripheral resistance and blood pressure, ensuring that perfusion of other more essential areas is maintained.

SPLANCHNIC BLOOD FLOW

Under these conditions sympathetic nerves also constrict the splanchnic veins, increasing venous return and thus cardiac output. The considerable importance of this mechanism reflects the large splanchnic contribution to the total capacitance of the systemic circulation.

SPLANCHNIC BLOOD FLOW

In contrast to these vasoconstrictor and venoconstrictor responses, vascular resistance falls and gastrointestinal blood flow is increased following ingestion of a meal. This depends on locally produced dilator hormones such as secretin, cholecystokinin and vasoactive intestinal polypeptide, intestinal kinins, rather than the direct action of nerves.

FETAL CIRCULATION

• The placenta is a major organ in the fetus and receives more than 50% of the combined output of both ventricles.

• At birth, marked changes occur in both the systemic and pulmonary circulations as the placental blood supply is lost.

• Changes in pulmonary circulation primarily result from a 90% decrease in pulmonary vascular resistance as the lungs inflate after birth. The decreased resistance causes a large drop in the afterload for the right ventricle and lowers the right atrial and ventricular pressures.

FETAL CIRCULATION

• The drop in the right-sided pressures causes pressure in the left atrium to exceed the pressure in the right atrium.

• This pressure reversal causes the flap-like valve to close over the FORAMEN OVALE.

• The valve then normally fuses to the interatrial septum over the next few days.

• Closure of the foramen ovale prevents the right-to-left flow of venous blood and improves the oxygenation of the systemic arterial blood.

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