Physiology: Cardiac Output and Blood Pressure

Arteries and Veins

• Arteries: Accept blood from the ventricles of the heart.

• Arterioles: Small arteries (control flow)

• Micro circulation: connects arterioles to venules

• Venuels: small veins, collect blood from micro circulation

• Veins: delivers blood to the atria of the heart.

Systemic vs Pulmonary

Pulmonary

• circulculation from the right side of the heart and gains oxygen from the lungs

• Pulmonary arteries have deoxygenated blood

• Pulmonary veins have oxygenated blood

Systemic:

• Delivers O2 to the rest of the body

• Systemic arteries have oxygenated blood

• Systemic veins have deoxygenated blood

Mean Arterial Pressure:

• the arterial pressure averaged over the cardiac cycle

• Pulse pressure: Difference in systolic and diastolic pressure.

• Diastolic pressure + 1/3(Pulse Pressure)

Blood Pressure

• Diastolic pressure: 80mmHg - relaxation of the heart

• Systolic pressure: 120mmHg - contraction of the heart

• Blood Pressure: systolic/diastolic

Cardiac Output

• Heart rate: Number of contractions per unit time (70 beats/min)

• Stroke Volume: Volume pumped by a ventricle in one contraction (70-80ml)

• Cardiac Output = Heart Rate x Stroke Volume pumped

Frank-Staring law

• Increased venous return stretches the ventricle and increases force production until cardiac output matches venous return.

Heart Beat (Electrical signal)

1. Action Potential (AP) starts at the SA node

2. AP conducted through atrial muscle

3. AP is delayed at the AV node before entering the Bundle of His. (Conduction through the Bundle of His and Purkinje fibres is extremely rapid).

4. The ventricle depolarise from endo to epicardium and from apex to base.

Autorhythmicity

• Some cells have an intrinsic rhythmicity which generates a pacemaker potential.

Cardiac Pacemakers

• The sinoatrial node - fastest pacemaker (90-100 beats/min), is the normal pacemaker

• The atrioventricular node - (40-60 beats/min)

• Bundle of His - (15-30 beats/min)

Neural Control of Heart Rate

• Agentd that alter heart rate are chronotopic

• Positive chronotropic agents increase heart rate. (Adrenaline + Noradrenaline act on B-adrenergic receptors on the heart)

• Negative Chronotropic agents slow the heart rate. (Acetylcholine cts on M-cholinergic receptors on the heart).

Heart Valves

• Two valves between the atria and ventricles: atrioventricular valves.

• Valves between the ventricles and arteries: semilunar valves

• Heart valves prevent back flow

Venticricular Pressure - ECG

• After the P wave atrial contraction lightly increases ventricular pressure

• Following the QRS complex ventricular contraction greatly increases ventricular pressure.

• Sound one occurs during ventricular pressure rise

• Sound 2 occurs during fall in ventricular pressure.

Ventricular Volume

• During Diastole the ventricle fill with blood.

• Atrial systole adds more blood

• Isovolumic contraction: when all valves are closed.

Transport across capillaries

• lipid soluble substance pass across the cell membranes (O2, CO2, ethanol)

• Small-medium sized water soluble substances diffuse between endothelial cells. (Water, Na+, Cl-, Ca+, ure, glucose, ADH, insulin).

• Large substances can be moved by pinocytosis.

Plasma Filtration

• Fluid (hydrostatic) pressure inside the capillaries pushes plasma out.

• Plasma proteins stay in capillary so total solute concentration is higher in plasma than in interstitial fluid.

• Colloidal osmotic pressure pulls interstitial fluid in

Plasma Osmotic Pressure

• Total osmotic pressure of blood includes all solutes including salts: 6000mmHg. (Both interstitial fluid and plasma)

• Colloid osmotic pressure of blood is due to the osmotic activity of proteins mostly albumin = 25mmHg. (Plasma)

Fluid Pressure

• Fluid pressure difference between the capillary (Pc) and interstitial fluid (Pif) pushes fluid out

• Oncoming (colloid osmotic) pressure between the capillary (πp) and interstitial fluid(πif), pulls fluid in.

Starling Equilibrium

Increased filtration vs Increased absorption

Increased filtration:

• vasodilation

• Arterial Hypertension

• Venous Hypertension

• Increased plasma leakage

• Plasma protein deficiency

Increased Absorption

• Vasoconstriction

• Arterial hypotension

• Venous Hypotension

• Dehydration

• Haemorrhage