CV - Specialised circulations

Question 1

Describe auto regulation:

Answer 1

describes the ability to maintain relatively constant blood flow in the face of changes in perfusion pressure

This occurs independently from extrinsic factors and includes particularly the myogenic mechanism whereby an increase stretch of a vessel wall causes a vasoconstriction and vice versa

Intrinsic factors - myogenic, metabolic and endothelial

As prefusion pressure drops below or rises above 60 to 160 mmHg, there is a marked decreased or increased flow, and organ function can be compromised

Question 2

Describe the features of coronary blood flow:

Answer 2

High basal flow - 80 mL/min/100g

High basal O2 consumption - 10-25 mL/min/100g

High myocardial O2 extraction - 70-80%

high density of myocardial capillaries

O2 transport increases by myoglobin in myocytes
little O2 in reserve if requirements increases

Question 3

Describe the control of coronary blood flow:

Answer 3

Local/ metabolic control:

• Low O₂, high CO₂, adenosine, NO, H⁺, K⁺, lactate, prostaglandins (PGs) → vasodilation
• NO released from endothelial increases vasodilation in response to higher flow
• ensure blood flow matches metabolism

Sympathetic nervous system:

• sympathetic vasoconstrictor tone is present
• Increased sympathetic activity to the sinus node → higher HR → increased metabolism → indirect vasodilation

Compression of coronary arteries:

• during systole coronary arteries are compressed + flow is reduced
• most blood flow occurs during early diastole aided by aortic recoil

Question 4

Describe blood flow to skeletal muscle:

Answer 4

At rest skeletal muscle receives 18% of CO, during exercise can increase to 70% of CO

Neurohormonal Control:

• at rest sympathetic vasoconstriction dominates via α-adrenoceptors
• during exercise adrenal medulla releases adrenaline binding to binds β₂-adrenoceptors
• induces vasodilation
• sympathetic vasoconstriction is overridden by local metabolites = sympatholysis

Local metabolic Control:

• Vasodilators: K⁺, PO₄³⁻, ↓ pH, hypoxia, lactic acid (H⁺)
• More capillaries open → Shorter diffusion distance for O₂ & nutrients.
• action of the muscle pump also enhances flow during rhythmic contraction

Question 5

Describe cerebral blood flow:

Answer 5

Brain receives ~15% of cardiac output

Autoregulation:

• Maintains CBF between 50-150 mmHg mean arterial pressure
• Vasodilation when pressure decreases
• Vasoconstriction when pressure increases

Metabolic control:

• ↑ CO₂ (hypercapnia) → vasodilation → ↑ blood flow
• ↓ CO₂ (hypocapnia) → vasoconstriction → ↓ blood flow

brain enclosed in the skull - intracranial pressure influences cerebral perfusion

Question 6

Describe blood flow to the splanchnic:

Answer 6

Supplied by the celiac artery, superior mesenteric artery, and inferior mesenteric artery

Blood drains through the portal vein into the liver before reaching the systemic circulation

Regulation:

• parasympathetic control which causes vasodilatation
• hormonal control, vasodilators increase blood flow after eating
• local factors/ autoregulation, local metabolite including

Regulation during hypotension or exercise:

• nervous control, sympathetic vasoconstrictor nerves e.g. - ↑ SV & CO.
• capacitance function of splanchnic system

Question 7

Describe pulmonary circulation:

Answer 7

The circulation of blood between the heart and lungs for gas exchange

Features:

• Low pressure, low resistance system
• High compliance allows it to accommodate large blood volume changes with minimal pressure changes

Control:

Hypoxic pulmonary vasoconstriction - low oxygen levels cause vasoconstriction to redirect blood to better ventilated lung regions

Autonomic Nervous System: Sympathetic activation has limited effect; parasympathetic stimulation induces mild vasodilation

Local gas tensions
allows perfusion/ventilation matching

Question 8

Describe cutaneous circulation:

Answer 8

Blood supply to the skin, critical for thermoregulation

Key features:

• extensive venous plexus
• High arteriovenous anastomoses (AVAs), which allow rapid changes in blood flow
• functions include delivering nutrients & regulating body temperature
• can increase 15 x

Sympathetic Control:

• Vasoconstriction (via α1 receptors) reduces heat loss
• Vasodilation (inhibition of sympathetic tone) increases heat dissipation

Local metabolic control

Non acral skin (trunk/ upper limbs):

• nervous control = hypothalamus output to sympathetic nerves
• when hot: decrease nerve activity, vasodilatation
• when thermoneutral/cold: increased nerve activity, vasoconstriction

Acral skin (extremities of the hands and feet, ears)

• when hot: AVAs open, flow in venous plexus near skin surface loses heat
• when cold: AVAs close, moving blood away from venous plexus, conserves heat

Question 9

Describe the influence of intracranial pressure on cerebral blood flow:

Answer 9

The cranial cavity contains the brain (1400g), blood (75ml), and cerebrospinal fluid (CSF, 75ml)—these volumes must remain constant.

Raised ICP compresses blood vessels, reducing cerebral blood flow (CBF).

Causes of increased ICP:

• Brain oedema → Swelling increases brain volume.
• Hydrocephalus → Excess CSF.
• Increased cerebral blood volume → Venous blockage or excessive vasodilation.

Question 10

Describe the effect of posture on the circulation (relate to fainting):

Answer 10

Standing from Lying/Sitting:

Gravity causes blood pooling (~400ml) in lower limbs.

Reduced venous return → ↓ Stroke Volume (SV) → ↓ Cardiac Output (CO) → ↓ Arterial Blood Pressure (BP).

Cerebral perfusion drops, which could cause fainting.

Compensatory Mechanisms to Maintain BP:

Baroreceptor Reflex:
Detects ↓ BP, triggers ↑ HR (tachycardia).
Arterial vasoconstriction → ↑ Total Peripheral Resistance (TPR) to restore BP.
Venoconstriction → Improves venous return.

Muscle Pump: Contraction of leg muscles aids venous return.

Hormonal Response (RAAS system): Increases blood volume (longer-term).

Question 11

Explain fainting in relation to cerebral ischaemia, blood pressure & cerebral vascular resistance

Answer 11

Fainting (syncope) occurs when cerebral blood flow (CBF) is insufficient due to:

• ↓ BP (e.g., from standing, dehydration, or heat-induced vasodilation).
• ↑ Cerebral Vascular Resistance (CVR) failing to adapt
• failure/delay in compensatory mechanisms (e.g., baroreflex impairment in diabetes, heart failure, or medications).

Cerebral Autoregulation:

• drop in BP → Cerebral vasodilation to lower resistance and restore flow
• metabolic factors (↑ CO₂, ↓ O₂) also promote vasodilation
• if compensation is inadequate, syncope occurs

Recovery Position:

• lying down aids venous return and restores cerebral blood flow
• raising legs further helps by using gravity to assist blood return.