Properties of special circulations (Physiology)

A surgical procedure used to treat coronary heart disease. It diverts blood around narrowed or clogged parts of the major arteries to improve blood flow and oxygen supply to the heart

Two coronary arteries originate from the left side of the heart.

They are the Left Main Coronary Artery (LMCA) and the Left Anterior Descending Artery (LAD).

The LMCA arises from the left coronary sinus of the aorta.

The LAD branches off from the LMCA and runs along the interventricular groove toward the apex of the heart.

Carry blood with a poor level of oxygen from the myocardium to the right atrium

Most of the blood of the coronary veins returns through the coronary sinus

-Requires a high basal supply of oxygen, approximately 20 times more than resting skeletal muscle

-It also needs to increase oxygen supply in proportion to increased demand or cardiac work

High capillary density and a large surface area for oxygen transfer

-The high capillary density and large surface area for oxygen transfer in coronary circulation reduce the diffusion distance to myocytes

-This reduction in diffusion distance is crucial because diffusion time is proportional to distance squared, allowing for fast oxygen transport to the heart muscles

-Fibre diameter 50 μm-Capillaries 400 / mm2

-Fibre diameter 18 μm-Capillaries 3000 / mm^2

-During normal activity, coronary circulation experiences high blood flow, approximately 10 times the flow per weight of the rest of the body

-It also has relatively sparse sympathetic innervation, high nitric oxide release leading to vasodilatation, and high oxygen extraction (75%), compared to the average of 25% in the body

During increased demand, coronary blood flow increases in proportion to demands

During increased demand, the production of vasodilators such as adenosine, potassium ions (K+), and acidosis out-compete relatively low sympathetic vasoconstriction

Circulating adrenaline dilates coronary vessels during increased demand due to the abundance of β2-adrenoceptors

Is a measure of the uptake and removal of oxygen by hemoglobin under different partial pressures

Shows how much oxygen pressure leads to what percentage of oxygen saturation of blood hemoglobin

Increased affinity

Decreased affinity

The Bohr shift

-The Bohr shift occurs when coronary sinus blood returning to the right atrium from myocardial tissue has a greater carbon dioxide content due to high capillary density, surface area, and small diffusion difference

-This high CO2 and low pH shift the oxygen dissociation curve to the right, meaning that hemoglobin has less affinity for oxygen, leading to more oxygen being released to the myocardial tissues

The myocardium is able to extract 75% of the oxygen, whereas typically, other tissues extract around 25% of the oxygen

An increased oxygen requirement produces increased blood flow

Extraction of oxygen is near maximal during normal activity, so to provide more oxygen during demand, increasing blood flow is necessary

-Myocardium metabolism generates metabolites that produce vasodilatation, leading to increased blood flow, a phenomenon known as metabolic hyperaemia

-Examples of such metabolites include adenosine, produced by ATP metabolism and released from cardiac myocytes, as well as increases in pCO2, H+, and K+ levels

-Functional end-arteries are arteries where decreased perfusion in one of them can cause major problems because they do not have adequate collateral circulation

-This is significant in Ischaemic Heart Disease because decreased perfusion in one of these arteries can lead to severe consequences

-The heart is susceptible to both sudden and slow obstructions

-Sudden obstructions, such as acute thrombosis, can produce myocardial infarction

-Slow obstructions, such as atheroma (sub-endothelial lipid plaques) causing chronic narrowing of the lumen, produce angina

Systole obstructs coronary blood flow

Thrombosis refers to the total occlusion of the left anterior descending coronary artery

Occlusion in this artery leads to obstruction of blood flow to the anterior (front) left ventricle, resulting in myocardial infarction

Myocardial infarction leads to various consequences such as ischemic tissue, acidosis, pain (stimulation of C-fibres), impaired contractility, sympathetic activation, arrhythmias, and cell death (necrosis)

Type of X-ray used to check blood vessels

Chest pain caused by reduced blood flow to the heart muscle

Mechanical factors that reduce coronary flow during diastole include shortening diastole (e.g., high heart rate), increased ventricular end-diastolic pressure (e.g., heart failure, aortic stenosis, stiffening of the ventricle), and reduced diastolic arterial pressure (e.g., hypotension, aortic regurgitation).

High heart rate shortens diastole, which can reduce coronary blood flow.

Heart failure increases ventricular end-diastolic pressure, thereby reducing coronary blood flow during diastole

Hypotension reduces diastolic arterial pressure, which can decrease coronary blood flow during diastole

Include defence against the environment, the Lewis triple response to trauma leading to increased blood flow, and temperature regulation.

Cutaneous circulation serves as a defence mechanism against the environment by responding to trauma with the Lewis triple response, which involves increased blood flow to the affected area

By delivering heat from the body core through conduction, radiation (infrared proportional to skin temperature), convection (heat carried away by the air), and sweating (latent heat of evaporation)

The skin is considered poikilothermic because it can tolerate a wide range of temperatures (from 0°C to 40°C briefly) without damage, unlike homeothermic organisms which maintain a constant temperature

Skin temperature depends on factors such as skin blood flow and ambient temperature

Organisms whose body temperature varies with the temperature of their environment

Organisms that regulate their body temperature internally to maintain a relatively constant temperature regardless of external conditions.

