Topic 1

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Heart & Circulation

The main purpose of the circulatory system is to move substances around the body. Multicellular organisms rely on a mass transport system to move substances by mass flow.

They are too large for diffusion to move substances fast enough

Substances move in unidirectional tubes due to pressure

Open Circulatory System - Insects

A heart pumps blood into cavities surrounding organs and substances can diffuse between the blood and cells. The heart muscles relax and blood is drawn back into the heart through small valve openings.

Closed Circulatory System

Blood is enclosed within blood vessels. This generates higher pressure by flowing in narrow channels. Blood travels faster so is more efficient at delivering substances. Animals with closed circulatory systems have single or double circulation.

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The Cardiac Cycle

Chambers of the heart alternatively contract (systole) and relax (diastole) in a rhythmic cycle. One complete sequence of filling and pumping blood is a cardiac cycle or a heartbeat.

There are 3 phases

Phase 1: Atrial Systole

Blood returns to the heart due to skeletal and muscular actions. Blood under low pressure flows into left and right atria from pulmonary veins and vena cavae. Increasing pressure in atrioventricular valves pushes them open and blood flows into ventricles. Atrial walls contract to force more blood into ventricles.

Phase 2: Ventricular Systole

Ventricles contract from the base upwards, increasing pressure which forces semilunar valves open and pushes blood up and out through pulmonary arteries and aorta. The pressure of blood against atrioventricular valves closes them and prevents backflow.

Phase 3: Cardiac Diastole

Atria and ventricles then relax during diastole, elastic recoil of the heart walls lowers pressure here. Blood under high pressure in aorta and pulmonary arteries drawn back to ventricles, closing the semilunar valves and preventing more backflow.

Pressure changes and valves determine the flow of blood here

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Blood Clotting Cascade

Blood clots when in contact with air

This process reduces the risk of infection and loss of blood

Internal blood clots are dangerous - can cause blocked vessels

The process

Platelet breakdown and tissue damage causes thromboplastin to be released (a protein molecule) to be released into the blood

Thromboplastin converted to prothrombin, this is soluble

prothrombin converted to thrombin which is an insoluble enzyme

This causes the release of fibrinogen which is converted to fibrin, a large scale insoluble. This forms a mesh cover which traps substances such as red blood cells forming a blood clot.

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Structure of Blood Vessels

Arteries

Thick elastic walls - stretch to accomodate the blood in systole . Thick walls can withstand the high pressure as blood is forced against the walls

Capillaries

Veins