8 Transport in mammals

The circulatory system

It’s called closed circulation as the blood remains within blood vessels.

Double Circulation

DOUBLE CIRCULATION
SYSTEMIC CIRCULATION	PULMONARY CIRCULATION
left ventricle → AORTA → body (except lungs) → VENACAVA	right ventricle → PULMONARY ARTERIES → lungs → PULMONARY VEIN→ left atrium

Detailed Diagram of veins and arteries

Outline the structre of the artery

• transports oxygenated blood at high pressures to tissues

• pulmonary artery and aorta have semilunar valves in the beginning

• tunica intima/interna – very smooth, single layer of flat cells

• tunica media – smooth muscle, collagen fibres, elastic fibres

• tunica media contains large amounts of elastic fibres to allow the artery wall to stretch as blood surges through at high pressure

• tunica media is the thickest in arteries

• tunica externa – collagen fibres, elastic fibres

• depending on the pressure, thickness of arteries’ walls

  differs

• artery wall can recoil inwards if the pressure drops

• as blood at high pressure enters, it can widen,

  reducing pressure slightly and vice versa

• arteries branch out into arterioles

• arteriole walls have more smooth muscle which can contract, narrowing the diameter and reducing blood flow

Outline the structre of the vein

• tunica intima – flat cells, smooth and not crinkly

• tunica media – smooth muscle, collagen, and elastic fibres, tunica media in veins is thin

• tunica externa – elastic and collagen fibres, the thickest in veins

• blood is transported at low pressures, no need for thick walls

• contain semi-lunar valves (formed from their endothelium)

• large lumen

• irregular shape

Outline the structure of capillaries

• takes blood really close to cells allowing exchange of materials

• network of capillaries is called the capillary bed

• wall made of endothelial cells and is one cell thick

• gaps are present between individual cells that form the endothelium

• gaps allow some components of blood to seep through into spaces between cells (tissue fluid)

What are blood plasma & tissue fluid

• as blood flows through capillaries within tissues, some plasma leaks out due to the pressure and seeps out into places between the cells of the tissues

• this plasma that leaks out is called tissue fluid

• if blood pressure is too high, too much fluid may be forced out of capillaries and the fluid may accumulate, this results in oedema

• it’s through tissue fluid that the exchange between cells and blood occurs

Outline the lymphatic system

• it is the drainage system

• it is digestive (assimilation of fatty acids)

• immunity – produces lymphocytes

• lymphatics are tiny, blind-ended vessels

• they contain valves which allow tissue fluid to flow in but not out

• walls are wide enough to allow larger protein molecules to pass through

• fluid inside lymphatics is called lymph

• lymph is transported to subclavian vein

• lymph vessels have smooth muscle in their walls which contract to push lymph along

Outline the red blood cells (erythrocytes)

• contain haemoglobin which gives red colour and transports oxygen

• produced in the bone marrow

• have a biconcave, disc shape – dent increases surface area in relation to volume

• spongy and flexible – have specialised cytoskeleton made of protein filaments that allow them to be squashed

• have no nucleus, endoplasmic reticulum, mitochondria – more space for haemoglobin, maximising amount of oxygen which can be carried

• broken down in spleen
  

Outline the platelets (thrombocytes)

Outline the structre of the heart

• consists of 2 atria and 2 ventricles

• right and left side separated by septum

• made of cardiac muscle

• papillary muscles contract to pull on valve tendons to prevent inversion of valves during systole

• atria and ventricles have valves between them called atrioventricular valves: RIGHT SIDE – TRICUSPID, LEFT SIDE – BICUSPID / MITRAL

• 2 types of valves: ATRIOVENTRICULAR – TENDONS, SEMI-LUNAR – POCKETS

Outline the cardiac cycle

• The cardiac cycle is the series of events that take place in one heart beat, including muscle contraction and relaxation
  

  • The contraction of the heart is called systole, while the relaxation of the heart is called diastole

• One cardiac cycle is followed by another in a continuous process

  • There is no gap between cycles where blood stops flowing

Outline the process of Atrial systole

• The walls of the atria contract

  • Atrial volume decreases

  • Atrial pressure increases 

• The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

• Blood is forced into the ventricles

  • There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria

• The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole

Outline the process of Ventricular systole

• occurs about 0.1s after atria contract

  The walls of the ventricles contract

  • Ventricular volume decreases

  • Ventricular pressure increases

• The pressure in the ventricles rises above that in the atria

  • This forces the AV valves to close, preventing back flow of blood

• The pressure in the ventricles rises above that in the aorta and pulmonary artery

  • This forces the semilunar (SL) valves open so blood is forced into the arteries and out of the heart

• During this period, the atria are relaxing; atrial diastole coincides with ventricular systole

  • The blood flow to the heart continues, so the relaxed atria begin to fill with blood again

Outline the process of Ventricular diastole

• The ventricles and atria are both relaxed

• The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

• The atria continue to fill with blood  

  • Blood returns to the heart via the vena cava and pulmonary vein

• Pressure in the atria rises above that in the ventricles, forcing the AV valves open

• Blood flows passively into the ventricles without need of atrial systole

• The cycle then begins again with atrial systole

• force produced in the right ventricle must be relatively small as –

  1.  blood goes only to the lungs which are at a shorter distance + less resistance to overcome

  2. if a too-high pressure was developed, tissue fluid would accumulate in lungs hampering gas exchange

Outline the cardiac cycle

Cardiac muscles are myogenic which means it naturally contracts and relaxes without receiving impulses from a nerve.