-The special structural features include sympathetic vasoconstrictor fibres: releasing noradrenaline acting on α1 receptors

-Sudomotor vasodilator fibres: releasing acetylcholine acting on endothelium to produce nitric oxide, and arterio-venous anastomoses (AVAs)

Sympathetic vasoconstrictor fibres release noradrenaline, which acts on α1 receptors to induce vasoconstriction in cutaneous blood vessels

Sudomotor vasodilator fibres release acetylcholine, which acts on the endothelium to produce nitric oxide, leading to vasodilation in cutaneous blood vessels

Temperature regulation nerves in the hypothalamus drive the activity of sudomotor vasodilator fibers and sympathetic vasoconstrictor fibers, influencing cutaneous circulation

Arterio-venous anastomoses are direct connections between arterioles and venules that expose blood to regions of high surface area, facilitating heat exchange through convection, conduction, radiation, and evaporation in cutaneous circulation

The special functional features include responsiveness to ambient and core temperatures, aiding in both heat loss and heat conservation

Cutaneous circulation increases vasodilation and venodilation in response to an increase in ambient temperature, facilitating heat loss from the body

Cutaneous circulation increases vasoconstriction and venoconstriction in response to a decrease in ambient temperature, helping to conserve heat within the body.

Severe cold can sometimes trigger paradoxical cold vasodilatation, where blood vessels in the skin dilate despite the cold temperature, possibly as a protective mechanism to prevent tissue damage.

Core temperature receptors in the hypothalamus control sympathetic activity to the skin, thereby regulating skin blood flow in response to changes in core temperature.

Conserves heat• Sympathetic nerves react to local cold by releasing noradrenaline, which binds to a2 receptors on vascular smooth muscle in skin.

• a2 receptors bind NA at lower temperatures than a1receptors

Protects against skin damage• Caused by paralysis of sympathetic transmission• Long-term exposure leads to oscillations ofcontract/relax

Increased core temperature stimulates temperature receptors in the anterior hypothalamus

This leads to:

Sweating and increased sympathetic activity to sweat glands, mediated by acetylcholine

Vasodilation due to increased sympathetic sudomotor activity

Acetylcholine acts on endothelium to produce nitric oxide (NO), dilating arterioles in the extremities.

Resulting in increased cutaneous perfusion

Increased core temperature stimulates:

Sweating and enhances sympathetic activity to sweat glands, both mediated by acetylcholine

Vasodilation in the extremities due to increased sympathetic sudomotor activity

This leads to increased cutaneous perfusion

The anterior hypothalamus contains temperature receptors.

Stimulation by increased core temperature initiates responses such as:

Sweating

Increased sympathetic activity to sweat glands

These contribute to enhanced cutaneous perfusion

Increased sympathetic sudomotor activity causes:

Acetylcholine to act on endothelium, producing nitric oxide (NO)

NO dilates arterioles in the extremities

This results in vasodilation and increased cutaneous perfusion

Baroreflex/RAAS/ADH-stimulated vasoconstriction of skin blood vessels occurs during conditions such as hemorrhage, sepsis, and acute cardiac failure.

This redirects blood to more important organs/tissues following loss of blood pressure.

Mediated by sympathetic vasoconstrictor fibers, adrenaline, vasopressin, and angiotensin II.

This response is responsible for the pale, cold skin observed in patients in shock

During hemorrhage, warming up the body too quickly may reduce cutaneous vasoconstriction.

Cutaneous vasoconstriction is essential for redirecting blood flow to vital organs/tissues rather than the skin

Cutaneous circulation plays a role in emotional communication, such as blushing

Blushing involves activation of sympathetic sudomotor nerves

The Lewis triple response is a response to skin injury

It involves three stages: redness (due to arteriolar dilation), flare (surrounding vasodilation), and wheal (localized edema)

This response demonstrates the complex involvement of cutaneous circulation in the body's response to injury

The Lewis triple response is the skin's response to trauma.

It involves three stages:

Redness, caused by capillary vasodilation

Flare, a redness in the surrounding area due to arteriolar dilation mediated by axon reflex

Wheal, exudation of extracellular fluid from capillaries and venules

The Lewis triple response facilitates increased delivery of immune cells and antibodies to the site of damage caused by trauma

This helps the body deal with invading pathogens effectively

Prolonged obstruction of flow by compression can lead to severe tissue necrosis, such as bed sores in areas like heels, buttocks, and weight-bearing areas

These problems can be avoided by shifting position or turning, causing reactive hyperemia upon the removal of compression

The skin has high tolerance to ischemia

Postural hypotension and edema can occur due to gravity, especially when standing for long periods in hot weather

This can lead to a decrease in central venous pressure (hypotension) and increased capillary permeability (edema)

Standing for long periods in hot weather can lead to symptoms such as feeling faint and tighter rings on the fingers