My integer for my player health seems to be only returned once

I can't get my health sprite to change when my player health is at a certain level. I am really not sure how to fix it or explain what is going on. When I run my game, it shows on the console that my health is full. However when I take one damage it doesn't show that my health has lowered.

Code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 using System;
 
 public class HealthSystem
 {
     public int health;
     public int maxHealth;
     public Image hpImage;
     private Sprite hp;
 
     public HealthSystem(int maxHealth)
     {
         this.maxHealth = maxHealth;
         health = maxHealth;
     }
     public int GetHealth()
     {
         return health;
     }
 
     public void Damage(int damageAmount)
     {
         health -= damageAmount;
         if (health < 0)
         {
             health = 0;
         }
     }
     public void Heal(int healAmount)
     {
         health += healAmount;
         if (health > maxHealth)
         {
             health = maxHealth;
         }
     }
     
 }
 ______________________________________________________________________________________________________
 public class Player_UI : MonoBehaviour
 {
     private bool plusOneDmg;
     private bool healOneHeart;
     private bool padHit = false;
     public Image healthImg;
 
 
     HealthSystem healthSystem = new HealthSystem(4);
 
     private void Start()
     {
         
 
         Debug.Log("Health:" + healthSystem.GetHealth());
 
 
     }
     private void Update()
     {
     
     }
 
 
     private void OnCollisionEnter2D(Collision2D collision)
     {
         if (collision.gameObject.tag == "Lava" && padHit == false)
         {
             healthSystem.Damage(1);
             Debug.Log("Health:" + healthSystem.GetHealth());
             padHit = true;
         }
         else if (collision.gameObject.tag == "HealCube" && padHit == false)
         {
             healthSystem.Heal(1);
             Debug.Log("Health:" + healthSystem.GetHealth());
             padHit = true;
         }
     }
     private void OnCollisionExit2D(Collision2D collision)
     {
         if (collision.gameObject.tag == "Lava" && padHit == true)
         {
             padHit = false;
         }
         else if (collision.gameObject.tag == "HealCube" && padHit == true)
         {
             padHit = false;
         }
     }
 }

 [RequireComponent(typeof(Image))]
 public class HealthBar : MonoBehaviour
 {
     public Sprite Hp4, Hp3, Hp2, Hp1, Hp0;
     public Image image;
 
     HealthSystem healthSystem = new HealthSystem(4);
     private void Start()
     {
 
         hpBar();
 
         image = gameObject.GetComponent<Image>();
 
 
     }
     private void Update()
     {
         
     }
     private void hpBar()
     {
 
 
         if (healthSystem.health == healthSystem.maxHealth)
         {
             Debug.Log("FullHp");
             image.sprite = Hp4;
         }
         if (healthSystem.health == 3)
         {
             Debug.Log("LessHp");
             image.sprite = Hp3;
         }
         if (healthSystem.health == 2)
         {
             Debug.Log("LesserHp");
             image.sprite = Hp2;
         }
         if (healthSystem.health == 1)
         {
             Debug.Log("EvenLesserHp");
             image.sprite = Hp1;
         }
         if (healthSystem.health == 0)
         {
             Debug.Log("NoHp");
             image.sprite = Hp0;
         }
     }
 }