1.  SAN (sinoatrial node)/pacemaker sends out waves of excitation which stimulates atria to contract

2. non-conducting tissue between atria and ventricles prevents atria and ventricles from contracting at the same time

3. AVN (atrioventricular node) delaying the impulse allows it to flow from atria into ventricles

4. AVN sends an impulse down to the bundle of His and along purkine fibres

5. this causes ventricles to contract from the base upwards

Outline the Oxygen dissociation curve

• once an O₂ molecule combines with haemoglobin, it becomes easier for more molecules to combine therefore, the curve rises very steeply

• a small change in the partial pressure O₂ causes a very large change in amount of O₂ carried by haemoglobin

Outline the process of the Bohr shift

• The shift in the curve of oxyhaemoglobin due to concentration of CO₂ at a given partial pressure of O₂ is the Bohr effect

• the amount of O₂ haemoglobin carries is affected by the partial pressures of both O₂ and CO₂

• the presence of high partial pressure of CO₂ causing Hb to release O₂ is the Bohr’s effect

  In the cytoplasm of red blood cells, CO₂ is catalysed by carbonic anhydrase enzyme when it reacts with water to form carbonic acid (REFER TO DIAGRAM)

• When the carbonic acid dissociates; haemoglobin combines with H⁺ ions forming haemoglobunic acid (HHb) and releases the O₂ it’s carrying

• Haemoglobin combining with H² ions maintains blood pH as if the ions were left in solution, pH of the blood would’ve been less and turns acidic

• presence of high partial pressures of CO₂ causes haemoglobin to release O₂

• high concentration of O₂ is found in respiring tissues which need O₂

• high concentration of CO₂ causes Hb to release O₂, curve lies below and to the right

• 85% of CO₂ – diffuses out of RBC into blood plasma and are carried in solution

• 5% of CO₂ – CO₂ that hasn’t dissociated and remains as CO₂ dissolves in blood plasma

• 10% of CO₂ – CO₂ diffuses to RBC and combines directly with amine groups (–NH₂) of some haemoglobin molecules forming carbaminohaemoglobin

What is formed when oxygen binds with haemoglobin?

Oxygen + Haemoglobin ⇌ Oxyhaemoglobin

4O₂ + Hb ⇌ Hb4O ₂

• The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind; this is cooperative binding

• The reverse of this process happens when oxygen dissociates in the tissues

Outline the Chloride shift

• The movement of chloride ions into red blood cells that occurs when hydrogen carbonate ions are formed

How are hydrogen carbonate ions formed?

• Carbon dioxide diffuses into red blood cells 

• The enzyme carbonic anhydrase catalyses the combining of carbon dioxide and water to form carbonic acid (H₂CO₃)

CO₂ + H₂O ⇌ H₂CO₃ 

• Carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions

H₂CO₃ ⇌ HCO₃^- + H⁺

What happens to negatively charged hydrogencarbonate ions

• Negatively charged hydrogencarbonate ions formed from the dissociation of carbonic acid are transported out of red blood cells via a transport protein in the membrane

• To prevent an electrical imbalance, negatively charged chloride ions are transported into the red blood cells via the same transport protein

  • If this did not occur then red blood cells would become positively charged as a result of a build up of hydrogen ions formed from the dissociation of carbonic acid

How and where is waste carbon dioxide excreted?

• Waste carbon dioxide produced during respiration diffuses from the tissues into the blood 

• This waste carbon dioxide is transported around the body in different ways

  • In the blood plasma in the form of hydrogen carbonate ions (HCO₃^-); around 85 % of carbon dioxide is transported in this way

  • Around 5 % of carbon dioxide dissolves directly in the blood plasma

  • Bound to haemoglobin as carbaminohaemoglobin; this accounts for around 10 % of carbon dioxide transport in the blood

Outline Carbon Dioxide in the plasma

• Carbon dioxide released as a waste product from respiring cells diffuses into the cytoplasm of red blood cells

• Inside red blood cells, carbon dioxide combines with water to form H₂CO₃

CO₂ + H₂O  ⇌  H₂CO₃

• Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water

  • Without carbonic anhydrase this reaction proceeds very slowly

  • The plasma contains very little carbonic anhydrase hence H₂CO₃ forms more slowly in plasma than in the cytoplasm of red blood cells

• Carbonic acid dissociates readily into hydrogen ions (H⁺) and hydrogen carbonate ions (HCO₃^-)

H₂CO₃  ⇌  HCO₃^– + H⁺

• Hydrogen ions can combine with haemoglobin, forming haemoglobinic acid and preventing the H⁺ ions from lowering the pH of the red blood cell

  • Haemoglobin is said to act as a buffer in this situation

• The hydrogen carbonate ions diffuse out of the red blood cells into the plasma to be transported in solution