Circulation

Circulation 1 

• Every organism exchanges substances with its environment 

• Cellular level of exchange occurs through crossing the plasma membrane 

• Unicellular organisms exchange substances via diffusion:  

• - diffusion plays an important role where distance are short (<100um) 

• Most multicellular organisms require a specialized transport system 

Fricks Law = rate of diffusion is directely proportional to surface area times concentration difference over thickness of membrane

Special transport system in animals = the circulation 

Circulatory system – connects the fluid that surrounds the cells with the organs that exchange gases, absorb nutrients and dispose of waste products 

2 types of circulatory system:  

•all vertebrates and humans have a closed circulatory system called the cardiovascular system consisting of blood, interconnecting vessels (arteries, capillaries, veins) and heart 

Arteries→ arterioles→ capillaries→ venules → veins 

Human double circulation: 

Contains 2 systems -  

1. Pulmonary circulation 

2. Systemic circulation 

Two atria have relatively thin walls & serve as collection chambers of blood returning to the heart. The ventricles have thicker walls and contract more forcefully 

The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle:  

• The contraction is called systole 

• The relaxation and filling of the heart is called diastole 

Right atrium – receives deoxygenated blood from body (systemic circulation)                              

right ventricle – pumps deoxygenated blood to lungs via the pulmonary arteries (pulmonary circulation)                                       

 left atrium – receives oxygenated blood from the lungs via the pulmonary veins (pulmonary circulation)                                                                          

left ventricle – pumps oxygenated blood to the body via aorta (systemic circulation) 

Four heart valves ensure blood flows the correct direction- 

1. Atrioventricular valves  

• Tricuspid: right atrium & right ventricle 

• Mitral (bicuspid): left atrium & left ventricle 

2. Semilunar valves 

• Aortic: left ventricle and aorta 

• Pulmonary: right ventricle & pulmonary trunk

The Heart: 

Autorhythmic- contract and relax repeatedly without any signal      

The heart contains an ‘S-A node’ (sinoatrial or pacemaker), this sets the time and the pace of the cardiac muscle contract. S-A node produces electrical impulses that spread rapidly within the heart tissue-----(can be measured on ECG) 

1. The signals (yellow) from SA node spread through the atria, this can be seen on the beginning of the ECG. 

2. Signals then become delayed at AV nodes, this is seen on the ECG as the flat line 

3. Bundle branches (the orange nerves) pass the signal to the heart apex (bottom of the heart), this is seen as the slight downwards line on the ECG 

4. Finally signals spread through the ventricles, this is the large spick on the ECG 

Heart rate can be controlled by physiological reasons as well as nerve impulses, e.g. 

1. Hormones 

2. Body temperature 

Autonomic nervous system 

Cardiac cycle contains:  

 • 0.4 secs of blood filling in, then 0.1 secs of atrial systole to move the blood into the    ventricles while it relaxes, then 0.3 secs is when the blood is pumped out the heart through ventricle systole and atrial diastole.  

• cardiac output is the volume of blood pumped into the systemic circulation per minute 

•stroke volume is the amount of blood pumped in a single contraction 

•blood pressure is determined by cardiac output and peripheral resistance (the resistance it faces when being pumped into arteries then into capillaries) 

Blood vessel:  

All blood vessels contain a central lumen. This endothelium is smooth and minimises resistances. 

LEFT: ARTERY        RIGHT: VEIN 

Atherosclerosis- deposition of fatty substances in arterial wall, causes heart attack 

Deep vein thrombosis (DVT): pooling of blood in deep veins leading to clot formations, can be caused by long time standing 

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blood flow resistance in narrow diameters of capillaries dissipates much of the pressure 

pulse pressure = systole-diastole 

High resistance & large total cross-sectional area →reduce velocity                                                                                                              

High resistance → reduce pressure 

Pressure is highest in arteries during ventricular systole (systole pressure), while Diastole pressure is the pressure in the arteries during diastole 
 

Homeostatic mechanisms regulate blood pressure by altering the diameter of arteries 

1. Vasoconstriction – contraction of smooth muscles in artery wall (increases pressure) 

2. Vasodilation – relaxation of muscles in arty walls (decrease pressure) 

Circulation 3 

Movement in and out of capillaries: 

•passively move through thin capillary endothelial cell wall 

•actively move by bulk flow through intercellular spaces between endothelial cells 

• active transport in the brain capillaries where there are tight junctions that close intercellular spaces forming the blood-brain barrier 

Fluid exchange: 

Blood pressure tends to drive fluid out of capillaries 

Blood proteins, creating osmotic pressure, tends to pull fluid back into capillaries 

net pressure 32-22=10mmHg - fluid flows out 

blood pressure drops along the capillary, but osmotic pressure stays the same.                                                                 

  Net pressure 15-22=-7mmHg   →fluid flows in 

While we have 10mmHg flowing out but -7mmHg flowing in, we have a Net fluid of 3mmHg. This fluid goes to the lymphatic system. Fluid lost by the capillaries is called lymph. 

Human respiratory system: 

Lungs → 

1. Fill up most of your chest 

2. Pleural membrane with parietal & visceral layers (pleura) 

3. Pleural space is a potential space between these layers containing pleural fluid 

4. Pleural fluid prevents friction and causes layers of pleural membrane to adhere together causing surface tension 

Alveoli → 

3. Form honeycombs creating a large surface area 

4. Surrounded by dense capillary networks 

5. Diffusion rate depends on  

1. Surface area 

2. Distance (short) 

3. Concentration difference (high to low) 

Inhalation is when lungs expand to fill the enlarged thoracic cavity, known as negative pressure breathing. 

The opposite occurs for exhalation, the rib muscle and diaphragm relax and the elasticity of the lungs drives the air out of them. 

control of breathing 
 

Haemoglobin: 

Found in all vertebrate animals and is contained within red blood cells 

It is a protein containing 4 subunits consisting of a polypeptide chain and a haem group 

Each haem group contains an iron atom 

When all sites have been bound to oxygen, haemoglobin is said to be 100% saturated 

Haemoglobin combines reversibly with oxygen and its readiness to do this depends on surrounding oxygen concentration 

Haemoglobin dissociation curve: 

pO2 in the lungs is ~100mmHg (haemoglobin is 98% saturated with O2)                                                                                     

pO2 in tissues at rest is -40mmHg (O2 is unloaded but haemoglobin is still 75% saturated with O2                             

pO2 in tissues during strenuous exercise can be <20mmHg (haemoglobin has given up most of its O2 to tissue and is now <20% saturated with O2) 
 

there is cooperativity between haem subunits, thus when one subunit unloads o2 the others do as well. This is caused by the changes in their shape decreasing affinity 

Carbon dioxide transfer:  

1. 5% as dissolved as carbon dioxide 

2. 5% attach to haemoglobin & other blood proteins – forms carbamino compounds by attaching to terminal NH2 group 

3. 90% as bicarbonate ions