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Heartbeat

can you tell me a bit about how a heart works

Emma

Answers

Your heart has 4 chambers: 2 atria and 2 ventricles.

jon.ellis

26 April 2011

The walls of these chambers are made of muscle. The chambers have to squeeze in the right order for the heart to pump blood correctly with each heartbeat.

jon.ellis

26 April 2011

A normal heartbeats involves an electrical impulse from the sinoatrial node (SA node), 60-80 a minute at rest. This controls the heart rate. Each impluse spreads across both atria, and makes them contract and pump blood through one-way valves into the ventricles.

jon.ellis

26 April 2011

This electrical impluse goes to the atrioventricular node (AS node), which acts like a junction box. A think band of conducting fibres called the atrioventricular bundle acts like wires and carried the impluse from the AV node to the ventricles.

jon.ellis

26 April 2011

The AV bundle has a right and left branch, and split into tiny fibres (Purkinje system) which conducts the electrical impulse through the ventricles, making them contract and pump blood through one way valves into large arteries.

jon.ellis

26 April 2011

The heart then rests (diastole) for a short period, with blood returning form the large veins filling the atria. The veins coming into the left atria bring blood from the lungs (which are full of oxygen) and right atria bring blood from the body (requiring oxygen).

jon.ellis

26 April 2011

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help with human biology

Emma

Muscles

why do muscles have to work in pairs?

Emma

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What's the difference between cell mediated response and humoral response? are they both a type of specific immune response?

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which type of blood vessel in the circulatory system has a pulse?

Emma

How does tissue fluid return back into the circulatory system? ( AQA AS biology)

Q - How does tissue fluid return to the circulatory system?

In my textbook, it says the main reason is that it re-enters the capillaries by the loss of hydrostatic pressure within them, so by the time the blood gets to the venous end of the capillary the hydrostatic pressure is less within the capillary than outside them in the tissue fluid, so the tissue fluid is forced back into the blood in the capillary. However, when I look at mark schemes it says that it mainly re-enters by osmosis? How does it move in by osmosis? (I know that the remaining moves back into the bloodstream via the lymphatic system)

Answer

As the blood moves through the capillary it is getting further from the heart and pressure is being split between branches. This means hydrostatic pressure, and the force that attempts to push the fluid through the walls, decreases, However, osmotic pressure (basically how big the gradient of water potentials is) must get higher than hydrostatic pressure to ‘over power’ it. With nutrients moving out with the water, this means water potential would hardly be affected. However, there are special proteins in the plasma of the blood called plasma proteins. Being soluble (unlike may proteins) they alter water potential and being large (like many proteins) they are unable to cross the semi-permeable cell membranes of the capillary wall. As water moves out, the concentration of these proteins increases, and the water potential in the capillaries decreases. The gradient of water potential from outside to inside gets higher, as does the osmotic pressure. Eventually, osmotic pressure is higher than hydrostatic pressure, and fluid is forced back in! Any excess is returned via the lymphatic system, entering dead-ended lymphatic capillaries. This system of vessels eventually drains the excess fluid back into the circulatory system.

Emma