Human Body Systems: Circulation, Excretion, and Homeostasis

1. Body Fluids

Our body is composed of approximately 60% water, distributed as follows:

• Intracellular fluids: 40%
• Extracellular fluids: 20%

Interstitial Fluid

All cells in our body are surrounded by fluid to obtain nutrients and expel waste. Interstitial fluids are essential for this process.

2. The Circulatory System

The circulatory system is responsible for transporting nutrients and waste, regulating temperature, and maintaining homeostasis. It consists of:

• Liquid: Blood
• Conduits: Arteries, veins, and capillaries
• Pump: The heart

2.a Blood

Blood is the liquid component of the circulatory system, composed of water, cells, and dissolved substances.

• Blood Plasma: Composed of 92% water, plus nutrients, gases, and waste.

2.b Capillaries

These are tiny vessels connecting veins and arteries. They have single-cell, thin, permeable walls where the exchange of nutrients, gases, and waste takes place.

2.c The Heart

The heart is a muscular organ (myocardium) located inside the rib cage, slightly shifted to the left. Its primary function is to pump blood.

3. Human Circulation

Human circulation is characterized by three main features:

• Double: There are two distinct circuits.
• Complete: Oxygenated blood and deoxygenated blood never mix.
• Closed: Blood remains within vessels unless an injury occurs.

4. Heart Movement

A heartbeat consists of three distinct phases, including systole.

5. The Excretory System

This system consists of mechanisms to remove and eliminate waste products from the body.

5.1 Kidneys

Located in the backside of the abdomen, kidneys are specialized in filtering blood and producing urine.

5.2 Nephrons: Anatomical and Functional Units

1. Filtration: Blood flows into the kidneys and enters the glomerulus under pressure, which filters waste and nutrients into the Bowman's capsule.
2. Reabsorption: Necessary nutrients are reabsorbed into the bloodstream.
3. Return: Clean blood returns to the bloodstream.
4. Excretion: Urine produced by the kidneys travels to the bladder for elimination.

5.3 Sweat Glands

Our skin contains millions of sweat glands that produce sweat, which is similar in composition to urine but less concentrated. They are primarily located in the palms, armpits, and feet. Their functions include producing sweat and regulating body temperature.

Review Questions

Differences Between Arteries and Veins

• Artery: Transports blood from the heart to various body parts under high pressure.
• Vein: Transports blood from various body parts back to the heart under low pressure.

Blood Cells

• Red Blood Cells (RBC): Contain hemoglobin for oxygen transport.
• White Blood Cells (WBC): Essential for immune protection.

Excretion vs. Egestion

Excretion involves the kidneys removing waste products from the blood, while egestion refers to the digestive system removing undigested waste.