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How does a single circulatory system work?
In single circulation, blood passes though the heart once in each complete circuit and blood flows under reduced pressure directly from the gas exchange organs to other organs.
The heart ventricle pumps blood to the gill capillaries, where gas exchange takes place. O2-rich blood converges into a vessel that carries it to capillary beds throughout the body. Blood then returns to the 2-chambered heart.
Mechanical resistance causes blood pressure to drop when blood flows through the capillary beds in the gills, limiting the rate of blood flow through the rest of the body.
Blood pressure stays high enough to circulate the blood though the body, aided by: Muscular contractions during swimming: neutrally buoyant water gravity has little effect on blood flow
A two-chambered heart is sufficient: an atrium that receives blood and a ventricle that pumps it out.
What have land-dwelling vertebrates evolved, and why?
Land-dwelling vertebrates have evolved two separate pumping circuits
Gravity opposes blood flow to elevated parts of the body.
However, the capillaries and alveoli would not be able to withstand the high pressures needed to counteract gravity.
Double circulation provides a vigorous flow of blood to the brain, muscles, etc. because the heart repressurises the blood after it passes through the capillary beds of the gas exchange organs.
- Pulmonary circulation is a lower-pressure circuit to and from the lungs
- Systematic circulation is a higher-pressure circuit to and from the rest of the body
A multichambered heart is required. The pulmonary and systematic circulations are completely separated in the 4-chambered heart of birds and mammals
What happens in double circulation?
In double circulation, there are two blood flow circuits, and two pumps fused into a multi-chambered heart.
The right side of the heart pumps O2-poor blood to the capillary beds of the gas exchange tissues, for net movement of O2 in and CO2 out of the blood. - Pulmonary circuit (lungs only, e.g. mammals, reptiles) or Pulmocutaneous circuit (lungs and skin e.g. amphibians)
The left side of the heart pumps O2-rich blood throughout the tissues of the body, for exchange of O2, CO2, nutrients and waste, and then back to the heart. Systematic circuit.
What type of double circulatory system do amphibians have?
Amphibians have a 3-chambered heart and a pulmocutaneous circuit.
The heart has 2 atria and a ventricle. A ridge in the ventricle diverts most (90%) of the O2-poor blood from the right atrium into the pulmocutaneous circuit, and most of the O2-rich blood from the left atrium into the systematic circuit.
Underwater, blood flow to the lungs is largely shut off, and flow continues to the skin as the only site of gas exchange.
What type of double circulatory system do turtles, snakes and lizards have?
Turtles, snakes and lizards have a 3-chambered heart with an incomplete ventricular septum.
The heart has 2 atria and a single ventricle.
An incomplete septum partially divides the ventricle into left and right chambers, further reducing mixing between O2-rich and O2-poor blood.
Crocodilians have a complete septum and therefore a 4-chambered heart, however the circuits connect where the arteries exit the heart.
A bypass vessel in many reptiles connects the right side of the ventricle directly to the systematic circulation, enabling arterial valves to shunt blood away from the lungs when underwater
What type of double circulatory system do mammals and birds have?
Mammals and birds have a 4-chambered heart.
The heart has 2 atria and 2 completely separated ventricles.
The left side of the heart receives and pumps only O2-rich blood, while the right side receives and pumps only O2-poor blood.
As endotherms, birds and mammals need about 10X more fuel/O2/CO2/waste exchange than comparable ectotherms.
This is made possible by separate and independently powered systematic and pulmonary circulations and by large hearts that pump the necessary volume of blood.
4-Chambered hearts arose independently in the ancestors of birds and mammals (convergent evolution
How does blood flow through a 4-chambered heart?
1. Blood returns to heart from body, enters right atrium.
2. Blood enters right ventricle.
3.Blood is pumped from right ventricle to lungs
4. Blood returns to left atrium from lungs.
5. Blood enters left ventricle
6. Blood is pumped from left ventricle to body.
What is blood pressure?
Blood pressure is the hydrostatic pressure blood exerts on the blood vessel walls.
Blood pressure is highest in the aorta and other arteries, and flows at its fastest rate (48 cm/s) in the aorta.
Blood flow is much slower in the capillaries (0.1 cm/s) because the total cross-sectional area of the capillaries is much greater than in any other part of the circulatory system. This slow flow is essential to allow gas and nutrient exchange to take place by diffusion.
Blood flow speeds up as it enters the venules and veins, as they have a smaller total cross-sectional area than the capillaries.
What problems to animals with long necks face?
Gravity opposes blood flow to elevated body parts, so animals with long necks face particular challenges
In a standing human, blood must be pumped about 35cm above the heart, with a drop in arterial blood pressure of 27mm Hg from a normal systolic pressure of 120mm Hg.
In a giraffe, blood must be pumped 2.5m above the heart, requiring a systolic pressure of more than 250mm Hg near the heart to neable blood to reach the head.
The giraffe's heart can weigh up to 10 kg and can be 60cm in length to generate this presure. 150 beats per minute.
What adaptions are needed to prevent high blood pressure from damaging the brain in tall animals?
One-way valves in the jugular veins prevent blood flowing back into the head.
Pressure sensors in the neck detect any increase in blood pressure and signal to reduce cardiac output.
Muscular artery walls contract to reduce blood flow to the head.
Carotid artery subdivides into a rete mirabile ('wonderful net') in the upper neck. The elastic walls expand to cope with the increased pressure when the head is lowered, and contract when the head is raised.
A branch from the carotid artery directs much blood straight into the vertebral artery before it even reaches the rete mirabile when the head is lowered
What other adaptations are needed to prevent blood pooling in the legs or profuse bleeding on injury for giraffes?
Rhythmic contractions of smooth muscles in the walls of veins and venules aid movement of low pressure blood.
Skeletal muscle contractions during exercise squeeze blood towards the heart.
Change in pressure in the thoracic cavity during inhilation causes the venae cavae and other large veins near the heart to expand and fill with blood.
Tight, tough skin and inner fascia (connective tissue) prevent blood pooling.
All leg vessels are deep inside, and the capillaries that reach the skin are very narrow.
Red blood cells are 1/3 the size if human rbcs - high SA:V means very rapid gas exchange.
What extreme challenges did long necked dinosaurs face?
Necks of up to 10m in length.
Estimated required systolic pressure to pump blood to the brain when fully raised: 760mm Hg
Heart would have needed to be 10-15% of body weight (c.f. 2% in giraffe, 0.5% in human).
Other adaptations needed to counteract pressure in the brain when head lowered.
It has been suggested that these dinosaurs fed close to the ground, rather than on high foliage.
Where are gases, nutrients and wastes exchanged?
Gases, nutrients and wastes are exchanged between the blood and tissues in the capillaries.
Substances cross the endothelium in vesicles (exo/endocytosis), by diffusion (e.g. O2, CO2), or by bulk flow through microscopic pores (fluid, sugars, salts, urea).
Hydrostatic pressure tends to drive fluid out of capillaries.
Osmotic pressure tends to drive fluid into the capillaries, because the concentration of non-penetrating solutes in the blood is higher than that in the interstitial space.
Blood proteins (albumin) maintain an almost constant osmotic pressure from arteriole end to venule end.
Fluid not recovered by the capillaries is transported out of the lymph.
What does the lymphatic system do?
The lymphatic system drains excess interstitial fluid not reclaimed by the capillaries.
The remaining fluid diffuses into the lymphatic system - a network of vessels that intermingle with the capillaries.
Rhythmic contractions of the vessel walls help to draw fluid in, and skeletal muscle contractions help fluid flow.
Valves prevent backflow.
Lymph drains into large veins at the base of the neck.