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.

The capillary hydrostatic pressure (Pc) is normally between 15 and 30 millimetres of mercury. Pc Decreases along the capillary’s length, mirroring the arteriolar and venule pressures proximally and distally.Pc is determined by the ratio of arteriolar resistance (RA) to venular resistance (RV).When the RA/RV ratio is high, the pressure drop across the capillary is modest, and Pcis is close to venule pressure.When the ratio of RA/RV is low, the pressure drop across the capillary is considerable, and Pcis is close to arteriolar pressure.Pcis closer to the venule pressure and thus more responsive to changes in venous pressure than arteriolar pressure when RA/RV is high.Pcis the major force behind fluid pushing out of the capillary bed and into the interstitium.It is also the most variable of the forces affecting fluid transport at the capillary, partly because sympathetic-mediated arteriolar vasoconstriction varies.

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:ErythromycinAmiodaroneQuinidineMethadoneProcainamideSotalolTerfenadineTricyclic antidepressantsJervell-Lange-Nielsen syndrome (autosomal dominant)Romano Ward syndrome (autosomal recessive)HypothyroidismHypocalcaemiaHypokalaemiaHypomagnesaemiaHypothermiaRheumatic carditisMitral valve prolapseIschaemic heart disease

Starling’s equation for fluid filtration describes fluid flow at the capillary bed.Filtration forces (capillary hydrostatic pressure and interstitial oncotic pressure) stimulate fluid movement out of the capillary, while resorption forces promote fluid movement into the capillary (interstitial hydrostatic pressure and plasma oncotic pressure). Although the forces fluctuate along the length of the capillary bed, overall filtration is achieved.At the capillary bed, there is fluid movement.The reflection coefficient (σ), the surface area accessible (S), and the hydraulic conductance of the wall (Lp) are frequently used to account for the endothelium’s semi-permeability, yielding:Volume / min = LpS [(Pc- Pi) –  σ(πp– πi)]Volume /min = (Pc-Pi) – (πp–πi) describes the fluid circulation at the capillaries.Where:Pc= capillary hydrostatic pressurePi= interstitial hydrostatic pressureπp= plasma oncotic pressureπi= interstitial oncotic pressure

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.

There are 4 classes of haemorrhage. Classification is based on clinical signs and physiological parameters.In CLASS I:Blood loss (ml) is < or = 750Blood loss(% blood volume) < or = 15%Pulse rate (bpm) is 30Pulse pressure is normal or increasedSystolic BP is normalCNS/mental status patient is slightly anxious In CLASS II:Blood loss (ml) is 750 – 1500Blood loss(% blood volume) is 15 – 30%Pulse rate (bpm) is 100 – 120Respiratory rate is 20-30Urine output (ml/hr) is 20-30Pulse pressure is decreasedSystolic BP is normalCNS/mental status patient is mildly anxiousIn CLASS III:Blood loss (ml) is 1500 – 2000Blood loss(% blood volume) is 30- 40%Pulse rate (bpm) is 120 – 140Respiratory rate is 30-40Urine output (ml/hr) is 5-15Pulse pressure is decreasedSystolic BP is decreasedCNS/mental status patient is anxious, confusedIn CLASS IV:Blood loss (ml) is >2000Blood loss(% blood volume) is >40%Pulse rate (bpm) is >140Respiratory rate is >40Urine output (ml/hr) is negligiblePulse pressure is decreasedSystolic BP is decreasedCNS/mental status patient is confused, lethargic

The causes of a prolonged QT interval include:HypomagnesaemiaHypothermiaHypokalaemia HypocalcaemiaHypothyroidism Jervell-Lange-Nielsen syndrome (autosomal dominant)Romano Ward syndrome (autosomal recessive)Ischaemic heart diseaseMitral valve prolapseRheumatic carditisErythromycinAmiodaroneQuinidineTricyclic antidepressantsTerfenadineMethadoneProcainamideSotalol

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).At the conclusion of expiration, the CVP should be measured with the patient resting flat. The catheter’s tip should be at the intersection of the superior vena cava and the right atrium. An electronic transducer is installed and zeroed at the level of the right atrium to measure it (usually in the 4th intercostal space in the mid-axillary line).CVP is a good predictor of preload in the right ventricle. Hypovolaemia is indicated by a volume challenge of 250-500 mL crystalloid eliciting an increase in CVP that is not sustained for more than 10 minutes.CVP is influenced by a number of factors, including:Mechanical ventilation (and PEEP)Pulmonary hypertensionPulmonary embolismHeart failurePleural effusionDecreased cardiac outputCardiac tamponadeCVP is reduced by the following factors:Distributive shockNegative pressure ventilationHypovolaemiaDeep inhalation

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 musclesCO2 and K+ are the most potent vasodilators in cerebral circulation.

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.