Short vessels called arteriovenous anastomoses (AVAs) link tiny arteries and veins. They have a large lumen diameter. The strong and muscular walls allow AVAs to completely clog the vascular lumen, preventing blood flow from artery to vein (acting like a sphincter). When the AVAs open, they create a low-resistance connection between arteries and veins, allowing blood to flow into the limbs’ superficial venous plexuses. There is no diffusion of solutes or fluid into the interstitium due to their strong muscle walls.

AVAs are densely innervated by adrenergic fibres from the hypothalamic temperature-regulation centre. High sympathetic output occurs at normal core temperatures, inducing vasoconstriction of the AVAs and blood flow through the capillary networks and deep plexuses. When the temperature rises, sympathetic output decreases, producing AVA vasodilation and blood shunting from the artery to the superficial venous plexus. Heat is lost to the environment as hot blood rushes near to the skin’s surface.
AVAs are a specialized anatomical adaptation that can only be found in large quantities in the fingers, palms, soles, lips, and pinna of the ear.

Hyperaemia is the process where the body adjusts blood flow to meet the metabolic needs of different tissues in health and disease. Vasoactive mediators that take part in this process include K+, adenosine, CO2, H+, phosphates and H2O2. Although the mechanism is not clear, all these mediators likely contribute to some extent at different points.

Specific organs are more sensitive to specific metabolites:
K+ and adenosine are the most potent vasodilators in skeletal muscles

CO2 and K+ are the most potent vasodilators in cerebral circulation.

The causes of a prolonged QT interval include:

Hypomagnesaemia
Hypothermia
Hypokalaemia
Hypocalcaemia
Hypothyroidism
Jervell-Lange-Nielsen syndrome (autosomal dominant)
Romano Ward syndrome (autosomal recessive)
Ischaemic heart disease
Mitral valve prolapse
Rheumatic carditis
Erythromycin
Amiodarone
Quinidine
Tricyclic antidepressants
Terfenadine
Methadone
Procainamide
Sotalol

The pressure measured in the right atrium or superior vena cava is known as central venous pressure (CVP). In a spontaneously breathing subject, the usual CVP value is 0-8 cmH2O (0-6 mmHg).

The structure of the CVP waveform is as follows:
The CVP’s components are listed in the table below:
Component of the waveform
The cardiac cycle phase.
mechanical event
mechanical event Diastole 
Atrial contraction
a wave 
C  wave 
v wave
Early systole
The tricuspid valve closes and bulges 
Late Systole 
Filling of the atrium with systolic blood 
x descent
y descent
Mid systole
Relaxation of the atrium 
Early diastole
Filling of the ventricles at an early stage

Capillaries are designed with a small diffusion distance for nutrition and gaseous exchange with the tissues they serve. Because oxygen and carbon dioxide are both highly soluble in lipids (lipophilic), they can easily diffuse along a concentration gradient across the endothelial lipid bilayer membrane. In contrast, glucose, electrolytes, and other polar, charged molecules are lipid-insoluble (hydrophilic). These chemicals are unable to pass through the lipid bilayer membrane directly and must instead travel through gaps between endothelial cells.
Capillaries are divided into three types: continuous, fenestrated, and sinusoidal. Each of these capillary types contains different sized gaps between the endothelial cells that operate as a filter, limiting which molecules and structures can pass through.

The permeability of capillaries is affected by the wall continuity, which varies depending on the capillary type.
Skeletal muscle, myocardium, skin, lungs, and connective tissue all have continuous capillaries. These capillaries are the least permeable. They have a basement membrane and a continuous layer of endothelium. The presence of intercellular spaces allows water and hydrophilic molecules to pass across. Tight connections between the cells and the glycocalyx inhibit passage via these gaps, making diffusion 1000-10,000 times slower than for lipophilic compounds. The diffusion of molecules larger than 10,000 Da, such as plasma proteins, is likewise prevented by this narrow pore system. These big substances can pass through the capillary wall, but only very slowly, because endothelial cells have enormous holes.

