a=2;b=1;c=1;d=2;e=1;f=0.5;
x=-1:0.01:2.5; y=0:0.02:5;
[X,Y]=meshgrid(x,y);
U=-a*X+b*Y+c;;
V=d*Y-e*X.*Y-f;
cvals=[0 0];
hold on
contour(x,y,U,cvals,'k--');
contour(x,y,V,cvals,'k');
[T1,Y1] = ode45('insu18',[0 100],[2.5 .7]);
[T2,Y2] = ode45('insu18',[0 100],[.5 2.5]);
[T3,Y3] = ode45('insu18',[0 100],[2.5 1]);
[T4,Y4] = ode45('insu18',[0 100],[0.1 .3]);
option=odeset('AbsTol',1e-9,'RelTol',1e-9);
plot(Y1(:,1),Y1(:,2),'b','LineWidth',1);
plot(Y2(:,1),Y2(:,2),'r','LineWidth',1);
plot(Y3(:,1),Y3(:,2),'r','LineWidth',1);
plot(Y4(:,1),Y4(:,2),'b','LineWidth',1);
xlim([0 2.5]);
title('Insulin-glucose interaction','FontSize',12)
xlabel('insulin'),
ylabel('glucose');
ylim([0 4]);