6.2 ~ The Blood system

Plasma

• The liquid part of the blood

• Leaks out from the capillaries to form the tissue fluid

• Tissue fluid contains O2, glucose and other parts of the plasma except large protein molecules

• Movement of tissue fluid in the capillary bed is determined by the balance of the hydrostatic and osmotic pressure

Osmotic and hydrostatic pressure

• The pressure due to osmosis

• Pressure is exerted on the wall of the vessel due to the fluid inside (called hydrostatic pressure)

• Due to high pressure in the vessel at the rate rial end, tissue fluid is forced out of the capillaries

• Tissue fluid surrounds cells within a tissue (intersitium)

  • Substances are exchanged between the tissue fluid and the cells in the tissue

• Hydrostatic pressure is lower at the venous end of the of the capillary bed

  • Due to osmotic pressure, tissue fluid then re enters capillary at the venous end of the capillary bed

• Capillary permeability changes between tissues, over time, and in response to different signalling molecules

Arteriole end

Hydrostatic pressure > osmotic pressure

= tissue fluid moves out of the capillary

Venule end

Hydrostatic pressure < osmotic pressure

= tissue fluid moves into capillary

Blood pressure

• Drops significantly as it passes through capillary bed

  • Therefore, blood pressure in the veins is much lower and doesn’t travel as fast

• Because blood is at lower pressure, the walls do not need to be as thick as the arteries - they have less smooth muscles and elastin fibres in the walls

• Venous return is facilitated by skeletal muscle contractions: veins run close to muscles so, as they contract, the muscles squeeze on the adjacent veins and act like a pump

Blood pressure in veins

• Is, at times, so low that it could actually flow backwards and, therefore, impact the return of the blood to the heart

• To prevent back flow, the veins have valves made up of 3 cup-shaped flaps of tissue

• The valve only opens one way (towards heart), ensuring unidirectional flow of blood

• The valve only opens one way (towards heart), ensuring unidirectional flow of blood:

  • If blood is flowing towards the heart, the flaps of the valve are pushed against the wall of the veins and out of the way

  • If blood is flowing backwards, it gets caught in the flaps of the valve which fill up and block the lumen of the vein, thus preventing back flow

Arteries

• Carry blood away from the heart to the organs

• High pressure: peak pressure is called systolic and low pressure diastolic

• Small lumen diameter: can be greater than 10mm

• Generally have thicker walls with narrow lumens

• 3 main layers

  • Tunica interna (endothelium lining the inside of the artery)

  • Tunica media (smooth muscle and elastin fibres)

  • Tunica adventita (tough connective tissue)

• No valves

Muscles and elastic fibres in arteries

• Smooth muscles + elastin fibres - smooth blood flow

• Elastin fibres in the wall of the artery stretch when blood passes into the vessel, storing elastic energy - recoil helps blood propel down the artery

• Smooth muscle helps control the diameter for the vessel-smaller pressure will increase blood pressure and velocity of blood flow

• Enables the withstanding of high pressure without bursting or bulging

Veins

• Carry blood back to the heart

• Pressure lower than arteries; slower blood flow

• Large lumen diameter - can be greater than 10mm

• Generally have thinner walls with larger lumens

• 3 main layers

  • Tunica interna (endothelium lining the inside of the artery)

  • Tunica media (smooth muscle and elastin fibres)

  • Tunica adventita (tough connective tissue)

• Thin muscle & elastic fibre compared to arteries

• Have valves (at interval) + skeletal muscles next to veins

Capillaries

• Exchange of materials with other cells

• Transport blood through almost all tissues in the body - all active body cells are close to a capillary

• Low pressure

• Lumen about 5um

• Walls with thick layer of endothelium with pores between cells - very permeable

• Leaky walls which forms tissue fluid around them - allows exchange of materials

• 1 layer - 1 cell thick

• No muscles and elastic fibres

• Have no valves

Separate circulation for the lungs

• Humans (and other mammals) have a double circulation

  • One pumps blood from the heart to the body (systemic circulation)

  • Other pumps blood from heart to the lungs (pulmonary circulation)

Reasons for separate circulation

• Lungs are very close to the heart and so do not need blood pumped at high pressure as that needed for the body

• After passing through the lungs, the blood is going to be at very low pressure

  • Therefore needs to pass back through the heart to be pumped at high pressure around the body

• The heart is therefore divided into 2 halves; the left side pumps blood around body’s dn the right side pumps blood to the heart

• Two sides separated by a thick septum to prevent the mixing of oxygenated and deoxygenated blood

• Right side: deoxygenated blood vs Left side: oxygenated blood

Heartbeat

• Cardiac cells under go myogenic contractions

  • Generate within the muscle cells and are not caused by inner action by motor neurons

• There are a group of cells in the wall of the right atrium called the sinoatrial (SA) node that initiate the contraction of other cardiac muscle cells

• These cells depolarise which activates adjacent cells to depolarise

  • Therfore, cardiomyocotes in the atria contract synchronously

Membrane potential and depolarisation

• Cell membrane have an electrochemical gradient across them

  • This means that there is a difference in charge inside the cell compared with the outside