The kidneys, gut, and exocrine and endocrine glands all have fenestrated capillaries. These are specialized capillaries that allow fluid to be filtered quickly. Water, nutrients, and hormones can pass via windows or fenestrae in their endothelium, which are connected by a thin porous membrane. They are ten times more permeable than continuous capillaries due to the presence of these fenestrae. Fenestrated capillaries have a healthy basement membrane.
The spleen, liver, and bone marrow all have sinusoidal capillaries, also known as discontinuous capillaries. Their endothelium has huge gaps of >100 nm, and their basement membrane is inadequate. They are highly permeable as a result, allowing red blood cells to travel freely.

In skeletal muscle, blood flow is closely related to metabolic rate. Due to the contraction of precapillary sphincters, most capillaries are blocked off from the rest of the circulation at rest and are not perfused. This causes an increase in vascular tone and vessel constriction. As metabolic activity rises, this develops redundancy in the system, allowing it to cope with greater demand. During exercise, metabolic hyperaemia, which is induced by the release of K+, CO2, and adenosine, recruits capillaries. Sympathetic vasoconstriction in the active muscles is overridden by this. Simultaneously, blood flow in non-working muscles is restricted, preserving cardiac output. During exercise, muscle contractions pump blood through the venous system, raising the pressure differential between arterioles and venules and boosting blood flow via capillaries.

Capillary angiogenesis is evident when muscles are used repeatedly (e.g. endurance training). It is a long-term effect, not a quick fix for increased blood flow.

The local partial pressure of alveolar oxygen is the primary intrinsic control of pulmonary blood flow (pAO2). Low pAO2 promotes arteriole vasoconstriction and vice versa. The hypoxic pulmonary vasoconstriction (HPV) reflex allows blood flow to be diverted away from poorly ventilated alveoli and towards well-ventilated alveoli in order to maximize gaseous exchange.

Syncope and sudden death due to ventricular tachycardia, particularly Torsades-des-pointes is seen in prolongation of the QT interval.

The causes of a prolonged QT interval include:
Erythromycin
Amiodarone
Quinidine
Methadone
Procainamide
Sotalol
Terfenadine
Tricyclic antidepressants
Jervell-Lange-Nielsen syndrome (autosomal dominant)
Romano Ward syndrome (autosomal recessive)
Hypothyroidism
Hypocalcaemia
Hypokalaemia
Hypomagnesaemia
Hypothermia
Rheumatic carditis
Mitral valve prolapse
Ischaemic heart disease

Cardiac output (CO) is calculated by multiplying stroke volume (SV) by heart rate (HR):
CO = HR x SV

As a result, both stroke volume and heart rate are exactly proportional to cardiac output. There will be an increase in cardiac output if the stroke volume or heart rate increases, and a reduction in cardiac output if the stroke volume or heart rate lowers.

The terminology cardiac output refers to the amount of blood pumped by the heart in one minute. Women’s rates are around 5 L/min, whereas men’s rates are somewhat higher, around 5.5 L/min.
Cardiac output (CO) is calculated by multiplying stroke volume (SV) by heart rate (HR):
CO = HR x SV

As a result, both stroke volume and heart rate are exactly proportional to cardiac output. There will be an increase in cardiac output if the stroke volume or heart rate increases, and a reduction in cardiac output if the stroke volume or heart rate lowers.

Short vessels called arteriovenous anastomoses (AVAs) link tiny arteries and veins. They have a large lumen diameter. The strong and muscular walls allow AVAs to completely clog the vascular lumen, preventing blood flow from artery to vein (acting like a sphincter). When the AVAs open, they create a low-resistance connection between arteries and veins, allowing blood to flow into the limbs’ superficial venous plexuses. There is no diffusion of solutes or fluid into the interstitium due to their strong muscle walls.

AVAs are densely innervated by adrenergic fibres from the hypothalamic temperature-regulation centre. High sympathetic output occurs at normal core temperatures, inducing vasoconstriction of the AVAs and blood flow through the capillary networks and deep plexuses. When the temperature rises, sympathetic output decreases, producing AVA vasodilation and blood shunting from the artery to the superficial venous plexus. Heat is lost to the environment as hot blood rushes near to the skin’s surface.
AVAs are a specialized anatomical adaptation that can only be found in large quantities in the fingers, palms, soles, lips, and pinna of the ear.