The Blood System

Draw a diagram of the heart

Chambers

• Two atria (singular = atrium) – smaller chambers near top of heart that collect blood from body and lungs

• Two ventricles – larger chambers near bottom of heart that pump blood to body and lungs [Left ventricle has a thicker myocardium [=wall of heart]]

Heart Valves

• Atrioventricular valves (between atria and ventricles) – bicuspid valve on left side ; tricuspid valve on right side

• Semilunar valves (between ventricles and arteries) – aortic valve on left side ; pulmonary valve on right side

Blood Vessels

• Vena cava (inferior and superior) feeds into the right atrium and returns deoxygenated blood from the body

• Pulmonary artery connects to the right ventricle and sends deoxygenated blood to the lungs

• Pulmonary vein feeds into the left atrium and returns oxygenated blood from the lungs

• Aorta extends from the left ventricle and sends oxygenated blood around the body

Identify components of a Dissected Sheep heart

Outline William Harvey’s discovery regarding the circulation of blood

• Arteries and veins were part of a single connected blood network (he did not predict the existence of capillaries however)

• Arteries pumped blood from the heart (to the lungs and body tissues)

• Veins returned blood to the heart (from the lungs and body tissues)

Outline the double-circulatory system

The human heart is a four chambered organ, consisting of two atria and two ventricles

• The atria act as reserviors, by which blood returning to the heart is collected via veins (and passed on to ventricles)

• The ventricles act as pumps, expelling the blood from the heart at high pressure via arteries

The reason why there are two sets of atria and ventricles is because there are two distinct locations for blood transport

• The left side of the heart pumps oxygenated blood around the body (systemic circulation)

• The right side of the heart pumps deoxygenated blood to the lungs (pulmonary circulation)

There is therefore a separate circulation for the lungs (right side of heart) and for the rest of the body (left side of heart)

• The left side of the heart will have a much thicker muscular wall (myocardium) as it must pump blood much further

Define the Systole

• Blood returning to the heart will flow into the atria and ventricles as the pressure in them is lower (due to low volume of blood)

• When ventricles are ~70% full, atria will contract (atrial systole) after signal from SA node, increasing pressure in the atria and forcing blood into ventricles

• As ventricles contract, ventricular pressure exceeds atrial pressure and AV valves close to prevent back flow (first heart sound)

• With both sets of heart valves closed, pressure rapidly builds in the contracting ventricles (isovolumetric contraction = No blood is ejected, and blood volume in ventricles remains constant)

• When ventricular pressure exceeds blood pressure in the aorta and pulmonary artery, the aortic/pulmonary valve opens and blood is released into the aorta/pulmonary artery

Define the Diastole

• As blood exits the ventricle and travels down the aorta/PA, ventricular pressure falls

• When ventricular pressure drops below aortic/PA pressure, the aortic/PA valve closes to prevent back flow (second heart sound) = End of systole/Beginning of Diastole

• When the ventricular pressure drops below the atrial pressure, the AV valve opens and blood can flow from atria to ventricle

  • Ventricles relax with all valves closed = Drop in pressure, no change in volume

• Throughout the cycle, aortic pressure remains quite high as muscle and elastic fibres in the artery wall maintain blood pressure

• Ventricular filling begins when Ventricular pressure drops below atrial pressure = Opens AV valve

  • Passive flow of blood from Atria to ventricles [as in first stage of systole]

Outline pressure changes during systole and diastole

1. Atrial Contraction (Ventricular filling)

2. AV valve closes

3. Isovolumetric Contraction of Ventricles

4. Semi-Lunar Valves open (Ventricular pressure exceeds aortic/PA pressure)

5. Ventricular ejection into Aorta/Pulmonary Artery

6. Semi-Lunar valves close

7. Isovolumetric Relaxation of the Ventricles (Atrial Filling)

8. AV valve opens (once Atrial pressure > Ventricular)

9. Passive filling of Ventricles

(repeat)

Outline the specialized structure of arteries to carry blood at high pressure away from the heart

• They have a narrow lumen (relative to wall thickness) to maintain a high blood pressure (~ 80 – 120 mmHg)

• They have a thick wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure

• The arterial wall also contains an inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract and stretch)