• The measure of this difference in charge called the membrane potential

• Most cells have a resting membrane potential of around -70mV

  • This means the inside of the cell is more negative than the outside

• This membrane potential is set up through distribution of positive (e.g. Na+ and K+) and negative (e.g. Cl-) ions

• Depolarisation of a membrane means that positive ions (cations) flood into the cell, making the membrane potential less negative

• Repolarisation is where the cell rebalances the memrbane potential by redistributing positive and negative ions

• Depolorisaton/ repolarisation are controlled by ion channels in the membrane opening and closing

The sinoatrial node

• The SA node initiates the contraction of the cardiac muscle cells and, hence, the heart beat

• Therfore the rate at which the SA node initiates contractions determines the pace of heart beat

  • As such, the SA node acts as a pacemaker

• The SA node sends electrical signals at regular intervals to caus ethe heart to beat around 60-70 times a minute in a normal healthy heart

• An artificial pacemaker can be used if the SA node stops working correctly

Propagation of contraction

• Cardiomyocytes (cardiac muscle cells) are connected by gap junctions and intercalated discs

  • These act like branches, connecting cardiomyocytes to each other

• As cells in the SA node depolarise, cations speed to adjacent cells via the gap junctions and depolarises them

• Thus, this electrical signal spreads through the wall of both atria very rapidly

• There is a short delay (0.1 seconds) before the signal is transmitted to the ventricles

  • This is to allow time for the atria to contract and pump bloo d into the ventricles

• The electrical signal is then transmitted to the walls of the ventricles via the atriventricular (AV) node

  • The AV node transmits the signal first down the inter ventricular septum and then to the cariomyocytes in the walls of the ventricles via fibres called purkinje fibres

• The ventricles then contract, pumping blood out to the arteries

SA key points

• Sinoatrial node is a specialised group of muscle cells located in the right atrium

• SA node generates the hear beat, starting the cardiac cycle

• The SA node sends out electrical signal which stimulates the contractions of heart muscle

• The signal passes through walls of atria to AV node and then through walls of the ventricles

The heart rate

• Can be increased or decreased by impulses brough to the heart through two nerves from the medulla of the brain

• The medulla oblongata (brain stem) is part of the brain involved in controlling autonomic responses (e.g. blinking, breathing, heart rate)

• Two nerves originating from a region in the meduall oblongata called the cardiovascular centre can increase or decrease the heart rate

• They act on the SA node, increasing or decreasing the frequency of the heart beats

• The cardiovascular centre receives inputs from receptors that monitor blood pH and blood pressure

  • Blood pH reflects carbon dioxide concentration

  • Low blood pressure, oxygen concentration or pH all suggest that the heart rate needs to increase blood flow to the tissues

  • High blood pressure, oxygen concentration or pH all suggest that the heart rate needs to slow down

Epinephrine

• Increases the heart rate to prepare for vigorous physical activity

• Epinephrine (adrenaline) also acts on the SA node to increase heart rate

• This hormone is produced in the adrenal glands and its release is controlled by the brain

• This hormone is often called the ‘fight or flight’ hormone as it is secreted in preparation to either attack or run-away both requiring rapid, vigorous physical activity

Diastole blood pressure

• Atria and ventricles relaxed

• Blood flows into heart from veins

• AV valves open

• SL valves closed (heart sounds 2)

Atrial systole

• Atria contract

• Ventricles relax

•  Blood pushed into atria

• AV valves open

• SL valves closed

Ventricular systole

• Atria relaxed

• Ventricles contract

• Blood pushed into arteries

• AV valves closed (heart sond 1)

• SL valves closed 

Atherosclerosis

• Where fatty deposits called at hero mas accumulate in the artery wall

• This is triggered by accumulation of low-density lipoproteins (LDL) which damage the endothelium

• Phagocytes respond to signals from the endothelium and engulf the fatty deposits

  • In turn, these phagocytes then accumulate and smooth muscle cells migrate over the site forming a tough cap

• This is called a plaque

  • Over time, this plaque may grow larger and larger and eventually block or occlude the artery, thus restricting blood flow

• If atherosclerosis occurs in the coronary arteries (the arteries that supply the heart with oxygenated blood) then these arteries may be partly or completely blocked

• The cardiac tissue supplie by the artery will be starved of oxygen and will start to die

  • This leads to a myocardia infraction (heart attack)

Consequences of atherosclerosis

• If atherosclerosis occurs in the coronary arteries (the arteries that supply the heart with oxygenated blood) then these arteries may be partly or completely blocked

• Coronary occlusion is narrowing of the coronary arteries that means oxygen and nutrients cannot adequately get to the cells

  • Anoxia (lack of oxygen) leads to angina (pain in te chest) and increase heart rate as the heart tries to get enough oxygen around the body

• Increased heart rate further increases blood pressure and increases risk of further plaque and clot formation

• Plaques can rupture causing blood clots that can further occlude or block the arteries

• Risk factors for atherosclerosis include:

  • High LDL cholesterol levels

  • High blood pressure

  • High blood glucose levels (due to diabetes or obesity)

  • Consumption of trans fat