Outline how muscle and elastin fibres aid in maintaining pressure between pumps

The muscle fibres help to form a rigid arterial wall that is capable of withstanding the high blood pressure without rupturing

• Muscle fibres can also contract to narrow the lumen, which increases the pressure between pumps and helps to maintain blood pressure throughout the cardiac cycle

The elastic fibres allow the arterial wall to stretch and expand upon the flow of a pulse through the lumen

• The pressure exerted on the arterial wall is returned to the blood when the artery returns to its normal size (elastic recoil)

• The elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between pump cycles

Outline the specialized structure of veins to carry blood at low pressure to the heart atria

• They have a very wide lumen (relative to wall thickness) to maximise blood flow for more effective return

• They have a thin wall containing less muscle and elastic fibres as blood is flowing at a very low pressure (~ 5 – 10 mmHg)

• Because the pressure is low, veins possess one-way valves to prevent backflow and stop the blood from pooling at the lowest extremities

What’s the relationship between skeletal muscle and valves of veins

Veins typically pass between skeletal muscle groups, which facilitate venous blood flow via periodic contractions

• When the skeletal muscles contract, they squeeze the vein and cause the blood to flow from the site of compression

• Veins typically run parallel to arteries, and a similar effect can be caused by the rhythmic arterial bulge created by a pulse

Outline the function of capillaries

The function of capillaries is to exchange materials between the cells in tissues and blood travelling at low pressure (<10mmHg)

• Arteries split into arterioles which in turn split into capillaries, decreasing arterial pressure as total vessel volume is increased

• The branching of arteries into capillaries therefore ensures blood is moving slowly and all cells are located near a blood supply

• After material exchange has occurred, capillaries will pool into venules which will in turn collate into larger veins

Outline the specialized structure of capillaries to perform material exchange

• They have a very small diameter (~ 5 µm wide) which allows passage of only a single red blood cell at a time (optimal exchange)

• The capillary wall is made of a single layer of cells to minimise the diffusion distance for permeable materials

• They are surrounded by a basement membrane which is permeable to necessary materials

Explain how capillary structure may vary based on location in the body and its role

• The capillary wall may be continuous with endothelial cells held together by tight junctions to limit permeability of large molecules

• In tissues specialised for absorption (e.g. intestines, kidneys), the capillary wall may be fenestrated (contains pores)

• Some capillaries are sinusoidal and have open spaces between cells and be permeable to large molecules and cells (e.g. in liver)

Explain material exchange in the capillaries

• Hydrostatic Pressure → Pressure exerted by blood on capillary walls (or fluid on cell walls)

  • Higher within capillaries at arterial end (hence fluid moves out)

  • Hydrostatic Pressure > Osmotic at arterial end

• Osmotic Pressure → Pressure to move water towards/into itself

  • E.g. Movement of water into cell/capillary

How to identify vessels

• Arteries have thick walls composed of three distinct layers (tunica)

• Veins have thin walls but typically have wider lumen (lumen size may vary depending on specific artery or vein)

• Capillaries are very small and will not be easily detected under the same magnification as arteries and veins

What is the cause of Coronary Occlusion

Atherosclerosis is the hardening and narrowing of the arteries due to the deposition of cholesterol

• Atheromas (fatty deposits) develop in the arteries and significantly reduce the diameter of the lumen (stenosis)

• The restricted blood flow increases pressure in the artery, leading to damage to the arterial wall (from shear stress)

• The damaged region is repaired with fibrous tissue which significantly reduces the elasticity of the vessel wall

• As the smooth lining of the artery is progressively degraded, lesions form called atherosclerotic plaques

• If the plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow

• If the thrombus is dislodged it becomes an embolus and can cause a blockage in a smaller arteriole = Blockage of coronary arteries

Define Coronary Arteries

• Coronary arteries are the blood vessels that surround the heart and nourish the cardiac tissue to keep the heart working

• If coronary arteries become occluded, the region of heart tissue nourished by the blocked artery will die and cease to function

What are the risk factors of coronary heart disease [A Goddess]

• Age – Blood vessels become less flexible with advancing age

• Genetics – Having hypertension predispose individuals to developing CHD

• Obesity – Being overweight places an additional strain on the heart

• Diseases – Certain diseases increase the risk of CHD (e.g. diabetes)

• Diet – Diets rich in saturated fats, salts and alcohol increases the risk

• Exercise – Sedentary lifestyles increase the risk of developing CHD

• Sex – Males are at a greater risk due to lower oestrogen levels

• Smoking – Nicotine causes vasoconstriction, raising blood pressure

What are the consequences of Coronary Occlusion

Atherosclerosis can lead to blood clots which cause coronary heart disease when they occur in coronary arteries

• Myocardial tissue requires the oxygen and nutrients transported via the coronary arteries in order to function

• If a coronary artery becomes completely blocked, an acute myocardial infarction (heart attack) will result

• Blockages of coronary arteries are typically treated by by-pass surgery or creating a stent (e.g. balloon angioplasty)

What does it mean when ‘Heart contraction is myogenic’

The signal for cardiac compression arises within the heart tissue itself

• In other words, the signal for a heart beat is initiated by the heart muscle cells (cardiomyocytes) rather than from brain signals

Explain the Electrical Conduction of the heart

• The sinoatrial node sends out an electrical impulse that stimulates contraction of the myocardium (heart muscle tissue)

• This impulse directly causes the atria to contract and stimulates AV node at the junction between the atrium and ventricle

• This second node – the atrioventricular node (AV node) – sends signals down the septum via a nerve bundle (Bundle of His)

• The Bundle of His innervates nerve fibres (Purkinje fibres) in the ventricular wall, causing ventricular contraction

What are the Accelerator and Decelerator nerves [Brain’s role in heart rate regulation]

• Nerves running from medulla (brain stem. Most primitive part of brain, regulating autonomic functions] to Sino-Atrial node

• Stimulation of Accelerator - Increases heart rate

  • releases the neurotransmitter noradrenaline (a.k.a. norepinephrine)

• Stimulation of Decelerator - Lowers heart rate

  • Release of neurotransmitter acetylcholine

What else may control the heart rate

• Nerve signals from the brain can trigger rapid changes, while endocrine signals can trigger more sustained changes

• Changes to blood pressure levels or CO concentrations (and thereby blood pH) will trigger changes in heart rate

How is heart rate affected by adrenaline

• The hormone adrenaline (a.k.a. epinephrine) is released from the adrenal glands (located above the kidneys)

• Adrenaline increases heart rate by activating the same chemical pathways as the neurotransmitter noradrenaline

• Results in : Faster heart rate and glucose metabolism. Draws blood away from surface to core organs, resulting in hyper-awareness

What are the consequences of interference of heart’s pacemakers

The interference of the pacemakers will lead to the irregular and uncoordinated contraction of the heart muscle (fibrillation)

• When fibrillation occurs, normal sinus rhythm may be re-established with a controlled electrical current (defibrillation)

What is blood composed of

Plasma

• Consists mainly of water, which dissolves materials and functions as a transport medium

• Contains electrolytes (minerals that carry a charge), which are important for maintaining fluid balance and blood pH

• Proteins in the blood plasma maintain osmotic potential (albumin), transport lipids (globulin) and help clot (fibrinogen)

• Blood plasma also functions to transport various materials needed by the body and wastes produced by body cells

Red Blood Cells

• Red blood cells (erythrocytes) are responsible for transporting oxygen around the body

• Oxygen is bound to haemoglobin at the lungs and released from the red blood cell at respiring body tissues

Buffy Coat

• The buffy coat is the fraction of a blood sample that contains white blood cells and platelets

• White blood cells (leukocytes) are involved in the body’s immune defence (eliminating infections)

  • Phagocytes - Engulf and destroy foreign particles

  • Lymphocytes - Synthesize antibodies

• Platelets (thrombocytes) are involved in blood clotting (repairing damaged vessels to prevent blood loss)

Outline the composition of blood plasma

Nutrients – needed by cells to make chemical energy (e.g. glucose)

Antibodies – involved in pathogen identification and elimination

Carbon dioxide – waste produced as a by-product of cell respiration

Hormones – chemical messengers that move through the bloodstream

Oxygen – required by body tissues in order to respire aerobically

Urea – compound that is excreted to remove nitrogen from the body

Heat – not a molecule, but still an important component of blood plasma

Mnemonic:  Nacho, Uh!